Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Sat Jul 20 2024 Valid 12Z Tue Jul 23 2024 - 12Z Sat Jul 27 2024 ***Heat wave continues across the West, and showers/storms with heavy rainfall expected from the Four Corners to portions of the East*** 19Z Update: The models and ensembles are in very good agreement across the Continental U.S. through Thursday, and a general deterministic model blend suffices as a good overall starting point in the forecast process. Looking ahead to Friday/Saturday, the latest GFS is still more progressive than the slower ECMWF/CMC solutions across southern Canada with the upper low, but not to the same degree observed in earlier guidance. The ensemble means still support the idea of a slower solution, so the model blend was hedged more in the CMC/ECMWF direction for the end of the forecast period. The GFS also remains stronger with the Western U.S. upper ridge through Friday. The ensemble means accounted for about 30-40 percent of the fronts/pressures forecast for Friday into Saturday, but still relatively good overall forecast confidence for those time periods. The previous forecast discussion is appended below for reference. /Hamrick -------------------- ...General Overview... Expect the amplitude of next week's upper pattern to become somewhat less pronounced toward the end of next week. A strong ridge extending from the Southwest U.S. into west-central Canada (producing hazardous heat from the West into the northern High Plains) should begin to weaken and get pushed east/southeast ahead of a Pacific upper low tracking into western Canada and trailing trough that settles near the West Coast. Monsoon conditions will promote daily episodes of showers/storms over the Four Corners states and vicinity under and near upper ridging over that part of the country. Meanwhile, one or more wavy fronts will be on the leading side of Great Lakes into southern Plains mean troughing aloft, leading to multiple days of rain/thunderstorms with areas of heavy rainfall from the southern Plains into the Mid-Atlantic and parts of New England. The Great Lakes and Northeast should eventually trend drier late week as the northern part of the trough moves eastward. Consensus still shows the Atlantic upper ridge building into the Southeast for a time, peaking in strength around Wednesday-Thursday. ...Guidance/Predictability Assessment... The most prominent forecast issue in terms of synoptic-scale features that have some degree of predictability in the extended range is with the ultimate progression of Great Lakes troughing late in the week after Canadian energy feeds into it by Wednesday. By Friday-Saturday the spread becomes fairly dramatic between the slower ECMWF/ECMWF mean and fast GFS runs (though with the 12Z GFS not as fast as the newer 18Z/00Z versions). ECMWF-initialized machine learning (ML) models continue to favor an intermediate to moderately progressive solution, not as fast as latest GFS runs but suggesting low probability of the slow ECMWF scenario. The new 00Z ECMWF has indeed nudged somewhat faster than prior runs. There are some amplitude differences as well, but without prominent clustering one way or the other. The other forecast consideration is that 12Z/18Z GFS runs strayed to the fast side of the spread for upper low energy crossing southern Canada, while ML models at least support a farther north/northeast upper low than advertised by the 12Z ECMWF by next Saturday. The ensemble means and an average of the models fit this general idea. The updated forecast started with a composite of 12Z/18Z operational models early and then gradually increasing incorporation of latest GEFS/CMCens/ECens means late, reaching 50 percent total by next Saturday. By the latter half of the period the blend kept ECMWF/ECens weight a little lower than would normally be the case to account for preferences with the Great Lakes/Northeast upper trough. ...Weather/Hazards Highlights... Expect dangerous heat over the West to extend into next week with high temperatures reaching the 90s and 100s and warm overnight lows providing only limited relief. There should be a fairly broad area of highs 5-15F above normal through midweek or so from California into the far northern Rockies. Gradual cooling will likely move in from the Pacific Northwest (where highs may be slightly below normal by late week) as an upper low tracking into British Columbia and trough to its south/southwest helps to lower heights aloft. Meanwhile the most anomalous heat should shift into Montana/northern High Plains during the mid-late week period, in particular Wednesday-Thursday when Heat Risk shows the most prominent signal for major impacts. The prolonged period of dry heat over the West could also result in enhanced wildfire danger. In contrast, the persistent upper trough/weakness over the central U.S. will promote below normal highs from the central-southern Plains into the Ohio/Tennessee Valley and southern Appalachians around midweek, eventually becoming more confined to Texas and vicinity by next Saturday. Monsoonal moisture will linger over the Four Corners region and promote daily episodes of showers/thunderstorms through the period. Isolated to scattered instances of flash flooding will be possible, especially near steep terrain and burn scars. The Days 4-5 Excessive Rainfall Outlooks covering Tuesday-Wednesday maintain Marginal Risk areas, with potential for embedded Slight Risk upgrades as we enter the short range period depending on how guidance clusters relative to each other and sensitive burn scar areas/regions with wettest ground conditions. The upper trough extending from the Great Lakes into southern Plains along with a couple leading wavy fronts will support a fairly broad corridor of locally heavy shower and thunderstorm potential from the southern Plains northeastward next week. At the moment, the best guidance signals in terms of coherence and rainfall magnitude (with ample moisture and instability) exist over southern/southeastern Texas during the Tuesday-Wednesday period covered by the Days 4-5 Excessive Rainfall Outlooks. Both days of these outlooks maintain a Slight Risk area and reflect a modest south/southeast drift of heaviest activity with time. Surrounding Marginal Risk extends northeastward through the Mid- Atlantic and parts of the Northeast both days, with some embedded Slight Risk areas possibly emerging as guidance refines the most likely regions for heavy rainfall potential in the shorter term. The existing Marginal was trimmed back for the updated Day 4 ERO from Kentucky to western New York where models have trended lighter with overall expected QPF. Meanwhile a Marginal Risk area over the Upper Midwest into Great Lakes Tuesday- Wednesday accompanies an area of rainfall generated by an upper shortwave and surface wave/frontal system. Recent guidance runs have tempered the higher end of the envelope for rain rates but the dynamics/surface features still suggest potential for some locally heavy rainfall. By Thursday- Saturday, the northeastern quadrant of the lower 48 should trend drier as upper troughing moves eastward to some degree while showers/storms persist over the southern Plains and Southeast. 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