Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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 continues to show the best agreement with the upper high initially atop the southern Rockies generally trending eastward with time, with some minor model differences regarding potential interaction/combination with ridging farther east. A general blend of models and ensembles was able to handle that sufficiently. The flow pattern farther north has been much more uncertain, generally low amplitude with small-scale features having low predictability. Fortunately in the last few model cycles overall there seems to be better agreement as larger scale troughing deepens slightly into the Northeast with time. There are still some differences in the depth of the trough on each day, affecting the frontal and QPF placement, but run-to-run continuity has been relatively better, especially with the GFS and ECMWF runs. The 12Z UKMET was not favored as the western Atlantic ridge seemed to overpower the trough pressing into the East and thus did not deepen the trough like other guidance, while the 12Z CMC ended up looking too fast/east with the eastern trough next week. Then there are still some notable differences with flow farther west into the eastern Pacific and southwestern Canada where shortwaves are likely, with the 12Z UKMET looking off and now the newer 00Z GFS looking a bit suspect with how fast a shortwave tracks east and erodes ridging in the northern Rockies or so early next week. So this is an area that still remains uncertain. The WPC forecast preferences favored an 18Z GFS and 12Z EC and CMC blend early on with smaller components of the ensemble means, removing the CMC with time and gradually increasing the ensemble means to half by the latter part of the period. QPF again broadened amounts across the north-central U.S. and eastern U.S. compared to the NBM using a multi-model and ensemble blend to provide some depiction of the rainfall potential, though more changes are likely yet to come with time. ...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 should 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 into early next week as the cold front moves south. The north-central U.S. can also expect scattered storms starting late week and possibly increasing in coverage/rain amounts into early next week as shortwaves track through the region and moisture increases. 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 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 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. Cooler trends over the Northeast under upper troughing could nudge highs a little below normal by early next week. 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