Extended Forecast Discussion NWS Weather Prediction Center College Park MD 304 AM EDT Sun Aug 13 2023 Valid 12Z Wed Aug 16 2023 - 12Z Sun Aug 20 2023 ...Heat wave to prevail across the south-central U.S. through the period... ...Hazardous heat across the Pacific Northwest and the Southwest to wane by late week... ...Overview... The upper flow pattern should be fairly amplified across the northern tier and into parts of the East this week as an initial shortwave exiting the Northeast on Wednesday may give way to a vigorous system through the Upper Midwest, Great Lakes, and the Northeast later this week. Meanwhile, hazardous heat will continue across the Pacific Northwest as well as the Southwest into the southern Plains much of this week. Amplified troughing down the West Coast by next weekend should quickly bring temperatures back to normal or below normal across the West and Southwest, but heat will continue across the southern Plains/Gulf Coast and expand northward into the central states. ...Guidance Evaluation/Predictability Assessment... As the medium range period begins Wednesday, model guidance is in good agreement regarding initial energy/troughing exiting the East and strong ridging from the southern Plains to the Northwest, as well as with a lingering broad closed low off the California coast. The greatest uncertainties arise as early as Thursday as shortwave energy (originating from the far north Pacific) rounds the top of the ridge into central Canada. Through the 18z/12z (Aug 12) model runs, which were the latest runs available at forecast generation time, the GFS was significantly flatter and faster with this system (a known bias of the GFS) barely skirting the northern U.S. and quickly shifting through eastern Canada Friday into Saturday. Meanwhile, the non-NCEP guidance were all much more amplified, splitting the energy allowing it to dive south and east across the Great Lakes/Midwest with suggestions that a closed low cuts off over the Ohio Valley or Northeast later this week. Uncertainty increased even more though with this system in the latest 00z runs as the CMC is significant faster than its previous run and more like the GFS, which did slow down somewhat with its new 00z run. The ECMWF and UKMET remain amplified like their previous runs. It's interesting to note the GFS has been very consistent over the past several runs with this fast solution, while the ECMWF is more variable regarding speed and strength. Based on spaghetti plots with various ensemble solutions and deterministic solutions, there is still more support for something slower/more amplified, thus the WPC forecast for tonight leaned in that direction. However, given latest 00z runs, it's a highly uncertain forecast and wouldn't be surprised to see the forecast trend faster with time. The WPC forecast leaned very heavily on the ECMWF through the period given it was more in the middle with the shortwave as it moves into the Northeast later in the week. Early period used some smaller parts of the UKMET and CMC, favoring the ensemble means (more ECENS mean) late in the period. Overall, this resulted in a fairly consistent forecast (albeit a bit deeper/more amplified) with previous WPC continuity. ...Weather/Hazards Highlights... Fairly quiet in terms of sensible weather through at least mid to late week. Showers and storm associated with a lingering frontal boundary over the Southeast, combined with anomalous moisture, may result in pockets of heavy to excessive rainfall along the Southeast Coast and over the Florida Peninsula. As such, a marginal risk in in place for this region on both the Day 4/Wednesday and Day 5/Thursday Excessive Rainfall Outlooks. Florida looks to remain wet into next weekend too. To the north, showers and storms should accompany the shortwave through the Great Lakes into the Northeast Wednesday-Friday, but highly uncertain on exact amounts and impacts. Monsoonal moisture will continue to stream into the Four Corners region through mid week, but may increase with rainfall expanding in coverage across parts of the Great Basin and the West later this week and next weekend as amplified troughing becomes established over the region. The big story during the medium range period though 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. The heat wave across the Southwest and Pacific Northwest will remain through about mid-week with daytime highs 10 to 20 degrees above normal across the Northwest 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 by Friday. See Key Messages issued by WPC for additional information and graphics related to these two heat waves. By late next week, troughing coming into the West should push the ridge east, promoting warming temperatures across the Central U.S. by the weekend. Daytime highs in the 90s to near 100 looks to build into the central Plains and Midwest. 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