Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 PM EDT Sat Jul 09 2022 Valid 12Z Tue Jul 12 2022 - 12Z Sat Jul 16 2022 ...Heat wave to last through early next week across the southern Plains and Mid-South as a heavy rainfall threat emerges for the central Gulf Coast states... ...Guidance Evaluation/Predictability Assessment... On the large scale, the guidance shows fairly good agreement on the upper pattern during the medium range through at least day 5. There appears to be better agreement on QPF placement near the central Gulf Coast where an upper disturbance/moisture from the Gulf of Mexico will interact with a stalled front to produce multiple days of heavy rainfall. The GFS/GEFS remain farther west with the heavier QPF both on days 4 and 5, but much of the remainder of the guidance now shows better consensus farther east towards southern LA/MS/AL, even into western FL Panhandle, which is where the WPC QPF is heavily weighted towards today. For the mass fields, a consensus of deterministic guidance worked fine through day 5. By days 6 and 7, there is some more larger scale differences in timing that arise around amplified troughing across the Eastern U.S., as well as a compact closed low into the Pacific Northwest/southwest Canada. In the East, the GFS gets much quicker with the trough exiting the east coast, where as the ECMWF/CMC and the ensemble means show the trough/front lingering across the East Coast region. In the West, the CMC is farthest north with a compact closed low moving in towards the Pacific Northwest, but the GFS and the ECMWF are farther south. The ensemble means support something in the middle of these two camps. Forecast uncertainty isn't huge, but it is notable for these later time periods, so a blend towards the ensemble means and ECMWF seemed best for today. This approach also maintains good WPC product continuity from overnight. ...Overview and Weather/Hazards Highlights... A mean upper trough position will develop off the Pacific Northwest by early next week with lead Midwest upper troughing also digging into the East where associated frontal passages will focus rain and thunderstorms. However, deeper moisture is expected to trail from the eastern Mid-Atlantic to the Southeast along a wavy stalled lead front forecast to interact with an upper low from the sub-tropics to support heavy rain potential near the central Gulf Coast. This system could focus slow-moving/repeat cells with very heavy downpours leading to a local runoff risk. The northern Plains to Upper Midwest should also see chances of rain and strong thunderstorms as ejecting impulses round the ridge. A small slight risk was added to the day 5 experimental Excessive Rainfall Outlook across eastern TN/central Appalachians where moisture and instability may pool along a frontal boundary with at least locally heavy rainfall over an area with recent rainfall and higher susceptibility to new rains. Meanwhile, a persistent monsoonal moisture flow into the Southwest offers chances for some locally heavy downpours. Very hot weather is expected to persist across the southern Plains/Lower Mississippi Valley underneath lingering upper ridging early next week, before easing slightly following a frontal passage. Early week high temperatures near or exceeding 100F are expected, which is between 5 and 15 degrees above normal. Combined with the humidity, daily maximum heat indices upwards to 105-110F+ are likely and the repeating days of high heat/humidity and lack of appreciable nighttime relief (lows in the upper 70s to low 80s for some areas) will increase the potential for impacts from this heatwave to the more vulnerable populations. Several daily record high temperatures are possible, particularly for warm lows, but a few highs may be close to records in Texas. As the upper ridge drifts westward towards the Four Corners by late week, anomalous heat is forecast to develop and overspread the northern Plains as upper ridging builds over the northern Rockies ahead of an upper trough advancing into southwestern Canada. Downstream, the East may initially see warm temps/humidity on Tuesday in between fronts, but can expect slightly cooler than normal temperatures behind the cold front, before warming to near/just above normal by the end of the week. Santorelli/Schichtel 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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