Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 AM EST Sun Dec 8 2024 Valid 12Z Wed Dec 11 2024 - 12Z Sun Dec 15 2024 ***Heavy rain threat for the East Coast continuing into Wednesday*** ...Overview... With typical localized adjustments, latest guidance continues to show the potential for heavy rainfall from the Mid-Atlantic into especially the Northeast on Wednesday, as a dynamic upper trough supports rapidly deepening low pressure that tracks from the Mid- Atlantic northward into Canada. Mean flow across the lower 48 still looks to trend more zonal behind this system, so the amplified upper trough over the East should lift out after Thursday while predictability of system specifics should decrease late week through the weekend. The pattern will favor at least a couple episodes of rain and higher elevation snow along parts of the West Coast, with latest guidance starting to show a stronger eastern Pacific system around Friday-Saturday which would support relatively greater precipitation totals in that time frame. Below normal temperatures will spread across the East late this week, replacing above normal readings near the East Coast on Wednesday. Then expect near to above normal readings over most of the country by next weekend, with warmest anomalies over the Plains and vicinity. ...Guidance/Predictability Assessment... Latest guidance continues to refine details near the East Coast Wednesday into Thursday, with improving consensus as the past two UKMET runs have finally joined the majority cluster for strongly deepening Mid-Atlantic into eastern Canada low pressure in response to approaching vigorous upper trough dynamics. Dynamical/machine learning (ML) guidance on average shows the surface low depth reaching at least as low as the 970s mb by early Thursday. There has been a slight westward nudge since yesterday, a nod to earlier ML runs that were westward. 12Z ML runs were a little more varied for longitude. Upstream features continue to be difficult to resolve. ML models show a bit more coherence with a Thursday West Coast shortwave continuing eastward thereafter, approaching the Northeast or vicinity by next Sunday with varied degrees of surface reflection, and then a better defined eastern Pacific system by Saturday moving into the West/High Plains by Sunday. Most dynamical guidance is showing less of a surface reflection than the ML models (or at least slower, holding onto stronger eastern U.S. high pressure) with the leading shortwave. The dynamical and ML models are gradually showing an improved signal for what could be a fairly strong eastern Pacific system around Friday-Saturday. ML models are generally more agreeable with bringing a moderately amplified upper trough into the West by next Sunday, while dynamical guidance varies--albeit with the means at least suggesting some troughing. The updated forecast started with a composite of 12Z/18Z operational models, and then transitioned toward a blend of the 18Z GFS, past two ECMWF runs, and 18Z GEFS/12Z ECens-CMCens means as those solutions provided the most coherent evolution when compared to each other and ML models. ...Weather/Hazards Highlights... Expect elongated low pressure to reach the Mid-Atlantic/New York state region by early Wednesday. Strong upper dynamics becoming negatively tilted will support deepening/consolidation of this system into the Northeast by late Wednesday followed by continued strengthening as it continues north into Canada into early Thursday. Accompanying this strong development, guidance still shows a pronounced surge of moisture into the Northeast, with precipitable water values reaching 3+ standard deviations above normal for a time, supporting a broad area of heavy rainfall. Snow cover or wet ground will lead to increased sensitivity over the Northeast while persistence of model guidance and strengthening signals from first-guess fields add to the confidence in the potential for runoff issues. Thus the Day 4 Excessive Rainfall Outlook maintains a Slight Risk area covering much of New England while a surrounding Marginal Risk area extends southward to the southern Mid-Atlantic where some instability could enhance rainfall totals. The first-guess fields actually suggest potential for some embedded probabilities at least in the higher half of the Slight range over New England, so this will merit continued monitoring. Both risk areas have been expanded a little westward from continuity to correspond with latest guidance trends. Meanwhile, the western edge of the precipitation shield may contain some snow and the deepening storm 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 forecast pattern from the Pacific into the West will favor at least a couple episodes of rain and higher elevation snow over the Pacific Northwest, possibly extending into parts of California. The progressive nature of the mean flow continues to temper confidence in some details, though guidance is starting to cluster around the idea of a fairly strong eastern Pacific system helping to produce the highest totals of the period and greatest southward extent of precipitation around Friday-Saturday. This storm could produce areas of brisk to strong winds depending on its depth and track. Some moisture may extend into the Rockies, while leading shortwave energy/surface feature(s) may produce areas of light precipitation over parts of the central/east-central U.S. The amplified pattern over the East will lead to above normal temperatures near the Atlantic coast on Wednesday while trailing cold from the Upper Midwest into the South pushes eastward into late week. Coldest anomalies should be from the Upper Midwest into Great Lakes/Ohio Valley Wednesday-Thursday, at least 10-20F below normal. The transition toward flatter mean flow aloft will lead to a warming trend over the Rockies and Plains late this week with above normal temperatures (some pockets of plus 10F or greater anomalies) persisting through the weekend. The East will trend closer to normal by next weekend. 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