Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Aug 14 2023 Valid 12Z Thu Aug 17 2023 - 12Z Mon Aug 21 2023 ...Heat wave to prevail across the southern Plains and build into the central U.S. by this coming weekend... ...Hazardous heat across parts of the West to wane after Thursday... ...Overview... The upper level pattern during the Medium Range period (Thursday-Monday) will trend more amplified with time. A vigorous shortwave will shift across the Great Lakes and Northeast later this week as strong upper level ridging holds strong across the southern tier into the Northwest. Shortwave energy and lowering heights should erode the ridge over the Northwest after Thursday with ridging then beginning to build across the central U.S. and troughing overtaking the West. Hazardous heat across the South will expand northward into the central Plains by this weekend, with cool and wet conditions across the West, possibly further enhanced by highly uncertain eastern Pacific tropical development and associated moisture. ...Guidance Evaluation/Predictability Assessment... Model agreement continues to improve compared to previous days with regards to the strong shortwave through the Great Lakes and Northeast and Friday. There remains some detail differences including strength and timing through the Northeast, but a general model blend seems to mitigate these now more typical differences for a day 3-4 lead time. Model guidance is also very agreeable with an upper low meandering gradually closer to and perhaps over the West Coast by next weekend. Its potential to join with the northern stream flow later in the period will depend on flow details over Canada, which remain rather uncertain. Last few runs of the ECMWF are stronger with troughing through central Canada allowing it to quickly become out of phase with other guidance. This affects additional energy sliding through the Northwest, staying separate from the West Coast upper low (as suggested by yesterdays runs of the CMC and GFS). The ECMWF however is stronger with the next energy dropping out of the Gulf of Alaska and is more aggressive with combining with the West Coast low late period compared with the GFS and CMC runs. The ensemble means also suggest a solution more like the ECMWF (just much weaker) and it's interesting to note that the new 00z GFS run (and to some extent the 00z CMC as well) have trended in this direction as well. This is compounded in the GFS and CMC however by a potential tropical system (with a high chance of formation within 7 days per latest NHC forecasts). Both the GFS and CMC (including their new runs tonight) have been consistent lifting this system northward along the Baja California coast reaching southwest California around day 7/Monday in a much weaker state, whereas the ECMWF and the UKMET pull this system westward out into the eastern Pacific. Ensemble guidance also suggests a more west/northwest track. The eventual track of this system may influence evolution of the flow pattern over the Western U.S. late period. It's obviously at this point a very volatile and uncertain forecast and will definitely need to be monitored in the coming days to see what, if any, impacts this will have on sensible weather across the Southwest U.S. this weekend and early next week. The WPC blend for tonight used a general model blend of the GFS, CMC, and ECMWF for the first half of the period. Given extreme uncertainty late period over the Western U.S., quickly trended towards a majority blend of the ensemble means to help mitigate these differences until details become more clear. ...Weather/Hazards Highlights... A weakening frontal boundary along the Southeast Coast will allow anomalous moisture to linger over Florida where a marginal risk is in place over much of the Florida peninsula for the Day 4/Thursday Excessive Rainfall Outlook. To the north, showers and storms should accompany the shortwave and surface frontal system through the Great Lakes into the Northeast Thursday-Friday. It seems any heavy rain should be fairly localized and will move pretty progressively to preclude any excessive rainfall risk, but the risk could be nonzero across the northeastern U.S. Thursday-Thursday night especially if prefrontal convection forms and is slower moving than the rain accompanying the front. Beginning Thursday, but especially into this weekend, monsoonal moisture will increase across the Southwest and expand northward into the Great Basin/Rockies as upper troughing builds over the region. For the Southwest later in the weekend and early next week, amounts are highly uncertain and dependent on potential tropical development and track in the eastern Pacific. Should the guidance trend towards a more northerly track as suggested right now by the GFS and CMC, this would bring significant moisture and rainfall into parts of of the Southwest. At this point, the WPC QPF trends wetter with time over the region (and the West as a whole), but remains on the more conservative side regarding rainfall amounts and impacts as this very late period forecast is likely to change in the coming days. Some models have been consistent with the more northerly track now over the past day or two so this potential needs to be monitored. Meanwhile, oppressive heat will continue through the period across the southern Plains to central Gulf Coast with numerous daily record highs and warm overnight lows likely. 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 Northwest should continue into Thursday with temperatures around 10-20 degrees above average across the Northwest and around 5-15 degrees above average in the central valley of California into the Desert Southwest. This equates to highs in the 90s/100s and even localized 110s, which could set temperature records (both daytime highs and warm overnight lows). See Key Messages issued by WPC for additional information and graphics related to these two heat waves. The heat wave across the West should come to an end on Friday as upper troughing takes hold over the region this weekend. This will also push an upper ridge east, serving to warm up the central U.S. by late this week. Daytime highs in the 90s to near 100 look to build into the central Plains to Midwest, with humidity levels bringing heat indices even higher. Hazardous heat is becoming more and more likely across this region starting this weekend and lasting into next week. 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