Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Tue Feb 21 2023 Valid 12Z Fri Feb 24 2023 - 12Z Tue Feb 28 2023 ...Major Northern Tier U.S. winter storm to track away from New England Friday... ...Record cold possible in the West while record warmth is possible in the Southeast... ...Overview... Guidance still agrees on the large scale pattern, consisting of an amplified mean trough aloft near the West Coast and strong progressive flow downstream, between an upper low north of Hudson Bay and an upper ridge gradually sagging south from southern Florida to Cuba. A significant winter storm will be winding down Friday as the progressive eastern U.S. flow pushes the system offshore the Northeast. To the west, heavy coastal rain and higher elevation/inland snow will spread across parts of California and the Southwest late this week into the weekend as temperatures drop with an upper low tracking southward near the California coast Friday-Saturday and reaching into the Southwest by Sunday. As the West Coast upper low emerges into the Plains early in the coming week, expect rain and thunderstorms to become widespread from the Southern/Central Plains through the Mississippi Valley with some severe weather possible. Meaningful precipitation will likely continue to the East Coast. Mixed precipitation including ice could be a threat along the rain/snow transition zone draped across portions of the Midwest/Great Lakes. Wintry precipitation may also extend into New England. Behind this system, precipitation will rebound again over the Pacific Northwest/Northern Rockies and then push south into California next weekend into early next week as another system approaches. The amplified pattern will lead to significant temperature extremes on both sides of the spectrum, with above average max/min temperatures over the East and below average max/min temperatures for the West--especially into Friday. ...Guidance/Predictability Assessment... Individual models have been shuffling around a little more in recent runs for the timing and track of the upper low initially near California. Latest GFS runs are still a bit on the faster side and the ECMWF a bit slow to bring the feature into the Southwest, but then switch places farther inland due to differences in weak shortwave energy arriving from the Pacific (ahead of a stronger amplifying trough). 06Z/12Z GFS runs are also a bit on the southern side of the spread for the upper low track over the Plains but the new 12Z UKMET clusters with those runs. Model/ensemble solutions also advertise a fairly strong surface low by Monday, with the majority at least as deep as the 980s mb. Then guidance suggests that a triple point wave should form near New England by next Tuesday. As for other features, the upstream trough nearing the West Coast by Tuesday shows better than average agreement for a day 7 forecast. The main difference of note is that the 06Z/12Z GFS and 12Z GEFS runs lean a bit eastward of most other solutions. Guidance has trended somewhat broader with upper trough energy crossing the northeastern quadrant of the lower 48 during the weekend, leading to a weaker trend for associated low pressure. Overall an average of the latest operational models, with a little ensemble mean input late, provided a good intermediate representation of individual systems while offering stable continuity as well. GFS input tilted more toward the 00Z run versus the 06Z cycle late based on comparisons with other guidance, while the ensemble component came from the 00Z ECMWF mean since it was deeper than the GEFS mean for the Plains through Northeast storm early next week. ...Weather/Hazard Highlights... A winter storm will already have impacted the Great Lakes region and parts of the Northeast prior to the the start of the forecast and will linger over the Northeast into Friday, with some gusty winds possible. The next system of note will be dropping south near the California coast late this week, bringing the potential for multiple feet of snow in some higher elevations across California and Interior West. The flow ahead of the upper low may produce the best focus for enhanced moisture into favored terrain along the southern California coast. Given the potential for at least a couple of inches of rainfall with the potential for runoff, the experimental Day 4 and Day 5 Excessive Rainfall Outlooks maintain a Marginal Risk for excessive rainfall. The upper low will spread less extreme amounts of rain/higher elevation snow into Arizona and parts of the Rockies. Then expect this system to spread a broad area of precipitation from the Plains through the East. Currently expect highest totals to be centered over the Midwest but there is still some uncertainty in specifics given the model differences for timing and track. This system will not have as much cold air to work with versus its predecessor, so there may be less coverage of wintry precipitation or at least a potential transition among precipitation types. Probabilities are somewhat higher for enough cold air to be in place for a period of meaningful snow over New England, depending on the details of coastal low development around next Tuesday. This system may produce some very strong and gusty winds through its existence, from California and the Southwest into the High Plains and through the eastern half of the country. Expect another episode of precipitation to arrive over the Pacific Northwest/Northern Rockies by next weekend into early next week, with the best focus for rain and mountain snow shifting into California by next Monday-Tuesday. The amplified pattern will support a pronounced temperature contrast across the country Friday when Northern Rockies to High Plains locations may see readings 20-40F below normal at the same time the East sees temperatures 20-30F or so above normal (through Friday morning). Frontal passage will bring colder temperatures leading to 10-20F below normal highs over New England Friday into Saturday. However the strong upper high over southern Florida/Cuba will keep temperatures warm over the South with some record highs possibly lingering into early next week. The system expected to affect the central/eastern U.S. early next week may bring another northward surge of unseasonably warm temperatures (plus 10-25F anomalies) to areas east of the Rockies. From the weekend into next week, the Southwest will tend to see the coldest anomalies for daytime highs within a generally cool pattern for most of the West. Rausch/Campbell 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml