Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM 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... Overall, the model guidance this cycle showed fairly good agreement, at least through day 5, with the large scale pattern. The exception is with the shortwave energy originating from the Gulf of Alaska and how that interacts with the troughing over the Western U.S. (or lack of) this weekend into early next week. The 00Z/06Z GFS was a faster and less phased outlier, taking the main shortwave energy through southern Canada and amplifying a downstream trough over the Great Lakes and Northeast by days 6-7. This solution, while less preferred, does have some support from previous runs and now also the 12Z model cycle, and would be a notable change to the end of the forecast period. Another potentially significant sensible weather event lies with the moisture anomalies possibly lifting northward through the Southwest and Great Basin/Northern Rockies later this weekend through early next week, with some connection to the tropical system currently being tracked by the National Hurricane Center, which depicts a high chance of development. The latest run of the GFS lies on the right side of the model spread and while not completely ruled out, was less preferred than the model consensus. Greater model differences also lie with the potential tropical system in the eastern Pacific, and its interaction with the existing troughing and how much moisture will be able to be pulled northward. A blend of the ECMWF/GFS ensemble means was the model consensus, though its worth noting that the track has edged further east this cycle, with the latest GFS consistently being the further east solution compared to the other deterministic models. The further east solutions would provide a more direct feed of the moisture into the Southwest U.S., so something that will need to be closely monitored over the next several days. The NBM was initialized for the QPF grids this cycle, with some inclusion of the latest multi-model ensemble bias corrected QPF, which largely trended well with continuity, with some minor increases across portions of Arizona, southern California, and southern Utah. ...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, by Day 5 (Saturday into Saturday Night) there's a growing signal that a Slight Risk could be needed in future updates across portions of western/southern Arizona, southern Nevada, and potentially southwest Utah. 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 expected. 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 as a result of the increased cloud cover and threat of heavy precipitation. Santorelli/Taylor 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