Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Mon Feb 17 2025 Valid 12Z Thu Feb 20 2025 - 12Z Mon Feb 24 2025 ...Intense Central to Eastern U.S. Arctic Blast this week... ...Overview and Guidance/Predictability Assessment... Latest guidance agrees upon moderate progression of individual shortwaves aloft within an evolution toward a larger scale mean pattern consisting of a western ridge and eastern trough by the start of next week. This transition will bring the best potential for meaningful precipitation to the Pacific Northwest from the weekend into early next week, along with a pronounced warming trend over the western-central U.S. after the very cold temperatures forecast east of the Rockies into late this week. The southern tier may see some precipitation but with much lower confidence for specifics, due to the lower predictability of medium to smaller scale leading shortwaves and associated surface features. The updated forecast started with an operational model early in the period (more 06Z GFS/00Z ECMWF weight relative to the CMC/UKMET) and then trended toward a model/ensemble mean mix (06Z GEFS/00Z ECens for the means) with the increase of detail uncertainty later in the period. Gulf into western Atlantic details will depend on specifics of shortwave energy emerging from the West and a separate northern shortwave dropping into the Great Lakes by Saturday, with some possible indirect influence of an upstream shortwave reaching western North America at that time. Machine learning (ML) models generally reflect the dynamical model spread at the surface during the weekend, with the sharper subset for the western energy yielding a better defined Gulf Coast surface wave (comparable to the 00Z CMC and latest ICON runs) versus a more diffuse/suppressed wave. By next Monday there is a diffuse signal for waviness over the western Atlantic but poor agreement for the details, with a range of possibilities including suppression of the Gulf wave while there is a mere front off the Southeast coast or instead progression of the Gulf wave into the Atlantic. GFS runs have been leaning on the western side of the envelope for their defined surface system. There is decent agreement on the series of fronts expected to affect the Pacific Northwest, though the fast flow pushing the fronts along will lead to some decrease in predictability. ...Weather/Hazards Highlights... An exiting coastal storm with lift robustly offshore the East Coast Thursday as mainly a maritime threat. High winds and surf will impact eastern New England, while recent trends are decreasing the potential for meaningful snow. In the wake of this storm, quite an impressive Arctic airmass is taking hold over the central and eastern U.S. this week with strong Canadian high pressure covering Plains as of Thursday. Entrenched cold air will maintain some lake effect snows and offer a widespread Extreme Cold threat. During Thursday into Friday, widespread record temperatures for morning lows/cold daytime highs are likely from the southern two- thirds of the Plains through the Mid-Lower Mississippi Valley and The South. The most extreme temperature anomalies will be on Thursday, ranging from 30 to 40 degrees below normal over the Central Plains/Mid-Mississippi Valley and 10-30 degrees below normal over much of the eastern U.S. The airmass will slowly moderate late week through the weekend while above normal temperatures spread into the western and eventually central U.S. By next Monday, expect most locations west of the Mississippi River to be above normal with decent coverage of plus 10-25 degree anomalies. Best potential for highest anomalies will be over and near the northern Plains. Temperatures over the East should finally reach near normal by next Monday. The West will see a leading Thursday system bring a swath of terrain/mountain enhanced snow over the north-central Intermountain West/Rockies. Accumulations should not be especially heavy though, with probabilities for 0.25 inch liquid in the form of snow in the 10-50 percent range. From Saturday onward the pattern becomes increasingly favorable for longer fetch moisture feed to fuel rounds of moderate to heavy low elevation rain and mountain snow primarily into the Pacific Northwest, but also inland to the northern Rockies. There is some uncertainty over the exact southward extent of this precipitation but the best potential for highest totals currently exists over the favored terrain and coastal areas of western Washington and northwestern Oregon. Farther east, no consensus exists for any strongly organized storm system during Thursday-Monday but multiple uncertain impulses feeding into an eventual larger scale eastern U.S. mean trough aloft may produce some light and unheralded but locally disruptive wintry weather in the Arctic airmass. One such feature is emerging western U.S. energy that may support periodic moderate rainfall over the Gulf and possibly extending into the Gulf Coast states in association with a persistent and slow moving wavy frontal zone. Most precipitation should be rain, but though a little wintry weather could be possible in the northern fringe of the moisture shield if it extends far enough north. Confidence is low regarding how the waviness may evolve over the Gulf during the weekend, and what the surface pattern may look like off the East Coast by next Monday. At that latter time the majority (though not all) of guidance suggests that most precipitation should stay offshore the Mid-Atlantic/New England coast. Meanwhile a series of waves may spread light precipitation (mostly snow) across parts of the Great Lakes into western New England from the weekend into early next week. Rausch/Schichtel 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