Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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... Latest guidance maintains 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, possibly drifting inland by next Friday. 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 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. However, there have been are two prominent uncertainties in the forecast during Tuesday-Wednesday: details of a potentially impactful storm system near the East Coast, and northeastern Pacific/western Canada evolution aloft that will affect upper flow and precipitation specifics over the West. Then as the upper ridge rebuilds near the West Coast late in the week, question marks arise regarding details of central-eastern U.S. troughing and how it will affect the surface pattern over the East. The models still agree in showing shortwave energy pivoting through the southern side of the broad mean trough serving to form a surface low near the East Coast Tuesday into Wednesday. Consensus maintains an elongated area of low pressure along or just offshore the Southeast Coast by early Tuesday but then displays increasing east-west spread in the track going into Wednesday. These track differences reflect sensitivity to lower predictability detail issues with the upper shortwave. Among the 00Z/06Z cycle of guidance valid into Wednesday, the GEFS/ECens/CMCens means were all fairly close in track and timing near the middle of recent model spread and just offshore New England. The 06Z GFS was closest to the means and the new 12Z CMC has adjusted that way as well after being on the western side of the spread in the 00Z run. Latest UKMET runs have also represented the western side of the spread. On the other hand, recent ECMWF runs have strayed farther east with the surface low track due to faster timing for the northern part of the supporting shortwave. The 00Z ECMWF-initialized machine learning models generally followed a track favored by the ensemble means, though with most hinting at somewhat faster northward progression by early Wednesday. The new 12Z GFS/GEFS mean trended a little farther offshore. Most of the observed run-to-run variability is fairly typical for forecasts several days out in time but still leads to significant differences for precipitation amounts (and in some cases, type) along the East Coast. Preferences remains with an intermediate solution closest to the ensemble means. Meanwhile the question mark farther west involves what could be a fairly brief but complex evolution over the northeastern Pacific/western North America Tuesday-Wednesday. Through the 00Z/06Z cycles, the GFS/GEFS had been steadfast in maintaining a strong upper ridge over the East Pacific/West Coast. In contrast most other models/ensemble means show a shortwave dropping south over the Alaska Panhandle/British Columbia into the Pacific, along with a separate piece of energy breaking off from a Pacific upper low and tracking underneath the northern shortwave. Even among the ECMWF cluster there is considerable spread for the associated surface system's track and depth. ECMWF-initialized machine learning models have shifted to a greater majority in favor of the ECMWF cluster compared to 24 hours ago, so the latest forecast tilted toward a weaker version of that grouping. The new 12Z GFS has finally switched to the majority scenario but with slower southward progression of the northern shortwave, thus yielding a farther north surface wave. At least the spread is narrowing but differences continue to be significant for refining expected precipitation coverage and totals over the West. While guidance returns to better agreement for the West Coast upper ridge by next Thursday-Friday, detail differences within the downstream trough affect the progression of the cold front expected to reach the central-eastern states as well as location and depth of low pressure. By the end of the period next Friday, there appears to be a general trend over the past 12-24 hours toward somewhat more potential low pressure development near or a little inland from the East Coast but with no agreement for specifics. A model/ensemble mean blend nudged the forecast to reflect latest trends while downplaying specifics of any particular solution. An average of latest operational guidance during the first half of the period helped to maintain continuity for the system near the East Coast and modestly incorporate trends toward the majority cluster along the West Coast. Then a mix of the models and 06Z GEFS/00Z ECens means (reaching 40 percent weight late) represented preferences well later in the forecast. ...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 air may reach the northern Plains by Thursday and expand to include most areas between the Rockies and Appalachians Thursday. Expect less extreme temperature anomalies than those forecast earlier in the period, but Friday may still see lows 10-20F below normal and highs 10-25F 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. The Day 4 Excessive Rainfall Outlook for Monday maintains a Marginal Risk area, with only minor adjustment from previous issuance. The northern edge of the guidance spread for highest totals has been drifting south over the past day while latest ECMWF runs remain on the southern side. As Gulf to western Atlantic low pressure develops along the front and pivots northeast, snow could spread across the Mid-Atlantic and 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. Latest solutions before the 12Z cycle led to a slight increase in probabilities along a similar axis. However snow coverage/amounts will remain 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. Then a front and possible low pressure 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-Wednesday, but still with fairly low confidence in specifics of precipitation totals and southward extent. 12Z GFS trends at least support leaning away from the drier side of the prior guidance spread. 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. Midweek winds near the Pacific Northwest coast are also very uncertain, given differences for low pressure that may affect the region. Rausch/Tate 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