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We are available 24 hours a day, 7 days a week to alert you to the potential for significant weather events dealing with heavy rainfall or snowfall, to discuss quantitative precipitation forecasts and model differences relating to general weather and precipitation forecasts, and to provide forecast guidance into the medium range period (days 3 to 7). Most of the forecasters at the WPC have extensive experience at quantitative precipitation, heavy snow and medium range forecasting. </font> </td></tr> </table> </center> <p> <hr><a NAME="qpf"></a> <center> <table border=0 width="90%"> <tr><td> <h3> <font color="black">Quantitative Precipitation Forecasts (QPFs)</font></h3> <center><b><a href="webinar/presentation_html5.html">WPC QPF Program Overview Webinar</a></b></center><br><br> <img src="../qpf/fill_94qwbg.gif" height="375" width="500" alt="QPF example" border=0> <p> Forecasters at the WPC and its predecessor organizations have been making Quantitative Precipitation Forecasts since 1960. Quantitative Precipitation Forecasts, or QPFs, depict the amount of liquid precipitation expected to fall in a defined period of time. In the case of snow or ice, QPF represents the amount of liquid that will be measured when the precipitation is melted. Precipitation amounts can vary significantly over short distances, especially when thunderstorms occur, and for this reason QPFs issued by the WPC are defined as the expected "areal average" (on a 20 x 20 km grid) in inches. <p> Methods for producing QPFs are similar to other meteorological forecasts. First, meteorologists analyze the current state of the atmosphere. Then they use model forecasts of pressure systems, fronts, jet stream intensity, etc., to form a conceptual model of how the weather will evolve. The WPC has unique access to the full suite of operational and ensemble model guidance from modeling centers in the U.S., Canada, and Europe (the foreign models are global models, so they also make predictions over the U.S.), including many high-resolution or convection-allowing models that come close to depicting individual thunderstorm cells (or mountaintops in the case of orographic precipitation. WPC also stores output from several consecutive runs of all of these models, allowing for trend analysis of model QPFs. And watching every model every day across the entire continental U.S. domain, WPC forecasters become very attuned to the strengths, weaknesses, and biases of each model. During a given cycle, forecasters determine which models are showing a reasonable amount of precipitation in roughly the correct place and time. Those choices serve as the starting point for QPFs, but forecasters also make manual adjustments based on their experience. <p> WPC forecasters often engage in discussion with some of the 122 local National Weather Service Forecast Offices, 12 River Forecast Centers, the National Water Center, and other national centers such as the Storm Prediction Center and National Hurricane Center. In fact, per a 2005 agreement, the WPC provides the rainfall forecast (known as a rainfall statement) that the National Hurricane Center inserts into each tropical cyclone advisory it issues for the Atlantic and eastern Pacific basins. The WPC is also co-located with NOAA's National Environmental Satellite, Data, and Information Services (NESDIS) Synoptic Analysis Branch (SAB). The SAB provides information on satellite trends which helps refine short range QPFs. All of these factors make WPC forecasts generally more accurate than any individual model, and consecutive versions of WPC forecasts do not differ as much as consecutive model forecasts sometimes do, although the very fine scale detail at any given point may vary more than anticipated - owing to automated techniques that scale the WPC forecast to the very local level. <p> <span style="font-size:11pt;font-weight:bold;">Technical Description of QPFs:</span> <p> The QPF contours (isohyets) are drawn to encompass areal average amounts of 0.01, 0.25 inch, 0.50 inch, 1 inch, 1.50 inches, and 2.00 inches. Any values greater than 2.00 inches are drawn in one-inch increments. It is important to note the valid time period when viewing each product. Specifically, for the Day 1, 2, and 3 forecasts, QPFs are manually created for 6-hour periods and an accumulated 24-hour total QPF is also issued. For Days 4-7, forecasters manually create QPFs for 24-hour periods (for a total of four forecasts corresponding to Day 4, 5, 6, and 7, respectively). Computer programs then take advantage of model forecasts of the timing of precipitation to break the WPC forecast down into 6-hourly QPFs. Forecasts issued on the day shift correspond to periods ending at 0000Z (for issuance times between 1815Z and 2215Z), and those issued on the night shift correspond