Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Dec 07 2023 Valid 12Z Sun Dec 10 2023 - 12Z Thu Dec 14 2023 ...Pattern Overview... A period of broad cyclonic mean flow aloft from the short-term time frame into early next week should transition by Wednesday-Thursday toward a ridge building over the northwestern then central U.S., a closed low forming over or near the Southwest, and the northern stream trough becoming more confined to the northeastern quadrant of the lower 48. A leading amplified upper system/wavy surface front will bring significant precipitation from the Gulf Coast to the Great Lakes and Northeast Sunday into early Monday. Much of the country will trend drier after passage of the eastern system and another feature initially bringing precipitation to the Northwest. The southern High Plains and Florida will have the best potential for organized precipitation by midweek. ...Guidance Evaluation/Predictability Assessment... Guidance continues to be on its general path toward better agreement and continuity for evolution of the Sunday-Monday system affecting the East with weak frontal waviness as of early Sunday becoming steadily stronger as it crosses New England by early Monday and then continues into Canada. The new 12Z CMC looks like a notable extreme in being somewhat weaker and suppressed versus consensus from early Monday onward. Pacific shortwave energy supporting the initial Northwest precipitation event has generally been gravitating toward the slower/stronger side of the prior spread. Then operational models are maintaining recent signals for immediately trailing energy to form a low over or near the Southwest by midweek, with ensemble mean trends supporting the operational model average. However by next Thursday the past couple ECMWF runs (00Z and new 12Z versions) are more aggressive than other guidance with another shortwave reaching the West Coast and helping to eject/open up the Southwest upper low. Thus at that time preference leans toward the majority that maintains a better defined upper low. A forecast blend starting with more 06Z GFS/00Z ECMWF input relative to the 00Z CMC/UKMET provided a good balance of latest trends and continuity for the first half of the period. With the means supporting the operational guidance in principle later in the period, aside from the aforementioned ECMWF question marks over the West, days 6-7 Wednesday-Thursday needed to incorporate only a moderate input of the 06Z GEFS/00Z ECens means. The blend included some 12Z/06 ECMWF input late to temper the more recent ECMWF ideas over the West. ...Weather/Hazards Highlights... Precipitation over the Pacific Northwest should extend into Sunday, with recent trends favoring potential for some meaningful totals. The Day 4 Excessive Rainfall Outlook period starting early Sunday still has some enhanced deep moisture but which departs very quickly, while most guidance thus far suggests 1-3 hour rain rates low enough not to be hazardous. Therefore the outlook maintains no risk area. Will continue to monitor trends given the wet conditions leading into the outlook period and the wetter end of the guidance spread possibly meriting a risk area. In the East, a broad area of significant precipitation will stretch from the Florida panhandle through the Mid-Atlantic to the Northeast on Sunday as a strong wavy front passes through. The Day 4 ERO maintains a Marginal Risk of excessive rainfall in the update issuance. The progressive nature of this system will tend to keep rainfall totals from being too extreme. However, anomalous moisture and sufficient instability (over southern areas) should still support rain rates high enough to produce some flooding concerns, and there may also be thunderstorm potential in the warm sector over the Southeast. Within the Marginal Risk area, there may be some relatively greater potential for runoff issues in the northern Mid-Atlantic/Northeast urban corridor and over parts of the Northeast due to wet ground/melting snow cover (before or during the event). Guidance suggesting that not too much instability reaches northern areas, plus some ongoing differences for exact location/magnitude of heaviest rainfall, preclude any embedded Slight Risk area at this time. Enhanced rainfall may extend into Maine early in the Day 5 ERO period on Monday but with signals not yet pronounced enough to merit introduction of a Marginal Risk area. Regarding other aspects of the forecast, the Sunday-Monday event over the East will likely contain some wintry weather on the back side of the precipitation shield. Best potential for meaningful snowfall (at least 30 percent probability for 0.25 inch liquid in the form of snow) should be over the central Appalachians, to the lee of the eastern Great Lakes, and far northwestern New England. A period of brisk to strong winds will also be possible along the East Coast. The precipitation over the Northwest early in the period will include snow over portions of the Cascades into the northern Rockies, followed by a drier trend. By midweek one precipitation areas of note should be over the southern Rockies/High Plains as an upper low settles over the Southwest and moisture interacts with a front. Some of this precipitation may be snow, with typical elevation/coverage uncertainty. Meanwhile easterly flow setting up to the south of strong eastern U.S. high pressure should promote periods of rain over Florida. Finally, light precipitation may reach the Pacific Northwest by next Thursday. Temperatures near the East Coast will be well above normal on Sunday, and continuing over New England into Monday, ahead of the approaching cold front. There will be decent coverage of at least plus 10-20F anomalies and a few locations in the northern Mid-Atlantic/southern New England could challenge daily record highs. Temperatures behind the front will trend to near or somewhat below normal, with the coolest anomalies likely to progress across the southern tier Sunday-Tuesday. The evolving pattern will favor a warmer trend over the northern-central parts of the central U.S. next week. Anomalies should become most pronounced by next Thursday with most northern areas reaching 10-20F above normal. Rausch/Dolan 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