Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Fri Dec 6 2024 Valid 12Z Mon Dec 09 2024 - 12Z Fri Dec 13 2024 ***Heavy rain threat across the central Gulf Coast and Southeast early next week*** 19Z Update: The 12Z models are in excellent synoptic scale agreement with the amplifying deep trough over the Central U.S. through the middle of the week, and the evolving storm system and cold front that will affect the East Coast. There are more questions that remain regarding Pacific shortwave energy reaching the West Coast and the eventual pattern evolution across the Western U.S. for the end of next week, and how the ridge axis over the West decays and becomes more zonal. The ECMWF and GFS are seemingly at odds by Friday, with the GFS maintaining the ridge axis longer and the ECMWF bringing in a well defined trough. An examination of the ML guidance at the time of forecast preparation did not strongly support the operational ECMWF with regards to trough placement, and therefore a solution closer to the CMC/GFS was preferred going into Thursday and Friday, with below average forecast confidence by this time. Ensemble means were gradually increased to about half by Friday amid growing uncertainty. The previous forecast discussion is appended below. /Hamrick ----------------------- ...Overview... The South and East will be the focus for potentially significant rainfall during the early-middle part of next week as a northern Plains into western U.S. upper trough as of early Monday heads into the east-central U.S. The leading wavy cold front will likely reach the East Coast around Wednesday with potential for stronger wave development over/near the Northeast later Wednesday into early Thursday. Upper troughing should linger over the East into next Friday but with a weakening trend by then. While most of the East dries out behind this system (aside from snow to the lee of the Great Lakes), expect the Pacific Northwest to see increasing precipitation after midweek as a mean trough aloft approaches. System progression will lead to an area of well above normal temperatures in the warm sector ahead of the central-eastern U.S. cold front, followed by an area of below normal readings and then a rebound to above normal over the Plains late in the week. ...Guidance/Predictability Assessment... Very early in the forecast, 12Z/18Z guidance as well as the new 00Z runs have trended stronger/faster with low pressure now forecast to reach the Canadian Maritimes by early Monday. As a result the trailing front pushes farther south along the Northeast coast with more high pressure wedging into New England. Latest consensus suggests the GFS could be a little overdone with its southward extent of the front though. For the upper trough progressing into the East and associated surface front/wave development, guidance agrees well in terms of the large scale evolution but shortwave differences lead to some variability for wave details and frontal timing. Still, there is decent model clustering toward expected Mid-Atlantic through New England wave development by Wednesday into early Thursday, with the majority of 12Z machine learning (ML) models also showing this low reaching just north or northeast of Maine by early Thursday. Some ensemble means are not as defined with this low yet due to spread among individual members. Within the upper trough, one notable questionable detail is the 12Z ECMWF's upper low that drops into the northern Plains/Midwest on Wednesday, with most other guidance and the new 00Z ECMWF run showing a more sheared depiction of energy digging south from Canada. Guidance spread increases noticeably over the eastern Pacific and western U.S. during the latter half of the week. The most common theme among dynamical and ML guidance at the moment is for a possible weak system or two to arrive from the Pacific and then for a better defined eastern Pacific upper trough to approach the West Coast by next Friday, with a surface front and anchoring surface low of very uncertain latitude. The ensemble means, 12Z and 00Z GFS, and 00Z/05 ECMWF plus a relative majority of ML models fit into this majority cluster. Other model runs and some ensemble members show different ideas so confidence in specifics is below average for now. Guidance comparisons among the 12Z/18Z runs led to the updated forecast starting out with an operational model blend followed by a transition toward some ensemble mean input (18Z GEFS/12Z ECens) as well as a late-period shift of the ECMWF component more toward the 00Z/05 run and GFS toward the 12Z run due to better comparisons to the ensemble means/ML guidance along the West Coast. ...Weather/Hazards Highlights... Early next week, upper trough dynamics reaching the West will support a brief period of enhanced snow over the central Rockies, primarily in Colorado. This energy will swing eastward within a larger scale upper trough eventually reaching the East, supporting a wavy cold front and eventual stronger Northeast surface development by around late Wednesday into early Thursday. Leading moist flow from the Gulf of Mexico will interact with the upper trough/front to generate an area of potentially heavy rainfall from the central Gulf Coast into Southeast/southern Appalachians Monday-Tuesday and continuing into parts of the Mid-Atlantic and Northeast on Wednesday. During the Monday-Tuesday night period covered by the Days 4-5 Excessive Rainfall Outlooks, locations along and a little inland from the central/east-central Gulf Coast are within a Slight Risk area for both days, given a favorable combination of anomalous moisture and some instability plus a brief period of slower frontal progression as western U.S. dynamics approach--thus leading to the possibility of some training/repeat convection. The Day 5 Sight Risk area also extends a little farther northeast into north-central Georgia. The updated Day 5 outlook has the Slight Risk area adjusted about a row of counties eastward compared to the initial overnight issuance to account for latest trends in the 12z guidance. The Storm Prediction Center is monitoring the potential for severe weather across this region. As the midweek wave develops near the East Coast, the far northwestern part of the moisture shield may contain some snow. Then expect the cold air arriving behind this system to produce another episode of lake effect snow during Wednesday-Thursday. Otherwise expect mostly dry conditions over the East late in the week. Rain and higher elevation snow should return to the Pacific Northwest and perhaps into the northern Rockies later in the week as one or more system affect the region. There is a lot of guidance spread for specifics at the moment, so it will take some time to get better resolution of details in terms of precipitation coverage and amounts. An area of well above normal temperatures will spread across the eastern half of the country early next week in the warm sector of the wavy cold front likely to reach the East Coast around Wednesday. Greatest anomalies should be for morning lows, reaching plus 15-20F or more at some locations. Then an area of below normal temperatures (generally negative 5-15F anomalies) will progress from the central-southern Rockies into the East. Then the West and Plains will rebound to above normal, with parts of the Plains seeing plus 10-15F anomalies by late next week. 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