to periods ending at 1200Z (for issuance times between 0615Z and 1015Z). <p> <span style="font-size:11pt;font-weight:bold;">Multi-day Accumulation Products:</span> <p> Through simple addition, WPC is able to provide multi-day precipitation accumulation forecasts. Days 1-3, Days 1-5, and Days 1-7 accumulations are made available, and can be powerful planning tools for many applications. <p> <span style="font-size:11pt;font-weight:bold;">Schedule of QPF Issuance:</span> <p> <table width="500" border=1> <tr> <td align=center width=85> <font color="green"><b>Valid Period</b></font> </td> <td colspan=2 width=120> <font color="green"><b>Issuance Time</b><br> Preliminary      Final</font> </td> <td width=200 align=center> <font color="green"><b>Valid Period</b></font> </td> <td> <font color="green"><b>Length of<br>Valid Period</b></font> </td> </tr> <tr> <td><font color="brown"><i>Night Shift*</i></font></td> <td> </td> <td> </td> <td> </td> <td> </td> </tr> <tr> <td>Day 1</td> <td>0530Z</td> <td> </td> <td>06Z Today (Day 1) - 12Z Tomorrow (Day 2)</td> <td>30 hours</td> </tr> <tr> <td>Day 1</td> <td> </td> <td>1000Z</td> <td>12Z Today (Day 1) - 12Z Tomorrow (Day 2)</td> <td>24 hours</td> </tr> <tr> <td>Day 2</td> <td>0530Z</td> <td>1000Z</td> <td>12Z Tomorrow (Day 2) - 12Z Day 3</td> <td>24 hours</td> </tr> <tr> <td>Day 3</td> <td>0530Z</td> <td>1000Z</td> <td>12Z Day 3 - 12Z Day 4</td> <td>24 hours</td> </tr> <tr> <td>Day 4</td> <td>N/A</td> <td>0400Z</td> <td>12Z Day 4 - 12Z Day 5</td> <td>24 hours</td> </tr> <tr> <td>Day 5</td> <td>N/A</td> <td>0400Z</td> <td>12Z Day 5 - 12Z Day 6</td> <td>24 hours</td> </tr> <tr> <td>Day 6</td> <td>N/A</td> <td>0400Z</td> <td>12Z Day 6 - 12Z Day 7</td> <td>24 hours</td> </tr> <tr> <td>Day 7</td> <td>N/A</td> <td>0400Z</td> <td>12Z Day 7 - 12Z Day 8</td> <td>24 hours</td> </tr> <tr> <td><font color="brown"><i>Day Shift</i></font></td> <td> </td> <td> </td> <td> </td> <td> </td> </tr> <tr> <td>Day 1</td> <td>1730Z</td> <td> </td> <td>18Z Today (Day 1) - 00Z Tomorrow Evening (Day 2)</td> <td>30 hours</td> </tr> <tr> <td>Day 1</td> <td> </td> <td>2200Z</td> <td>00Z This Evening (Day 1) - 00Z Tomorrow Evening (Day 2)</td> <td>24 hours</td> </tr> <tr> <td>Day 2</td> <td>1730Z</td> <td>2200Z</td> <td>00Z Tomorrow Evening (Day 2) - 00Z the Evening of Day 3</td> <td>24 hours</td> </tr> <tr> <td>Day 3</td> <td>1730Z</td> <td>2200Z</td> <td>00Z Day 3 - 00Z Day 4</td> <td>24 hours</td> </tr> <tr> <td>Day 4</td> <td>N/A</td> <td>1600Z</td> <td>00Z Day 4 - 00Z Day 5</td> <td>24 hours</td> </tr> <tr> <td>Day 5</td> <td>N/A</td> <td>1600Z</td> <td>00Z Day 5 - 00Z Day 6</td> <td>24 hours</td> </tr> <tr> <td>Day 6</td> <td>N/A</td> <td>1600Z</td> <td>00Z Day 6 - 00Z Day 7</td> <td>24 hours</td> </tr> <tr> <td>Day 7</td> <td>N/A</td> <td>1600Z</td> <td>00Z Day 7 - 00Z Day 8</td> <td>24 hours</td> </tr> </table> <p> * Forecasts made on the night shifts are verified versus observed rainfall. The valid time of 1200z to 1200z for QPFs issued by the night shift coincides with the 24-hour observation cycle for National Weather Service Cooperative Observers and many other rainfall networks such as CoCoRaHS. <p> All Times shown are in Zulu (Z), or Greenwich Mean Time, and correspond to the time in Greenwich, England, as seen on a 24-hour clock. <p> <hr><a NAME="excessrain"></a> <p> <span style="font-size:14pt;font-weight:bold;">Excessive Rainfall Outlooks (EROs)</span> <p> <img src="excessive_rainfall_example.gif" height="360" width="451" alt="Excessive Rainfall Example" border=0> <p> In the Excessive Rainfall Outlooks, the Weather Prediction Center (WPC) forecasts the probability that rainfall will exceed flash flood guidance (FFG) within 40 kilometers (25 miles) of a point. Gridded FFG is provided by the twelve NWS River Forecast Centers (RFCs) whose service areas cover the lower 48 states. WPC creates a national mosaic of FFG, whose 1, 3, and 6-hour values represent the amount of rainfall over those short durations which it is estimated would bring rivers and streams up to bankfull conditions. WPC estimates the likelihood that FFG will be exceeded by assessing environmental conditions (e.g. moisture content and steering winds), recognizing weather patterns commonly associated with heavy rainfall, and using a variety of deterministic and ensemble-based numerical model tools that get at both the meteorological and hydrologic factors associated with flash flooding. The ERO is a highly collaborative product and benefits from the input of meteorologists and hydrologists among the WFOs, RFCs, and National Water Center. <p> The risk of excessive rainfall is expressed both probabilistically and categorically according to the table below. <p> <table border=0 width=350> <tr> <td><b><u>Risk Category</u></b></td> <td width=200 style="padding-left:15px;"><u>Probability of Rainfall Exceeding