Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Jul 25 2023 Valid 12Z Fri Jul 28 2023 - 12Z Tue Aug 01 2023 ...Heat continues across the Southwest while above normal temperatures are also likely from the Midwest to the East Coast into late week, becoming more limited to the central U.S. by next week... ...Overview... The persistent upper high that has stayed atop the southern Rockies for weeks now is forecast to combine with ridging coming in from the western Atlantic and drift eastward atop the south-central to southeastern U.S. into next week. This should finally bring temperatures in the Desert Southwest closer to normal (though still hot) as the period progresses. The flow pattern of ridging stretching north will also promote above normal temperatures across the central Plains to Mid-Mississippi Valley with the Southeast also likely to warm above normal, with high heat indices. Late week warm temperatures are forecast into the Ohio Valley to the Northeast/Mid-Atlantic, ahead of a cold front and trough axis pressing into those regions by early next week. This front should also focus convection moving through the north-central to eastern U.S., with some risk of heavy rainfall and flash flooding, though confidence remains low in the details. Diurnally driven showers and thunderstorms will be possible over and near the Four Corners states through late week into next week with monsoonal moisture incoming under the ridge. ...Guidance Evaluation/Predictability Assessment... Model guidance indicates overall summertime upper-level pattern continues with a persistent high across portions of the southern tier of the country and stronger upper-level flow shunted to the north. Height rises look to increase more broadly initially over the Southeast/Mid-Atlantic as an upper-level high over the Atlantic combines with a stubborn upper-level high over the southern Rockies/Plains late week/early weekend. Stronger upper-level flow should then shift back southward over the Northeast/Mid-Atlantic rotating around an upper-level low over Hudson Bay. Upper-level ridging will then build a bit northward late this weekend into early next week over the eastern Great Basin/Rockies/Plains as the overall pattern amplifies ever so slightly with deeper troughing over the Northeast/Mid-Atlantic. The upper-level high centered over the southern Rockies/Plains also looks to shift eastward with time. Shortwave energy over the northeastern Pacific will drop southward over the Pacific Northwest before upper-level ridging looks to expand even further northwestward into the region next week as well. Subtle shortwave impulses will likely impact the location/coverage of organized convection in the northwesterly flow centered over the Northern Plains/Midwest/Northeast, but have very low predictability at this time range. A general deterministic guidance blend of the 00Z ECMWF/CMC/UKMET and 06Z GFS is used for the updated WPC forecast given general large-scale agreement between the models and good run-to-run continuity over the last few cycles. Contributions from the 00Z ECens mean and 06Z GEFS mean are increased as the forecast time-limited UKMET is removed by the middle of the period. The contribution from the 00Z CMC is also reduced given a bit more amplification with the flow over the northern tier that is an outlier compared to both the deterministic GFS/ECMWF and their respective system means. Given the generally good larger-scale agreement in the guidance this update follows continuity from the prior forecast. ...Weather/Hazards Highlights... Showers and thunderstorms look likely across parts of the north-central and much of the eastern U.S. through the latter part of the week and into next week in generally westerly to northwesterly flow on the north side of the upper high, with upper troughing edging into the East and a frontal boundary or two helping to focus convection. Even with the frontal systems, there has been considerable uncertainty over the precise east-west timing and southward extent--keeping confidence lower than desired. But for Friday and Saturday for the Days 4 and 5 EROs, the ingredients are in place for some focused heavy rainfall that could cause flash flooding despite the location differences. Convection will be more strongly forced in the East with shortwaves aloft and focused near the cold front with a general west-east orientation allowing for training/repeat activity in a moist and unstable environment. Given the location differences, relatively broad Marginal Risks are in place Friday-Saturday in the EROs as a starting point, which may be able to narrow in future forecasts if models come into better agreement with the frontal and thus the QPF timing, and future Slight Risks cannot be ruled out. The focus of heavy rainfall in the East should drop southward somewhat each day this weekend and into early next week as the cold front slowly progresses. The north-central U.S. can also expect scattered storms as shortwaves track through the region and moisture increases. Any heavy rain/flash flooding threat will likely be tied to the potential for organized convective systems in northwesterly flow, with this potential apparent overnight Friday and an associated Day 4 Marginal Risk ERO. Rainfall amounts/coverage look to increase early next week after a lull over the weekend. Elsewhere, the Four Corners states are likely to see diurnally driven convection each day as monsoonal moisture comes in underneath the southern Rockies upper ridge. The EROs depict a Marginal Risk area on both Days 4 and 5. As the ridge shifts eastward next week, this may increase moist southerly flow and allow for shortwave energy to come in, so rainfall amounts may become locally heavier by Sunday-Monday. Florida and parts of the Gulf Coast should see some scattered showers and thunderstorms as well, and Marginal Risks of flash flooding are in place for South Florida for Friday and Saturday as precipitable water values will be well above an already high average and convection could produce heavy rainfall rates, with urban areas particularly sensitive. The plume of moisture should shift west with time. The persistent upper high centered over the southern Rockies will keep temperatures in the Desert Southwest around 5-10F above average for highs in the 110s through late week. By the weekend into early next week, highs may finally ease to near normal there with the center of the upper high forecast to drift east. Farther northeast, temperatures approaching or over 100F are likely across the central Plains to the Middle Mississippi Valley with anomalies for highs of 10-20F above normal, while plus 10-15F anomalies are likely across the Ohio Valley and Mid-Atlantic/Northeast on Friday. Some record temperatures for highs and warm lows are possible and may be most focused across the East. Temperatures in most of these areas should moderate by the weekend as upper-level troughing begins to shift southward and cold fronts push through the northern tier, though warmer than normal temperatures may continue from the central/south-central Plains into Mississippi Valley depending on exactly where the northern tier cold front ultimately stalls. The Southeast/Gulf Coast could see increasing temperatures into the weekend and early next week under ridging aloft that builds west from the Atlantic and southeast from the Plains, and heat indices could rise above 110F as the warmth combines with high dewpoints. Details of convection and residual cloud cover will add an extra element of uncertainty to the forecast of daytime highs over many areas. Mean troughing near the Pacific Northwest will give that region the most persistent area of potentially below normal temperatures, though still with highs no cooler than a few degrees below average, and temperatures look to rebound a bit early next week as upper-level ridging builds northward. Cooler trends over the Northeast under upper troughing could nudge highs a little below normal by early next week. Putnam/Tate 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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