Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Wed Mar 03 2021 Valid 12Z Sat Mar 06 2021 - 12Z Wed Mar 10 2021 ...Overview and Guidance Evaluation... Eastern Pacific mean troughing aloft will gradually expand eastward over the course of the period, eventually including portions of the western U.S. Features within and ejecting from this trough will focus multiple episodes of precipitation along the West Coast with somewhat more moderate totals over the Interior West and Rockies. By next Tuesday-Wednesday one ejecting shortwave may generate a significant Plains system along with a broad precipitation area over the central into east-central U.S. Meanwhile a shortwave entering the West to start the weekend will become more diffuse as it traverses the lower 48 though a mean ridge, temporarily flattening flow slightly which will help push along an amplified upper trough over the East Coast during the weekend. Expect a mean ridge to reach the East by next Wednesday. As for guidance details, the 12Z model consensus suddenly jumped toward the somewhat faster GFS for the shortwave entering the West by the start of the period early Saturday and the new 00Z runs are holding fairly close to that idea. The average of guidance is still a little slower than the GFS but differences are trending closer to the noise range. Upstream there is reasonable clustering for a deep-layer low to settle just off the Pacific Northwest from about late weekend into the first half of next week, with gradually increasing uncertainty for the eastward extent of the low and details of a possible reinforcing feature late in the period. Recent GFS runs have been a bit on the fast/strong side with shortwave energy ejecting into the West and Plains early-mid week. Based on recent operational model runs there is still at least a moderate signal for what could be a fairly strong system over the Northern Plains but with a fair amount of variability/spread for specifics. Overall an operational model blend trending to a model/mean mix (including some 18Z parallel GFS to tone down some operational run specifics especially late in the period) represented consensus evolution well. This yielded a solution perhaps slightly on the conservative side for the late period Plains system while awaiting improved agreement/stability on specifics. ...Weather/Threats Highlights... Systems carried within the mean trough aloft near the West Coast will support an unsettled period with multiple episodes of rain and higher elevation snow from the Pacific Northwest into northern California and Sierra Nevada. Precipitation could be moderate to locally heavy at times with highest five-day totals (perhaps reaching several inches) most likely over southwest Oregon into northern California. The Interior West and Rockies should also see periods of precipitation though with less extreme totals. Expect precipitation to expand across the Plains and east-central U.S. with a potential Tuesday-Wednesday system, with rain and thunderstorms in the warm sector and some wintry weather possible over the extreme northern tier depending on how the system evolves. Weakening Southern Plains upper low energy dropping toward the Gulf Coast by early Saturday and then becoming absorbed into the longwave East Coast trough will support fast moving Gulf of Mexico into Atlantic low pressure. Most associated rainfall should be over the Florida Peninsula with moderate to perhaps locally heavy rainfall possible over northern/central areas with the best interaction of daytime heating, available moisture, and lift. This wave will then deepen and move well offshore while cool high pressure builds over the eastern U.S. Much above normal temperatures over the Northern Plains this weekend, reaching 10-25F above normal, will gradually extend eastward into the Upper Midwest and Great Lakes with less extreme warmth also reaching into the Northeast by late in the period. Some daily records for highs and/or warm lows will be possible. By next Tuesday-Wednesday well above normal readings for morning lows in particular will extend well southward over the Plains and Mississippi Valley as Gulf inflow increases. Elsewhere, a cooling trend toward moderately below normal readings will spread across the western U.S. during the period. East Coast states will be chilly during the weekend with highs 5-10F or so below normal and then trend warmer. Rausch 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