Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 PM EST Thu Feb 18 2021 Valid 12Z Sun Feb 21 2021 - 12Z Thu Feb 25 2021 ...Active weather continues across the Pacific Northwest and northern Rockies through the period... ...Overview and Guidance Evaluation... Latest guidance generally agrees that an eastern Pacific mean ridge aloft (which may briefly extend into parts of the West late weekend/early next week) should retrograde enough to promote a developing mean trough over the western U.S. from the middle of next week onward. The progressive pattern in place ahead of this evolution and then a late period frontal system expected to drop into the Northwest will likely produce multiple days of active weather over the Pacific Northwest into the northern Rockies. Meaningful precipitation could extend farther south over parts of the West late in the period depending on specifics of shortwave energy. Systems carried along in the mean flow downstream will produce areas of precipitation to the east of the Rockies but likely with light to moderate totals. The forecast remains fairly agreeable and consistent for about the first half of the period when a composite of 00Z/06Z operational runs (with least weight on the CMC) depicts the features of interest well. A leading wave should track from the central Plains through the Great Lakes, most likely followed by triple point/coastal development leading to a surface low near Downeast Maine or Nova Scotia by early day 5 Tuesday. This evolution has been consistent in the ensemble means in addition to most models. The 00Z CMC was weaker/sheared with this system versus other guidance but the new run has adjusted to consensus. Then a strong jet pushing across the northern tier states and southern Canada will support Canadian low pressure and frontal system crossing the northern U.S. during the first half of next week. The majority cluster is now more agreeable on a defined frontal wave near northeastern Montana by early Tuesday, a trend toward recent ECMWF runs. Models/ensembles increasingly diverge for energy expected to descend into the West around Tuesday-Wednesday. GFS/GEFS runs have tended to be flatter with the 00Z runs becoming particularly progressive. Other models and ensemble systems have also varied with the handling of this energy. The 06Z GFS/GEFS mean made a significant adjustment back toward the 00Z ECMWF/ECMWF mean, with a blend of these models and means ultimately providing a good intermediate solution that is still a bit eastward of the 12Z/17 ECMWF and latest CMC runs that are on the slower/closed side of the spectrum. Interestingly there seems to be somewhat better agreement and continuity for the upstream feature dropping into the mean trough late in the week. Farther east the consensus hints at the aforementioned Montana wave continuing eastward through the Upper Midwest/New England, with the trailing front pushing somewhat farther southeast than continuity. ...Weather/Threats Highlights... Expect areas from the Pacific Northwest into the northern Rockies to see the heaviest precipitation during the period. The highest totals are likely to be from late weekend into early next week with strong westerly flow enhancing rain/mountain snow over western Washington and eventually reaching into parts of Oregon. Northern Rockies snowfall will also be heavy but should not be quite as extreme as over the Cascades. Heading into midweek precipitation should trend lighter as it spreads southeastward, though some areas of meaningful precipitation could extend into or near the central Rockies depending on shortwave/surface evolution details. Then a frontal system dropping in from the northeastern Pacific should bring another episode of rain/mountain snow to the Pacific Northwest by next Thursday. Some locations along the most favored terrain in western Washington may see several inches liquid equivalent over the five-day period. Meanwhile the pattern will support breezy to windy conditions over areas from the Northwest into the northern Plains through the first part of next week. Central Plains through New England low pressure and associated frontal system will produce generally moderate wintry precipitation during the first part of the week, most likely from the Midwest into the Northeast. Areas of light to moderate rain are likely farther south. Some lake effect snow will be possible in the chilly cyclonic flow behind this system with locations to the lee of Lake Ontario seeing the best potential for meaningful accumulations. After midweek another front should reach the Plains/east-central U.S. with possible precipitation development depending on still-uncertain details of shortwave energy ejecting from the West and the amount of low level Gulf inflow ahead of the front. The pattern evolution during the period will provide some variability to temperatures. On Sunday moderately below normal temperatures (generally 5-15F below normal) will linger over the East Coast while similar negative anomalies spread across the Interior West/Rockies into the Plains behind the wave tracking out of the central Plains. Aside from a modified form of this latter airmass reaching the east-central U.S. Monday-early Tuesday, temperatures over the South/East should trend to near or somewhat above normal levels over the course of the week. Northern tier areas will see a pronounced warming trend during the first part of the week with the northern Plains most likely to see plus 10-20F anomalies for both max/min readings while min temps should be more likely to reach that threshold farther east and west. The Pacific upper ridge that may extend into the West early in the week should support a brief period of warmth over the Great Basin/Southwest, peaking with some highs 10-15F above normal on Tuesday. By next Wednesday-Thursday expect upper troughing that develops over the West to spread colder temperatures (up to 10-15F below normal) from the Northwest/northern Rockies into the central Rockies/Plains. Areas to the south will see readings trend closer to normal. 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