Flash Flood Guidance Within 40 km (25mi) of a Point</u></td> </tr> <tr> <td><b>Marginal (MRGL)</b></td> <td align=center>At Least 5%</td> </tr> <tr> <td><b>Slight (SLGT)</b></td> <td align=center>At Least 15%</td> </tr> <tr> <td><b>Moderate (MDT)</b></td> <td align=center>At Least 40%</td> </tr> <tr> <td><b>High (HIGH)</b></td> <td align=center>At Least 70%</td> </tr> </table> <p> Flash floods are rare events at any one specific location, and, therefore, the probability of a flash flood is low even when forecasters are confident that flash flooding will occur within the region. When forecasters declare risk areas by placing a contour on an Excessive Rainfall Graphic they are expecting at least some flash flooding to occur, and are sometimes expecting very organized heavy rainfall and flash flooding to affect numerous locations. In an ideal case, Slight, Moderate, or High Risk may be advertised 2 to 3 days in advance of an event. In other cases, as confidence increases (usually as lead time decreases) the category may be updated from Marginal to Slight and so forth. In other cases, usually involving small or less well-defined weather systems, risk areas may be introduced quite suddenly and with short lead time - flash flood forecasting is still one of the most difficult aspects of meteorology. <p> On the Excessive Rainfall Outlook graphics a closed contour with an arrowhead delineates the probability forecasts, with risk areas defined to the right of the direction of the arrowhead. <p> If conditions are not expected to become favorable for flash flooding anywhere in the lower 48 states then text across the center of the graphic will read, "The probability of rainfall exceeding flash flood guidance is less than 5 percent". <p> <p> Note that FFG may be difficult to apply to a Day 2 or Day 3 forecast (or even late in Day 1 if it is already raining). Forecasters must make an educated guess as to how FFG will change in response to exiting precipitation systems (FFG recovery) or incoming precipitation systems (lowering of FFG). Increased variability in model guidance and the inability of the models to resolve mesoscale features results in greater uncertainty in forecasting excessive rainfall for the Day 2 and Day 3 periods. The product definition, "probability of rainfall exceeding flash flood guidance," does represent our best attempt to forecast <i>flash</i> flooding from heavy rainfall, which is almost always driven by convective rain rates or persistent orographic lift. Some events, however, do border on <i>(or transition to)</i> inundation flooding or main stem river flooding occurring when the causative rain event is of longer than a 6-hour duration and/or is not especially heavy except when summed over longer periods of time. Forecasts of main stem river flooding are made by the RFCs out to 5 day lead time, and are packaged by WPC into a national mosaic in the Flood Outlook Product. <p> The ERO is now being experimentally issued for Days 4 and 5, and those outlooks only include contours at the Slight and Moderate Risk levels. The risk categories are produced and labeled in ways that are consistent with the Day 1 to 3 ERO, with the same definitions, colors, and nomenclature. When no risk category is assigned for a given Day 4 or Day 5 outlook, there will be a text label placed on the map that says "The probability of rainfall exceeding flash flood guidance is less than 15 percent". <p> <span style="font-size:11pt;font-weight:bold;">Schedule of Excessive Rainfall Outlook Issuance:</span> <p> Day 1 Excessive Rainfall Outlooks (graphic and associated discussion) have scheduled issuances three times per day: 01, 09, and 15 UTC. Day-2 and Day-3 versions are issued twice daily. The valid times vary as noted in the table below. <i>Unscheduled, event-driven updates may be issued as needed.</i> <p> <table width="550" border=1> <tr> <td width=85> <font color="green"><b>Valid Period</b></font> </td> <td> <font color="green"><b>Issuance Time</b></font> </td> <td> <font color="green"><b>Valid Period</b></font> </td> </tr> <tr> <td>Day 1</td> <td>0100Z</td> <td> 01Z - 12Z Day 1 (11 hours)</td> </tr> <tr> <td>Day 1</td> <td>0830Z</td> <td> 12Z Day 1 - 12Z Day 2 (24 hours)</td> </tr> <tr> <td>Day 1</td> <td>1600Z</td> <td> 16Z Day 1 - 12Z Day 2 (20 hours)</td> </tr> <tr> <td> </td> <td> </td> <td> </td> </tr> <tr> <td>Day 2</td> <td>0830Z</td> <td> 12Z Day 2 - 12Z Day 3</td> </tr> <tr> <td>Day 2</td> <td>2030Z</td> <td> 12Z Day 2 - 12Z Day 3 (an update to the earlier Day 2 forecast)</td> </tr> <tr> <td> </td> <td> </td> <td> </td> </tr> <tr> <td>Day 3</td> <td>0830Z</td> <td> 12Z Day 3 - 12Z Day 4</td> </tr> <tr> <td>Day 3</td> <td>2030Z</td> <td> 12Z Day 3 - 12Z Day 4 (an update to the earlier Day 3 forecast)</td> </tr> <tr> <td> </td> <td> </td> <td> </td> </tr> <tr> <td>Day 4</td> <td>0830Z</td> <td> 