Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Wed Aug 16 2023 Valid 12Z Sat Aug 19 2023 - 12Z Wed Aug 23 2023 ...Heat wave expanding across the central to east-Central U.S. this weekend and early next week... ...Heavy rainfall possible for parts of the Southwest associated with potential tropical development... ...Overview... A vigorous shortwave with embedded closed low will shift out of the New England this weekend as an anomalously strong upper ridge expands across the central to east-central U.S. bringing a resurgence of excessive heat through at least the middle of next week. Troughing over the West coast looks to remain in tact through the period renewed by shortwaves out of the Gulf of Alaska and into southwest Canada. Meanwhile, likely tropical development in the east Pacific and eventually lifting northward off the Baja California coast may bring a threat for heavy to excessive rainfall into parts of the Southwest U.S. and California Saturday-Monday. ...Guidance Evaluation/Predictability Assessment... Models show good agreement with the exiting system out of the Northeast on Saturday as well as expansion of the ridge over much of the eastern two thirds of the country. West Coast trough evolution and details remain highly uncertain, even as early as Sunday. The 12z/Aug 15 ECMWF and UKMET are much more amplified with a shortwave into the Northwest, related to a stronger/blockier ridge across the central states. The GFS and CMC (including the new 00z runs) track this system eastward across southern-central Canada, eventually amplifying it again through the Great Lakes and Northeast early next week. Given this split in the guidance, opted to stick close to continuity which falls closer to the ECMWF, but not nearly as strong, as is consistent with the ensemble means. Plenty of additional late period uncertainties with the flow upstream moving out of the Gulf of Alaska early next week as well. Given these uncertainties, the WPC blend for tonight trended strongly towards the ensemble means for the latter half of the period, which maintains good continuity with the previous WPC forecast. Possibly the most significant threat for sensible weather however lies with highly anomalous monsoonal moisture into the Southwest U.S. starting this weekend. Models continue to suggest this moisture should get enhanced by a potential tropical system in the East Pacific (with a high probability of development in the next day or two per the National Hurricane Center). The guidance has come into much better agreement compared to the last several days on tracking this system north-northwestward just off the Baja California coast this weekend into early next week. Deterministic guidance suggests it could come close to the Southwest U.S., although the ensemble means are farther west lowing confidence in any one individual solution. A further east track would provide a more direct feed of moisture into the Southwest, so this will need to be closely monitored. The WPC QPF this cycle stuck close to the NBM and latest multi-model ensemble bias corrected QPF, which largely trended well with continuity. ...Weather/Hazards Highlights... Monsoonal moisture streaming northward ahead of West Coast troughing will keep much of the interior West wet through the period. Rainfall amounts over especially the Southwest remain highly uncertain and dependent on potential tropical development and track in the eastern Pacific. However, models continue to trend towards a heavy to excessive threat and confidence/model agreement was high enough tonight to include a slight risk across across portions of southeast California, southern Nevada, and western Arizona on both the days 4 and 5 (Saturday and Sunday) Excessive Rainfall Outlooks. Should the forecast hold, it is likely at least a moderate risk will be needed eventually, but opted to hold off given the system in question has not formed yet and exactly placement of the heaviest QPF axis will be highly dependent on eventual track of this system. Heavy rainfall may also lift across south-central California as well early next week. To the north, moisture will interact with a frontal boundary across the Northwest/northern Rockies providing some enhancement of rainfall to that region. Broad marginal risks are depicted on the Excessive Rainfall Outlooks. Much of the rest of the country looks to remain mostly dry under the expansive upper ridge, with the exception of Florida and the Gulf Coast as tropical moisture shifts westward across the Gulf of Mexico. This could also bring some rainfall into southern Texas next Tuesday. A heat wave will build over the southern and central U.S. into the Midwest with daytime highs 10-20 degrees above normal in some places. This equates to daytime highs near 100 degrees possible as far north as the Upper Midwest states, with daily record highs possible, and some locations in the Midwest seeing their hottest day of the year so far. Humidity levels will send heat indices into the 100s for many. See key messages issued by WPC for additional details and graphics related to this heat wave. Heat may also build back into the Southeast by the middle of next week, and possibly beyond per the latest forecasts from the Climate Prediction Center. Elsewhere, upper troughing out West should keep highs generally 5-15 degrees below normal with parts of the Southwest 15-20 degrees below normal especially Sunday and Monday as a result of the increased cloud cover and threat of heavy precipitation. 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