Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM 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 through the Northeast Tuesday into Wednesday will 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 mid week before lower heights move back into the region. ...Guidance Evaluation/Predictability Assessment... Increasing model agreement with the initial shortwave through the Northeast Tuesday-Wednesday allowed for a equal blend of the latest deterministic guidance through day 5. By day 5 into 6, with the next shortwave moving across the Great Lakes region, the 18z/Aug 11 GFS was faster than the rest of the guidance, and the new 00z run even faster. Given this is a typical bias for the GFS, WPC favored a blend of the slower ECMWF and CMC models for this system (along with the ensemble means). The ECMWF and CMC are more consistent through day 7, but with plenty of uncertainty in the details. Trended more towards the ensemble means for day 7. This approach also worked well later in the period across the Northwest where models are consistent in showing lowering heights pushing the ridge more into the central Plains, but with some uncertainties in timing. ...Weather/Hazards Highlights... A quick moving cold front 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. This boundary should slow/stall as it weakens across the Southeast into the middle of the week, but anomalous moisture and instability along the boundary and across Florida could bring at least a localized heavy rainfall/flash flood risk across portions of this area Tuesday into Wednesday. The next system later in the week should bring showers and thunderstorms back to parts of the Midwest/Great Lakes into the Northeast, while Florida remains unsettled through the week with diurnal showers and storms. Monsoonal moisture will continue into the Southwest/Four Corners through about Tuesday, with marginal risks 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 moderate Wednesday as troughing drops into the region. Meanwhile, a second area of heat will build into parts of the Northwest this weekend into mid next week, 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. Troughing across the Western U.S. should push the ridge east, with heat beginning to build back into the central Plains by next Friday. 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