12Z Day 4 - 12Z Day 5</td> </tr> <tr> <td>Day 4</td> <td>2030Z</td> <td> 12Z Day 4 - 12Z Day 5 (an update to the earlier Day 4 forecast)</td> </tr> <tr> <td> </td> <td> </td> <td> </td> </tr> <tr> <td>Day 5</td> <td>0830Z</td> <td> 12Z Day 5 - 12Z Day 6</td> </tr> <tr> <td>Day 5</td> <td>2030Z</td> <td> 12Z Day 5 - 12Z Day 6 (an update to the earlier Day 5 forecast)</td> </tr> </table> <div style="line-height:100%;"> </div> <span style="font-size:11pt;font-weight:bold;">Forecast Discussions/Text Products:</span> <p> National Weather Service forecast discussions have been used for decades to summarize our assessment of the upcoming weather pattern and associated impacts. Discussions are also used to convey potential worst case scenarios and the forecasters' confidence in both the available model output and the manual forecast. There is a lot of good information in the discussion products which can be difficult to depict in a gridded or graphical product. Reading the WPC discussions is just about the closest thing to calling up the forecaster on the phone and asking what they think is going to happen. <p> Discussions related to excessive rainfall and QPF include the following: <ul> <li>Excessive Rainfall Discussion (QPFERD) (Day 1; Day 2; Day 3 all available)</li> <li>Days 4/5 and 6/7 QPFs are produced by the WPC medium range desk which issues a medium range forecast discussion (PMDEPD).</li> <li>Mesoscale Precipitation Discussion (FFGMPD) – see full description below</li> </ul> <p> <span style="font-size:11pt;font-weight:bold;">Tropical Rainfall Statement</span> <p> Per an agreement between the National Hurricane Center (NHC), National Water Center (NWC), and WPC, tropical rainfall and flooding statements for all public advisories for tropical cyclones, and key messages, are collaborated between WPC and NWC and provided to NHC. The statements are produced for active tropical systems that threaten land masses in the Atlantic and eastern Pacific basins, and they describe specific islands or continental regions expected to be impacted by heavy rain. The statements include a range of expected areal average rainfall as well as the maximum amounts expected at isolated locations, along with potential impacts from the rainfall. <p> <a NAME="qpfver"></a> <span style="font-size:11pt;font-weight:bold;">QPF Verification</span> <p> 6-hourly precipitation forecasts are verified using a point (station) method while 24-hour forecasts are verified using an areal method. Current graphs depicting WPC verification scores are available on the <a href="http://www.wpc.ncep.noaa.gov/html/hpcverif.shtml"><b><u>WPC Verification page</u></b></a>. For more details on the verification of WPC precipitation forecasts, read the <a href="http://journals.ametsoc.org/doi/pdf/10.1175/1520-0434(1995)010<0498%3AEOYOQP>2.0.CO%3B2"><b><u>article</u></b></a> by Olson, Junker and Korty in Weather and Forecasting. Volume 10, 1995, pgs. 498-511. <p> <hr><a NAME="mpd"></a> <div style="text-align:left;margin-left:1em;"> <div style="line-height:25%;"> </div> <div style="font-size:14pt;"><b> Mesoscale Precipitation Discussions</b> </div> <p> </div> <div style="text-align:left;margin-left:1.5em;"> On April 9, 2013, WPC began providing short term guidance during heavy rain events leading to a threat of flash flooding to the National Weather Service (NWS) Weather Forecast Offices (WFOs), River Forecast Centers (RFCs), the media, emergency managers and interested partners. Guidance is given in the form of Mesoscale Precipitation Discussions (MPDs), that are ideally issued 1-6 hours ahead of time, averaging an area equal to roughly half the size of the state of Kansas. Each MPD consists of a graphic indicating the area of concern and any pertinent meteorological features as well as a brief text discussion focused on the mesoscale features supporting the anticipated heavy rainfall. The potential for flash flooding within the area of concern will be highlighted by one of three headlines:</p> <p><b>FLASH FLOODING LIKELY</b> High confidence exists that environmental conditions are favorable, or will become favorable, for heavy rainfall that will result in flash flooding.</p> <p><b>FLASH FLOODING POSSIBLE</b> Environmental conditions are favorable, or will become favorable, for heavy rainfall, but there are questions about how the event will evolve and/or whether flash flooding will occur.</p> <p><b>FLASH FLOODING UNLIKELY</b> High confidence exists that environmental conditions are unfavorable, or will become unfavorable, for heavy rainfall that will result in flash flooding. (typically issued toward the end of an event)</p> <p>While flash flooding is caused by a variety of factors (e.g., intense rainfall, dam failure, ice jams), WPC's Metwatch desk will only focus on flash floods triggered by intense rainfall that occur over a sufficient areal coverage. Localized flash flooding is not considered. </p> <p>The MPD appears under WMO Header: AWUS01 KWNH<br> and AWIPS header: KWNH FFGMPD </p> </div> <p> <hr><a NAME="medrange"></a> <center> <table border=0 width="96%"> <tr><td> <h3> Medium Range/Extended (3-7 days)</h3> <table BORDER=0 cellpadding=2 WIDTH="100%"> <tr> <td><font color="#000099"><b>Product</b></font></td> <td><font color="#000099"><b>Time</b></font></td> </tr> <tr> <td><a href="/medr/medr500.shtml">Day 3-7 500mb Height Forecasts</a></td> <td>0200 UTC and 1400 UTC</td> </tr> <tr> <td><a href="/medr/medr.shtml">Day 3-7 Fronts/Pressure Graphics for North America</a></td> <td>0430 UTC and 1630 UTC</td> </tr> <tr> <td><a href="/medr/medr_min.shtml">Day 3-7 Min/Max/PoPs</a></td> <td>0400 UTC and 1600 UTC</td> </tr> <tr> <td><a href="/5km_grids/5km_gridsbody.html">CONUS Grids</a></td> <td>0400 UTC and 1600 UTC</td> </tr> <tr> <td><a href="/discussions/hpcdiscussions.php?disc=pmdepd">Forecast Discussion</a></td> <td>0700 UTC and 1900 UTC</td> </tr> <tr> <td><a href="/qpf/day4-7.shtml">6-hour QPFs for Days 4, 5, 6, and 7</a></td> <td>0400 UTC and 1600 UTC</td> </tr> <tr> <td><a href="/wwd/pwpf_d47/pwpf_medr.php">Day 4-7 Winter Weather Outlook</a></td> <td>0500 UTC and 1700 UTC</td> </tr> </table> </td></tr> </table> </center> <div style="line-height:100%;"> </div> <div style="text-align:left;margin-left:1.5em;"> The medium range graphical forecast products include: <ul> <li>Surface pressure patterns, circulation centers, fronts, and 500mb heights for days 3-7 into the future <li>Daily maximum and minimum temperatures and anomalies for days 3-7 <li>Daily precipitation probabilities for days 3-7 <li>Four 24-hour QPFs encompassing days 4-7 <li>5-Day Total QPF for Days 1-5 and a 7-Day Total QPF for days 1-7 <li>Winter Weather Outlooks for Days 4, 5, 6, and 7 </ul> The surface pressure and fronts graphics cover the continental U.S. (CONUS) and adjacent areas of southern Canada and northern Mexico and are generated twice per day at 0430 UTC and 1630 UTC. The 500mb forecasts and Min/Max/PoP graphics are primarily focused on the CONUS and issued twice per day. <p>In addition to the graphical forecasts, the forecasters prepare two daily written discussions. They highlight medium-range model differences, provide weather solution preferences, a measure of uncertainty, forecaster reasoning and highlight any significant weather expected to impact the CONUS during the Day 4-7 time frame. Forecasters also provide a separate discussion describing guidance differences and preferences across Hawaii by 1230 UTC. <p>The workflow consists of three shifts per day. Two meteorologists focus on the contiguous U.S., one during the overnight hours (0000-0900 UTC) and one on the day shift (1130-2030 UTC). They generate a set of North American 3-7 day pressure systems/fronts and 500mb forecasts, 3-7 day sensible weather grids, 16 6-hour QPFs covering Days 4-7, winter weather outlooks for Days 4-7, and a discussion. In addition, the overnight forecaster prepares a Hawaiian discussion focusing on numerical forecast guidance and significant weather threats. The third meteorologist, who works from 1500-0000 UTC primarily focuses on Alaskan forecasting. See the <a href="/html/fam2.html#alaska">Alaska Medium Range</a> section below for more information. All forecasters routinely use output from the GFS, ECMWF, and UKMET medium range models and also consider the Canadian, the Navy's NAVGEM model, and ensemble guidance from the GFS, ECMWF, Canadian, and North American Ensemble Forecast System (NAEFS). <p>In addition, during hurricane season, at 1700 UTC on a daily basis since June 1, 1997, the medium range pressure dayshift forecaster also participates in a conference call with the NHC via the Hurricane Hotline to discuss current and potential tropical activity in the Atlantic and Eastern Pacific oceans and how the medium range models are handling the situation. </div> <p> <hr><a NAME="alaska"></a> <center> <table border=0 width="90%"> <tr><td> <h3> <font color="black">Alaska Medium Range (Days 4-8)</font></h3> <table border="1" width="100%" cellspacing="3" cellpadding="3"> <tr> <td><b>Product</b></td> <td><b>Time</b></td> </tr> <tr> <td>Day 4-8 Fronts and Pressure Graphics</td> <td>2030 UTC</td> </tr> <tr> <td>Day 4-8 500 hPa Height Graphics</td> <td>1915 UTC</td> </tr> <tr> <td>Alaska Medium Range Discussion</td> <td>0000 UTC</td> </tr> <tr> <td>Day 4-8 Grids</td> <td>2300 UTC</td> </tr> </table> <p></p> To accommodate a request for support from the NOAA National Weather Service (NWS) Alaska Region, the NCEP/Weather Prediction Center's (WPC) Alaska Medium Range Desk is issuing the following products: <ul> <li>Day 4-8 Surface Fronts and Pressures graphics</li> <li>Day 4-8 500 hPa Height graphics</li> <li>Alaska Medium Range Forecast Discussion</li> <li>Day 4-8 Maximum/Minimum Temperature grids</li> <li>Day 4-8 12-hour Probability of Precipitation grids</li> <li>Day 4-8 derived Dewpoint Temperature, Cloud Cover, Precipitation Type, and Wind Speed/Direction grids</li> </ul> Surface graphics depict surface pressure patterns , high and low pressure circulation centers and fronts for days 4-8.  