Extended Forecast Discussion NWS Weather Prediction Center College Park MD 311 PM EST Fri Feb 19 2021 Valid 12Z Mon Feb 22 2021 - 12Z Fri Feb 26 2021 ...Overview and Guidance Evaluation... Latest guidance continues to show progressive mean flow over the lower 48 at the start of the week followed by a transition toward western U.S. mean troughing in response to strengthening/amplification of an upper ridge over the East-Central Pacific. To the east of the Rockies expect the mean flow to be nearly zonal, or perhaps slightly west-southwesterly as upper ridging over the southern Gulf of Mexico/far northwestern Caribbean strengthens somewhat around midweek. The pattern will support active weather over parts of the West (especially the Pacific Northwest into the Rockies) and likely bring a couple precipitation-producing systems across the eastern half of the country. The greatest model/ensemble discrepancies still involve the separate bundles of energy dropping into the developing western mean trough, one that arrives Tuesday-Wednesday and then ejects eastward thereafter followed by a second that arrives later in the week. For the leading feature a guidance average has held up well over the past day or so as the previously slow/closed CMC has accelerated a bit in the 00Z run while the 06Z GFS again (like yesterday) trended slower than the 00Z run to join the ECMWF. By early Thursday the 12Z GFS returned to a slightly faster solution (joined by the UKMET) while the 12Z CMC is still close to the 00Z ECMWF/06Z GFS. The 12Z GEFS mean is close to this latter cluster's timing as well and the new 12Z ECMWF is fairly consistent. The timing and shape of this shortwave become increasingly uncertain as progressive flow carries it across the central/eastern U.S. late in the week. Upstream energy arriving into the West Thursday-Friday has generally been better behaved over the past day or so but still with some detail/timing differences. 12Z models arriving thus far hint at slightly slower timing than some 00Z/06Z guidance but differences are still well within typical guidance spread/error ranges at that time frame. To the east of the Rockies, consensus has held up well for the early week shortwave trough crossing the East and associated surface low evolution/trailing front. Upstream northern tier U.S. and southern Canada flow will bring one frontal system across northern areas and then one or more trailing waves will anchor the rest of the front that makes its way through most of the central/eastern U.S. Tuesday-Thursday. Guidance shares this general depiction but has varied with frontal wave details after Tuesday and confidence is modest given dependence on smaller scale shortwave details. The shortwave ejecting from the West after midweek may form a frontal wave over the southeastern quadrant of the lower 48 by late Thursday/Friday but again with fairly low confidence on specifics. A blend of operational 00Z/06Z runs represented majority ideas well for the first half of the period, while a composite of varied 00Z ECMWF/CMC and 06Z GFS weights actually compared rather favorably to the ensemble mean pattern into days 6-7 Thursday-Friday as well. Thus the latter part of the forecast required only about 1/3 total input from the 06Z GEFS/00Z ECMWF means to account for detail uncertainty. The resulting blend provided a good middle ground for representing potential southeastern waviness late in the week, between the operational runs and ill-defined means. ...Weather/Threats Highlights... Over the West expect the highest precipitation totals over the Pacific Northwest as strong westerly flow aloft and a wavy cold front support one episode of enhanced rain/mountain snow early in the week and a northeastern Pacific upper trough/frontal system bring more precipitation after midweek. Moderate to heavy snow over the northern Rockies should continue into the early part of the week followed by a lighter trend and southeastward progression. Snowfall could become heavier again for a time over or near the Colorado portion of the Rockies after midweek depending on specifics of ejecting western shortwave energy. The moisture shield associated with the late week system may bring another area of snow to the northern Rockies. A storm/frontal system crossing the East early in the week will produce mainly light to moderate precipitation, with some snow most likely from the northern Mid-Atlantic to New England and rain to the south. A few pockets of heavier snowfall could be possible. Flow behind the early week system and around the frontal system right behind it will promote some lake effect/terrain-enhanced snowfall. Expect highest snowfall totals to be over upstate New York, to the lee of Lake Ontario. Otherwise fronts affecting northern areas to the east of the Rockies Tuesday-early Thursday should produce relatively light/scattered precipitation. Shortwave energy ejecting from the West may lead to a frontal wave and some heavier precipitation over parts of the central/eastern U.S. late in the week, along with snow along the northern fringe of the precip shield. However confidence is currently low for resolving the precise timing/coverage/intensity of this precipitation. The greatest warm anomalies for temperatures will progress from west to east across the northern tier states from Monday into early Thursday. Most areas should see one or more days with morning lows 10-20F above normal while such anomalies for highs may be more confined to the Northern Plains during Monday-Tuesday. Meanwhile during mid-late week a push of colder air behind a front will bring temperatures down to as much as 10-20F or so below normal from portions of the West through the southern half of the Plains. Coldest readings compared to average may be for highs over the central/southern High Plains on Thursday. The Southwest will be moderately above normal early in the week (perhaps exceeding plus 10F anomalies at some locations Tuesday) before a cooling trend. The East will see temperatures oscillate up to 5-10F or so on either side of normal in response to system progression. 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest, Mon, Feb 22. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Mon-Tue, Feb 22-Feb 23 and Thu-Fri, Feb 25-Feb 26. - Heavy snow across portions of the Pacific Northwest, Thu-Fri, Feb 25-Feb 26. - Heavy snow across portions of the Northeast, the Great Lakes, and the Northern Rockies, Mon-Tue, Feb 22-Feb 23. - Heavy snow across portions of the Central Rockies and the Central Plains, Wed, Feb 24. - Flooding possible across portions of the Southeast, the Mid-Atlantic, and the Pacific Northwest. - Flooding occurring or imminent across portions of the Southeast and the Mid-Atlantic. - Flooding likely across portions of the Southeast and the Mid-Atlantic. - High winds across portions of the Central Rockies, the Central Plains, the Central Great Basin, the Northern Plains, and the Northern Rockies, Mon, Feb 22. 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