Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Jan 13 2025 Valid 12Z Thu Jan 16 2025 - 12Z Mon Jan 20 2025 ...Strong Arctic surge pushing into the central U.S. by the weekend and gradually expanding eastward... ...Overview... Models and ensembles still show the mean pattern evolving toward a very amplified regime consisting of a ridge extending from the eastern Pacific well northward and a Hudson Bay into northern Mexico mean trough by the weekend. As a result expect very cold air to drop into the central U.S. then, with below normal temperatures expanding into the eastern half of the country by next Sunday-Monday. Although the large scale pattern has well above average confidence/predictability, there are embedded shortwave details that guidance has been showing significant difficulty in resolving. As a result confidence is below average for any associated surface waves that could have a significant influence on sensible weather over the East (with rain south and potential wintry weather farther north) from the weekend into early next week. There are also uncertainties with a leading shortwave crossing the southeastern quadrant of the lower 48 around Thursday and a lower latitude Pacific upper low whose energy should open up and eject across the lower 48. ...Guidance/Predictability Assessment... Early in the period on Thursday, there are still notable differences with a shortwave forecast to cross the southeastern quadrant of the country. In particular, the 00Z GFS is a deep and slow extreme relative to other new guidance and this results in the model run spreading much more rainfall across Florida. While the majority of guidance is signaling a weaker/faster shortwave with a corresponding drier forecast over Florida, there have been previous random solutions with a GFS-like scenario. Preferences side with the majority scenario but with the understanding that a low- probability risk still exists for the alternative. Aside from the GFS which is recovering after a couple weak runs, guidance has been nudging a little stronger with the system forecast to track near northern Minnesota/Lake Superior around Friday and continue into eastern Canada thereafter. Amplifying dynamics will push the trailing cold front south and east, with notable spread/continuity changes for shortwave energy forecast to drop through the West. The 12Z and new 00Z ECMWF runs have become somewhat more progressive than a number of its prior runs (trending a little in the GFS/GEFS direction) while the CMC/CMCens are persistent in showing the greatest amplitude/sharpness of western energy. Early on some of these differences influence the details of shearing Pacific upper low energy (with some influence on surface waviness/southern tier rainfall farther east). Then shortwave differences have a more significant influence on potential surface development toward next Sunday-Monday. Latest ECMWF runs and some machine learning (ML) models show potential for some combination of shortwave energy to support a Mid-Atlantic wave that tracks into or south of the Canadian Maritimes. The 18Z GFS fit into this cluster as well, but 12Z and new 00Z runs show a more inland surface low track that seems to have minimal support from other guidance. CMC/ICON runs show a flatter evolution due to holding the sharpest upper troughing farther west. Finally, there is a fair amount of spread among dynamical/ML models for the strength and location of high pressure over the central U.S. by next Monday, with no dominant clustering evident at this time. In order to reflect a combination of prevailing themes and accounting for spread/guidance variability, the updated forecast incorporated parts of the 12Z and 00Z/12 ECMWF runs, 18Z GFS, and small input from the ensemble means (18Z GEFS, 12Z CMCens, and 00Z/12 ECens since the 12Z cycle was not available locally for blending). ...Weather/Hazards Highlights... After a chilly start to Thursday over parts of the East, expect temperatures to moderate from late week into the weekend as above normal temperatures over the Plains/Mississippi Valley on Thursday-Friday (lows up to 15-25F above normal over the Upper Midwest) then reach the East with less pronounced anomalies ahead of a cold front. Behind the front, expect the Northwest to see a cooling trend on Friday, but then during the weekend the eastern Rockies into the Plains will see a pronounced surge of Arctic air. Expect increasing coverage of temperatures that are 15-35F below normal, with expansion of the cold air bringing highs down to 10-20F below normal over the East by next Monday. The most extreme anomalies are likely to be over the central High Plains and vicinity, while coldest temperatures on an absolute basis should be over far northern areas where some subzero highs are possible. Much of the lower 48 should be fairly dry on Thursday. The best organized activity should be across parts of the Great Lakes into the central Appalachians with periods of snow from a combination of lake effect and a weakening frontal system passing through the region. By Friday or Friday night, approaching shortwave energy and a combination of fronts may start to generate some enhanced rainfall across the southern tier. However differences in shortwave details lead to a fair amount of spread for location and magnitude of amounts. Therefore the Day 5 Excessive Rainfall Outlook does not reflect any risk areas at this time. Potential exists for much of the East to see at least some precipitation in the Saturday into Sunday or Monday period, but guidance differences for potential surface development keep confidence low for details of precipitation coverage and amounts. Best potential for wintry weather will be from the Mid-Atlantic northward, with rain farther south and a possible transition zone in-between. 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 enhanced accumulations over some areas. 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