Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Aug 26 2024 Valid 12Z Thu Aug 29 2024 - 12Z Mon Sep 02 2024 ...Overview... Expect the strong upper ridge lingering over the East into Thursday to give way to mean troughing that settles over the northeastern quadrant of the lower 48 by the weekend and early next week, after a strong and sharp upper trough crosses the northern Plains/Upper Midwest late this week. Meanwhile an upper ridge should drift across the West while a weak upper low eventually nears the West Coast. A cold front pushing across the eastern half of the country will bring areas of showers/storms late this week into the weekend. The southern Rockies and High Plains may see increasing rainfall after Thursday and episodes of rainfall will be possible through the period near the west-central Gulf Coast and over Florida. The forecast pattern will favor moderation and suppression of mid-late week eastern U.S. heat by the weekend while the Plains/Rockies will tend to see near to below normal highs. Upper ridging will support above normal highs over the Pacific Northwest into far northern Rockies late week through the weekend. ...Guidance/Predictability Assessment... From Thursday into Saturday the guidance is fairly well clustered as the initial trough/upper low tracking near the Canadian border opens up and upstream energy begins to broaden the mean trough, while the associated cold front progresses eastward from the central U.S. A 12Z/18Z operational model composite offers a good representation of consensus for this part of the forecast. Various areas of spread arise in the guidance by Sunday-Monday, while most guidance on averaged has nudged a little eastward with the overall eastern North American trough. Within this mean trough, during the weekend solutions diverge for the character of shortwave energy dropping into the western side. 12Z machine learning (ML) models suggest a little stronger shortwave brushing the Great Lakes than some dynamical guidance, and especially discount latest CMC runs that stray to the flat/progressive side with southern Canada flow. At the same time the GFS becomes stronger than nearly all other dynamical and ML models with potential shortwave energy evolving over the southern Rockies and vicinity. Remaining solutions generally lean toward greater persistence of ridging over the region and less northward extent of moisture over the Plains. The 12Z ECens mean does offer the potential for rainfall to extend into the central Plains by late in the period, but consensus would at least suggest that GFS max amounts there are overdone. Finally, the ensemble means and ML models agree upon slower progression of the East Pacific upper low than seen in the 18Z GFS/12Z ECMWF. This combination of considerations led to trending the late part of the forecast more toward the ensemble means with some lingering input from the ECMWF (split between more 00Z/25 run than the 12Z/25 version). ...Weather/Hazards Highlights... Hazardous heat will persist over portions of the eastern U.S. into late this week, with some locations seeing highs up to 10-15F above normal and potential for daily records. Experimental HeatRisk shows a broad area of Moderate to Major (and localized embedded Extreme) level impacts Thursday with coverage of these categories narrowing on Friday. Weakening of the eastern U.S. upper ridge and approach/arrival of a cold front from the west should bring temperatures closer to normal by the weekend, confining most of the above normal readings over the South. Upper ridging will produce above normal temperatures (by up to 10-15F) over the Pacific Northwest into far northern Rockies late this week into the weekend, with an eventual cooling trend from west to east. In- between, the Plains and most of the Rockies will tend to see near to below normal highs through the period. During the Thursday-Friday period covered by the Days 4-5 Excessive Rainfall Outlooks, the cold front progressing eastward from the central U.S. will be one feature producing areas of potentially heavy convection. The Day 4 Outlook maintains a Marginal Risk area over the Upper Midwest where activity may be progressive but short-term rain rates could be fairly high. A modest increase in clustering/QPF totals could open the door to an embedded Slight Risk upgrade. By Friday the best signal for relatively greater rainfall exists over and south of the eastern Great Lakes, favoring a Marginal Risk area. The Day 4 Marginal Risk area over the Mid- Atlantic has been maintained close to continuity as guidance still shows potential for locally heavy and slow-moving convection in the vicinity of a stalling surface front. Convection over the southern Rockies and High Plains may increase in coverage and intensity on Friday relative to Thursday as low level upslope flow provides an added contribution. Thus the Day 5 outlook depicts a Marginal Risk for this activity. Convection will be possible along the remainder of the Midwest/Great Lakes into southern Plains front but with lesser clustering for location and amounts in addition to the central Plains locations having dry antecedent conditions. The west-central Gulf Coast will be another region that requires monitoring. For multiple days guidance has provided varying signals for rainfall in this region but with no clear consensus for precise amounts and how much will be over land versus just offshore. Beyond early Saturday, expect eastern U.S. rainfall to continue progressing eastward along a leading (and eventually weakening) front while a second drier likely arrives by early next week. Episodes of convection should continue along the Gulf Coast and over Florida. Southern Rockies and High Plains rainfall should continue through the weekend, though with increasing uncertainty over coverage and amounts by Sunday-Monday. 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 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