Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Wed Jan 10 2024 Valid 12Z Sat Jan 13 2024 - 12Z Wed Jan 17 2024 ...Another major storm to impact the Eastern U.S. into Saturday with snow/rain/wind threats followed by an arctic blast through much of the Central U.S.... ...Overview... A deep upper low closing off over the Great Lakes by early Saturday, supporting a strong storm affecting the East at the start of the forecast period, will likely settle near Hudson Bay and become the new anchor of a large scale mean trough covering much of the lower 48 through the period. At the same an eastern Pacific mean ridge could get close to the West Coast for a time. A leading system passing through the ridge will affect parts of the West this weekend with another one possibly arriving around the middle of next week. The forecast large scale pattern will promote very cold temperatures over the central U.S. from the weekend into next week with some locations potentially breaking daily records. By next week the eastern states will see a modified form of this cold air while some wintry precipitation may spread from the south-central Plains/Mississippi Valley and into parts of the East. However the precipitation/surface system details affecting the East during the first half of next week remain very unclear at this time. ...Guidance/Predictability Assessment... Early in the period, consensus looks quite good for the deep storm tracking from the Great Lakes into eastern Canada during the weekend. The average of recent guidance has held up well for depicting the storm's depth which may be similar to the preceding storm currently taking a similar track. Meanwhile the guidance spread is gradually narrowing for the system arriving into the Pacific Northwest/central West Coast. The GFS has consistently been on the northern side of the spread (but with a subtle southward trend over the past day) while the farther south UKMET has adjusted northward somewhat. The average of 00Z ECMWF-initialized machine learning models supports an intermediate surface low track into Oregon. Heading into early next week, the shortwave energy moving into/through the West should shear out/accelerate underneath more pronounced northern tier U.S. energy. Dynamical models and ensembles plus machine learning models continue to diverge considerably for the details of how this northern tier energy may evolve as it reaches the eastern U.S. This leads to a remarkably broad spread for the strength and track of any low pressure system(s) from the eastern U.S. into the western Atlantic/Canadian Maritimes. For example, forecasts valid early next Tuesday show the potential for best-defined low pressure to be anywhere from the Southeast U.S. (12Z/09 ECMWF and a couple machine learning models) to the Canadian Maritimes (06Z GEFS/00Z CMCens means). Surface low depth from Tuesday into Wednesday is just as diverse, with ensemble member spread leading to a weak depiction in the means, while other solutions range from moderate to very strong surface development. The combined high uncertainty of surface low track and strength at any particular valid time continues to favor a conservative approach for the deterministic forecast (a north-south mean frontal zone with embedded waves) until better clustering arises. By late in the period some operational guidance and machine learning models suggest another weak feature may pass through the eastern Pacific mean ridge aloft, but with typical uncertainty for the specifics. Recent ECMWF runs have been oscillating for the relative strength of mean ridging versus southwestern Canada shortwave elongation (00Z runs favoring the latter and without great support from other guidance). Also of note, already by Monday the new 12Z UKMET becomes an amplified/western extreme with the Pacific ridge, throwing its downstream trough out of sync with the model/mean consensus. Forecast considerations based on 00Z/06Z guidance led to starting with a composite of 00Z/06Z operational models for the first half of the period, followed by a transition to a model/mean blend along with splitting the ECMWF input between the 00Z and 12Z/09 runs. ...Weather/Hazards Highlights... The Mississippi Valley to East Coast storm will be well underway by Saturday with snow, rain and wind threats likely from the Midwest to the East Coast. An occluded Great Lakes surface low in the 970s mb as of early Saturday will produce heavy snow and strong winds over the Midwest and Great Lakes with northwesterly flow over Lakes Huron, Superior and Michigan early on in the day followed by lake effect snow downwind of Lakes Erie and Ontario in the afternoon through Sunday as the system lifts northward into eastern Canada. Lingering snow showers are also possible over upslope areas of the Central Appalachians on Saturday. Sustained winds of 20-30 mph and gusts of over 40 mph are possible from the Midwest to the Northeast on Saturday. Winds calm down considerably on Sunday across the East. Ahead of the storm's frontal system, expect heavy rainfall to continue northeastward across the New England coast Saturday morning into afternoon with the Day 4 Excessive Rainfall Outlook depicting a Marginal Risk area. In the West, shortwave energy and associated low pressure will focus moisture over the Northwest/central West Coast on Saturday. Heavy rainfall is possible for portions of the Oregon coast down into northern California where a Marginal Risk of Excessive Rainfall is in effect. There is still some north-south uncertainty for the axis of heaviest precipitation but this risk area depicts the best overlap of guidance and prior extremes have been gradually trending closer. Heavy snow is likely over the Cascades of Oregon, southern Washington, the Klamath Mountains and the Shasta Siskiyous. Cold air in place over parts of the Northwest could also support significant snowfall at low elevations. This system should also bring heavy snow into the Intermountain West and Northern/Central Rockies Saturday night into Sunday morning. Arctic high pressure spilling into the Great Plains during the weekend will build into the Midwest/East later in the week. Extremely cold overnight temperatures are likely from the Northern Rockies into the Central Plains with many nighttime lows struggling to get out of the negative digits through Tuesday at the earliest. Low temperatures in the -20s and -30s will represent negative 30-50 degree temperature anomalies over parts of northern Idaho, Montana and Wyoming this weekend into early next week. High temperatures in the negative to single digits will represent similar anomalies down into the Southern Plains early next week. The cold blast expands through the Mississippi Valley and into the Gulf Coast on Monday and Tuesday when low temperatures may drop below freezing in many places. Tuesday morning lows may bottom out into the 20s for south Texas and the Central Gulf Coast Monday night. These anomalous freezing temperatures will be dangerous, particularly for those without adequate heating or shelter. Within the core of coldest anomalies from the northern High Plains into the Southern Plains/Lower Mississippi Valley, daily records for cold morning lows/daytime highs will be possible. Guidance is gradually increasing the signal for some wintry weather from the south-central Plains into Lower Ohio Valley during Sunday-Sunday night with the combination of shortwave energy and southern tier surface waviness. Latest WPC probabilities of at least 0.25 inch liquid in the form of snow reach at least 30 percent along an axis from southern Missouri/northern Arkansas into the Lower Ohio Valley during that time frame. Thereafter, expect some rain to across portions of the Gulf Coast/Southeast but the coverage and accumulations of winter weather (and any other effects such as wind) farther north over the East remain very uncertain. At the very least, the colder pattern should lead to a rain-snow line farther southeast than with previous systems. 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