Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Apr 07 2022 Valid 12Z Sun Apr 10 2022 - 12Z Thu Apr 14 2022 ...Active weather likely across a large part of the country next week... ...Overview... Latest guidance continues to show an amplified pattern in which one upper trough departs from the East after Sunday while energy digging in from the northeastern Pacific reinforces western U.S. mean troughing that should finally emerge into the Plains by around midweek. There is a decent signal in the guidance that this pattern should produce active weather over a significant portion of the lower 48 next week--including rain/snow over the West, eventually a vigorous storm system over the Plains/Upper Midwest with some snow to the northwest of its track, and a threat for areas of heavy rainfall plus strong to severe convection over some southern areas of the central U.S. However within this broad theme there are a number of uncertainties for details that temper confidence in specifics of the forecast, not only for the main area of focus over the western-central U.S. but also frontal position over the East due to differences in how leading shortwave energy initially ejecting across the Plains ultimately evolves. ...Guidance Evaluation/Predictability Assessment... Aside from the fairly stable handling of the upper trough departing from the East early in the period, latest models and ensembles are generally still finding it challenging to resolve some important aspects of the forecast. Depiction of leading shortwave energy crossing the northern tier U.S. and southern Canada has recently become an increasingly problematic part of the forecast, leading to some wide differences in the southern extent of a front over the East. 12Z and new 00Z UKMET runs are on the extremely amplified side of the spread with the shortwave energy and thus push the front quite far south. On the other hand the 12Z ECMWF deflected the energy around the eastern North America ridge, keeping the front farthest north. Given the current spread, preference was to emphasize the 12Z ECMWF and the 18Z GEFS/12Z ECMWF means which lean toward the northern side of the spread for the front but represent a modest southward adjustment from continuity. Further changes can be made if newer guidance improves confidence. Meanwhile farther west, guidance is still working on how to depict Gulf of Alaska energy that should flow southeastward into the western U.S., with modest differences in timing and exactly when a closed low forms having significant influence on the character of low pressure nearing the central/northern West Coast by early Monday. The manual forecast shows a better defined system than the weak and suppressed 12Z (and new 00Z) ECMWF but not as strong as the GFS/UKMET/CMC given the currently low predictability of this evolution. By late in the period the ensemble spaghetti plots are extremely messy over the eastern Pacific and western U.S. so detail confidence remains quite low. In light of this uncertainty, there is a positive consideration that the 18Z/00Z GFS and latest CMC/ECMWF runs all cluster decently in principle for how the upper trough/low ejects into the Plains/Midwest to produce what would be a significant storm system with general support from the ensemble means. Trailing flow could bring another system toward the Pacific Northwest but again with below average confidence. The new 00Z GFS has trended favorably toward the ensemble means for the eastern Pacific/western U.S. upper pattern by the end of the period, in contrast to the strong ridge or southern/elongated trough of the 12Z and 18Z runs respectively. As mentioned above, the first half of the forecast emphasized the 12Z ECMWF and 18Z GEFS/12Z ECMWF means to yield the desired frontal alignment over the East, with only 30 percent total input of the 18Z GFS and 12Z CMC/UKMET. Later in the period a more balanced model/mean blend captured a good intermediate starting point for the system that may track into the Plains and Upper Midwest by next Wednesday-Thursday. ...Weather Highlights/Threats... Early in the period expect the best rain/mountain snow focus to be over the Pacific Northwest and central West Coast. However the details of location and intensity are unusually uncertain for a forecast only 3-4 days out in time as guidance varies widely for the strength and track of arriving low pressure. Lighter activity should prevail eastward through the northern Rockies. Then some moisture should extend southward into the Sierra Nevada, with rain and mountain snow extending across central and possibly some southern parts of the West. By Tuesday the southern Montana into Colorado Rockies and north-central High Plains should begin to see a favorable combination of upper trough dynamics and low level upslope flow that would increase precipitation over those areas, with the High Plains possibly seeing some springtime snow in addition to the Rockies. Potentially strong low pressure that may track from the Plains into the Upper Midwest would extend snow farther northeast across the northern tier along with strong winds. Meanwhile in the warm sector, there will be increasing coverage and intensity of rainfall over the eastern Plains/Mississippi Valley with the establishment of moist low level flow from the Gulf of Mexico. Persistence of the Gulf inflow could lead to a couple days or so with a heavy rain threat over or near the Lower Mississippi Valley. Also the Storm Prediction Center is monitoring the potential for severe weather over some southern areas in the central U.S. Check their latest outlooks for more information. Farther north, a front draped over the Midwest could also provide a focus for rainfall though likely with lower totals given less moisture/instability. Lack of confidence in some specifics of eastern Pacific/western U.S. flow maintains the potential for some changes in the above most likely scenario. Colder air moving into the West will initially bring highs down to as much as 10-20F below normal over the Northwest/northern Rockies as of Sunday. An area of minus 10-25F anomalies for highs should then reach into the Intermountain West, Rockies, and northern-central Plains by next Tuesday-Wednesday. The northern half of the Plains should still see highs around 15-20F below normal on Thursday. Sunday should provide the best potential for some locations in the Northwest to see max temperatures near or below current record cold highs but Monday could still produce a few lingering records. Meanwhile most of the eastern half of the country should trend from mostly below normal temperatures (minus 5-15F anomalies) on Sunday, and potential for a late frost across parts of the southeastern quadrant of the country, to mostly above normal readings (plus 5-15F anomalies for highs and perhaps slightly warmer for morning lows) spreading from west to east after the weekend. However confidence is lower for exact temperatures over the northern half of the East due to uncertain position of a front during the first half of next week. Rausch/Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml