Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM EST Thu Dec 12 2024 Valid 12Z Sun Dec 15 2024 - 12Z Thu Dec 19 2024 ...Overview... Today's models and ensemble means still show a change in pattern from a sequence of progressive shortwaves spaced a couple days or so apart during the weekend and first half of next week toward a larger scale mean configuration consisting of a West Coast upper ridge and east-central U.S. trough. Currently the guidance has diverse ideas for some details of this transition around Wednesday-Thursday before clustering actually seems to improve a day or two later. Individual shortwaves and associated surface systems will produce rain and higher elevation snow over parts of the West and then develop precipitation over the southern Plains and expanding across the east-central U.S. then continuing into portions of the East. Most of the West will trend drier mid-late next week as the upper ridge builds in. Much of the country should see near to above normal temperatures during the period with a few pockets of moderately below normal readings. However temperature and precipitation details become more uncertain over the eastern half of the country late in the period. ...Guidance/Predictability Assessment... The compact shortwave/upper low crossing the Great Lakes Sunday into Monday still exhibits decent clustering with a small number of stray solutions. Latest GFS/CMC runs have generally been best, with ECMWF runs becoming too far south by Monday and 12Z UKMET diverting the feature too far north. Both models trend to consensus in the new 00Z run. For the trailing upper trough reaching the Plains by Monday, guidance seems to be attempting to converge toward an intermediate solution by way of previously slow ECMWF runs trending faster and prior fast GFS runs plus ECMWF-initialized machine learning (ML) models nudging back a little. However there are still fine scale differences that affect surface low development Monday into Tuesday, with a general theme of one potential low near the Canadian border and a separate wave farther south near the Midwest. Both dynamical and ML models differ over which low ultimately develops more strongly, or perhaps with some merging of the two waves. Thus the confidence in this detail of the forecast is fairly low, but will affect sensible weather over parts of the Midwest/western Great Lakes. Note that the 00Z CMC is a southern extreme with the surface low by Tuesday, while the 00Z UKMET is slow with the upper trough. Minus the 00Z UKMET, there is now decent clustering for the upper trough and surface system off the West Coast as of early Monday and moving through the western half of the country Tuesday-Wednesday. By Wednesday some differences with northern stream flow arise, with recent ECMWF/ECens means in particular leading to strong high pressure over the Plains versus some degree of surface waviness/frontal system in the GFS/GEFS and CMC. Thus far the ML models have favored at least some Plains waviness (though weaker than some earlier GFS runs) and indeed the new 00Z ECMWF is finally trending that way. From Wednesday into Thursday, ECMWF runs further diverge from most other guidance due to a stronger Pacific shortwave pushing through the West Coast mean ridge. However even among other dynamical/ML guidance there is a fair amount of spread for what stream interaction may occur. There is a slightly better ML theme (including the ECMWF-AIFS ensemble mean) for the Wednesday Plains waviness to track into the Great Lakes per latest GFS runs given sufficient inclusion of northern stream flow. There are other possibilities depending on upper trough timing and energy distribution. Based on guidance comparisons, the first half of the forecast consisted of half GFS (12Z/18Z split) and the rest divided between the 12Z ECMWF and CMC. Then the blend steadily shifted GFS input more toward the 18Z run (12Z run possibly straying a little fast with the eastern upper trough) while including more 18Z GEFS mean than 12Z ECens mean as well as maintaining some 12Z CMC. ...Weather/Hazards Highlights... A shortwave with an embedded compact upper low will likely track from the Midwest/western Great Lakes through or near the eastern Great Lakes during Sunday-Monday, producing an area of precipitation from the Great Lakes southward and then continuing eastward. Parts of the Appalachians/Northeast may see some wintry precipitation but it most should be light, as probabilities for 0.25 inch liquid in the form of snow remain below 30 percent. The next system will cross the Interior West and Rockies on Sunday, producing snow over the northern-central Rockies but with totals lower than forecast over the northern Rockies/Northwest on Saturday. Expect continued progression of the upper trough with leading cold front and incorporation of some Gulf moisture to develop an area of precipitation from the southern half of the Plains northeastward by Monday-Monday night. Most of this activity will be in the form of rain except for some scattered pockets of wintry weather in the far northern tier. Guidance continues to suggest greater instability with this system relative to its predecessor, leading to higher rainfall rates within portions of an area extending from the southern half of the eastern Plains into the Lower Ohio Valley. For the time being the Day 5 Excessive Rainfall Outlook depicts no risk areas, given a combination of current guidance spread for heaviest rainfall magnitude/location as well as generally dry antecedent conditions. However some rainfall expected in the short term may temper the initial dryness and improved clustering toward heavier rainfall would increase the potential for runoff issues. Faster movement of the cold front should lead to lower rainfall totals over the East on Tuesday. The northern half of the West Coast should see another episode of rain and higher elevation snow on Monday, with moisture extending east through the Rockies by Tuesday. Details of the surface pattern and corresponding precipitation coverage and amounts over the central/eastern U.S. become increasingly uncertain by next Wednesday-Thursday. Within a generally mild pattern across the lower 48, warmest anomalies should extend from the eastern slopes of the Rockies to just west of the Appalachians Sunday-Monday and reach the East on Tuesday ahead of a progressive cold front. Expect a decent area of highs 10-15F above normal and anomalies up to plus 15-25F in places for morning lows. Localized areas with slightly below normal highs should include the central West Coast Sunday-Monday, the far northern tier Tuesday-Wednesday, and possibly parts of the South by next Thursday. Rausch 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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