Extended Forecast Discussion NWS Weather Prediction Center College Park MD 114 AM EST Fri Jan 17 2025 Valid 12Z Mon Jan 20 2025 - 12Z Fri Jan 24 2025 ...Strong Arctic surge will move from the Central U.S. into the South/East next week... ...Overview... Models and ensembles continue to show a very amplified mean pattern through next week consisting of a deep trough over much of the lower 48 with an eastern Pacific ridge which may eventually edge into the Western U.S. late next week. Confidence remains high in a surge of Arctic air progressing from the Central U.S. to the East early next week. By midweek most of the Plains should see moderating temperatures while the cold will likely linger over the South and East. Embedded shortwaves within the overall mean trough have low confidence, but the details of which will play an important role in determining the coverage and amounts of potential wintry precipitation the South into next week. ...Guidance/Predictability Assessment... Latest models and ensembles generally show good agreement on the overall large scale pattern across the CONUS during the medium range. However, despite the synoptic agreement, there remains quite a bit of uncertainty in the details of individual shortwaves dropping into and through the mean trough. The first of these digs into the South around Tuesday and swings off the Southeast coast by Wednesday. There is considerable timing and amplitude spread in this which impacts how far south the cold air reaches and exact precipitation types even as far south as the Gulf Coast. The CMC and UKMET were the deepest/coldest (and slower) than the better consensus and were not the preferred solutions early in the period. The early period WPC leaned most heavily on the GFS and ECMWF. A compact and cut off closed low over the East Pacific may gradually get re-absorbed into the mean flow over the CONUS around mid-week but the details of this remains very uncertain. This could push another shortwave through the South mid to late week, with some increasing QPF across parts of the Southern Plains. Yesterdays 18z GFS run was most bullish with this, but the new 00z run tonight (which came in after the WPC forecast was generated) backed off. Another shortwave drops through the Rockies and surges south Thursday-Friday, and again the shape and amplitude of the mean trough at this time is very uncertain and has implications on precipitation across the South. Felt the 12z/Jan 16 ECMWF was the best middle ground consensus so this combined with the ensemble means formed a good late week starting point for the WPC forecast. ...Weather/Hazards Highlights... By Monday, a broad area of hazardously cold Arctic air will already cover the central to Eastern U.S. with daytime highs at least 20F below normal for most. The coldest anomalies are likely to be over the High Plains and vicinity where highs could be 30-40F below normal. Coldest temperatures on an absolute basis though should be over northern Plains and Upper Midwest where subzero highs and lows in the -20s are likely early in the week. 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 gusty winds) while anomalously 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 into the start of next week, with near normal and then slightly above normal readings slowly beginning to push eastward from the West Coast thereafter. Early in the week the pattern would favor Lake Effect snow across/downwind of the Great Lakes with some generally light and scattered rain across Florida associated with a lingering front. 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 wintry, 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. Moderate to locally heavy rain is possible across the central Florida Peninsula as well mid-week. 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 which may increase fire weather concerns. Continue to monitor Storm Prediction Center fire weather forecasts for further information. Santorelli 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