Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Tue Jan 09 2024 Valid 12Z Fri Jan 12 2024 - 12Z Tue Jan 16 2024 ...Another major storm to impact the East-Central to Eastern U.S. with snow/rain/wind threats followed by an arctic blast through much of the Central U.S.... ...Overview... Most guidance shows upper troughing aloft in the mean centered over the lower 48, helping to establish a cold pattern over the central U.S. and a gradual eastward expansion of below normal temperatures toward the eastern states next week. Within this pattern, a vigorous Plains shortwave as of early Friday should close off into a Great Lakes upper low by Saturday. This evolution will support a rapidly intensifying Mississippi Valley through Great Lakes surface low whose array of weather hazards will have some similarities to the strong system currently affecting the East. Meanwhile, a system passing through the East Pacific mean upper ridge should bring areas of heavy rain and snow to the Northwest this weekend. Arctic high pressure building in across the Central U.S. will bring a broadening area of temperatures 20F or more below normal. There is a potential for a combination of northern stream and/or Pacific-Western U.S. energy to produce some degree of surface low pressure that may reach near the East Coast by next Tuesday. Colder temperatures may lead to a farther southeast rain-snow line versus previous events for any associated precipitation but otherwise the details are very uncertain at this time. ...Guidance/Predictability Assessment... The most agreeable features during the forecast period are the intensifying storm tracking from the Mississippi Valley through Great Lakes into eastern Canada, and to a reasonable degree the longer term mean pattern involving a broad trough over much of the lower 48 and eastern Pacific mean ridge that eventually drifts to the West Coat. The Pacific system reaching the West during the weekend has considerably more spread, with combined uncertainties of this energy and northern stream flow leading to well below average confidence for potential low pressure/frontal system that consensus suggests will be near the East Coast by next Tuesday. Regarding the intensifying Friday into weekend storm affecting the eastern half of the country, a model composite seems to provide a reasonable starting point for the depth and track. Taken as a whole, the operational model forecasts valid early Saturday indicate this storm should be similar to or possibly even a little deeper than the current storm affecting the East through this Wednesday. Among the 00Z/06Z solutions, the ECMWF is on the deep side of the spread and the CMC weakest. Interestingly the 00Z ECMWF-initialized machine learning models are generally a few mb weaker with the Saturday storm than extended range runs were for the current storm valid Wednesday. As is typically the case for smaller scale features passing through a mean ridge, guidance varies considerably for exactly how elongated North Pacific shortwave energy as of early Fri evolves as it progresses through the ridge. ECMWF/GFS runs and 00Z UKMET/CMC agreed on at least brief closure of an upper low but with some latitude differences (GFS farthest north) while the new 12Z UKMET actually shears the energy to yield a very weak and suppressed surface wave. Ensemble means are more open aloft as expected, but favor a surface low track into the Pacific Northwest. 00Z ECMWF-initialized machine learning models vary but with a slight majority closest to the 00Z ECMWF/ECens track. Thus prefer an intermediate scenario at this time. By the latter half of the period there has been a lot of model/ensemble spread and run-to-run variability for details of the incoming western energy as well as northern stream energy, plus how flow may evolve over southwestern Canada (00Z ECMWF showing more westward elongation of shortwave energy than most other guidance). This leads to well below average confidence for details of potential surface low pressure/frontal system expect to be near the East Coast by next Tuesday--ranging between strong development per the past couple ECMWF runs and modest waviness per the GFS/CMC/ensemble means. The uncertainty favors maintaining a conservative approach closer to the means by next Tuesday. ...Weather/Hazards Highlights... Another intense Mississippi Valley through Great Lakes storm system late this week into the weekend will produce a swath of heavy snow and strong winds from Missouri through the Great Lakes on Friday, and into New England on Friday night with lake effect snow likely occurring behind the arctic cold front (likely heaviest downwind from Lakes Erie and Ontario on Saturday). Some light to moderate snow may occur down through the Central Appalachians on Friday night and Saturday as well. The threat for high winds across much of the northeastern quadrant of the East on Saturday will add to the hazardous conditions over areas where snow is falling. Meanwhile showers and scattered thunderstorms are likely from the Midwest to the Mid-Atlantic and Southeast/Gulf Coast within the warm sector of the storm on Friday. The Storm Prediction Center highlights a 15% probability of severe thunderstorms from far eastern Louisiana across the Southeast, Carolinas and into far southern Virginia. Some heavy rainfall is possible over parts of the central Midwest Friday afternoon where some instability may creep in, leading to the threat of excessive rainfall. This threat merits a Marginal Risk (at least 5%) in the Day 4 Excessive Rainfall Outlook. There will also be a focus for heavy rainfall potential from the eastern slopes of the Appalachians into southern New England Friday-Friday night, denoted by a separate Marginal Risk area, along with an embedded Slight Risk (at least 15%) for the area encompassing the I-95 corridor between Baltimore and western Rhode Island where favorable jet dynamics and moisture flux will be present. Wet conditions from melting of at least a portion of the existing snowpack in northeastern Pennsylvania into northern New Jersey, southern New York and southern New England plus the heavy rain from the current storm may also contribute to the flash flooding threat. The system expected to reach the Pacific Northwest during the weekend should also produce a period of heavy rain and mountain snow, but with tempered confidence for specifics due to guidance spread for the system's evolution and track. The Days 4-5 Excessive Rainfall Outlooks currently depict Marginal Risk areas, with the Day 4 period having the greater potential for a possible embedded Slight Risk at some point if guidance aligns better for highest totals. The combination of this energy and northern stream flow may promote some waviness over the far southern Plains/Gulf Coast to East Coast during the first part of next week, with snow possibly spreading from the central/south-central Plains into parts of the East with rain farther southeast. Precipitation and surface evolution details are still very uncertain at this time. The most extreme anomalies for the upcoming cold pattern over the central U.S. should be over the northern High Plains/Montana with some temperatures at least 40F below normal. Expect a much broader area of temperatures at least 20F below normal to become more established between the Rockies and Appalachians with time, leaving the immediate East Coast and California as the only places with near normal highs by next Tuesday. The East will see a brief period of warmth ahead of the late week storm system. 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