Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Fri Jul 26 2024 Valid 12Z Mon Jul 29 2024 - 12Z Fri Aug 02 2024 ...Hazardous heat to shift into the central Plains early on... ...Heat builds across the northern tier of the country late next week... ...Showers/storms with locally heavy rainfall possible over parts of the eastern half of the lower 48... ...Overview... Guidance continues to show the pattern evolving toward a strengthening southern Rockies/Plains upper ridge by the middle to late part of next week as a retrograding Gulf Coast ridge merges with the lingering ridge over the southern Rockies. This should help set up what should be a prolonged period of hot weather over many areas with the most anomalous heat expected over the Central Plains. Shortwave energy moving through the Midwest with eventual mean troughing that develops over the eastern U.S., will support a broad area of showers/thunderstorms drifting across the eastern half of the country. Shortwaves feeding into an initial mean trough aloft over the Pacific Northwest coast should bring some rainfall to the region early next week. This system should eject eastward mid-late week as larger scale troughing takes shape over the northeastern Pacific. Despite the overall pattern, heat increases with time across the Lower 48, with the largest positive anomalies temperature-wise migrating towards the US/Canadian border ...Guidance/Predictability Assessment... The guidance showed reasonable agreement, with the 00z ECMWF on the strong side of the guidance with an occluded low that it seems to want to develop a subtropical cyclone out of early on. There are some differences with a trough moving by the Great Lakes, with the 00z ECMWF a little out of sync with the rest of the guidance. The pressures/fronts, winds, and 500 hPa heights used a blend of the 06z GFS, 00z Canadian, 00z UKMET, and 00z ECMWF for the first half of the period which helps deals with details around New England effectively. After this, increased contributions from the 00z NAEFS and 00z ECMWF ensemble means was used, maximizing at 40% of the blend by late next week. The QPF was generally a slightly amped up version of the 13z NBM, except across the Northwest where some 12z GFS, 12z Canadian, and 12z UKMET were used per coordination with PDT/the Pendleton OR forecast office. Other grids were based more strongly on the 13z NBM. Overall, this maintained reasonable agreement with the previous WPC forecast. ...Weather/Hazards Highlights... By Monday, there will be a broad corridor of above-average precipitable water values extending from the Upper Midwest into the Southeast, gradually shifting eastward with time on the backside of a weakening deep layer cyclone. The Days 4 and 5 Excessive Rainfall Outlooks covering Monday- Tuesday night show broad marginal risks across portions of this region. There was enough agreement in the guidance for small slight risks across portions of the southern- central Appalachians where flash flood guidance is typically lower, but moreso lately due to recent rains and additional rainfall expected on Sunday, prior to the medium range period. Much of the Eastern third of the country should remain generally unsettled next week as a upper trough is slow to budge out of the region while it generally weakens. Elsewhere, above normal moisture associated with a shortwave should bring some modest rainfall to portions of the Pacific Northwest on Monday, with much lighter/more scattered rainfall as the shortwave shifts inland. Monsoonal convection over the Southwest should stay confined to southern areas through midweek, though it may eventually expand a little northward depending on the shape of the southern Rockies/Plains ridge and upper level energy rotating around its western periphery. The forecast pattern evolution will support an expanding area of warmer than average temperatures across the Lower 48, possibly into the Ohio Valley/Mid-Atlantic by next Thursday. The most persistent and extreme high temperature anomalies should be over the Central Plains early on where some locations should see multiple days with highs 10-15F or so above normal and high temperatures broach 105F. An incoming front will cause some easing of the heat late next week. The experimental HeatRisk likewise reflects an expanding area of Moderate to Major risks of heat- related impacts from the weekend through next Wednesday, and even some pockets in the Extreme category over the central U.S. Forecast temperature anomalies would yield highs in the upper 90s towards 110F in isolated spots over the central Plains and upper 80s to 90 or so farther northeastward. Climate Prediction Center forecasts indicate some variation of this pattern may persist into next weekend. Much of the West will likely see near to slightly below normal highs through the first half of next week before the strengthening ridge pushes temperatures above normal by next Thursday, and more significantly so next Friday with lower elevations seeing 100F+ high temperatures. The length of the northern tier of the country could see some highs reach 10F or so above normal during the first couple days of August/late next week, with the strongest anomalies expected in eastern WA and the ID Stovepipe. Lingering rainfall over the southern tier should support near to below normal highs, especially over southern/eastern Texas at the start of the week. Roth/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