Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 PM EST Sat Dec 7 2024 Valid 12Z Tue Dec 10 2024 - 12Z Sat Dec 14 2024 ***Heavy rain threat For the East Coast Tuesday into Wednesday*** 19Z Update: The 12Z models are in excellent synoptic scale agreement across the Continental U.S. through midweek, especially with the building deep trough over the Central U.S. and the strong cold front exiting the East Coast. Similar to yesterday, the greatest model differences emerge as the upstream upper ridge over the Western U.S. breaks down as shortwave energy from the eastern Pacific moves in. The ECMWF and UKMET solutions are stronger with the shortwave crossing the northwestern U.S. on Thursday, and model spread increases substantially going into next Saturday, with below average confidence by that time. An examination of the ML guidance is not in great alignment with the operational ECMWF with the timing of the shortwave passages, and therefore a solution closer to the CMC/GFS and the ensemble means was employed for the second half of the forecast period. The ensemble means were gradually increased to about 60% by Saturday, and the previous forecast discussion is appended below. /Hamrick --------------------------- ...Overview... Most guidance has been fairly consistent in showing the potential for heavy rainfall rainfall along parts of the East Coast during Tuesday-Wednesday, as central into eastern U.S. upper troughing initially pushes along a wavy front and then by Wednesday onward develops a stronger Mid-Atlantic into Northeast wave as the trough takes on a negative tilt. Upper troughing over the East will likely lift away after Thursday as mean flow from the eastern Pacific into the lower 48 trends more zonal. Such a low amplitude regime tends to decrease predictability, and indeed guidance shows a lot of spread for individual details from the Pacific eastward late week into next weekend. However the details turn out, there is at least better agreement with the more general theme of increasing rain and high elevation snow for the Northwest. The amplified evolution over the East will favor a day or so of warm temperatures in the followed by a couple chilly days, while above normal readings will become established over the Plains and vicinity by Friday-Saturday under the forecast zonal mean flow aloft. ...Guidance/Predictability Assessment... Regarding the amplified evolution over the East during the first half of the period, most guidance has maintained fairly good continuity with the idea of frontal wave intensification from the Mid-Atlantic into the Northeast and northward over Canada from Wednesday into Thursday. Most trends are adding some confidence to this being a fairly strong system, as the ensemble means have trended better defined plus the 12Z machine learning (ML) models have on average trended about 5-10 mb deeper by the time the surface low reaches north of Maine by early Thursday. The UKMET is the one misfit, as it hangs back the shortwave energy in the southern part of the upper trough, resulting in a weaker/eastward surface evolution. One ML model trait of note is that most track this wave a little farther westward than most of the dynamical models/means, so it will be worth watching to see if the dynamical solutions eventually display any trend in that direction. Model/ensemble spaghetti and other comparison plots show rapidly increasing divergence in specifics behind this system. A weak shortwave may reach the West Coast around midweek (with less southern stream continuation than depicted by the 18Z GFS). Then ML models cluster toward the idea of trailing energy arriving a day later, and then progressing eastward as a defined shortwave into the Plains and Great Lakes by Friday-Saturday (with an associated surface system). The 18Z GFS/GEFS mean come closest to this idea. Behind this feature, there is a signal for another feature or two, with best defined upper troughing ultimately near the West Coast by early next Saturday. Surface details will be very sensitive to the timing and energy distribution within the upper trough. In varying ways the 12Z ECMWF did not compare well to the majority guidance cluster late, though the new 00Z run has at least trended somewhat in the desired direction. The aforementioned array of guidance recommended emphasis on the 18Z GFS and 12Z ECMWF/CMC for about the first half of the period, followed by a transition toward a model/ensemble mean blend with greater weight on the 18Z GFS/GEFS mean. ...Weather/Hazards Highlights... Latest guidance continues to show a wavy cold front moving into the East by Tuesday-Wednesday, with stronger wave development from the Mid-Atlantic into the Northeast during Wednesday-Wednesday night as the supporting upper trough takes on a negative tilt. Signals have been fairly stable over the past day regarding a potential axis of enhanced rainfall over parts of the Southeast on Tuesday within an axis of anomalous moisture and some instability ahead of the front, favoring maintenance of a Slight Risk area in the Day 4 Excessive Rainfall Outlook, from central Georgia to the FL Panhandle. The Storm Prediction Center is still monitoring for the possibility of severe weather across this region. By Wednesday the majority of guidance shows a broader area of heavy rainfall over the Northeast in response to vigorous upper dynamics and surface development. Snow cover or at least wet ground will cause higher sensitivity across this region, so the Day 5 ERO maintains a Slight Risk area covering about the eastern two-thirds of New England. A Marginal Risk area extends from west of the Slight down into the southern Mid-Atlantic where some instability could help to enhance totals locally. The deepening wave may produce a period of brisk to strong winds in its wake. Also expect the cold air arriving behind this system to produce another episode of lake effect snow during Wednesday-Thursday. The developing pattern along the West Coast should lead to a return of rain and higher elevation snow across the Northwest from late this week into the weekend. There will likely be multiple shortwaves/frontal systems but there is still greater than average uncertainty in the specifics of these features. Thus it will take additional time to resolve the finer details of precipitation coverage, intensity, and timing. The East will see well above normal temperatures Tuesday into Wednesday ahead of the cold front pushing into the region. Greatest anomalies should be for morning lows, reaching plus 15-20F or more at some locations. Then an area of below normal temperatures (mostly negative 5-15F anomalies) will progress across the eastern half of the country Wednesday-Thursday. The West and especially the Plains will rebound to above normal from midweek onward, with parts of the Plains seeing plus 10-15F anomalies by late next week. Rausch/Hamrick 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