Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Mon Aug 29 2022 Valid 12Z Thu Sep 01 2022 - 12Z Mon Sep 05 2022 ...Much above normal temperatures and some record highs possible over a large portion of the West for multiple days... ...Heavy rain threat along portions of the Gulf Coast... ...Overview... The most stable feature during the Thursday-Monday period should be a strong western U.S. upper ridge that will promote very warm to hot temperatures over a majority of the region, with daily records for highs and warm lows possible each day. The other focus for significant weather will be over the South and Gulf Coast areas where the interaction of a front settling over the region and southeasterly to easterly low level flow from the Gulf, along with one or more upper level impulses/circulations, could produce several days of heavy rainfall and potential for some flash flooding. Currently expect highest totals to shift westward from near the Florida Panhandle to the Texas Gulf Coast over the course of the period. Meanwhile one upper trough departing from the Northeast late this week will be replaced by another one that reaches the area by Sunday-Monday. The latter trough should push a cold front from the northern tier into the East. ...Guidance Evaluation/Predictability Assessment... The first guidance difference of note involves the details of an eastern Pacific upper low whose energy brushes the Northwest as upstream troughing amplifies to the south of Alaska. Thus far the operational models have varied among each other and from run to run for timing. Slower 12Z CMC trend to the 00Z ECMWF has left an even split between that cluster and the faster GFS/UKMET (though with the 12Z UKMET slower than its prior run). Larger scale differences develop from the Pacific eastward by the latter half of the period as GFS/GEFS mean runs continue their recent tendency to bring in Pacific troughing more quickly than other guidance and in turn begin to shift the downstream ridge/trough configuration over southern Canada and into the lower 48 farther eastward. The 12Z GFS nudged a little slower than the 06Z run but is still somewhat out of phase with the majority. Associated with this difference, by late in the period the GFS/GEFS mean weaken the western upper ridge versus the ECMWF/CMC and their means, all of which still show a 594+ dm ridge centered over California and/or Nevada as of early Monday. Overall preference is to maintain a forecast that tilts at least two-thirds toward the ECMWF/CMC cluster. Favoring this scenario should in turn result in the leading feature ejecting a little slower than forecast by the GFS. The forecast near the Gulf Coast shows lower than desired predictability for specifics, given the short-term convective/mesoscale influence on areas of significant rainfall as well as the medium to smaller scale of upper features that may be involved. However there are some general signals. These include a stalling Gulf Coast frontal boundary that should interact with Gulf moisture carried by southeasterly to easterly low level winds, and a general westward shift in rainfall emphasis from the Florida Panhandle late this week toward the Texas Gulf Coast by the weekend and early next week. Guidance disagrees with the magnitude of anomalously high precipitable water values, with the ECMWF/ECMWF mean suggesting moisture could reach up to 2-3 standard deviations above normal over/near the central/western Gulf Coast. ...Weather/Hazards Highlights... The persistent upper ridge over the West will support a broad area of much above normal temperatures, with the highest anomalies likely to extend from California into the Interior Northwest/northern Rockies and at times into the northern High Plains. Expect most days to see plus 10-20F anomalies which could challenge daily records for both highs and warm lows. Meanwhile the other primary weather focus will be with showers and thunderstorms across the southern tier due to a stalling front likely interacting with ample instability and moisture as well as one or more upper level impulses/circulations between the central U.S. and the South. Initial moisture/energy could produce rainfall over parts of Texas. Otherwise the best surge of moisture should generally translate westward with time, from near the Florida Panhandle late this week and then continuing through the central Gulf Coast and western Gulf Coast/Texas during the weekend into early next week. Some of this rainfall may be heavy with multiple days of such activity leading to concerns for flash flooding and at least localized rainfall accumulations of several inches. Note that there are still meaningful guidance differences for specifics so detail confidence is lower than desired at this time. 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. How much rainfall the cold front produces as it continues farther eastward is still up for debate. The start of Meteorological Fall will indeed feel fall-like for parts of the eastern U.S. Portions of the Great Lakes/Northeast could see highs and/or lows up to 5-10F below normal Thursday-Friday. Eastern U.S. temperatures should trend to near or somewhat above normal after Friday. Clouds and rainfall will likely keep highs near to below normal across the far southern tier. Rausch/Taylor 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