Extended Forecast Discussion NWS Weather Prediction Center College Park MD 212 AM EST Fri Jan 12 2024 Valid 12Z Mon Jan 15 2024 - 12Z Fri Jan 19 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... Guidance is persistent with a deep and broad mean trough aloft over much of the lower 48 while the upstream mean ridge over the eastern Pacific drifts toward the West Coast through next 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 Monday and Tuesday, with numerous daily records for cold lows/daytime highs. In modified form, coldest anomalies should focus over the eastern half of the country by midweek while the West trends warmer. Low pressure forming on the southern periphery of the Arctic high may spread snow across portions of the Mid-South Monday and into the Mid-Atlantic/Northeast Tuesday. 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. However, there are two prominent uncertainties in the forecast, both during Tuesday-Wednesday: details of a potentially impactful storm system near the East Coast, and northeastern Pacific/western Canada evolution aloft that will affect precipitation specifics over the West. Recent models remain agreeable with showing shortwave energy pivoting through the southern side of the broad and deep trough serving to form a surface low near the East Coast Tuesday into Wednesday. 12/18Z and now the 00Z model guidance generally shows low pressure developing near the Southeast Coast by early Tuesday, but with some spread in the formation/track that is amplified by the fact that even relatively small differences would have considerable sensible weather differences. With the 12/18Z suite of models, the GEFS and EC mean showed similar low tracks northward across the western Atlantic and a fairly strong system reaching New England or the Canadian Maritimes by early Wednesday. 12/18Z GFS runs were most like the means, so favored that cluster for the WPC forecast. The 12Z and now the 00Z ECMWF deterministic runs are more offshore where hardly any precipitation is onshore of the East Coast. This seems unlikely based on other guidance and the EC ensemble mean and members, but unfortunately the ECMWF-initialized machine learning models did not seem to run for the 12Z cycle, so we lack that piece of information that could help with the model diagnostics. Meanwhile the 12Z CMC showed reasonable alignment in the track though a bit faster than preferred, but now the 00Z CMC has a track considerably inland that would spread rain to some Mid-Atlantic areas. The 00Z GFS also shows differences from its previous runs, with a dual structure to the low early Wednesday with some low pressure hanging near the DelMarVa, and not consolidating until later Wednesday. Overall, the forecast details remain low confidence as the storm track spread is still high. Meanwhile the question mark farther west involves the degree to which a trough crossing over/south of Alaska (and possibly energy to the south) ultimately dents the East Pacific/West Coast upper ridge around Tuesday-Wednesday, before consensus returns to a more agreeable upper ridge near the West Coast by next Thursday. Over the past couple days guidance has formed two pronounced clusters in principle, with the GFS/GEFS holding onto a persistent ridge versus remaining guidance showing varying degrees of height falls. This makes for a significant difference in how strong of a surface system may reach the Pacific Northwest and for precipitation and coverage amounts over the West, with the GFS/GEFS scenario much lighter and confined with what precipitation occurs. The best course of action still seems to be a compromise along the West Coast with modest denting of the upper ridge and associated weak low pressure. The WPC forecast began with a model blend of the 18Z GFS and 12Z ECMWF and CMC with a bit of GEFS and EC mean as well to smooth out some differences. By Day 5/Wednesday the ensemble means made up 40 percent of the blend but was actually able to hold this proportion steady through Days 6-7 given that the large scale pattern stabilizes during the latter part of next week, as upper ridging edges into the West and troughing remains farther east. ...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. as the medium range period begins Monday, supported by cold surface high pressure and amplified troughing aloft. Low temperatures in the -10 to -20 range and highs staying near or below 0F will be common across the northern half of the Plains and into the Upper Midwest, while farther west the coldest air should affect Montana with lows in the -20s and even -30s. 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 Monday-Tuesday. By Wednesday, temperatures finally look to moderate in the central U.S., though chilly temperatures (albeit in moderated form) will shift into the eastern third of the country before warming closer to normal on Thursday. Some reinforcing cold may reach the northern Plains by Thursday and the central Plains by Friday, but it should be less extreme than what is forecast earlier in the period, with anomalies more like 10-20F below normal. Along the southern fringe of the Arctic high, some limited moisture looks to overrun a Gulf Coast to Florida frontal boundary and could produce accumulating snow from portions of the Lower Mississippi Valley stretching into the Tennessee Valley and the southern/central Appalachians on Monday. Though there is still model spread, some convergence in recent guidance regarding the timing and placement of a surface low forming along this front suggests that snow potential farther northeast of those regions could hold off until Tuesday. Meanwhile farther south, convection is forecast that could focus near the stalled front in northern Florida with increasing deep moisture, which could produce heavy rainfall potential, and a Marginal Risk remains in place there in the ERO for Monday. Then as Gulf to western Atlantic low pressure develops along the front and pivots northeast, snow could spread across the Mid-Atlantic to Northeast to the west of the low track. The most recent WPC forecast continues to show a band of 30 percent or greater potential for 0.25 inch liquid in the form of snow from near the northern Mid-Atlantic coast through New England in the snow probabilities. Though this forecast stayed pretty consistent with the previous one, snow coverage/amounts will be very sensitive to storm track and strength, so additional changes in the most likely scenario are likely. Farther north, expect continued periods of lake effect snow with persistence of cold air and breezy conditions. By Tuesday-Thursday, moisture should increase over the Northwest around Tuesday-Wednesday, but with low confidence in specifics of precipitation totals and southward extent. 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. 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