Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Jul 25 2024 Valid 12Z Sun Jul 28 2024 - 12Z Thu Aug 01 2024 ...Hazardous heat to shift into and northeast of the central Plains... ...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. The strong ridging will set up what should be a prolonged period of hot weather over many areas east of the Rockies, with the most anomalous heat expected over the central Plains. Shortwave energy ejecting from the Plains/Mississippi Valley Sunday onward, and then mean troughing that develops over far eastern U.S., will support a broad area of showers/thunderstorms drifting across the eastern half of the country. Energy feeding into an initial mean trough aloft over the Pacific Northwest coast should bring some rainfall to the region early next week. This energy should eject eastward mid-late week as larger scale troughing takes shape over the northeastern Pacific. ...Guidance/Predictability Assessment... The latest forecast update based on 12Z/18Z guidance started with an operational model composite and then trended toward even weight of the models/means by the end of the period next Thursday, albeit with the 12Z ECMWF having less weight than the 18Z GFS or 12Z CMC by that time. Guidance is still working out the finer details of the complex evolution over the eastern U.S./western Atlantic into the first half of next week, as shortwave energy ejects from the Plains/Mississippi Valley and a forming weak upper low just off the East Coast may track into or near New England. An average of latest machine learning (ML) models and dynamical guidance provides a reasonable depiction of this evolution. Around the northern periphery of the building Rockies/Plains upper ridge (which has some modest spread for details with low predictability several days out in time), there is some spread but also improving clustering regarding shortwave details by the latter half of the period. In particular, during Tuesday-Thursday the ML models generally offer more support for the 18Z GFS/12Z CMC/12Z ECens mean which show a better defined shortwave ejecting from the Pacific Northwest than depicted by the 12Z ECMWF. Thus the latter part of the forecast blend underweighted the 12Z ECMWF. The new 00Z ECMWF shows a stronger depiction of this shortwave. Consensus agrees that Northwest heights should rise after midweek as this shortwave departs and larger scale troughing develops over the northeastern Pacific. ...Weather/Hazards Highlights... During the Sunday-Monday night period covered by the Days 4-5 Excessive Rainfall Outlooks, there will be a broad corridor of above-climatology precipitable water values extending from the Upper Midwest into the Southeast, with this moisture drifting gradually eastward. On Day 4 the guidance suggests two relatively greater areas of focus for heavy rainfall potential. One will be over the Upper Midwest where shortwave energy and a wavy surface front will interact with a moist and unstable environment, with the other centered over/near the eastern two-thirds of Tennessee. Both regions generally lean toward wet antecedent conditions. The southern area exhibits a bit more overlap in the guidance, allowing for introducing a Slight Risk (Level 2 of 4) area, while preference is to await somewhat better agreement before introducing a Slight Risk over the Upper Midwest. Note that locally heavy rainfall and some flash flooding issues will be possible elsewhere within the broad Marginal Risk area. This broad area of showers and thunderstorms will drift eastward into Day 5, with guidance signals not sufficiently coherent to support any Slight Risk areas within the still broad Marginal Risk area. Elsewhere, lingering activity over southern Texas merits a Marginal Risk on Day 4 with no change in continuity while monsoonal activity and an associated Marginal Risk on Days 4-5 will be confined to parts of Arizona/New Mexico. Upper troughing aloft will tend to maintain an unsettled pattern over the East after early Tuesday, with showers/storms possibly becoming a little lighter and more scattered by Wednesday- Thursday. Monsoonal convection over the West 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. A frontal system approaching/reaching the Pacific Northwest early in the week may produce a period of rainfall, though currently with a fair amount of spread in the guidance for intensity. The forecast pattern evolution will support an expanding area of hot weather over the Plains and to a lesser degree extending into the Great Lakes and Northeast Sunday onward, possibly including the Ohio Valley/Mid-Atlantic by next Thursday. The most persistent and extreme high temperature anomalies should be over the central Plains where some locations should see multiple days with highs 10-15F or so above normal. The Northeast could see some highs reach 10F or so above normal early in the week. The experimental HeatRisk likewise reflects an expanding area of Moderate to Major (Level 2-3 of 4) risks of heat-related impacts from the weekend through next Wednesday, and even some pockets in the Extreme category over the central U.S. by next week. Forecast temperature anomalies would yield highs in the upper 90s to 100s 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 well beyond next Thursday. Much of the West will likely see near to slightly below normal highs through the first half of next week. Then gradual strengthening of the southern Rockies upper ridge should begin to push temperatures somewhat above normal by next Thursday. Lingering rainfall over the southern tier should support near to below normal highs, especially over southern/eastern Texas at the start of the week. 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