Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Thu Feb 13 2025 Valid 12Z Sun Feb 16 2025 - 12Z Thu Feb 20 2025 ...Strong storm to bring heavy snow to the Northeast on Sunday with less extreme rainfall to the south compared to Saturday... ...Overview... A deep upper trough lifting through the East on Sunday will support a strengthening surface low that should track from the Upper Ohio Valley to the Canadian Maritimes Sunday into Monday. This storm will bring significant wintry precipitation and then strong winds to New England plus rain to the south--but with less extreme totals than forecast over the Tennessee Valley/central Appalachians on Saturday. Behind this system, elongated upper troughing with a possible embedded low will likely linger over the northern tier U.S. and southern Canada while a couple shortwaves/surface fronts track into the West. These features will bring low elevation rain and mountain snow into the Pacific Northwest and northern California with some precipitation spreading farther inland. The first system arriving on Sunday should ultimately spread precipitation of varying types across the southern two-thirds of the central-eastern U.S. Tuesday onward as Plains low pressure tracks near the Gulf Coast and into the Southeast or farther east into the Atlantic. This part of the forecast currently has below average confidence. Arctic high pressure expanding south and east from the northern Plains/southern Canada will lead to well below normal temperatures for most of the country east of the Rockies. ...Guidance/Predictability Assessment... The models continue to show gradual narrowing of spread for the system forecast to be over the Upper Ohio Valley at the start of the forecast early Sunday, with dynamical solutions continuing to be a few mb stronger than machine learning (ML) models. Note that 00Z/12 and the 18Z/12 AIFS were the most recent ML models available for review. There are still some timing issues to the northeast, with the new 00Z UKMET straying to the slow side by early Monday. The system reaching the Northwest Sunday-Monday exhibits small- scale detail differences that will likely take into the shorter range to resolve more finely. Into Tuesday guidance still agrees fairly well with the shortwave dropping into the West, underneath elongated upper troughing aligned along the Canadian border. However by Wednesday-Thursday the forecast becomes rapidly more uncertain due to the combined differences that arise with timing of the Western U.S. shortwave and what becomes of the elongated northern trough. As for the latter, solutions range between the trough remaining open or an upper low closing off somewhere between the Northern Plains and Great Lakes. ML guidance tends to favor an upper low position a little northeast of the dynamical model average. The end result is a wide spread for timing/strength/track of associated surface low pressure. The most agreeable scenario, comprising the 00Z/12 ECMWF, 12Z GEFS mean, and a fair number of the ML models, was for a surface track near the Gulf Coast on Wednesday and then off the Southeast Coast by early Thursday. Among new guidance, the 00Z CMC was closest to this idea (after being slow in the prior 12Z run). The 00Z GFS becomes an eastern extreme due to being sheared with the northern trough (after being fast but closer to the coast in 12Z/18Z versions). In contrast, the new 00Z ECMWF is just inland over the Southeast. Needless to say, confidence in this part of the forecast is below average at this time. With the next system reaching the Northwest around midweek, some ECMWF runs continue to pull off more of a closed low aloft than most other dynamical or ML guidance. The 00Z/12 ECMWF at least provided a more palatable option than the 12Z run. The updated forecast represented the best ideas of guidance and good continuity with greater emphasis on the 18Z GFS/12Z ECMWF relative to the 12Z UKMET/CMC early-mid period. As spread rapidly increased by Wednesday-Thursday, that part of the forecast trended a lot more toward the 12Z GEFS/ECens means with only lingering input from the ECMWF (more 00Z/12 run than 12Z) for the operational component by next Thursday. ...Weather/Hazards Highlights... The strengthening storm lifting northeastward from the Upper Ohio Valley on Sunday will produce some warm sector rainfall along the East Coast, but expect the threat of heavy rainfall to decrease considerably compared to Saturday as the low and its trailing cold front become more progressive, so the Day 4/Sunday Excessive Rainfall Outlook depicts no risk areas. There could be some lingering localized issues over the Mid-Atlantic and southern New England depending on rainfall details and existing ground conditions leading into Sunday, but it will take into the short range time frame to resolve these details. Locations from New York state through central and northern New England will see the best potential for heavy snow on Sunday, with a transition zone of wintry mixed precipitation between the snow area and rain to the south. Continue to monitor forecasts as the exact position of the rain/snow line or transition band becomes better resolved. There may be a period of strong and gusty winds over parts of the East behind this system, along with some lake effect snow. In the West, a Pacific system will likely continue to produce rain and mountain snow over the Pacific Northwest into Sunday. Precipitation amounts/rates are still forecast to stay below the criteria for an Excessive Rainfall Outlook area, but light to moderate precipitation will still be possible. Precipitation will push east/southeast through the Rockies early next week while tapering off in the Pacific Northwest. Another system should bring precipitation back to the Northwest Tuesday into Wednesday with another round of light to moderate totals. As the upper trough crossing the West early next week heads into the Central U.S. by Wednesday and flow to the north potentially interacts, expect precipitation to develop from the Central/Southern Plains eastward. Wintry weather will be possible in the northern part of the precipitation shield with rain over the South. Currently the relatively greater potential for wintry weather extends from the Central Plains to the Mid-Atlantic, with latest probabilities for 0.25 inch of liquid in the form of show reaching 30-50 percent or so over parts of Missouri and Kansas during Tuesday-Tuesday night. Probabilities become more diffuse farther eastward during the next day as guidance spread rapidly increases within a broad range between a progressive system with light amounts or a slower/westward system producing heavier snow and other wintry precipitation types. Well below normal temperatures will likely prevail through the period over much of the central U.S., with surges reaching into the Southern Plains on Sunday and again next Wednesday-Thursday. Recent colder trends in guidance suggest some locations could be at least 30 degrees below normal (especially for highs) with an increasing signal for some daily record lows/cold highs by around midweek. The East will also see significantly colder temperatures after a very brief episode of warmth along the East Coast ahead of the Sunday system's cold front. Coldest anomalies of 10-25 degrees below average over the East should extend over the Great Lakes/Ohio Valley into the Mid-Atlantic/southern New England. Expect temperatures in the West to remain fairly close to normal, with the Northwest tending to be slightly below and southern areas a little above. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw