Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 PM EDT Tue Mar 17 2020 Valid 12Z Fri Mar 20 2020 - 12Z Tue Mar 24 2020 ...Heavy Rain Threat across portions of the South... 1730 UTC Update... Overall, model consensus at the large scales is relatively good through the medium range forecast period. The pattern remains relatively unchanged into early next week with a couple upper-level lows continuing to drop southward along the West Coast, the second of which (early next week) is forecast to begin moving inland (models continue to differ on the exact speed at which the feature moves inland). Farther east, a shortwave is forecast to cross the Midwest/Great Lakes/Northeast Fri-Sat, with the associated cold front sweeping across the centeral/eastern states. In the wake of the front, a strong polar surface high (~1044 hPa) is expected to overspread the central/eastern states, eventually stalling across the northern Gulf of Mexico and promoting a heavy rainfall threat over portions of the Southern Plains and lower Mississippi Valley. Overall model consensus on these features was good, with relatively minor timing/intensity differences noted. A blend of the 00Z ECMWF/06Z GFS was used a starting point for this forecast update during days 3-4 (Fri-Sat), with a gradual shift to more weighting of ensemble means (ECENS/GEFS) during days 5-7 (Sun-Tue). Overall, changes to forecast continuity were relatively small. Ryan Previous Discussion (issued at 0658 UTC)... ...Overview... Multi-day means maintain the idea of East Pacific/West Coast troughing through the period with west-southwesterly flow tending to prevail downstream across the rest of the lower 48. Canadian flow should extend into the northern tier states at times. Guidance is still attempting to resolve individual features within the mean trough but overall supports the idea of increasing moisture over the western states by the first half of next week--albeit with a slower trend over recent days. Low pressure tracking away from the Great Lakes on Fri will bring a front through the South and East with the trailing part of the front settling into the Gulf. Strong trailing high pressure will build into the Plains and then move through the East on its way into the Atlantic. Parts of the South will see a potential for areas of heavy rainfall in association with the cold front initially crossing the region and then with the combination of shortwave energy aloft plus the front stalling over the Gulf and eventually lifting north as a warm front. ...Guidance/Predictability Assessment... The dominant focus of guidance spread continues to be within the East Pacific/West Coast mean trough aloft with differences eventually extending across the lower 48. The general sequence of features has remained the same, with an upper low dropping south well offshore the West Coast Fri through the weekend and ejecting a weaker southern feature toward southern California and northwest Mexico by Sat, followed by another amplifying trough whose embedded closed low should end up close to the northern half of the West Coast by early next Tue. There are still significant differences for the path of the initial upper low but at least the GFS is in the process of trending westward toward the remaining consensus that includes the GEFS mean. Then latest GFS runs are still faster than other guidance with the next digging trough. From late Sun into Tue the combination of this faster timing and the GFS being on the eastern side of the spread for the first/southern upper low result in a vigorous system crossing the lower 48 at a much faster rate than depicted in other model/ensemble guidance. By next Tue this difference is about one day, with 18Z/00Z GFS low pressure already east of the Mississippi River while consensus is over the Central Plains region. As a result forecast preferences emphasize the most common ideas of recent ECMWF runs along with the 12Z CMC/UKMET/ECMWF mean and 18Z GEFS mean. The primary adjustment noted within this cluster over the past couple days has been a slower trend for the initial upper low off the West Coast, which leads to a later arrival of moisture into the West. Elsewhere, models and ensembles show poor agreement for one or more shortwaves that may flow southeastward from western Canada into the northern U.S. between the Northwest and Northern Plains region by Fri-Sun. Thus the finer-scale surface details across this part of the country have fairly low confidence. Farther east there are still some lingering differences with the storm system that tracks away from the Great Lakes into eastern Canada on Fri, with the aforementioned blend based on guidance through the 18Z cycle providing a reasonable starting point and good continuity. Consult the Model Diagnostics Discussion PMDHMD for updated preferences valid through early Fri based on the new 00Z guidance. ...Weather/Hazard Highlights... Locations over and near an axis from eastern Texas into northern Alabama currently have the best potential for a period of heavy rainfall in association with the front crossing the region Fri-Fri night. Farther north expect a drier trend late week into the weekend, with a period of brisk winds in the strong gradient between the departing storm system/front and high pressure building into the central U.S.. As the surface high continues eastward and the front settles into the Gulf, expect some moisture to linger over the South with low-predictability shortwave impulses likely aiding periods of rainfall during the weekend--and possibly into early next week as the front begins to lift northward. Some of this activity may occur over areas affected by the first event. Precipitation coverage should generally increase over the eastern half of the country during the first half of next week with low pressure expected to develop over the Central Plains by next Tue. From late week into the weekend the West should see fairly limited coverage of precipitation. Shearing energy from a weakening upper low may produce mostly light-moderate snow over and west of the south-central Rockies while northern stream energy could bring some modest amounts into the northern Rockies. The combination of the initial system off the West Coast and another one dropping through the Northeast Pacific should promote increasing rain and higher elevation snow over the West during the first half of next week, albeit with a somewhat slower trend over the past couple days of forecasts. Resolution of important flow details will take additional time but the best clustering of guidance continues to indicate that highest totals through the end of the forecast period will be over California. Expect the most extreme temperature anomalies to be from late this week into the weekend. The warm sector ahead of the front approaching the East on Fri will bring temperatures of 10-30F above normal and perhaps locally higher. On the other hand cold high pressure building into the Plains will bring temperatures down to 10-25F below normal over the central U.S.. This cold air will modify as it heads into the eastern states. Locations over/near California and the Southwest will see persistently below normal highs but mostly by only 5-10F after somewhat more coverage of minus 10F or greater anomalies on Fri. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml