Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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 increasing over the central Rockies and High Plains... ...Overview and Guidance/Predictability Assessment... A robust upper trough with embedded closed low will track slowly across the Southwest and into the Plains where surface cyclogenesis appears increasingly likely. 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. Models and their ensembles have progressively trended toward more vigorous interaction between the upper trough exiting the Southwest and the pre-existing stationary front across the central Plains. This has led to the heaviest rainfall to shift westward along and north of the stationary front with less rain forecast for the Mid-Mississippi Valley into the Ohio Valley. The increased dynamic interaction has also led to an increasing chance for heavy snowfall from the Front Range to the central High Plains through the weekend. The chance of snow has also increased farther north into the northern Plains. Nevertheless, models continue to show significant run-to-run variations regarding the details of the cyclogenesis and the cyclone's subsequent downstream track and intensity. The WPC forecast package was based on the consensus of the 00Z ECMWF/EC mean, the 06Z GFS/06Z GEFS with a small from the 00Z CMC/CMC mean. This takes the system slowly eastward across the Plains early next week. This scenario agrees fairly well with the latest (12Z) from the GFS and the ECMWF. Behind this system, there is an emerging model signal for another upper trough to dig southeastward through the western U.S. 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. In general, the guidance signals are getting stronger for the potential of a significant central Rockies/High Plains heavy snow event, together with the tendency for the snow to extend farther northeastward over the north-central U.S. and Midwest from late weekend into early next week depending on system intensity and track. Model/ensemble consensus continues to highlight an area from the south-central Plains into the Lower Ohio Valley for highest 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. Kong/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