Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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 southern Rockies 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 remained mostly consistent on the overall summertime upper-level pattern through much of the medium range period, consisting of the upper high gradually moving east atop the south-central U.S. while a trough axis persists over the Northeast, and more progressive flow with multiple energies is likely from the eastern Pacific into the northwestern and north-central U.S. and southern Canada. While the general trend of the upper high eastward shows good agreement in model guidance, there has been some minor waffling with just how fast it will shift east, especially by around midweek. The 00Z guidance cycles the past couple of nights have tended to be slower than the 12Z cycles. These are relatively minor differences but they do affect the flow pattern farther north as an initially eastern Pacific shortwave or two track east across southern Canada, potentially suppressing the ridge stretching north. GFS runs have been on the faster side with with the shortwave/small closed low ejecting east from the Pacific Northwest around Monday-Tuesday compared to other models/ensembles. But beyond that all models diverge with not really clear outliers or favored solutions. Meanwhile the trough in the East is quite agreeable in the model guidance, though starts to show some differences by Wednesday-Thursday as the shortwaves approach from the west and possibly the northwest as well. It remains uncertain if this will push the initial troughing east and another trough axis forms, or if and when the shortwave energy gets absorbed into the original trough. Some small-scale energies coming into the West are looking more favorable by midweek, which will have implications on monsoon/overall precipitation activity over the region. With the broad reasonable agreement but with the smaller-scale differences noted above, the WPC forecast was based on a blend of the 12/18Z deterministic guidance early in the period, with increasing proportions of the ensemble means from the GEFS and EC to about half by the end of the period amid increasing uncertainty. ...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. There are some model differences with how far inland enhanced rainfall totals will make it, but the risk area stretches back into the southern Appalachians in case heavy rain falls in sensitive mountainous areas. But if model guidance continues to trend down with rain totals there, the Marginal may be able to be trimmed on the western side to focus 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 for portions of Georgia and Florida as there could be high rain rates possibly causing excessive rainfall. Another front is then likely to push through the Northeast and Mid-Atlantic by Tuesday-Wednesday, trending precipitation chances upward in the region at that point. 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 uncertain and vary somewhat among model guidance, but Marginal Risks cover this potential for early next week. 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. Thus rainfall totals may become 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 southern Rockies will finally 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 early next week and perhaps shifting into the Intermountain West closer to the middle of next week. The Northeast should also be slightly below normal for temperatures as a couple of cold fronts pass through. 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