Preliminary Forecasts
Issued by NWS
Issued by NWS
971 FXUS02 KWNH 100638 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Sun Aug 10 2025 Valid 12Z Wed Aug 13 2025 - 12Z Sun Aug 17 2025 ...Overview... Increased troughing into the Pacific Northwest later this week will linger through the upcoming weekend as above normal heights dominate much of the rest of the Lower 48. Slow- moving frontal boundaries in the East will spark convection and locally heavy rainfall amid seasonably warm to hot temperatures, with at least some signal for monsoon convection in the Southwest. ...Guidance/Predictability Assessment... Through the 12Z/18Z guidance, most of the deterministic runs offered reasonable clustering with the ensemble means except for the 12Z Canadian over western Canada. The ECMWF AIFS EPS was a bit slower/deeper than its dynamical counterpart which was more in line with the ECMWF/GFS. Still, timing the shortwaves across Canada exhibits lower confidence beyond day 3 as is typical in the summer. Trended toward an even split between the ensembles and deterministic GFS/ECMWF by the end of the period. ...Weather/Hazards Highlights... Near and ahead (and south) of a frontal boundary astride the Great Lakes, locally heavy rain is expected within a moist and unstable air mass. The western end of this front over the High Plains will push eastward as a warm front, igniting some locally heavier rain across the Upper Midwest Wednesday into Thursday. Over the Southwest, monsoonal moisture will make another attempt at pushing northward midweek. These areas are denoted with a Marginal risk of flash flooding (excessive rainfall) Wednesday-Thursday. For the rest of the period (Fri-Sun), rainfall chances will be modestly high over the Upper Midwest near a couple frontal boundaries, over the Southeast, and across the Southwest. Temperatures will start the period quite warm into the Northeast to near record levels Wednesday with highs into the 90s. This will push HeatRisk levels into the Moderate (level 2) to perhaps Major (level 3 out of 4) category from DC northward into New England. Temperatures will remain near to several degrees above normal over much of the CONUS east of the Rockies, especially over the Midwest into the Northeast/Mid-Atlantic (though likely below record levels). Over the Pacific Northwest into California, temperatures will trend cooler than normal as troughing digs into the West. Fracasso 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 $$