Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Aug 12 2023 Valid 12Z Tue Aug 15 2023 - 12Z Sat Aug 19 2023 ...Heat wave to prevail from the central Gulf Coast westward into the Southwest... ...Hazardous heat across the Pacific Northwest into mid-next week... ...Overview... The upper flow pattern should be fairly amplified across the northern tier and into parts of the East next week as a vigorous initial shortwave moving through the Northeast Tuesday into Wednesday could give way to another strong system into the Upper Midwest, Great Lakes, and the Northeast late week. This eastern U.S. troughing should finally erode heat across the Southeast, but hazardous heat is expected to persist through the southern Plains into the Southwest all of next week. A heat wave across the Pacific Northwest region starting this weekend should last through about midweek before lower heights move back into the region. ...Guidance Evaluation/Predictability Assessment... As the medium range period begins Tuesday, model guidance is in good agreement regarding the initial strong shortwave moving from the Great Lakes region quickly into the Northeast, the resultant trough axis pushing into the Southeast, and ridging persisting across the south-central U.S. while affecting the Northwest as well. However, some models significantly diverge with the potential for another possibly strong shortwave coming into the north-central to northeastern U.S. behind this first one. GFS runs specifically, instead of digging energy southeastward into the north-central U.S. and causing amplified troughing to shift slowly east like the non-NCEP guidance, track energy straight east across Canada much more progressively and end up out of phase. There are a few reasons to lean away from the GFS-type solutions. Some of these GFS runs have been flatter and more progressive with any troughing even compared to the majority of its ensemble members and the GEFS ensemble means. The GFS has also exhibited this faster/progressive type of bias with troughs generally over the years. Additionally, leaning away from the GFS leads to a forecast closer to continuity. However, the persistence of the GFS showing this for four or five runs does lead to some lowering of confidence in the eventual pattern. So does the origins of the energy that could form this trough feature, way out in the northern Pacific rounding an Aleutians upper low. Thus though it seemed prudent to prefer other guidance, confidence is not exactly high at this point. The WPC forecast favored a blend of the 00Z ECMWF, CMC, and UKMET (through its time period), with some 00Z GEFS mean component that wasn't quite as fast as the deterministic GFS runs. As usual increased the portion of GEFS and EC ensemble means with time. Incoming 12Z runs are much like each model's older 00Z ones, except that the 12Z CMC seems to become quite slow with the trough/low by late next week. ...Weather/Hazards Highlights... A low pressure system moving quickly through the Northeast on Tuesday may bring some threat for showers and thunderstorms ahead of it. Given sensitivity to rainfall across portions of the Northeast, a low end Marginal Risk remains in that region on the Day 4/Tuesday Excessive Rainfall Outlook. The associated cold front should slow/stall as it weakens across the Southeast into the middle of the week, providing a focus for anomalous moisture and instability to pool there and stretching southward into Florida, for some excessive rainfall potential. Marginal Risks are in place for parts of the central Gulf Coast, Southeast, and Carolinas Tuesday and Wednesday. Florida at least looks to remain in a wet pattern through much of the week. The next potential shortwave and surface low/frontal reflection could bring another round of rain and storms to the Midwest/Great Lakes into the Northeast. Given model uncertainty and the potential for any heavy rain to be fast-moving, held off on any excessive rainfall risk area for this activity. Meanwhile, monsoonal moisture is likely to stay enhanced across the Four Corners states through Tuesday, with a Marginal Risk indicated on the Day 4 ERO. Showers may expand across parts of the Great Basin and West later next week as weak upper troughing moves northward off California. The big story during the medium range period will continue to be oppressive heat lasting into next weekend from the central Gulf Coast into the southern Plains. Numerous daily record highs and warm overnight lows are likely, and the combination of heat and humidity will also result in extremely dangerous heat indices in excess of 110F in many locations. Hazardous heat this weekend into early next week across the Southeast should gradually moderate Tuesday with near normal temperatures by Wednesday as troughing drops into the region. Meanwhile, a second area of heat is forecast to last through Tuesday-Thursday across much of the Northwest, with daytime highs 10 to 20 degrees above normal through Thursday. High temperatures in the 90s, 100s, and even localized 110s possible are expected across Washington, Oregon, and the interior Valley of California. Erosion of the warm core ridge across the Northwest with time should begin to moderate temperatures on Thursday and especially Friday. See Key Messages issued by WPC for additional information and graphics related to these two heat waves. Record high maximum and minimum temperatures are likely to be widespread. By late next week, troughing coming into the West should push the ridge east, promoting temperatures around 10 to 15 degrees above average in the north-central U.S. Friday-Saturday. Tate/Santorelli 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