Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Tue Mar 09 2021 Valid 12Z Fri Mar 12 2021 - 12Z Tue Mar 16 2021 ...Late week/weekend heavy rain threat from the south-central Plains to the Lower Ohio Valley... ...Late week/weekend heavy snow threat over the central Rockies and High Plains... ...Overview and Guidance/Predictability Assessment... A robust upper trough with embedded closed low will track slowly through the Southwest and into the Plains and be a prominent focus of the forecast late this week into the start of next week. This system will support areas of enhanced rain and higher elevation snow over the southern Great Basin/Southwest and a favorable system track, deformation, and several days of fairly strong upslope flow suggest increasing potential for a significant heavy snow event with focus over the central Rockies and High Plains. This system offers good forecast confidence and continuity into 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. Guidance is all over the place with ejection and lifting of this main feature over the central U.S. and continuity is not well maintained days 6-7. 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. Given the uncertainty, the WPC favored blend reflects only a portion of the deepening trend for the incoming Pacific trough. This still leads to a northward lift for the Plains system into next week. However, the WPC solution leaned increasingly toward the ECENS mean that does not bring the Plains system as quickly northward as the 12/18 GFS or the 12 UTC ECMWF. The 00 UTC GFS has trended a bit slower, but the 00 UTC ECMWF is now a lot slower. 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. Overall, the WPC medium range product suite was primarily derived from a composite blend of best clustered guidance from the 18 UTC GEFS mean, 12 UTC ECMWF/ECMWF ensemble mean and the 01 UTC National Blend of Models Friday into Sunday before shifting blend emphasis increasingly toward the 12 UTC ECMWF ensemble mean by early next week. ...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 north-central U.S. and Midwest 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 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. 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