Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Tue Aug 15 2023 Valid 12Z Fri Aug 18 2023 - 12Z Tue Aug 22 2023 ...Heat wave to prevail across the southern Plains and build into the central U.S... ...Heavy rainfall possible for parts of the Southwest this weekend into early next week... ...Overview... The upper level pattern during the Medium Range period (Friday-Tuesday) will stay amplified as a vigorous shortwave shifts across the Great Lakes and Northeast late this week, with a potential embedded closed low tracking over or near northern New England around Saturday. Behind this feature an upper level ridge should gradually trend stronger as it shifts into the Plains/east-central U.S., while upper troughing steadily descends over the West Coast. An initial upper low off the California coast may weaken in favor of another possibly upper low over the Northwest Coast early next week. Hazardous heat across the South will expand northward into the central Plains by this weekend, while the West should trend cooler and wetter, possibly further enhanced by highly uncertain details involving eastern Pacific tropical development and associated moisture. Farther east, expect the evolving pattern to confine most rainfall to a brief episode over the Northeast Thursday-Friday as well as the Florida Peninsula and spreading westward along the Gulf Coast by the weekend or early next week. ...Guidance Evaluation/Predictability Assessment... Amplified shortwave/embedded closed low over the Northeast early period continues to show good agreement overall, although the CMC is a bit slower on Saturday. After Sunday, some differences in the details of upper troughing over the West Coast along with embedded closed low over the Pacific Northwest early next week. Also some question in amplitude of additional shortwave energy sliding through south-central Canada which impacts northward extent of the heat over the Upper Midwest. For all these features, generally favored a general model blend with some incorporation of the ensemble means by the second half of the period. Most impactful for sensible weather however, will be resolving how much moisture reaches the Southwest by the weekend and early next week, with a potential connection to an eastern Pacific tropical system for which the National Hurricane Center shows a high chance of formation within the next several days. The GEFS/ECENS means form a most reasonable track for now that is fairly well offshore the western Baja California coast but CMC and GFS run have consistently been farther east and would provide a more direct feed of significant moisture into the Southwest. The ECMWF and especially the UKMET are more west with this system, but the ECMWF has trended closer to the means/GFS compared to even just a day ago. Regarding the rainfall forecast over the Southwest during the latter half of the period, stayed closest to the NBM for days 5-7 but with somewhat tempered amounts given the lingering and still quite significant uncertainty. Compared to WPC continuity, did trend slightly wetter. ...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 indicate potential for heavy rainfall to impact portions of this region. Given the uncertainty though, the days 4 and 5 Excessive Rainfall Outlooks depict only a broad marginal over this region and northward into the Great Basin/Rockies. However, should model consensus continue to improve, it's likely at least a slight risk will be needed in future updates to the ERO. To the north, moisture will interact with a frontal boundary across the Northwest with some enhancement to rainfall in that region. Elsewhere, showers and storms will accompany the strong shortwave through the Northeast, and given sensitivity in that region to rainfall and anomalous moisture, a marginal risk was maintained on the Day 4/Friday ERO. Lingering moisture will keep locally heavy and diurnally driven rains across parts of Florida, and tropical moisture through the Gulf of Mexico late period may begin to spread rainfall along the Gulf Coast. 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. Heat will begin to build over the central Plains to the Midwest as anomalous ridging expands over this region. Daytime highs near 100 degrees are possible as far north as the Upper Midwest, which is 15 to 20 degrees above normal. Daily record highs are possible into the central Plains and Upper Mississippi Valley.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. The current heat wave out West should come to an end by the start of the medium range period with upper troughing keeping highs generally 5-15 degrees below normal. Parts of the Southwest could be 15-20 degrees below normal especially Sunday and Monday. 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