Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
696 FXUS02 KWBC 270744 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 344 AM EDT Sun Sep 27 2026 Valid 12Z Wed Sep 30 2026 - 12Z Sun Oct 04 2026 ...Major South-Central to East-Central U.S. Heavy Rain/Flooding threat with tropical moisture connections into mid-later week... ...Guidance/Predictability Assessment... Forecast spread has decreased over the past few cycles and recent guidance is now showing a reasonably similar larger scale pattern evolution through these medium-range time scales to bolster broad threat confidence, but not local focus details. This is due to numerous smaller scale embedded system timing issues and system/convection interactions. Accordingly favor a blend of best clustered guidance of the GFS/ECMWF and GEFS/ECMWF ensemble means, with emphasis on the models past midweek slowly shifting to the ensemble means later week into next weekend in line with with uncertainty growth for consistent threat messaging and products. ...Pattern Overview and Weather/Hazards Highlights... A closed upper trough/low with wavy frontal systems will shift slowly eastward from the Southwest U.S. to the South-central U.S. mid-later week in southern stream split flow. Excessive rainfall/flooding threats over the Southwest/Southern Rockies states through midweek will focus over the South-central U.S. and with Gulf coastal frontal genesis mid-later week into next weekend. The significant episode period will be fueled by tropical moisture connections from current East Pacific Hurricane Polo (set to landfall into Mexico) and also with increasingly deep and sustained lead Gulf inflow with some tropical influence. This expansive multi-day threat should prove slow to progress over the east- central U.S. late week/next weekend blocked between upper ridge developments over both the Southwest and Southeast U.S.. Accordingly, huge WPC Excessive Rainfall Outlook (ERO) Marginal Risk and embedded Slight Risk threat areas have been introduced for Day 4/Wednesday and Day5/Thursday over the South-Central U.S.. Meanwhile, systems within a more progressive/zonal mean northern stream flow will support passage of a series of wavy frontal systems across the U.S. northern tier of the nation from the Northwest through the Northeast this week. These will act to provide a northern rainfall shield focus for lifting moisture and also deliver periods of moderating post-frontal fall temperatures. 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, 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 $$