Extended Forecast Discussion NWS Weather Prediction Center College Park MD 241 AM 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, not surprising with the lower-predictability fine details involved with the upper system. Based on guidance through the 12Z/18Z cycles a majority cluster approach closest to the GFS/ECMWF and their means led to only modest adjustments from continuity. Latest GFS runs have returned to consensus after straying a bit to the north for a time around yesterday's 00Z cycle. The 12Z CMC/UKMET were slower with the CMC a bit closer in principle to other solutions. Thus the favored blend dropped the UKMET after day 3 Wed while maintaining only a modest CMC component. Once the system reaches the western Atlantic after Fri a model/mean blend provided a good intermediate timing between the fast GFS and slow ECMWF, with the latter becoming a notable extreme by the end of the period. New 00Z runs are better clustered into early day 4 Thu (near established consensus) but then the CMC/UKMET again start to trend slower while the GFS strays to the fast side. The forecast remains extremely uncertain for evolution of energy that starts to flow into the West late Wed-Thu. At least through the 12Z/18Z runs the past couple days of GFS runs had been more consistent in principle than the ECMWF for forecasts across the eastern half of the country next weekend. In particular the 12Z ECMWF ultimately took an extra day to amplify/sharpen the eastern trough aloft versus most other guidance--leading to offshore surface development by early next week--while the prior run was a bit slower than the average and spun up a very deep eastern Great Lakes low by day 8 Mon. The new 00Z GFS is quick to close off a system over the Mississippi Valley, in an evolution reminiscent of the 12Z ECMWF from two days ago. As of day 7 Sun the ensemble means were not far apart aloft given typical day 7 forecast differences/errors. However a very wide dispersion of ECMWF ensemble members led to an ill-defined surface pattern over the East while 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 close to an even model/mean 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 fairly conservative in light of the high uncertainty. Operational input came from the 18Z GFS/12Z CMC and offsetting 12Z-00Z/18 ECMWF runs while the 18Z GEFS mean had a tad more weight than the 12Z ECMWF mean. ...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 possibly 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. Monitor Storm Prediction Center outlooks for latest details of the severe weather threat as the specifics become better defined. 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. through the Mid-Atlantic as having relatively higher probability of meaningful totals with the Ohio Valley and New England not far behind. 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. 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. 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. 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