Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Mon Mar 08 2021 Valid 12Z Thu Mar 11 2021 - 12Z Mon Mar 15 2021 ...Late week through weekend heavy rain threat from the south-central Plains to the Lower Ohio Valley/Mid-South... ...Late week/weekend heavy snow threat over the central Rockies and High Plains... ...Overview and Guidance/Predictability Assessment... A vigorous upper trough with embedded closed low expected to track rather slowly from California/Nevada through the Southwest and into the Plains will be a prominent focus of the forecast late this week into the start of next week. This system will initially support areas of enhanced rain and higher elevation snow over southern California and portions of the Great Basin. Then a favorable system track, deformation, and possibly a couple days of fairly strong upslope flow suggest increasing potential for a significant heavy snow event over the central Rockies and High Plains. This system has decent confidence into at least the first part of the weekend. However rapidly increasing model/ensemble spread for evolution of energy within an upstream Pacific trough lowers confidence in how much upper troughing (and associated precipitation coverage/amounts) may ultimately reach the West late in the period. This issue in turn begins to affect the evolution of the Southwest system once it reaches the Plains. Farther east a departing Upper Great Lakes wave will anchor a cold front that crosses the eastern U.S. (with support from amplified northern stream upper trough passage) but settles into the central Plains/Southeast. Interaction of this front with Gulf moisture and instability--and then reinforcement as the Southwest system tracks into the Plains--will likely promote a multi-day threat for locally enhanced rainfall and training convection. Ahead of the cold front, upper ridging will support much above normal temperatures for the East into later this week. The updated forecast for the Southwest into Plains system is generally a mere refinement incorporating the best consensus of latest data. Recent GFS/GEFS trends have generally been more in favor of the slower ECMWF/ECMWF mean but the GEFS mean component favored the 00Z cycle as the 06Z version returned to a somewhat faster timing. Model/ensemble spread and recent behavior leads to rather low confidence for how flow with the approaching Pacific trough may separate and thus what the resulting shortwave (possibly upper low) looks like as it heads into the West late in the period. GFS/GEFS solutions have tended to favor bringing a deeper and more amplified trough into the West while ECMWF/ECMWF ensembles have been more inconsistent. It take some time to see whether it is mere coincidence that the 00Z and new 12Z ECMWF have ended up with an evolution similar to latest GFS runs. Given the uncertainty, the favored blend reflects only a portion of the deepening trend for the incoming Pacific trough--which also leads to a northward nudge for the Plains system late in the period (furthered in the 12Z GFS/ECMWF). For an alternative Pacific/western U.S. evolution, the past couple CMC runs pull off so much energy over the Pacific that the West stays under an upper ridge. Model consensus at the start of the period fairly quickly phased out 00Z CMC/UKMET input due to developing differences over the Pacific and elsewhere, leading into a 06Z GFS/00Z GEFS and 00Z ECMWF/ECMWF mean blend that also added a little 12Z/07 ECMWF late in the period. ...Weather/Threats Highlights... The system tracking through the Southwest and into the Plains should produce the heaviest snow over the central Rockies and High Plains toward the end of the week through the weekend, after first producing areas of locally enhanced to possibly heavy rain/higher elevation snow over the Great Basin/Southwest late this week. Typical guidance errors 4-5 days out in time suggest some ongoing uncertainty for exact track of the system and location of highest totals but in general the guidance signals are getting stronger for the potential of a significant central Rockies/High Plains heavy snow event. Some snow may extend farther northeastward over the central U.S. from late weekend into early next week depending on system evolution and track. Confidence for this latter part of the forecast is much lower though. Model/ensemble consensus is highlighting an area from the south-central Plains into and perhaps somewhat south of the Lower Ohio Valley for highest five-day rainfall totals during the period. Late this week into the weekend there will be a persistent flow of Gulf moisture into a front that stalls over the south-central Plains/Mississippi Valley while the rest of the front crosses the East. Arrival of height falls aloft from the Southwest and associated surface system will provide reinforcement for heavy rainfall potential later in the weekend/early next week. Meanwhile the Storm Prediction Center is monitoring the threat for severe thunderstorms over and a little east of the Southern Plains. Check the latest outlooks for more information as severe weather details come into clearer focus. Expect the Northwest to see an increase of rain/mountain snow and a cooling trend by the weekend into early next week with the arrival of an upper trough and cold front. The details of timing, coverage, and amounts are uncertain at this time given guidance spread for specifics of the upper trough. The warm sector ahead of the Plains into eastern U.S. front late this week will contain much above normal temperatures. The Great Lakes/Northeast may see plus 15-25F anomalies for daytime highs on Thursday while a broader northeast-southwest band of plus 20-30F anomalies for morning lows may exist Thursday-Friday. Daily records will likely be more numerous for morning lows (where they hold as the min through the end of the calendar day) than for daytime highs. Meanwhile the system tracking from the Southwest into the Plains will drop highs to as much as 10-20F below normal from about the southern half of the West into the central Rockies/High Plains. Rausch/Schichtel 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, 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