Extended Forecast Discussion NWS Weather Prediction Center College Park MD 401 PM EDT Tue Aug 30 2022 Valid 12Z Fri Sep 02 2022 - 12Z Tue Sep 06 2022 ...Prolonged and dangerous heat wave for portions of the Western U.S. through the holiday weekend... ...Heavy rain threat from Southern/Southeast Texas through the central Gulf Coast states... ...Overview... It remains the case that anomalously strong upper ridging to be ongoing at the start of the medium range period over much of the West and persist through early next week. This will support very warm to hot temperatures in that region, with potentially record breaking and dangerous high temperatures. The other focus for significant weather will be found from Southern/Southeast Texas through the Gulf Coast/South where interactions with a stalling frontal boundary and deeper Gulf moisture could spell a multi-day heavy rainfall period leading to some flash flooding. Some of that moisture and potential heavy rain may lift northward through the Lower Mississippi Valley by late in the weekend. Meanwhile, shortwave troughing will work through the Midwest/Great Lakes and Northeast this weekend into early next week, bringing another cold front with thunderstorms. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite of reaosnably well clustered mass field guidance from the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian along with WPC continnuity and the 13 UTC National Blend of Models (NBM) days 3-5 (Friday-Sunday). The main model QPF focus was more varied over Texas and the South, but favorable support and protracted threat potential exists on a elongated axis across the broad region given deep moisture pooled near a stalled and wavy front. Later, a blend of the ECMWF/ECMWF ensembles seems to have a better idea into day 6/7 (Labor Day/next Tuesday). somewhat on the more amplified side of the envelope with the southward digging of ample shortwave trough energies through the Midwest to the lee of the amplified upper ridge over the West. Accordingly, the 06/12 UTC GFS seem too warm over the region for this period and overall WPC QPF/PoPs were created as per a targetted change to the NBM to show much more areal coverage and some embedded convective maxes from the Midwest into the East. ...Weather/Hazards Highlights... An anomalously strong upper ridge for early September positioned over the Western U.S. will support much above normal temperatures Friday through early next week, with the greatest departures from normal expected from California through the Pacific Northwest, Great Basin, and the northern Rockies. Daytime highs between 10-20F above normal and nighttime lows between 15-20F above normal are likely. The holiday weekend looks particularly hot with highs from the Desert Southwest through the California Central Valley well above 100 with some readings greater than 110 while mid to upper 90s likely for portions of the northern Rockies. Widespread daily records are possible each day and for portions of Desert Southwest through California, a potentially dangerous and record breaking heat wave is possible. A residual frontal boundary draped across the Gulf Coast interacting with favorable Gulf moisture transport and a weak upper level disturbance will help focus widespread showers and thunderstorms through the weekend into early next week. For Friday, the best focus looks to be over the Florida Panhandle then the moisture/disturbance moves westward toward the Texas Gulf Coast by this weekend into early next week. Some of this energy may get drawn northward across the Lower Mississippi Valley, bringing the threat for heavy rainfall northward across portions of Mississippi and Alabama through the holiday weekend. Over the entire period, the greatest threat for multi-day totals reaching several inches looks to be across the Texas Gulf Coast and the Central Gulf Coast with slightly less confidence further inland/north across the Lower Mississippi Valley. A Slight Risk in the Day 5 (12Z Sat - 12Z Sun) Experimental Excessive Rainfall Outlook was introduced given the potential for both intense rain rates and longer duration heavy rain that could lead to flash flooding. Some of these areas have seen recent heavy rainfall with the latest soil moisture anomalies well above 90 percent, particularly across portions of the Lower Mississippi Valley. There does remain some meaningful model differences, so overall confidence in specific details and locations of heavy rain/flash flooding is a bit lower now, but the environmental ingredients do support the potential somewhere in the region. Elsewhere, the combination of a weakening warm front crossing the Plains and cold front pushing into the northern tier late this week may produce showers/thunderstorms of varying intensity from the Upper Great Lakes southwestward with some threat of localized heavy rainfall across the Upper Peninsula of Michigan on Friday where slightly higher moisture pools ahead of the advancing front. After starting Meteorological Fall with a drier airmass, there remains uncertainty in the forecast for the East Coast by late in the weekend and Labor Day as model guidance has trended wetter over the last couple of cycles. Deeper moisture being pulled northward into the region from the Gulf by Labor Day interacting with favorable upper level dynamics could bring greater chances of showers and thunderstorms. Schichtel/Taylor Hazards: - Heavy rain across portions of the Southeast and the Lower Mississippi Valley, Sat-Sun, Sep 3-Sep 4. - Heavy rain across portions of southern Texas, Sat-Tue, Sep 3-Sep 6. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Flooding likely across portions of the Lower Mississippi Valley. - Excessive heat across portions of the Pacific Northwest and the Northern Rockies, Fri, Sep 2. - Excessive heat across portions of the Pacific Northwest, the Northern Rockies, the Great Basin, California, and the Southwest, Fri-Tue, Sep 2-Sep 6. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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