Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sun Aug 25 2024 Valid 12Z Wed Aug 28 2024 - 12Z Sun Sep 1 2024 19Z Update: The latest GFS runs are now in better agreement with the model consensus with the leading trough across the Great Lakes region for the middle of the week, in contrast to the more progressive runs over the past few days. There remains good overall agreement on the large scale upper trough/closed low near the U.S./Canadian border, although the GFS is initially a little slower with its eastward progression. Model spread increases across southwestern Canada into the northwestern U.S. in the wake of that trough regarding how far north the upper ridge gets, but better agreement with the synoptic scale trough over the Northeast U.S. by Sunday. A general deterministic model blend, with slightly less weighting to the GFS, was used for the beginning of the forecast period, and then trended up on the ensemble means to about 40% by next weekend. The previous forecast discussion is appended below for reference. /Hamrick ------------------ ...Overview... An eastern U.S. upper ridge will remain strong enough to support anomalous heat in its vicinity mid-late week with some daily records possible, but temperatures should slowly moderate with time as the ridge gradually weakens and drifts southeastward. The heat should be notably more suppressed by the weekend. A strong trough/embedded upper low will track along the U.S.-Canadian border mid-late week with upstream energy likely to broaden the overall Canada/northern tier U.S. mean trough by the weekend. A couple cold fronts supported by the overall trough will bring areas of showers/storms to the central/eastern U.S., with a leading Northeast/Mid-Atlantic front also producing rainfall of varying intensity mid-late week. An upper ridge will likely build over more of the West by late week into next weekend after the initial trough's departure. Diurnally favored convection should continue over the southern Rockies and along the Gulf Coast. ...Guidance/Predictability Assessment... Recent adjustments in the GFS/GEFS closer to remaining guidance have improved overall clustering compared to some recent days. Therefore the updated forecast was able to employ a 12Z/18Z operational model blend early in the period, followed by a transition toward 40 percent ensemble mean input by next Sunday. Details that still require some fine-tuning include shortwave energy crossing the Northeast early in the period and exact southward extent of the front dropping down the East Coast, late week evolution of the vigorous trough tracking along the Canadian border, and specifics of incoming energy that should serve to broaden the overall mean trough (with latest GFS runs leaning a bit to the fast/amplified side). By next Sunday there is also a fair amount of spread and variability regarding whether upper ridging or shortwave energy/weak troughing will be more prevalent over the central-southern Rockies and vicinity. Most of the machine learning models favor a tilt toward ridging. Clustering is better than average for an upper low nearing the West Coast next weekend. ...Weather/Hazards Highlights... The strong upper ridge over the eastern U.S. as of Wednesday will support anomalous heat from the Lower Great Lakes/northern Mid- Atlantic into the South, with decent coverage of plus 10-15F anomalies for highs. Forecast trends over the past couple days valid on Wednesday have raised temperatures in the northern part of this area and increased the number of locations where readings may approach or reach daily records. After midweek, expect the slow weakening and southeastward progression of the upper ridge to yield a gradual moderating trend, especially by next weekend as an approaching fronts reaches closer to the East Coast. During Wednesday-Thursday in particular, experimental HeatRisk continues to show a broad area of Moderate to Major level impacts with some embedded pockets of Extreme. Some Moderate/Major coverage persists into Friday but then becomes much more suppressed by Saturday. Elsewhere, The northern Rockies into parts of the Plains will be the main focus for below normal temperatures during the period. The system exiting the Northwest around midweek will bring cool highs through the northern Rockies/northern High Plains Wednesday- Thursday. A trailing cold front may reinforce below normal highs over the northern and then central Plains next weekend. Meanwhile upper ridging that builds into the Pacific Northwest will bring above normal temperatures to the region late week into the weekend. The Day 4 Excessive Rainfall Outlook valid Wednesday-Wednesday night has maintained the Marginal Risk area extending from the eastern Great Lakes/Upper Ohio Valley eastward into southern New England and the northern Mid-Atlantic. While guidance QPFs are still not particularly well clustered, there is now a more coherent signal for an axis of instability and anomalous moisture to exist ahead of a west-east cold front sagging through the region. This setup appears favorable for locally high rain rates and possibly some areas of west-east training. Marginal Risk areas over the northern Plains/Upper Midwest (convection ahead of an advancing northern tier front) and over the southern Rockies (adequate moisture/instability, GFS/ECMWF QPF signals, and likely existence of weak shortwave impulses aloft) remain similar from the overnight issuance. The Day 5 ERO covering Thursday-Thursday night depicts a Marginal Risk area over the Upper Midwest, with continued eastward progression of the central U.S. front. Some locally heavy convection may also be possible in the Mid-Atlantic near the surface front that should be stalling over the region, and a Marginal Risk area is now planned for the Piedmont of Virginia and portions of adjacent states. Continued southward/eastward progression of the central U.S. cold front after early Friday should bring areas of rain and thunderstorms into the eastern part of the country by late week into the weekend, as well as extending southwest into the southern Rockies/High Plains where at least a brief period of low level upslope flow could help to enhance rainfall. Another front dropping into the Plains next weekend could eventually produce some showers/storms as well. Periods of convection will be possible along the Gulf Coast through the period, with guidance hinting at relatively better potential over west-central areas. Also Florida may see some increase in moisture and rainfall by late week or the weekend. Rausch/Hamrick 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