Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Thu Dec 5 2024 Valid 12Z Sun Dec 8 2024 - 12Z Thu Dec 12 2024 ***Heavy rain for the central Gulf Coast early next week*** 19Z Update: Overall, the 12Z model guidance suite has consolidated more compared to yesterday with the amplifying upper level pattern across the continental U.S. and the deep upper trough situated over the north-central states. The GFS is initially faster with a lead shortwave crossing the Ohio Valley on Monday, whereas the AI guidance and non-NCEP guidance suggest a slower solution, but matches up well elsewhere. Looking ahead to Wednesday, the ECMWF becomes stronger in bringing a reinforcing trough across the Upper Midwest and then across the Ohio Valley by Thursday. It is also stronger with the next offshore trough approaching the Pacific Northwest. The ensemble means were gradually increased to about 40-50% by next Thursday as mesoscale uncertainty increases. The previous discussion is appended below. /Hamrick ------------------- ...Guidance/Predictability Assessment... The WPC medium range product suite valid for Sunday and Monday was primarily derived from a composite of best clustered guidance of the 18 UTC GFS and 12 UTC ECMWF/UKMET. These have good ensemble support and continuity in an active pattern with above normal overall predictability despite lingering embedded shortwave timing issues mitigated by the blending process. Pivoted at longer time frames to the most compatible solutions of the 18 UTC GEFS mean and 12 UTC ECMWF/ECMWF ensemble mean to maintain best continuity as forecast spread grows to average levels. The 01 UTC National Blend of Models and latest guidance from the 00 UTC guidance cycle mainly seems in line with the forecast plan. ...Weather/Hazards Highlights... It remains well heralded in guidance that emerging moist return flow from the Gulf of Mexico as ambient high pressure over the South retreats to the east will support increasing coverage of enhanced to heavy showers and thunderstorms from the west-central Gulf Coast states to the southern Appalachians and parts of the Southeast this weekend through early-mid next week. In terms of the WPC Excessive rainfall outlook, the Day 4/Sunday Slight Risk area has been adjusted slightly to the east by approximately a row of counties/parishes to account for new trends in the 12z guidance over the ArkLaMiss region. The Day 5/Monday Slight Risk has been trimmed some to the south with most of the heaviest convective rainfall likely confined closer to the Gulf Coast. This region will be in a favored right entrance region of the upper jet pattern to support multiple rounds of convection, as strong moisture flux from the Gulf continues ahead of an approaching cold front. There may also be some severe weather that develops in the warm sector of this low pressure system, and this is being monitored by the Storm Prediction Center. Later in the forecast period, the moderate to heavy rain and convective focus is expected to encompass more of the Southeast and up across the East Coast with system translation through the middle of next week. Colder air on the northern periphery of this broad system and additional northern stream energies will likely herald the return to lake effect snow by Wednesday and into Thursday with strong cold air advection across the Great Lakes. Upstream, moderately heavy terrain enhanced precipitation is expected to develop this weekend over the Pacific Northwest with energetic upper trough passage and long lead moisture feed. Terrain enhanced snows will then spread increasingly inland across the Northwest to the north-central Rockies and eventually spread lighter winter precipitation over the Northern Plains/Upper Midwest with cold frontal passage over/out from the West into early-mid next week. Expect a warming trend ahead of the front out from the West this weekend/early next week prior to cold frontal and secondary seasonal surges across most of the central to eastern U.S. Schichtel/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