Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
781 FXUS02 KWBC 030716 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 AM EST Tue Dec 3 2024 Valid 12Z Fri Dec 06 2024 - 12Z Tue Dec 10 2024 ..,Return flow may fuel emerging rainfall from the west-central Gulf Coast to the Appalachians this weekend to early next week... ...Guidance/Predictability Assessment... Leaned on recent ECMWF/UKMET and trends from GEFS/ECMWF ensembles that hold main rainfall development closer to the west-central Gulf coastal regions late week until later weekend/early next week breakout up through the Appalachians. Inland QPF developemt is delayed compared to recent GFS/Canadian runs and the NBM. Across the nation, it is noteable that recent deterministic model guidance forecast spread and cycle to cycle continuity issues arise earlier than normal into the medium range time scales, lowering forecast confidence from the weekend into next week despite some common transition of the larger scale pattern. Accordingly, opted to pivot broadly toward a more compatible and run stable GEFS/ECMWF ensemble mean blend along with the National Blend of Models to maintain feasible WPC continuity amid uncertainty. ...Weather/Hazards Highlights... Arctic air that has persisted over the east-central U.S. will moderate this weekend as amplified upper troughing finally lifts out, but not before renewed lake effect and broader southern Canada to northern Northeast snows with windy clipper passage. Meanwhile, moist return flow from the Gulf of Mexico with retreating high pressure will support emerging enhanced rain chances set to spread from the west-central Gulf Coast states up through the Appalachians this weekend into early next week, albeit with uncertainty on the timing and inland extent of the rain. No threat areas are currently denoted on the WPC Day 4/5 (Fri-Sat) Excessive Rainfall Outlook areas, but this is predicated on ample variances in flow transition and with a notion that heavier inland focus may be delayed into later weekend/early next week as aided by uncertain southern stream upper energy ejection from the Southwest/Baja and effect of possible wavy approach of early next week cold front. Upstream, moderate to terrain enhanced precipitation will develop this weekend over the Pacific Northwest with energetic upper trough passage and lead moisture feed. Terrain enhanced snows will spread across the Northwest to the north-central Rockies with system and unsettling cold frontal passage over the West into next week. Schichtel 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 $$