Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 PM EDT Sun Jul 28 2024 Valid 12Z Wed Jul 31 2024 - 12Z Sun Aug 04 2024 ...Hazardous heat to shift from the south-central Plains early period to the Northwest later this week... ...Overview... Guidance continues to show a pattern evolving from a southern Rockies/Plains upper ridge through midweek to an amplifying ridge in the West later in the week. This would help set up a prolonged period of hot weather over many areas with the most anomalous heat expected over central parts of the Plains through midweek. Warmer than normal temperatures are forecast to expand north toward the U.S./Canadian border later in the week, despite shortwave energy moving through the Midwest and eventually developing mean troughing over the eastern U.S. that will support a broad area of showers/thunderstorms drifting across the eastern half of the country. ...Guidance/Predictability Assessment... The latest models and ensembles continue to show good agreement on the large scale regarding the initial southern tier ridge shifting and building over the West. The main model uncertainties still lie north of the ridge, particularly with the timing and strength of a shortwave through the northern tier. ECMWF runs have been on the slower side of the envelope with the trough's track through the Midwest/Great Lakes/Ohio Valley late week. Have been leaning away from solutions this slow because the ECMWF-based AI/ML models were much faster, but the 06Z GFS seemed overly fast so a middle ground seemed more reasonable. The 12Z GFS has since slowed down closer to the ECMWF though. This also affects the timing of a surface low moving through the Midwest. Model guidance is quite uncertain with the pattern in the northeastern Pacific next weekend, which could affect the northwestern U.S., which will have to be resolved in future forecasts. Guidance and ensembles continue to hint at possible tropical development in the Atlantic, which the latest outlook from the National Hurricane Center is highlighting. The operational ECMWF and its ensembles show a more eastern track (owing to stronger ridging into the Gulf), bringing a low northward off the coast of FL next weekend. The CMC and now the 12Z GFS (with some support from their ensembles as well) shows a more southerly track south of FL into the Gulf. A lot of uncertainty still on the track or even its existence as a defined tropical system, but this feature will need to be watched. The WPC forecast used a blend of 00Z/06Z deterministic guidance early in the period, ramping up the proportion of ensemble means to 60 percent by Day 7. Overall, this maintained good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... A wet pattern is likely from the Midwest to the Eastern Seaboard next week with ample moisture and instability in place as well as mean troughing aloft and surface frontal systems all supporting scattered thunderstorms. Thus the Days 4-5 Excessive Rainfall Outlooks covering Wednesday-Thursday night show fairly broad Marginal Risks across parts of the Midwest to Ohio Valley and the Northeast. Model guidance has been hinting at a likely mesoscale convective system with heavy rain rates moving on the cusp of the instability gradient in northwest mean flow that should affect parts of the Midwest Wednesday-Wednesday night. There is some placement spread with this, but antecedent wet conditions and the heavy rainfall potential seems to warrant a Day 4/Wednesday Slight Risk across the Midwest, though the area will likely be adjusted in future forecasts if/when models converge. This round and potentially additional rounds of storms may focus in Lower Michigan and vicinity by Thursday. Held off an embedded Slight Risk for Day 5 though, as there is additional spread in placement of heaviest rain amounts (and whether they affect areas with wet antecedent conditions or not) and uncertainty if multiple rounds overlap each other. Much of the north-central U.S. to the eastern third of the country should remain generally unsettled through late week as a upper trough is slow to budge out of the region. Elsewhere, monsoonal convection over the Southwest should stay confined to southern areas through midweek, and a Marginal Risk remains reasonable for the Day 4 ERO from eastern Arizona through much of New Mexico. Chances for convection may expand northeastward into later week depending on the shape of the southern Rockies/Plains ridge and upper level energy rotating around its western periphery. As such the Day 5/Thursday Marginal expands a bit northward into southern Colorado, and rain chances increase into the central High Plains by Friday. The forecast pattern evolution will support areas of warmer than average temperatures across the Lower 48 into next weekend. The most significant 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 near 105F. The experimental HeatRisk likewise reflects an expanding area of Major to even Extreme risks of heat-related impacts through Wednesday. An incoming front will cause some easing/southward shift of the heat late this week in that region, with some heat building across parts of the Southeast as well. Record warm minimum temperatures in the Southeast will exacerbate heat stress. Meanwhile, much of the West will trend warmer after Wednesday, with lower elevations seeing 100F+ high temperatures. Scattered daily record highs are possible. Periods 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. Tate/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