Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Sat Jul 27 2024 Valid 12Z Tue Jul 30 2024 - 12Z Sat Aug 03 2024 ...Hazardous heat to shift from the central Plains early period to the Northwest later next week... ...Overview... Guidance continues to show the pattern evolving toward a strengthening southern Rockies/Plains upper ridge which eventually will build back across the Western U.S. by later next week. 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 early to mid week. 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. Despite the overall pattern, heat increases with time across the Lower 48, with the largest positive temperature anomalies migrating towards the US/Canadian border by next weekend. ...Guidance/Predictability Assessment... The guidance continues to show good overall agreement on the large scale pattern, but plenty of uncertainty in the details. The greatest area of uncertainty is with the timing of a shortwave shifting out of the Northwest on Tuesday. The 18z GFS is quite a bit faster than the other operational models as it moves across the Upper Midwest mid week, but the new 00z run (available after the WPC forecast was generated) trended slower. This affects the overall shape of troughing over the Northeast later next week. Otherwise, there is good agreement on the initial central Plains ridge expanding Westward, but some question on timing and strength of the ridge by next weekend over the interior West. The WPC forecast for tonight used a non-GFS blend for the first half of the period, increasing the ensemble mean weighting the second half to help smooth out some of the details that will need more time to resolve fully. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks covering Tuesday- Wednesday night show fairly broad marginal risks across parts of the Midwest to the Northeast where above-average precipitable water values will be in place along weak frontal boundaries moving through the flow. There was enough agreement in the guidance for small slight risks across portions of the central Appalachians on each day where flash flood guidance is typically lower and antecedent conditions are wet. Much of the Eastern third of the country should remain generally unsettled the rest of the week as a upper trough is slow to budge out of the region while it generally weakens. Elsewhere, 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 through next Wednesday, and even some pockets in the Extreme category over the central U.S.. 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 into Saturday with lower elevations seeing 100F+ high temperatures. The Northeast should also trend warmer with time next week. 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