Extended Forecast Discussion NWS Weather Prediction Center College Park MD 353 PM EDT Sun Sep 14 2025 Valid 12Z Wed Sep 17 2025 - 12Z Sun Sep 21 2025 ...Overview... A generally blocky/slow moving upper air pattern develops over North America by midweek with ridging over the Mountain West and Midwest and troughing over the West Coast, north-central states, and Northeast lingering into this weekend. Wetter than normal conditions can be expected midweek over the Mid-Atlantic, South Florida, much of the Great Plains into the Midwest, and the Desert Southwest. ...Guidance/Predictability Assessment... The latest model and ensemble guidance agrees on a somewhat blocked pattern from a ridge shifting over central Canada Wednesday to Friday which maintains slow moving troughs over the West Coast, north- central States (with a likely closed low) and over the Northeast. The guidance shows better agreement with a closed low moving up the Mid-Atlantic coast early in the period, but some question on timing of that. Same story with the upper low over the Dakotas into the Midwest, agreement on the existence but timing uncertainties (GFS was notably slower). Also, there's been a trend for the low along the California coast Thursday/Friday to retrograde offshore Friday night then potentially shifting back to shore (CMC/GFS) or continuing out to sea (EC). Timing uncertainties also exist across the Northwest with a couple of weak shortwaves in the flow. Overall, was able to use a general model compromise for about the first half of the period with reasonable agreement. Trended towards greater weighting of the ensemble means later in the period (along with the ECMWF and the GFS). Did not use the CMC for days 6 or 7 due to it being more aggressive with bringing the upper low off the California coast inland this weekend. ...Weather/Hazards Highlights... Southward shifting of the upper trough/low down the Dakotas Wednesday will continue to bring enhanced Gulf moisture up the Plains with convergence ahead of a slow moving frontal system. Areas of at least locally heavy rain are likely to continue drifting east from the central Plains to the Midwest Wednesday/Thursday and warrant Marginal EROs for both Days 4 and 5. Increasing monsoonal moisture over New Mexico through the Texas Panhandle on Wednesday warrants maintenance of the Marginal ERO there. A notable Pacific moisture surge up into California and the lower Colorado River region into the Great Basin is on tap for Thursday as the upper ridge axis over CA early this week shifts east to the Intermountain West. Guidance is quite varied in QPF there, but diurnally driven convection initiating on terrain continues a Day 5 Marginal ERO for southern CA. A stalled front over the southern Florida Peninsula provides focus for much above normal moisture and combined with wet antecedent conditions supports both a Day 4 and Day 5 ERO. Elsewhere, the Southwest ridge axis shifting east to the Intermountain West on Wednesday focuses more interior Southwest heat later this week. An upper ridge will promote heat over the Southeast through late in the week temperatures centered on the Midwest and Mississippi Valley through at least mid-next week, with anomalies of 10-15 degrees above average. Warmer than average temperatures move from the Midwest to the Northeast in the later half of the week as well. Santorelli/Jackson 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