Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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 eventually 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... Latest model guidance has remained mostly consistent on the overall upper-level pattern through much of the medium range period. A broad upper-level high over the southern tier is forecast to consolidate once again over the southwest/south-central U.S., as has been the case for much of the last few weeks. Mean troughing will build in over the Northeast following this consolidation with another trough located along Pacific Northwest Coast, with the main upper-level flow over the U.S. over the northern tier. The upper high is expected to shift slowly eastward over the south-central U.S. mid-period and eventually towards the southeastern U.S. as the overall pattern amplifies a bit. Ridging will build somewhat northward stretching from the Pacific Northwest through the northern Rockies and Plains as the trough over the east dips southward. The northern stream has trended slightly more amplified during the forecast period, most noticeable run-to-run in the GFS, culminating with the 06Z run, and to a lesser extent the 00Z run. However, the guidance remains in generally good agreement overall on the large-scale through the mid-period, with the most noticeable differences related to the timing and strength of embedded waves in the flow regime across the West and northern tier of the country surrounding the southern upper high. Predictability decreases in the late period as disagreement arises over how far east the upper-high shifts, and consequently the evolution of mean troughing over the northeastern U.S., which would either weaken and/or shift eastward under this scenario. The movement of the upper high notably trended slower across the guidance run-to-run as well. The 00Z/06Z GFS runs are both furthest east with the upper high, with the 06Z GEFS and 00Z ECens means a bit further west, followed by the 00Z ECMWF and 00Z CMC. The GFS has also trended towards more embedded upper energy over the West compared to the ECMWF/CMC or ensemble means, which will have implications on Monsoon/overall precipitation activity over the region. With good overall large-scale agreement early in the period a general model blend is initially used for the updated WPC forecast. Contributions from the CMC and GFS are reduced given noted forecast differences later in the period, with the means and ECMWF offering a good compromise between the upper-high placement. This also avoids trending too much towards depicting embedded energy over the West as indicated by the GFS until it becomes a more reliable pattern across the guidance, unlikely until forecast lead-time decreases. The noted disagreement over the upper-high placement in the guidance led to the biggest differences with the prior forecast, and will have implications on precipitation chances and high temperatures over the Southeast, with temperatures trending cooler in this forecast cycle, particularly for the Carolinas. ...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 has been maintained across the Northeast to Mid-Atlantic for the Day 4 ERO (Saturday July 29). The risk area is broader/farther south into the Tennessee Valley/Carolinas 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. There is also still significant model disagreement in the location of higher rainfall amounts, with some strong signals present but significant areal displacement. 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. Another area of concern is over New England, especially if confidence grows for portions of interior New England that are sensitive given recent significant rainfall events. However, given the significant disagreement in areal coverage and high FFGs near the coast where some of the stronger QPF signals are present, a Marginal Risk remains sufficient for now. 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 the Carolinas 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, with precipitation chances trending upward across the region. 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 additional 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 and widespread record warm minimum temperatures likely. 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. Mean troughing over the Northwest will keep temperatures a bit below average through the weekend before upper-ridging begins to build in and temperatures push above average by the middle of 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