Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 PM EST Thu Jan 16 2025 Valid 12Z Sun Jan 19 2025 - 12Z Thu Jan 23 2025 ...Strong Arctic surge will push into the central U.S. this weekend and reach the East by early next week... ...Overview... Models and ensembles continue to show a very amplified mean pattern consisting of a deep trough from northeastern Canada through the lower 48 and an eastern Pacific ridge extending well northward. Thus confidence remains high in the forecast of Arctic air dropping southward over the central U.S. this weekend and expanding to include much of the East by early next week. By midweek most of the Plains should see moderating temperatures while the cold will likely linger over the South and East. In contrast, guidance is having a lot of difficulty in resolving important embedded shortwaves that will play an important role in determining the coverage and amounts of potential wintry precipitation across the East on Sunday and then the South into next week. ...Guidance/Predictability Assessment... There is still a frustrating amount of model spread with a shortwave and surface low/frontal wave impacting the East on Sunday that affects snow chances and amounts. CMC runs have been farther inland with the low track which in turn tracks QPF (snow) farther west into the Mid-Atlantic and Northeast and even keeps the coastal Mid-Atlantic with rain as the precipitation type. ECMWF runs have generally been more suppressed with the low track to keep snow in the southern Mid-Atlantic and offshore of New England farther north, with the exception of the 06Z run, and the 12Z run trended a little favorably. The EC-based AIFS on the other hand has been fairly persistent in its precipitation axis across the Mid-Atlantic and coastal Northeast, and the UKMET is similar. GFS runs had been suppressed but have gradually been trending northward. Thus the general trend is for increasing precipitation and snow probabilities across the urban corridor from DC to NYC to Boston with a low track that is in the middle of the model spread. But small shifts in the low track will cause dramatic differences in impactful snow, so continue to monitor forecasts as this system moves into the short range. Energy digging south through the western side of the broad trough early next week will also be a factor in model uncertainty on the periphery of the broad trough. Models as of about a day ago tended to hold more energy back in the Southwest to produce positively tilted troughing around Tuesday. Today's model guidance (with the exception of the CMC) is trending away from that pattern in favor of a small closed upper low to the west of the main trough in the eastern Pacific to Southwest or Great Basin. There are, of course, some model differences in when/how that feature gets absorbed back into the mean flow. These model differences have ramifications for how much precipitation (wintry in most areas) affects the south- central U.S. to Southeast. In a general sense, CMC ensemble members have more wintry precipitation across the South compared to the more suppressed GEFS and ECMWF members. Then by Wednesday- Thursday, the broad trough axis should finally lift and push eastward, while there is general model agreement for more troughing to drop into the northern Rockies or so. The WPC forecast was based on a multi-model and ensemble mean blend early in the forecast period given the model variations even at the beginning. Increased the proportion of the ensemble means to well over half by Days 6-7 due to the detail differences among the relatively predictable large scale pattern. ...Weather/Hazards Highlights... By Sunday a broad area of hazardously cold Arctic air will already cover the central U.S. and should expand to cover much of the east by Monday. Expect many areas to see highs at least 20F below normal, with the coldest anomalies likely to be over the High Plains and vicinity where highs could be 30-40F below normal. Coldest temperatures on an absolute basis should be over northern Plains and Upper Midwest where subzero highs and lows in the -20s are likely. Wind chills should be even colder and more dangerous. WPC is posting Key Messages regarding this cold outbreak. The northern-central Rockies/Plains should moderate considerably by Tuesday-Wednesday (though with a potential for high winds) while cold weather may persist into Wednesday and possibly to some extent Thursday across the South and East. The West will see a period of cool but not extreme temperatures from the weekend into the start of next week, with near normal and then slightly above normal readings slowly beginning to push eastward from the West Coast. The wave forecast to track quickly off the East Coast on Sunday is likely to produce some possibly impactful precipitation before its departure. As discussed in the above section, chances for plowable snow are trending up in the urban corridor with a compromise/middle ground forecast track for the low, but uncertainty remains. After this system exits, the pattern will be favorable for lake effect snow through midweek. Farther south, fronts pushing through Florida could provide some focus for rain. Farther west, a shortwave digging into the West early next week may produce some scattered snow depending on its evolution. What this shortwave does after Monday will determine how much precipitation emerges over the southern tier from Texas eastward on the southern periphery of the overall upper trough and surface high pressure. The southern extent of the cold air at the surface means that a majority of precipitation could be of a wintry variety, perhaps all the way to the Gulf Coast. Freezing rain/ice is also an increasing concern for parts of the South given the possibility of a warm nose atop Arctic cold air. Southern tier snow potential continues to show up in the winter weather probabilities for 0.25 inch liquid in the form of snow, 10-20 percent over an area just north of the western Gulf Coast for the early Monday- early Tuesday period and 10-40 percent across a much broader swath of the South into southern Mid-Atlantic early Tuesday into early Wednesday. Some precipitation potential lingers thereafter, but through this whole period confidence in details is below average due to considerable model differences and variability. Uncertainties in details aloft over the Southwest into next week make the surface pattern uncertain as well. However, there is some potential for a period of relatively stronger winds over parts of California and vicinity around Monday-Tuesday. Continue to monitor Storm Prediction Center fire weather forecasts for further information. Tate/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