The 500 hPa height graphics display the general flow pattern forecast for days 4-8, and the gridded guidance depicts various important meteorological variables for the forecast period. <p></p> A WPC meteorologist interprets available deterministic and ensemble model guidance (including the National Blend of Models) and collaborates with the WPC contiguous U.S. (CONUS) medium range forecasters.  The Alaska forecaster then uses the available model guidance and meteorological reasoning to depict the most likely scenario for days 4-8.   That meteorologist then composes a forecast discussion outlining deterministic and ensemble model differences, preferences and trends.   In addition, within the discussion the forecaster communicates confidence level, forecast uncertainty and any significant weather expected in the forecast period. <p></p> </td></tr> </table> </center>  <p> <hr><a NAME="sfcanalysis"></a> <center> <table border=0 width="90%"> <tr><td> <h3> <font color="black">Surface Analysis Products</font></h3> <p><font color="black">The following chart indicates the approximate issuance and web posting schedule for the WPC Surface Analysis. This product depicts the analysis of synoptic and sub-synoptic/mesoscale surface features including highs, lows, fronts, troughs, outflow boundaries, squall lines, and drylines. The analysis domain covers most of North America, the Western Atlantic and Eastern Pacific oceans, and the Gulf of Mexico.</font> <center> <p> <table BORDER=0 cellpadding=2 WIDTH="100%"> <tr> <td><font color="green"><b>Analysis Time</b></font></td> <td><font color="green"><b>Issuance Time</b></font></td> <td><font color="green"><b>Web Posting</b></font></td> </tr> <tr> <td><a href="../sfc/namsfc00wbg.gif">00Z</a></td> <td><font color="black">0127Z</font></td> <td><font color="black">0147Z</font></td> </tr> <tr> <td><a href="../sfc/namsfc03wbg.gif">03Z</a></td> <td><font color="black">0430Z</font></td> <td><font color="black">0450Z</font></td> </tr> <tr> <td><a href="../sfc/namsfc06wbg.gif">06Z</a></td> <td><font color="black">0733Z</font></td> <td><font color="black">0753Z</font></td> </tr> <tr> <td><a href="../sfc/namsfc09wbg.gif">09Z</a></td> <td><font color="black">1028Z</font></td> <td><font color="black">1048Z</font></td> </tr> <tr> <td><a href="../sfc/namsfc12wbg.gif">12Z</a></td> <td><font color="black">1329Z</font></td> <td><font color="black">1349Z</font></td> </tr> <tr> <td><a href="../sfc/namsfc15wbg.gif">15Z</a></td> <td><font color="black">1635Z</font></td> <td><font color="black">1655Z</font></td> </tr> <tr> <td><a href="../sfc/namsfc18wbg.gif">18Z</a></td> <td>1938Z</td> <td><font color="black">1958Z</font></td> </tr> <tr> <td><a href="../sfc/namsfc21wbg.gif">21Z</a></td> <td>2230Z</td> <td><font color="black">2250Z</font></td> </tr> </table> </center> <p><b><font color="#FF0000">PLEASE NOTE: </font></b> <font color="#FF6666">The Pacific Ocean analyses (East and West Pacific) are prepared by NCEP's </font><font color="#993366"><a href="http://www.opc.ncep.noaa.gov">Ocean Prediction Center</a>. </font> </td></tr> </table> </center> <p> <hr><a NAME="hvysnow"></a> <center> <table border=0 width="90%"> <tr><td> <h3> <font color="black">Probabilistic Heavy Snow/Icing Forecasts</font></h3> <table BORDER=0 cellpadding=1 WIDTH="100%"> <tr> <td style="vertical-align: top;"> <b>PRODUCT DELIVERY SCHEDULE</b> <p>The WPC Winter Weather Desk (WWD) is staffed from September through May.  The WWD forecaster routinely releases updated forecasts twice daily at 0900Z and 2100Z (4am/pm EST or 5am/pm EDT respectively). Forecasts may be updated if warranted by rapidly-changing situations. <p> </td> </tr> <tr> <td><b>PROBABILITY GRAPHICS FOR SNOW AND FREEZING RAIN</b> <p> These graphics indicate the <b>probability</b> (potential) for a location to receive specific thresholds of accumulated snow or ice.  <ul> <li> Snowfall - closed lines represent the probability (slight, moderate, and high) that enclosed areas will receive equal to or greater than a specific threshold accumulation (4", 8" or 12") of snowfall in a 24 hour period.</li> <li> Freezing Rain - depicts the probability in the same manner and time period as snowfall, but with an accumulation threshold of .25" (one quarter of an inch) of  freezing rain.</li> </ul> <div style="margin-left:3.3em;"> <p><i><font color="#ff0000">CAUTION ! The probability contours may appear to inordinately expand, contract or "jump" geographically after a scheduled update.  