Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 PM EDT Mon Jul 29 2024 Valid 12Z Thu Aug 01 2024 - 12Z Mon Aug 05 2024 ...Hazardous heat from the south-central Plains eastward gradually becoming more suppressed with time while building over the Northwest later this week... ...Overview... A strong upper ridge over the central U.S. into the Southeast as the period begins Thursday will continue to promote unseasonably warm temperatures over central parts of the Plains and into the Southeast. Then guidance shows upper ridging building over the West while a Midwest shortwave drifts into a developing mean trough over the East. Trailing northern stream flow should maintain a more shallow eastern trough by early next week. This sets up what may become a stagnant upper pattern supporting increasing heat across the West and unsettled weather over the East. The National Hurricane Center is also monitoring the potential for tropical development that could at least come close to affecting locations near the southeastern coast. ...Guidance/Predictability Assessment... Most guidance agrees fairly well for the large scale pattern but still displays important differences for some details. The most significant issues involve the upper trough crossing the northeastern quadrant of the lower 48 (plus amplitude of lingering mean troughing by next Monday) and potential tropical development that could track near the southeastern coast. Regarding the initial Midwest upper trough, incoming 12Z guidance generally maintains the spread from prior runs with the ECMWF leaning on the slower side and the GFS leaning faster. The 06Z/12Z GFS runs have adjusted somewhat slower to eject the feature than the 00Z run though. Machine learning (ML) models show some spread as well but on average lean somewhat faster than at least the operational ECMWF. By next Monday there is a significant majority of dynamical and ML models recommending more of a lingering trough across the Great Lakes/Northeast versus what the 06Z/12Z GFS runs advertise. The 00Z GFS ended up closer to consensus by next Monday. Most guidance continues to support the idea of possible tropical development in the Atlantic, as highlighted in the National Hurricane Center outlook. The operational ECMWF and its ensembles maintain a more eastern track, bringing a low northward off the coast of Florida/the Southeast by the weekend into early next week. The new 12Z CMC has adjusted eastward from its 00Z run, getting closer to the ECMWF idea. ML models (including the 18Z/00Z GraphCast GFS) generally support some degree of development with a track closer to the ECMWF cluster. Latest operational GFS runs have been very ill- defined with any possible system, with this scenario very much in the minority at the moment. The updated forecast based on 00Z/06Z solutions started with an operational model composite, with a later-period trend toward a model/ensemble mean mix that tilted a little more than average toward the ECMWF/ECens mean given WPC-NHC preferences toward that cluster of solutions for possible tropical development. Meanwhile this approach provided a reasonable intermediate solution for the Midwest through Northeast trough and reflected consensus for lingering eastern troughing by next Monday--with fairly good continuity relative to previous forecast. ...Weather/Hazards Highlights... A wet pattern is likely from the Midwest to the Eastern Seaboard late this week with ample moisture and instability in place while a slow moving upper trough and surface/frontal systems all support areas of showers and thunderstorms. The Days 4-5 Excessive Rainfall Outlooks covering Thursday-Friday night show fairly broad Marginal Risks across parts of the Midwest to Ohio Valley and the Northeast/Mid-Atlantic. Latest guidance continues to show potential for embedded mesoscale systems to produce locally heavy rainfall across parts of the above areas. However there is still a lot of spread for location of heaviest activity, plus dependence on still- uncertain shorter term rainfall and resulting soil sensitivity. Therefore preference remains to wait before introducing any Slight Risk areas. Also, ahead of the late week system, a departing shortwave and sufficient moisture/instability may produce locally heavy rainfall over northern New England where the Day 4 outlook depicts a Marginal Risk area. Much of the north-central U.S. to the eastern third of the country should remain generally unsettled through the weekend and into early next week with the aforementioned upper trough continuing northeastward and general mean troughing likely to persist in its wake. Locations near the southeastern coast should monitor latest forecasts regarding possible tropical development during the weekend/early next week time frame. Elsewhere, monsoonal convection over the Southwest continues to support Marginal Risk areas on the Days 4 and 5 EROs. Chances for convection may expand northeastward late this week depending on the shape of the southern Rockies/Plains ridge and upper level energy rotating around its western periphery. Thus the Day 4/Thursday Marginal Risk expands a bit northward into southern Colorado, and rain chances increase into the central High Plains by Friday. From the weekend into early next week, flow between the upper ridge and a weak trough setting up off the West Coast may support a northwestward/northward expansion of scattered convection. The heat wave over the Central Plains early this week should continue through Thursday, with daytime highs around 10F above normal and widespread major to locally extreme HeatRisk threats. An incoming front will bring relief to the Central Plains by Friday, but heat will continue across the Southeast through Friday or Saturday with warm overnight temperatures only exacerbating heat stress in that region. Meanwhile, much of the West will trend warmer, with lower elevations seeing 100F+ high temperatures. Scattered daily record highs are possible and appear most likely over the Interior Northwest on Friday-Saturday. Shorter bursts of above normal temperatures are also likely for the Midwest to Ohio Valley to the Mid- Atlantic/Northeast late this week, with perhaps some moderation next weekend after cold fronts pass through. Rausch/Santorelli 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