Extended Forecast Discussion NWS Weather Prediction Center College Park MD 209 AM EST Sat Jan 13 2024 Valid 12Z Tue Jan 16 2024 - 12Z Sat Jan 20 2024 ...Hazardous and record-breaking cold from an Arctic airmass will spread across the Northwest into the central and east-central U.S. next week... ...Potential for an impactful storm system near the East Coast next Tuesday-Wednesday... ...Overview... Latest guidance maintains a deep and broad mean trough aloft over much of the lower 48 through much of the week, ahead of the upstream mean ridge over the eastern Pacific tracking into the western U.S. late week. This upper pattern and Arctic high pressure filtering in at the surface will support very cold temperatures over areas from the Northwest through much of the Plains/Mississippi Valley into Tuesday, with numerous daily records for cold lows/daytime highs. By Wednesday-Thursday there should be some moderation and shift east of the cold temperatures before a resurgence in the central U.S. late week (but not quite as cold as the initial event). Precipitation-wise, meaningful snow is still possible in parts of the Mid-Atlantic/Northeast on Tuesday, though with continued uncertainty in amounts. The northern half or so of the West should also see increasing precipitation chances Tuesday and beyond. ...Guidance/Predictability Assessment... Model guidance is quite agreeable with the large and deep upper trough covering the eastern two-thirds of the nation next week, as well as the upstream ridge shifting into and across the western U.S. late week. Smaller-scale features show more differences that will be impactful for sensible weather though. The first of these more uncertain features is the surface low that could create an impactful storm system near the East Coast. Though models still agree in showing shortwave energy pivoting through the southern side of the broad mean trough serving to form this surface low, low predictability small detail issues with the shortwave must be producing some surface low differences in the track after consensus showing an elongated area of low pressure along or just offshore the Southeast Coast early Tuesday. In general, most model runs in the past couple of cycles have trended toward a more offshore solution with the surface low, with the 12Z and 00Z UKMET the main western/inland exception. CMC runs are also slightly west of the now agreeable GFS/ECMWF cluster but not to the same extreme. 12Z/18Z ensemble means generally lagged the low track more slowly than the deterministic runs, showing there is not only east-west spread but north-south too. The solution considered most likely was a blend of the GFS/ECMWF/CMC and the GEFS and EC ensemble means. Most of the observed run-to-run variability is fairly typical for forecasts a few days out in time but still leads to significant differences for precipitation amounts (and in some cases, type) along the East Coast. There has been some model convergence in another aspect of the pattern Tuesday-Wednesday, namely a shortwave and surface low diving toward the Northwest. After the GEFS/GFS over the past few days had been steadfast in maintaining a strong upper ridge over the East Pacific/West Coast, the more recent runs of the GFS/GEFS starting with 12Z have shown an indication of the shortwave/surface low, matching better with other guidance. Thus this forecast was able to show that low a little stronger and trend precipitation up compared to the previous forecast. However there are still some placement and strength differences with this, including in the newer 00Z models. Generally speaking this shortwave should get absorbed into the central U.S. trough during the latter part of the week, and could be a factor in an uncertain frontal system tracking through the east-central and eastern U.S. late week with some precipitation. The 12Z ECMWF seemed to be among the slowest solutions with this front and associated low pressure through the eastern U.S. The WPC forecast used a blend of the ensemble means and deterministic models even for the early part of the forecast period given the preference for an intermediate/ensemble mean solution for the western Atlantic low. Increased the proportion of the ensemble means to over half by Days 6-7 amid increasing uncertainty. ...Weather/Hazards Highlights... Very cold air originally stemming from the Arctic will be atop the northern Great Basin and Rockies and southeastward through the central U.S. on Tuesday, though starting to moderate a bit from the short range period. Still, widespread temperatures of 20 to 40 degrees below normal are forecast, with low temperatures in the -10 to -20 range across the northern half of the Plains and into the Upper Midwest, and locally even colder in higher elevations of the Rockies. High temperatures at least look to be above 0F in most areas by Tuesday. Any wind chills would be even colder and thus more dangerous. The cold air diving farther south into the southern half of the Plains and Mississippi Valley will not be as cold in terms of absolute temperatures, but temperature anomalies of 20 to 40 degrees below average are likely to set widespread records across the South into Tuesday. By Wednesday, temperatures finally look to moderate in the Rockies to High Plains, though chilly temperatures (albeit in moderated form) will shift into the eastern half of the country before warming closer to normal on Thursday. Some reinforcing cold air may reach the northern Plains by Thursday and expand to include most areas between the Rockies and Appalachians Friday and farther east by Saturday. Expect less extreme temperature anomalies than those forecast earlier in the period, but Friday and Saturday may still see temperatures 10-25F below normal. A low pressure system developing along a front in the western Atlantic will track quickly northward Tuesday into Wednesday. Snow could spread across the Mid-Atlantic and Northeast to the west of the low track, but snow amounts and placement/coverage will be very sensitive to this storm track and strength. The general model trend over the past couple of cycles has been for a farther east/more offshore low track, lessening the coverage of precipitation falling farther inland, but an area of 30 percent or greater potential for 0.25 inch liquid in the form of snow is still in place across eastern parts of the northern northern Mid-Atlantic coast through New England. Continue to monitor forecasts for additional changes. Farther south, moisture may focus along and ahead of the front that moves across Florida on Tuesday. There is perhaps a nonzero chance of heavy rainfall, but does not look impactful enough for an area in the Excessive Rainfall Outlook Tuesday. Back to the north, expect continued periods of lake effect snow with persistence of cold air and breezy conditions. Then a front and possible low pressure system reaching the East late in the week may produce some precipitation, but at this point confidence is very low regarding coverage/amounts as well as for where the rain-snow line will be. Expect moisture to increase over the Northwest around Tuesday, with somewhat increased confidence in low pressure just offshore Wednesday helping to enhance amounts. This event will likely see snow confined to higher elevations than in the shorter term, but areas where earlier cold air persists longer than forecast could still present some lower elevation winter weather issues. Precipitation is likely for the northern half or so of the West for Wednesday-Thursday, with generally more of a precipitation focus along coastal areas into late week. 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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