This is partially due to the frequency with which the products are updated.  The "new" 24 hour period covers the last 12 hours of the previous issuance AND the next 12 hours. Additionally two model cycles have passed since the last scheduled issuance.</font></i> </p> <p><font color="#000000"><b>Specific</b> (deterministic) accumulations for a particular location in the United States can be obtained via the <a href="http://www.weather.gov"><u>National Weather Service home page</u></a>.</font> </p> <p><b><i><font color="#009900">The probabilistic graphics combined with the deterministic forecasts provide a user both the most likely amount expected from an event and the potential the event will produce accumulations in excess of specific thresholds.<br> </font></i></b> </div> </td> </tr> <tr> <td> <p></p> <b><font color="#000000">PROBABILITY LEGEND</font></b> <p>The probability thresholds used are defined as follows:</p> <ul> <li> <b>SLGT</b> - 10% to 40% chance of occurrence within the outlined area.</li> <li> <b>MODERATE (MDT)</b> - 40% to 70% chance of occurrence within the outlined area.</li> <li> <b>HIGH</b> - 70% chance or greater of occurrence within the outlined area.</li> </ul> <p>To gain further insight into this forecast, please read the <a href="/discussions/hpcdiscussions.php?disc=qpfhsd"><u>Heavy Snow Discussion (HSD)</u></a> that accompanies these graphical products.</p><br> </td> </tr> <tr> <td> <p><b><font color="#000000">LOW TRACKS GRAPHIC<br> </font></b></p> <p style="text-align: left;"><font color="#000000">These graphics depict the forecast location of significant surface lows impacting the 48 contiguous United States in 12 hour increments out to 72 hours into the future.  It is provided in two formats:  non-technical and technical.</font></p> <ul> <li>The non-technical version depicts the low position and track forecast by the NCEP WPC meteorologist in white.  Each low position is accompanied by a lead time (Eastern Time).  The circle around each low represents a 75% probability the observed low will be located within the circle. Note: The probability is derived using previous season's verification data. For reference, existing surface lows are depicted with a red marker without yellow circles. </li> </ul> <ul> <li>The technical version depicts the low position and track forecast by the NCEP WPC meteorologist in black.  Each low is accompanied by a forecast central pressure. Additionally there are no fewer than 35 different computer model forecasts of low positions for a given lead time available to the WPC forecaster - these are depicted with symbols.  Both the central pressure and computer model forecasts are color coded according to lead time (Universal Time).  Together, the WPC forecast position of the low and computer-generated position provide a user <b><i>both</i></b> the preferred position and track of the low <b><i>and</i></b> a sense of the uncertainty with the forecast.</li> </ul> <ul> <li>For reference, existing surface lows are depicted with a red marker.  Surface lows can also be found on the WPC Surface Analysis. One can see the current location of surface lows and the forecast path of both existing surface lows and surface lows expected to develop within three days' time.<br> </li> </ul> <center><b><i><font color="#cc0000">Note - winter weather is not always associated with significant surface lows.</font></i></b></center> </td> </tr> </table> </td></tr> </table> </center> <p> <hr><a NAME="shortrng"></a> <center> <table border=0 width="100%"> <tr><td> <h3> <font color="black">Short Range Forecasts</font></h3> <center><table BORDER=0 cellpadding=2 WIDTH="100%"> <tr> <td><font color="#000099"><b>Product</b></font></td> <td><font color="#000099"><b>Issuance Time</b></font></td> </tr> <tr> <td><font color="black">6 and 12 hour forecasts</font></td> <td><font color="black">0200Z (Night Shift)</font></td> </tr> <tr> <td></td> <td><font color="black">1400Z (Day Shift)</font></td> </tr> <tr> <td><font color="black">18 and 24 hour forecasts</font></td> <td><font color="black">0430Z (Night Shift)</font></td> </tr> <tr> <td></td> <td><font color="black">1630Z (Day Shift)</font></td> </tr> <tr> <td><font color="black">30, 36, and 48 hour forecasts</font></td> <td><font color="black">0730Z (Night Shift)</font></td> </tr> <tr> <td></td> <td><font color="black">1930Z (Day Shift)</font></td> </tr> <tr> <td><font color="black">60 hour forecast</font></td> <td><font color="black">0800Z (Night Shift)</font></td> </tr> <tr> <td></td> <td><font color="black">2000Z (Day Shift)</font></td> </tr> <tr> <td><font color="black">Discussion</font></td> <td><font color="black">0900Z (Night Shift)</font></td> </tr> <tr> <td></td> <td><font color="black">2100Z (Day Shift)</font></td> </tr> </table></center> <p>The short range meteorologist prepares 6 through 60 hour forecasts for the continental U.S., southern Canada, and northern Mexico.   