Extended Forecast Discussion NWS Weather Prediction Center College Park MD 110 PM EDT Sun Apr 19 2020 Valid 12Z Wed Apr 22 2020 - 12Z Sun Apr 26 2020 ...Another Excessive Rainfall/Severe Weather Threat over the South Mid-Late Week... ...Overview... Models and ensemble means generally agree with the idea that progressive flow aloft will transition toward a ridge/trough pattern by next weekend as an eastern Pacific ridge moves into the West and shortwave energy that begins to flow into the West late this week amplifies downstream. Within this evolution, continue to expect a fairly vigorous system to track from the Southern Plains to East Coast Wed-Fri and off New England by Sat. Best potential for heavy rainfall and possible strong to severe convection will be across the South but meaningful rainfall may extend as far north as the Central Plains to New England. The energy initially coming into the West will bring a period of precipitation to areas from the Northwest into the northern-central Rockies. Farther east the guidance is still at odds over the precise character of the amplifying trough and associated surface reflection. Thus confidence remains low regarding the coverage and intensity of precipitation over the eastern half of the country by next weekend. ...Guidance/Predictability Assessment... Models/means have been shuffling around somewhat for the system tracking out of the Southern Plains (due in part to the lower-predictability fine details involved with the upper system) as evidenced by recent GFS runs. Based on guidance through the 00Z/06Z cycles a majority cluster approach closest to the 00Z ECMWF/UKMET and somewhat the 00Z GFS/Canadian (along with continuity) offered the most reasonable starting point in light of the quicker/slower solutions of the GFS/Canadian, respectively, on Wed. By Thu, 00Z models were clustered better though the 06Z GFS was notably quicker with the system over the Lower Mississippi Valley. As the system approaches the western Atlantic Fri, trend has been for a bit farther south track across the Mid-Atlantic/central Appalachians before turning the corner northeastward toward the 40/70 benchmark late Fri. Again preferred to remain between the quicker GFS and slower Canadian/UKMET, near the 00Z ECMWF/ECMWF ensemble mean and the 06Z GEFS mean. The forecast remains extremely uncertain for evolution of energy that starts to flow into the West late Wed-Thu. The 00Z/06Z GFS were quick to close off a system over the Mississippi Valley which was at or outside the multi-center ensemble envelope. As of day 7 Sun the ensemble means were not far apart aloft given typical day 7 forecast differences/errors though the ECMWF ensemble may be a victim of its 50-member spread which resulted in a more ill-defined surface pattern over the East. The GEFS/CMC means at least depicted a broad area of low pressure, which seems reasonable given the approach of an amplifying upper trough. Trending the days 6-7 Sat-Sun forecast toward a model/mean/continuity blend captured the consensus mean pattern aloft over the lower 48 and by Sun yielded the desired area of low pressure over the East, with the evolution/placement/strength fairly conservative in light of the high uncertainty. Operational input came from the 00Z GFS/ECMWF which, though different, gave some credence to a slower system. ...Weather/Hazard Highlights... The system forecast to track from the Southern Plains through Mid-Atlantic Wed-Fri still has some timing and detail uncertainty but there is a consistent signal for heavy rainfall potential and some strong to severe convection across portions of the South. Also fairly consistent is the idea that significant rainfall, even if less extreme than over southern areas, may extend northward into areas from the Central Plains through Ohio Valley and New England. Areas across the South will be quite sensitive to additional rainfall due to recent accumulations and this weekend's event. The Storm Prediction Center has highlighted an area over northeastern Texas on Wed with a 15% probability for severe weather (within 25 miles of a point). Severe threat may continue through the week in tandem with frontal progression. Farther north a weak wave/front may bring some light precipitation into the Upper Midwest/Great Lakes mid-late week. Areas from the Pacific Northwest into the northern-central Rockies will see a period of terrain-enhanced rain and higher elevation snow with shortwave energy crossing the region Wed-Fri. Precipitation will taper off from west to east leaving most of the West dry next weekend. An approaching Pacific front may bring some moisture into extreme northwestern locations by Sun though. As the shortwave energy crossing the West eventually amplifies downstream, confidence remains below average for the coverage and intensity of precipitation from the Plains into the East late this week through the weekend. What overlap exists among guidance hints at areas from the Midwest/south-central U.S. eastward as low pressure heads toward the coast. Despite being at the end of April next weekend, snow may be possible on the northern side of the precipitation shield given the chilly temperatures near the Canadian border. Locations from the Great Lakes/Ohio Valley into the northern Mid-Atlantic/Northeast will see morning lows and/or daytime highs up to 10-20F below normal on Wed followed by a trend to more modest negative anomalies (though perhaps again cooler at the end of the period). At the same time parts of the Northern Plains will see readings 10-15F above normal before a trend toward average. Over the West expect California and the Southwest to be most consistently above normal for temperatures during the period. Arrival of an upper ridge from the Pacific next weekend will likely lead to an expanding area of plus 10-15F and locally higher anomalies across the West. This may result in the warmest readings of the season (including near/over 100F in the deserts) which may be within a couple degrees of record highs. On the other hand there should be some below normal highs under the upper trough reaching the eastern U.S. but with low confidence in coverage and magnitude of cool anomalies. Fracasso/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