Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Jul 26 2023 Valid 12Z Sat Jul 29 2023 - 12Z Wed Aug 02 2023 ...The best potential for hazardous heat should shift into the central Plains/Mid-Mississippi Valley and the Southeast this weekend into 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. Ridging stretching north from the upper high 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. A "ring of fire" type of pattern is likely in the precipitation forecast, as monsoonal moisture leads to diurnally driven convection in the Four Corners states and vicinity, shortwaves push through northwest flow and spark thunderstorms in the north-central U.S., and a cold front pressing south and then stalling helps to focus rain and storms along the Eastern Seaboard. There is a risk of heavy rainfall and flash flooding in these areas where high rain rates occur and where storms train/occur over the same areas. ...Guidance Evaluation/Predictability Assessment... Model guidance agrees on a general summertime upper-level pattern consisting of upper ridging across the southern tier and the stronger upper-level flow shunted to the north, with some smaller scale differences. Recent models are pretty agreeable in ridging combining from the southwestern U.S. and from the western Atlantic to stretch across the southern tier as the period begins Saturday. With time the two may break apart and combine again and perhaps combine again, with less certainty. With the 12/18Z model cycle, though the 12Z CMC was a minor outlier in hanging the western part of the ridge/high back west compared to other guidance, but the incoming 00Z guidance as a whole is actually more like that with a slower drift of the high eastward. In the northern stream, models have good consensus with a slowly deepening trough axis over the East this weekend into early next week, but vary somewhat on how deep and with the associated frontal position. The 12Z CMC was especially deep with the trough but the newer 00Z run has backed off some. The GFS and ECMWF runs showed better consensus than a couple days ago certainly though. The eastern Pacific to the Northwest is another area that could see some troughing, though smaller-scale and thus less certain. Models have been waffling with how quickly to bring energy eastward into British Columbia or so this weekend, with newer 00Z runs looking slower to do so, which is also consistent with the upper high's slower movement with its ridge extending north just east of this energy. Thus this is still an area of uncertainty that will affect temperatures and perhaps precipitation forecasts in the northwestern and north-central U.S. The WPC forecast was based on a deterministic blend of the 12/18Z guidance favoring the ECMWF and GFS early in the period, gradually reducing and eliminating the CMC and the time-limited UKMET with time. Ensemble means from the GEFS and EC were used in their place and eventually ended up with an equal blend of the 18Z GFS and GEFS mean and the 12Z ECMWF and EC mean by day 7. ...Weather/Hazards Highlights... Showers and thunderstorms look likely across much of the north-central and eastern U.S. through the weekend 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 force convection. More specifically, a cold front pressing southward across the Northeast on Saturday and the Mid-Atlantic on Sunday should focus storms along and ahead of it. With shortwaves aloft and the cold front showing a general west-east orientation allowing for training/repeat activity, a broad Marginal Risk is in place across the Northeast to Mid-Atlantic as a starting point in the Day 4 ERO Saturday. The risk area is broader/farther south than just the frontal convection itself since the environment to the south of the front will be unstable and high rain rates are likely, plus the front will be moving south through the day. But the area covered by the Marginal may be able to narrow some with time if models come into better agreement with placement of higher QPF. Future Slight Risks cannot be ruled out within this broad Marginal. One such area where a Slight Risk was considered was across West Virginia, where upslope flow could enhance rain totals and it tends to be more sensitive in the varying terrain. By the Day 5 ERO timeframe Sunday, the Northeast should be post-frontal and dry while convection focuses farther south. A Marginal Risk covers Virginia and North Carolina generally, and back into the southern Appalachians in case heavy rain falls in sensitive mountainous areas. The cold front should push even farther south Monday for a convective focus over the Southeast, but the front lifting and/or reforming by Tuesday-Wednesday could put rain chances in the Northeast to Mid-Atlantic by then. Meanwhile, the north-central U.S. can also expect scattered storms as shortwaves track through the region and moisture increases. A heavy rain/flash flooding threat will likely be tied to the potential for organized convective systems in northwesterly flow. Day to day positions of heavy rain and MCSs are uncertain and vary somewhat among model guidance, but Marginal Risks cover this potential over the weekend. The Four Corners states are likely to see diurnally driven convection each day as monsoonal moisture comes in underneath the southern Rockies upper ridge. As the ridge shifts eastward next week, this may increase moist southerly flow and allow for shortwave energy to come in, so rainfall totals may become more widespread and locally heavier as the week progresses. Florida and parts of the Gulf Coast should see some scattered showers and thunderstorms as well, and a Marginal Risk is in place on Day 4/Saturday across the west coast of the Florida Peninsula before a plume of well above (already high) average precipitable water values that could produce heavy rainfall rates exits to the west. Urban areas will be particularly sensitive to heavy rain potential. 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 Saturday. By early next week, highs should finally ease to near normal there with the center of the upper high forecast to drift east, though this is still over 100F in many areas. Farther east, temperatures approaching or over 100F are likely across the central/southern Plains into the Middle/Lower Mississippi Valley for the weekend into next week with temperatures around 10-15F above normal. In some areas, the exact locations seeing near normal or above normal temperatures will depend on the axis of a front meandering from day to day, as well as details of convection and residual cloud cover. The same is true across the East--another warmer than average day looks likely on Saturday in the Ohio and Tennessee Valleys to Mid-Atlantic, with a few record highs possible but likely widespread record warm minimum temperatures. The cold front should put these areas closer to normal by early next week, and temperatures may even be a few degrees below normal in the Northeast. Meanwhile the Southeast/Gulf Coast should see greater persistence of above normal temperatures by 5-10 degrees while heat indices soar above 105F, even above 110F in some areas, as the warmth combines with high dewpoints. 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