Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Mon Jan 15 2024 Valid 12Z Thu Jan 18 2024 - 12Z Mon Jan 22 2024 ...Another Arctic airmass comes into the central U.S. late week, but not as extreme as the first one... 19Z Update: The model guidance is in above average agreement on most aspects of the forecast going forward to the end of the week and into the weekend, and a multi-deterministic model blend sufficed as a good overall starting point in the forecast process, with a little more weighting to the GFS/ECMWF given their close agreement to the respective ensemble means. The models still agree that the developing offshore low southeast of New England will be far enough from the coast to mitigate the potential impacts from this event Friday night, with the CMC being the closest solution to the coast. There is excellent agreement on the arctic surface high over the eastern half of the country this weekend, but more model differences emerge over the eastern Pacific that have impacts for the West Coast for the second half of the forecast period, mainly regarding timing and not intensity. The CMC began to stray from the consensus by early next weekend with the developing upper ridge axis over the central U.S. situated west of the ensemble means, so this was weighted less in the forecast forecast. In terms of QPF, the NBM was too light with expected lake effect snow for the Thursday-Saturday time period, so an equal blend of GFS/ECMWF/NBM was used as a baseline, and then this result was increased about 75% downwind of the Great Lakes to better account for lake effect banding. The previous forecast discussion, along with updated excessive rainfall outlook, are appended below for reference. /Hamrick -------------------------- ...Overview... The deep and broad mean upper trough that should dominate much of the lower 48 through late week will finally get displaced into the East and then the Atlantic over the weekend, as an upper ridge shifts from the West Coast Thursday across the western and then central U.S. by the weekend. Ahead of the ridge, mean northwesterly flow will bring in another Arctic surface high, producing much below normal temperatures across the central U.S. Thursday and spreading into the East late week. While temperatures are forecast to be generally be 15-30F below average, at most locations this second round of cold temperatures should be less extreme than the the first. Meanwhile, rounds of precipitation are expected for the West Coast, and in the East possible low development along a surface frontal system or two could produce some winter weather during Thursday-Saturday, but with continued significant uncertainty for details. ...Guidance/Predictability Assessment... Latest models/means continue to agree well for the large scale evolution through the period with more progressive flow than has been seen recently, as the upper ridge axis pushes the main trough through the East and an upstream trough comes into the West through the period. Details of the flow show some differences that are certainly within reason for the medium range timeframe, but have sensible weather impacts. One of these more variable systems is a shortwave consolidating in the east-central U.S. by Friday as energies from the northwest and north combine within the broad trough. This strong shortwave should pivot through the base of the trough and help produce western Atlantic low pressure on Friday, while additional low pressure/frontal systems may be farther west, making for a complex surface pattern before these features may combine. The surface low track Friday-Saturday has some east-west spread and also some north-south spread indicating timing differences. More specifically, CMC runs have been on the eastern side of the spread compared to consensus--but the new 00Z ECMWF has a similar eastern track farther from shore. These low track variations lead to meaningful differences as far as East Coast effects. Farther west, the ridge axis taking a steadily eastward track seems well forecast by the models. Mean troughing behind the ridge becomes more questionable with the details especially by early next week, as multiple shortwaves show variability with timing and amplitude in the models, affecting surface fronts as well. These will take additional time to resolve especially since they are tracking from the data-poor Pacific that causes models to struggle. The 12Z CMC seemed like the main outlier by early next week as it maintains troughing across the East Coast, serving to amplify the ridge more so than other guidance and not allowing for shortwaves to propagate from the West. The WPC forecast was based on a multi-model deterministic blend early in the forecast period. Gradually incorporated the GEFS and EC ensemble mean guidance while phasing out the CMC (and of course the time-limited UKMET) as the period progressed, ending up with an even blend of the 18Z GFS and GEFS mean and 12Z ECMWF and EC mean by Day 7. This smoothed out model differences but maintained some strength individual systems. ...Weather/Hazards Highlights... By the start of the period Thursday, most areas will have warmed up compared to the first surge of Arctic air currently affecting the nation, though with some lingering slightly below normal temperatures in the Southeast. However, another Arctic high will provide a reinforcing surge of cold air that is forecast to push into the north-central U.S. Thursday, well into the Plains/Mississippi Valley Friday, and into the East Saturday. Below normal temperatures by 15-30 degrees should be widespread, and this cold episode will still be notable even if not quite as extreme as the current/short range cold event over most areas. Temperatures should moderate closer to normal by early next week. Meanwhile west of the Rockies, ridging aloft will promote above normal temperatures increasing in magnitude and coverage late week into the weekend, and shifting into the north-central U.S. early next week. The West Coast states can expect rounds of precipitation for the latter part of this week into early next week. On Thursday possibly heavy precipitation looks to focus across western Washington, with heavy snow for the Cascades. The Olympic Peninsula and far northwestern Oregon may have just enough moderate to heavy rainfall to warrant a planned Marginal Risk area for Day 4. By late Friday, a round of precipitation looks to affect California along and ahead of a frontal system. Models indicate that rain rates could be over half an inch per hour as instability may sneak onshore, along with good dynamical support for heavy rain from a strong shortwave aloft. Therefore, the existing Marginal Risk area remains unchanged in the Excessive Rainfall Outlook for Friday-Friday night across portions of northern California. Precipitation chances should remain across the West Coast and also expand farther southeastward over the weekend. Some locally moderate/heavy totals are possible over favored terrain, depending on upper level and surface details that will be better resolved in coming days. Eventually by early next week, the pattern should allow for Gulf return flow to produce rain chances in the southern Plains/Lower Mississippi Valley. Farther east, expect continued periods of lake effect snow with persistence of cold air and breezy conditions into Saturday. Lake effect snow will likely taper off by Sunday as surface high pressure builds into the East. A frontal system with embedded low pressure reaching the East during the latter part of the week may produce some precipitation. This system could initially spread some light snow across the northern-central Plains by Thursday while some limited Gulf moisture comes into the east-central U.S., shifting to the East Coast Friday. The precipitation type in the Southeast should stay rain while generally from the Ohio Valley into the Appalachians and the northern Mid-Atlantic to Northeast should have snow. Areas in between are more uncertain with precipitation type, while uncertainty also remains with the coverage and amounts of precipitation, which will continue to be refined in future forecasts. The majority of guidance still suggests that snowfall will be fairly light, and the latest probabilities for 0.25 inch liquid in the form of snow are no more than 10-30 percent or so during the Thursday-Friday night time frame, though slightly higher perhaps in southern New England and back into the central Appalachians. 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