Extended Forecast Discussion NWS Weather Prediction Center College Park MD 220 PM EST Tue Jan 14 2025 Valid 12Z Fri Jan 17 2025 - 12Z Tue Jan 21 2025 ...Strong Arctic surge pushing into the central U.S. by the weekend and gradually expanding eastward... ...Overview... By this weekend the guidance continues to show the mean pattern evolving into a very amplified regime consisting of a deep trough extending from Canada through the lower 48 and a ridge whose axis extends over the eastern Pacific and well northward. As this occurs, very cold air will drop into the central U.S. with the most extreme anomalies likely to be during Sunday-Monday. The trough axis should nudge far enough east by next Tuesday to offer some moderation over the northern-central Plains while coldest anomalies become aligned more over the East and South. Within this highly confident large scale pattern, embedded shortwave uncertainties continue to pose issues for specifics of potentially enhanced rainfall over the South around the end of the week into the weekend as well as for East Coast into western Atlantic frontal waviness that could produce lighter precipitation of various types. Meanwhile expect an area of snow to push south through the Rockies/High Plains with the cold surge before tapering off next week. ...Guidance/Predictability Assessment... While models agree regarding upper troughing over the central/eastern CONUS and an Arctic high coming in at the surface, systems on the periphery of these features are more uncertain and could have sensible weather impacts. At the start of the period, a low pressure system forecast to track just north of Minnesota and Lake Superior and its trailing front will be the main feature of note. A multi-model blend seemed reasonable for this system as it pushes through the East and forms relatively weak (until perhaps strengthening once offshore) low pressure waves along it. Meanwhile there are likely to be multiple shortwaves digging south through the western side of the trough, creating some differences in the upper pattern from the eastern Pacific to the Rockies during much of the period. The 00Z CMC was aggressive enough with shortwave energy to reorient the trough axis to be more positively tilted over the weekend, unlike consensus. The newer 12Z CMC run trended more favorably toward the other models. Differences with an initially eastern Pacific upper low continue to cause model differences downstream as it pivots eastward. Deterministic and AI models and ensemble members vary quite a bit on whether/how much energy is held back into the Southwest by next Tuesday. The general trend over the past day seems to be for more energy to separate into the southern stream at least somewhat from the main trough. Trends will continue to be monitored because this will have considerable effects on forecast aspects like precipitation that could be wintry next week. The WPC forecast used a blend of the 00Z ECMWF/06Z GFS/00Z CMC/00Z UKMET in decreasing proportional order early in the period, but phased out the CMC and UKMET in favor of the GEFS and EC ensemble means, and increased the proportion of means to half Days 6-7 while maintaining some EC and GFS inclusion. ...Weather/Hazards Highlights... The forecast is fairly consistent with respect to the Northwest seeing a moderate cooling trend on Friday followed by a pronounced surge of Arctic air dropping south over the eastern Rockies into the Plains by this weekend. Expect increasing coverage of temperatures that are 15-35F below normal, with expansion of the cold air bringing highs down to 15-25F below normal as far east as locations between the Mississippi River and Appalachians by Monday and continuing toward the East Coast by Tuesday. The most extreme anomalies are likely to be over the central High Plains and vicinity during Sunday-Monday, while coldest temperatures on an absolute basis should be over far northern areas where some subzero highs and lows in the -20s are expected. Wind chills should be even colder and more dangerous. Key Messages are set to begin from WPC in part due to these cold temperatures. The northern-central Rockies/Plains should begin to moderate on Tuesday while cold weather is expected to linger farther south. The West will see a period of cool but not extreme temperatures from the weekend into the start of next week, with near normal readings eventually beginning to push eastward from the West Coast. Ahead of the cold front progressing south/east from the Plains Friday onward, an area of above normal temperatures will progress from the central U.S. on Friday to the East Coast by early Sunday. The Upper Midwest may see lows that are 20F or more above normal early Friday, with plus 5-15F anomalies more common elsewhere within this area of relative warmth. By Friday night, approaching shortwave energy and a combination of fronts may start to generate some enhanced rainfall over the Lower Mississippi Valley and Southeast. The reasonably fast frontal movement and moderate rain rates should keep the flash flooding potential pretty minimal for the Day 4 ERO time frame. This activity should continue across the Southeast during Saturday and Saturday night with the supporting wavy surface front lying down a bit to become more east-west oriented, possibly supporting some training. Thus there is some potential for locally heavier rainfall into the Day 5/Saturday ERO period in the Southeast and Florida Panhandle. As rain occurred in that general vicinity yesterday, soil moisture percentiles are actually showing some above average anomalies there currently, but soils should dry out by late week. Continued to hold off on introducing any ERO areas as there is uncertainty in whether the forecast rain amounts would cause flooding issues, but there is likely a nonzero threat especially if training or repeating showers and storms with heavy rates occur. Some rainfall may persist over parts of the Southeast, especially northern Florida, beyond Saturday. Much lighter precipitation of varying types will be possible farther north over the East during the weekend and possibly into early next week. There are still significant differences for potential frontal wave development, keeping confidence low for details of precipitation coverage and amounts. However what signals exist suggest that any waves should be fairly weak with relatively light precipitation (which could be of the wintry variety from the Mid-Atlantic/Appalachians northward). Probabilities of 0.25 inch liquid in the form of snow reflect this, with low coverage of 10 percent or greater values, though higher elevations like the Adirondacks have higher probabilities. Some locally heavier snow will be possible to the lee of the Great Lakes from the weekend into early next week, as well as across the Appalachians with upslope flow. Farther west, expect snow to move southward across the Rockies/High Plains late this week into the weekend as part of the Arctic surge, with upslope flow helping to enhance accumulations over some areas. Snow squalls are of concern Friday with the cold frontal passage. Into Monday or Tuesday of next week, it is uncertain how much precipitation emerges over the southern tier from Texas eastward on the periphery of the upper trough and surface high pressure. However, the southern extent of the cold air at the surface means that if there is precipitation it would mainly be in the form of wintry precip, perhaps all the way to the Gulf Coast, and freezing rain/ice is an increasing concern for parts of the South given the possibility of a warm nose atop Arctic cold air. Uncertainties in the setup aloft over the Southwest into next week make the pattern at the surface somewhat uncertain as well. However, there is a general trend for weaker winds over southern California Friday into the weekend, but offshore flow may strengthen again by Monday as 850mb winds increase (dramatically in some models). Keep an eye on SPC 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