Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Jan 11 2024 Valid 12Z Sun Jan 14 2024 - 12Z Thu Jan 18 2024 ...An Arctic airmass blankets much of the Northern and Central U.S. through early next week... ...Monitoring 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. This pattern will support very cold temperatures over areas from the Northwest through much of the Plains/Mississippi Valley into early next week 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. Some reinforcing cold may reach the northern Plains by next Thursday but it should be less extreme than what is forecast earlier in the period. 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 which will affect precipitation specifics over the West. ...Guidance/Predictability Assessment... Compared to previous days, latest guidance has displayed significantly improved clustering regarding the details of initial northern tier shortwave energy and its eventual eastward progression that should lead to strengthening surface low pressure near the East Coast Tuesday into Wednesday. Latest trends have been tilting toward the slower side of the prior broad spread with the upper shortwave, now tending to favor developing low pressure near the Southeast Coast by early Tuesday and a fairly strong system reaching New England or the Canadian Maritimes by early Wednesday (which is closest to some prior ECMWF runs and ECMWF-initialized machine learning model runs). 00Z ECMWF-initialized machine learning models fit this theme as well. Relative to this majority scenario, the 00Z CMCens appears a little fast on Tuesday while the past couple UKMET runs have been a bit on the western side of the envelope (but to a lesser degree in the new 12Z run). Recent GEFS means have been a bit farther offshore around Tuesday. The new 12Z GFS fits well between its slightly westward 06Z run and eastward 00Z run. 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 00Z CMC seemed to offer the least confident extreme among the ECMWF cluster, with more exaggerated amplitude of upper troughing and southward path of a separate feature to the south (leading to very far southward moisture coverage over the West). 00Z ECMWF-initialized machine learning models do not help in determining the relative potential of either scenario, being only 3-2 in favor of the ECMWF scenario. The updated forecast started the first part of the period with a blend emphasizing the 00Z/06Z GFS and 00Z plus 12Z/10 ECMWF runs with less weight on the 00Z CMC/UKMET, followed by some incorporation of 06Z GEFS/00Z ECens means and removal of the CMC. During the less confident Tuesday-Wednesday period, this approach provided a reasonable intermediate solution along the East Coast and a compromise along the West Coast with modest denting of the upper ridge and associated weak low pressure. ...Weather/Hazards Highlights... A weather highlight during the period will be the Arctic blast likely to impact the Northern Rockies, Northern Great Basin, Great Plains, Mississippi Valley and Midwest. The synoptic setup featuring a strengthening upper ridge in the eastern Pacific and amplified downstream trough will support the descent of surface high pressure through the Central U.S. this weekend through early next week. The coldest part of this airmass will be over portions of northern Montana where low temperatures could bottom out around 40 degrees below zero Sunday morning. Portions of the Northern Rockies down through the Central Plains and Middle Mississippi Valley may drop to below zero degrees Saturday, Sunday, and Monday nights. This will make for very dangerous outdoor conditions. Minimum temperature anomalies will likely be between 20-40 degrees below average for these areas. Freezing temperatures will also make their way to the Gulf Coast Monday and Tuesday night. Temperatures will begin to rebound over the Northern Plains Tuesday-Tuesday night and elsewhere over the Plains by Wednesday as a disturbance enters the Northern Plains/Upper Midwest and shifts surface winds out of the south or west. Another push of cold air may reach the northern tier by next Thursday but with less extreme 10-15 degree below normal anomalies. Parts of the Intermountain West into south-central Rockies should see heavy snow on Sunday, a continuation of the moist system affecting locations to the west before the start of the forecast period. Snow probabilities remain high for the Wasatch, Uinta, Colorado Rockies, San Juan and Sangre de Cristo Mountains. A band of snow may also spread across the south-central Plains into the Lower Ohio Valley/Tennessee Valley Sunday into Monday. With a slight southward adjustment over the past 24 hours, the latest probabilities for 0.25 inch liquid in the form of snow highlight an axis of 30 percent or greater values aligned from eastern Oklahoma to the Mississippi River. Then with the development of northern Gulf to East Coast/western Atlantic low pressure, parts of the Southeast may see showers and thunderstorms early next week. With a leading stalled front as a focus and guidance showing increasing deep moisture with potential for a band of heavy rainfall, the Day 5 Excessive Rainfall Outlook covering Monday-Monday night has introduced a Marginal Risk area covering parts of northern Florida and far southeastern Georgia. Meanwhile locations west of the surface low track will see the potential for meaningful snowfall. Latest snow probabilities depict 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. 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. 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. Rausch/Kebede 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