Extended Forecast Discussion NWS Weather Prediction Center College Park MD 439 PM EDT Fri Aug 18 2023 Valid 12Z Mon Aug 21 2023 - 12Z Fri Aug 25 2023 ...Hilary to spread rare and dangerous lead Excessive Rainfall Threat through California and the Great Basin early next week... ...Major heat wave for the Central to Southeast U.S. into mid-later next week... ...Heavy rain threat may emerge over southern Texas by next midweek... ...Synoptic Overview... A highly anomalous lead surge of deep tropical moisture associated with the inland track of Hilary will spread north across the Southwest and California, and then northward across much of the Intermountain West into the Rockies, to fuel an elevated and widespread heavy rainfall and flooding potential. An impressive 600dm upper ridge will be anchored in place across the Midwest and Central Plains for the beginning to middle of next week with widespread excessive heat and humidity accompanying it. This ridge may start to decay going into the end of next week and displaced towards the south as shortwave energy impinges upon it from the north, but the high heat and humidity will likely continue for much of the central and southern states. Across the Northeast U.S., and upper level trough in place through at least next midweek will shield the region from the excessive heat. An increase of tropical moisture underneath the aforementioned strong upper ridge is expected to work westward across the Gulf of Mexico and inland across Texas to fuel widespread coverage of showers and thunderstorms likely to focus across southern Texas going into Tuesday and spread further westward in earnest into Wednesday/Thursday. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of best clustered guidance from the 06 UTC GFS and 00 UTC ECMWF/UKMET along with the 13 UTC National Blend of Models and WPC/WFO continuity Monday into Wednesday. Gravitated to the more compatable 06 UTC GEFS mean and the 00 UTC NAEFS/ECMWF ensemble means later Wednesday into Thursday amid growning forecast spread, especially with the split of upper troughing into the eastern Pacific/West Coast or more inland to southwest Canada. The ECMWF offers less than stellar run to run continuity for these features. The GFS was also discounted by mid-later next week given uncertainties with temperatures fields that again do not seem in line with most other guidance, especially over a hot north-central U.S. and Midwest/Great Lakes. ...Weather/Hazards Highlights... A rare and dangerous heavy rainfall episode will continue to be a threat across central and southern portions of California into Monday, and also including portions of south-central Nevada, as the moisture plume steadily lifts north across this region. There is a Slight Risk of excessive rainfall for the Day 4 period Monday, with the majority of the rain expected from 12Z Monday to 00Z Tuesday. Some 2-3 inch rainfall maxima are possible across portions of the central southern Sierra, and 1-2 inches for the Transverse Mountains, albeit with some risk for heavier terrain enhanced amounts locally. Farther to the north, lead tropical moisture well ahead of Hilary will interact with a frontal boundary across the Northwest/northern Rockies providing enhancement of rainfall, and therefore a Slight Risk is in effect for Day 4 from eastern Oregon to northwestern Montana, with additional fuel for activity up through the Great Basin well downstream of Hilary in deeply moist flow. Meanwhile, expect an expanding rainfall area to work inland from the Gulf of Mexico through the western Gulf Coast starting Tuesday as deepened tropical moisture advects westward. Possible tropical connection and inland surge favored the issuance of a Marginal Risk excessive rainfall threat through midweek in the Day 5 ERO for southern Texas. This threat area has also been expanded westard and aims at the Big Bend/SW Texas into Thursday given guidance and now greater NHC western Gulf low potential. A major heatwave is expected across much of the Midwest and extending across the central and southern Plains through about Thursday with the massive upper ridge in place and suppressing cloud development. Widespread 100+ degree high temperatures are expected from southern South Dakota and points southward, and some overnight lows may stay above 80 degrees, especially from central Texas to the Florida coast. These readings across the Midwest are on the order of 10-20 degrees above late August averages, with the core of the anomalies centered mainly over Iowa through mid-week. There will likely be some abatement to the later summer heatwave across the Midwest by Friday as the ridge is suppressed to the south. The opposite holds true from California to Nevada and western Utah with temperatures running 15 to 25 degrees below average owing to expansive cloud cover and widespread rainfall, perhaps setting some record low max temperatures across this region. Temperatures slowly moderate going into the second half of the week across this region, whilst still remaining slightly below average. Hamrick/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 Hazards: - Heavy rain across portions of the northwestern U.S. from eastern Oregon into central Idaho, Mon, Aug 21. - Heavy rain across portions of southern Texas, Tue-Wed, Aug 22-Aug 23. - Hazardous heat across much of the mid-section of the country including the lower and mid-Mississippi Valley, portions of the Ohio Valley, the Southeast, and into northern Florida, Mon-Fri, Aug 21-Aug 25. - Hazardous heat across the inner coastal plain of the Carolinas, Tue, Aug 22 and Fri, Aug 25. 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