These products are issued twice daily using numerical model output from the National Weather Service's (NWS) Global Forecast System (GFS) and North American Mesoscale model (NAM), as well as guidance from the European Centre for Medium-Range Weather Forecasts (ECMWF), the United Kingdom's Meteorology Office (UKMET), the Meteorological Service of Canada, including ensembles.   Coordination with the surface analysis, model diagnostics, quantitative precipitation, winter weather, and tropical forecast desks is also performed during the forecast process.   <p></p> The short range forecast products include surface pressure patterns (isobars), circulation centers and fronts for 6-60 hours, and a depiction of the types and extent of precipitation that are forecast at the valid time of the chart.  The primary goal is to depict accurately the evolution of major weather systems that will affect the continental U.S. during the next 60 hours.   In addition, discussions are written on each shift and issued with the forecast packages that highlight the meteorological reasoning behind the forecasts and significant weather across the continental United States. <p></p> Please note that at this time isobars are not included on the 6-hour forecast and precipitation is not included on the 60-hour forecast chart. </td></tr> </table> </center> <p> <hr><a NAME="intldesk"></a> <center> <table border=0 width="100%"> <tr><td> <h3> <font color="black">International Desks</font></h3> <a href="/international/intlhist.shtml">Please click here to find out more about this WPC function</a>.  </td></tr> </table> </center> <p> <hr> <a name="winwxsum"></a> <h3> <b><font color="black">Storm Summaries</font></b></h3> Storm summaries provide both a summary of the significant weather which has occurred, and an WPC general forecast of the storm system over the next 1 to 2 days. Storm summaries serve as a central source for storm information which would otherwise have to be gleaned from a number of NWS Forecast Office websites. <p>Storm summaries are issued for significant large-scale storms which: <ul> <li>Affect multiple NWS Forecast Office areas of responsibility <li>Are likely to be of media interest <li>Impact large population areas, or major transportation systems, or otherwise make a significant impact upon the nation's or a region's commerce <li>Are usually snow and/or ice storms, but which may be rainfall events if they are causing widespread flash flooding, mudslides, etc. </ul> If two or more separate storm systems are occurring simultaneously, a storm summary is issued for each individually. <p> <hr> <a name="tropadvisories"></a> <h3>Tropical Public Advisories</h3> The WPC will issue public advisories after the <a href="http://www.nhc.noaa.gov"><b>National Hurricane Center</b></a> (NHC) discontinues its advisories on subtropical and tropical cyclones that have moved inland, but still pose a threat of heavy rain and flash floods in the conterminous United States or adjacent areas within Mexico which affect the drainage basins of NWS River Forecast Centers. The last NHC advisory will normally be issued when winds in an inland tropical cyclone drop below tropical storm strength, and the tropical depression is not forecast to regain tropical storm intensity or re-emerge over water. WPC advisories will terminate when the threat of flash flooding has ended. <p> <TABLE cellSpacing="0" cellPadding="0" width="96%" border="0"> <TBODY> <tr> <td colSpan="3"><hr></td> </tr> <tr vAlign="top"> <td align="left" nowrap="nowrap" class="gray"> <a href="http://www.noaa.gov"><span class="gray">NOAA/</span></a> <a href="http://www.nws.noaa.gov/"><span class="gray">National Weather Service</span></a><br> <a href="http://www.ncep.noaa.gov/"> <span class="gray">National Centers for Environmental Prediction</span></a><br> Weather Prediction Center<br> 5830 University Research Court<br> College Park, Maryland 20740<br> <a href="/mail_webmaster" target="_new"><span class="gray">Weather Prediction Center Web Team</span></a></td> <td nowrap="nowrap"> <a href="http://www.nws.noaa.gov/disclaimer.php"><span class="gray">Disclaimer</span></a><br> <a href="http://www.weather.gov/credits.php"><span class="gray">Credits</span></a><br> <a href="http://www.weather.gov/glossary/"><span class="gray">Glossary</span></a><br> </td> <td align="right" nowrap="nowrap"> <a href="http://www.weather.gov/privacy.php"><span class="gray">Privacy Policy</span></a><br> <a href="http://www.weather.gov/admin.php"><span class="gray">About Us</span></a><br> <a href="http://www.weather.gov/careers.php"><span class="gray">Career Opportunities</span></a><br> </td> </tr> <TR vAlign="top"> <TD class="gray" colSpan="3"> Page last modified: Wednesday, 24-Jul-2024 12:01:03 UTC </TD> </TR> </TBODY> </TABLE> </TD> </TR> </TBODY> </TABLE> </BODY> </HTML>