Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1253 PM EST Mon Mar 1 2021 Valid 12Z Thu Mar 4 2021 - 12Z Mon Mar 8 2021 18Z Update: The 12Z model guidance, along with the 00Z ECMWF, are in above average agreement on the synoptic scale pattern through Friday across the continental U.S., and a general deterministic model blend sufficed as a starting point in the forecast process. By Saturday, the operational GFS becomes notably more progressive with the shortwave crossing the Intermountain West compared to the other guidance, including the GFS parallel, and the 12Z CMC trended more in line with the slower ECMWF, although not as slow as its 00Z run. Since the GFS parallel version was closer to the ensemble means, it was given more weighting for the second half of this forecast period than the regular GFS, and more contributions from the EC and GEFS means by next Monday. In terms of sensible weather, the heaviest precipitation appears most likely for portions of the Pacific Northwest early in the forecast period, and rainfall chances increase for the Florida Peninsula for this weekend as the Gulf of Mexico low affects the region. This low is expected to track far enough offshore to keep most of the East Coast region dry, but if future forecasts trend more towards the northwest, then precipitation chances could increase for coastal regions. The previous discussion is appended below for reference. /Hamrick ...Overview and Guidance Evaluation... Latest models and means continue to show a fairly amplified large scale upper pattern featuring a west-central U.S./Plains ridge between mean troughs over the eastern Pacific and East Coast. Embedded within this pattern will be an upper low that should reach the southwestern U.S. and then the Southern Plains after midweek, followed by rather pronounced suppression of the upper energy and surface reflection beneath the eastern mean trough aloft. The axis of this trough should be far enough eastward to keep any developing Atlantic low pressure by the weekend/early next week well offshore. The eastern Pacific mean trough will feature two primary features, one likely reaching the West Coast early in the weekend (with leading wavy front) and then ejecting inland followed by amplifying upstream energy that should bring another system toward the Northwest toward late weekend/early next week. Guidance has stabilized since runs from 24 hours ago had adjusted slower for the upper low tracking into the southwestern U.S. and Southern Plains, followed by suppression of most moisture/surface waviness around the southern periphery of high pressure that builds over parts of the east-central/eastern U.S. and Southern Plains. Hopefully this evolution represents something close to a "final answer" after multiple days of wildly diverse and inconsistent solutions, first for whether an upper low would form to begin with and then the low's track/timing. The primary forecast discrepancy upstream regards how the leading trough off the West Coast ejects inland after guidance maintains good consensus into Friday. Thus far there have generally been two persistent clusters, a slower/sharper ECMWF-led majority that ultimately brings the remaining shortwave energy eastward/east-northeastward as it continues across the lower 48, and the faster/flatter GFS-GEFS mean that take more of the energy around the ridge. CMC runs through 12Z have been somewhat of a compromise. A blended approach still looks reasonable, leaning two-thirds or so toward the majority cluster near the West Coast into Saturday while toning down the strength of this energy relative to the 12Z ECMWF as it continues into the large scale mean ridge. The 00Z GFS/GEFS mean have adjusted a little slower than their prior runs, the CMC has adjusted close to the ECMWF idea, and the new ECMWF has become less amplified as the energy ejects inland--in sum maintaining general support for current preferences. The next system that reloads the East Pacific mean trough aloft shows fairly good large scale agreement in most models and means (00Z CMC could be somewhat flat). Thus a model/ensemble mean composite represents the large scale pattern well while downplaying low-predictability details during the latter part of the forecast. ...Weather/Threats Highlights... The upper low likely to reach the Southwest/Great Basin area by early Thursday will produce locally enhanced rain and higher elevation snow over the Four Corners states with moisture then extending eastward into the Plains. Some moderate to heavy rainfall is possible over the Plains but likely over a fairly small area. Then most of the rainfall should trend lighter as it gets suppressed along and south of the Gulf Coast, though activity could rebound again over parts of the Florida Peninsula during the weekend. Continue to expect any low pressure development over the western Atlantic from the weekend into early next week to be well offshore. From late week into the weekend the eastern Great Lakes could see some lake effect activity in prevailing cool cyclonic flow. Precipitation associated with the first eastern Pacific upper trough/surface front will focus primarily over Vancouver Island and into the Olympics at first and then extend southward toward the central West Coast and possibly farther south over California. A frontal wave nearing the coast may support a band of heavier precipitation by Saturday. Rain and mountain snow will trend lighter as the upper shortwave continues inland, with mostly light totals over the Interior West and Rockies. The next system should bring another episode of rain and mountain snow to the northern half or two-thirds of the West Coast. Expect temperatures over the Northern Plains and vicinity to remain well above normal through the period with plus 10-25F anomalies for highs and only slightly less extreme for morning lows. The amplified pattern aloft will also bring a more moderate warming trend to the West including warmest anomalies of plus 10-15F for highs from portions of the Interior West into the Southwest and eventually southern Rockies. Meanwhile the upper trough near the East Coast will keep that part of the country on the chilly side. Highs of 5-15F below normal are likely to be most widespread Friday-Sunday. 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