Extended Forecast Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Thu Jul 27 2023 Valid 12Z Sun Jul 30 2023 - 12Z Thu Aug 03 2023 ...Hot temperatures and high heat indices are likely across the south-central Plains and Mississippi Valley into the Southeast next week... ...Overview... The persistent upper high that has stayed atop the southwest/south-central U.S. for weeks now is finally forecast to drift eastward next week, finally bringing temperatures in the Desert Southwest closer to normal (though still hot). 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 at least the southern half of 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 continued to indicate the medium range period will begin in the middle of moderate flow amplification as a stubborn upper-level high over the southwest/south-central U.S. builds a bit northward and troughing builds southward over the Upper-Midwest/Northeast. This upper high is expected to shift eastward through the period, and with this evolution there may consequently be some corresponding northward adjustment to the troughing along the East Coast. This evolution has also remained the more notable source of guidance disagreement over the past couple days. The recent runs of the ECMWF, including the 00Z, indicate the upper-high will combine once again with an upper-high over the western Atlantic following a brief split as the upper-trough builds southward over the East Coast. Heights over the western Atlantic had been a bit lower in the GFS, though the 00Z run did briefly show a similar pattern to the 00Z ECMWF, before reversing a bit at 06Z. The 00Z ECens and 00Z GEFS mean follow a similar evolution to their respective deterministic counterparts. Otherwise, shortwave energy traversing northwest flow over the Northern Plains/Midwest will likely be related to organized convection chances during the period, though with very low predictability at this time. The upper high building north also looks to shift mean troughing over the Pacific Northwest coast north/westward as the pattern over the West begins to amplify a bit as well, with southwest flow in place over portions of the West overall. Subtle shortwave energy may translate through and have subsequent effects on precipitation chances in a Monsoonal pattern though any details are expectedly hard to predict at this time range. The more recent runs of the CMC and GFS have been the most energetic, but this is noted in the ECMWF as well. The upper high over the southern tier looks to build a bit back over the Southwest/northern Mexico in the latest 00Z and 06Z GFS/00Z ECMWF runs and 00Z ECens/GEFS means, but not so much in the CMC. Heights overall remain the most expansive across the entire southern tier in the 00Z ECMWF/00Z ECens mean and 00Z GFS compared to the other guidance (00Z CMC/00Z GEFS mean) at the end of the period. Good clustering of the guidance through the early period lends itself to a general model blend in the updated WPC forecast. The 00Z GFS is favored over the 06Z given the noted similarities to the ECMWF and ECens mean with respect to heights over the Southeast. The contributions from the means are naturally increased towards the end of the period with the 00Z GFS/00Z ECMWF and 00Z ECens favored in the blend over the 00Z CMC and 00Z GEFS mean given the better consensus on the overall expansion of the upper high over the southern tier compared to the other guidance. It should be noted the updated 12Z GFS was more similar to the 06Z in terms of heights over the Southeast, though was still more expansive across the southern tier compared to the 00Z CMC/00Z GEFS mean. ...Weather/Hazards Highlights... Shortwave energy in northwest flow aloft from the north-central U.S. to the eastern U.S. will carry showers and thunderstorms across those regions early to mid-next week. In the East, by Sunday the Northeast should be post-frontal and dry, while a cold front pressing slowly south in the Mid-Atlantic to Carolinas should help spark convection there. A Marginal Risk is in place for this more focused activity in an unstable and moist environment. Forecast QPF has remained rather low over western, mountainous areas of the region, and despite heightened sensitivity to any locally heavy downpours, low confidence in spatial converge has led to an updated Marginal Risk area focused farther east. The cold front should push even farther south Monday for a convective focus over the Southeast. A Marginal Risk has been drawn up in the Day 5 ERO for portions of Georgia and Florida as there could be high rain rates possibly causing excessive rainfall. The combination of an additional frontal passage as well as the southern front retreating back a bit northward is then likely to bring precipitation chances back up the East Coast mid next week. 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 the northwesterly flow. Day to day positions of heavy rain and MCSs are very uncertain at this point with notable QPF footprint changes run-to-run in the guidance. Broad Marginal Risks have been included on both Days 4 and 5 to cover this potential, with the expectation that their areal coverage may likely still need some further adjustment, and eventual more concentrated Slight Risks possible. The Four Corners states and into Wyoming are likely to see diurnally driven convection each day as monsoonal moisture comes in, and as the upper high shifts eastward with time this may increase moist southerly flow and allow for additional shortwave energy to come in. There is also a notable trend of QPF becoming more widespread and locally heavier as the week progresses, and Marginal Risks are in place and become broader with time. By early next week, the persistent upper high that has been centered over the southwest/south-central U.S. will start drifting eastward, allowing for high temperatures in the Desert Southwest to finally ease to near normal. Highs should still be over 100F in many areas but at least there should be minor relief compared to the past few weeks. Meanwhile with the core of the upper high/ridge tracking east, temperatures are likely to increase to above normal with it. The most persistent/greatest anomalies should be in the central Plains to the Mid-Mississippi Valley, where above normal temperatures by 5 to 15 degrees are likely as actual temperatures near or exceed 100F. The southern Plains to Lower Mississippi Valley and Southeast may see slightly lower anomalies of 5-10F above normal, but the heat combined with high dewpoints will lead to heat indices soaring above 105F, even above 110-115F in some areas. Record hot highs and warm minimum temperatures are possible in these regions. Slightly cooler than average temperatures are possible in the Pacific Northwest this weekend/early next week before shifting into the Intermountain West closer to the middle of next week as upper-ridging builds northward and precipitation chances increase over the Intermountain West. The Northeast should also be slightly below normal for temperatures as a couple of cold fronts pass through. 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 Hazards: - Hazardous heat across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Sun-Thu, Jul 30-Aug 3. - Hazardous heat across portions of the Central Plains, the Middle Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Tue-Thu, Aug 1-Aug 3. - Hazardous heat across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Sun-Mon, Jul 30-Jul 31. 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