Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Tue Aug 30 2022 Valid 12Z Fri Sep 02 2022 - 12Z Tue Sep 06 2022 ...Prolonged and potentially dangerous heat wave for portions of the Western U.S. this holiday weekend... ...Heavy rain threat along portions of the Gulf Coast... ...Overview... Anomalously strong upper ridging is expected to be ongoing at the start of the medium range period 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 across the Gulf Coast and 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, initially over portions of the Florida Panhandle then moving westward toward the Texas Gulf Coast this weekend. 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 Great Lakes and Northeast this weekend into early next week, bringing another cold front through the East. Some unsettled weather may develop along that front for Labor Day across the Mid-Atlantic and Northeast, but there remains some uncertainty. ...Guidance Evaluation/Predictability Assessment... The initial synoptic model differences this cycle remain with the eastern Pacific upper low and the strength/amplitude of the energy that brushes up the Northwest early on in the forecast period. By Friday, a sharp shortwave trough axis centered near 35N/135W quickly lifts northeast as it opens up over British Columbia. While earlier runs of the deterministic guidance showed a bit of timing variability, it seems this cycle things have clustered better, aside from the CMC which continues to be a slower solution. This energy rounds the top of the Western U.S. ridge and then eventually helps to carve out a deeper trough over the Great Lakes by Labor Day. Here, the model differences are more notable with greater uncertainty in the sensible weather. The latest deterministic have trended toward a deeper trough over the East Coast by 12Z Tuesday though vary on the depth and position of that trough. The forecast challenges near the Gulf Coast are more subtle with less synoptic forcing and more convective/mesoscale driven influence on the areas of significant rainfall. A disturbance and interactions with the stalling frontal boundary in the area should focus the greatest threat of heavy rainfall initially over the Central Gulf Coast to Florida Panhandle Friday then the deeper moisture moving westward and easterly flow pushes the threat toward the Texas Gulf Coast by this weekend. Beyond that, some indications that a weak jet streak over the South and southerly flow may pull some of this deeper moisture through the Lower Mississippi Valley and potentially as far north as the Ohio Valley into Mid-Atlantic by Labor Day and Tuesday next week. The ECMWF has been more aggressive pulling moisture northward through much of the Eastern U.S. early next week while the GFS had been on the drier side, though the latest 00Z run seems to be trending toward the ECMWF. For the WPC blend overall, a split between the ECMWF/GFS was generally used with some inclusion of the UKMET/CMC early on then transitioned toward the ECENS/GEFS means by day 6-7 given some of the model uncertainty and spread. ...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. Taylor/Rausch 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