Extended Forecast Discussion NWS Weather Prediction Center College Park MD 442 PM EDT Tue Aug 23 2022 Valid 12Z Fri Aug 26 2022 - 12Z Tue Aug 30 2022 ...Overview... Expect troughing across eastern North America Friday to lift away for the weekend, while a couple of shortwaves enhance mean troughing in the central U.S. that shifts toward the Midwest/Great Lakes by Monday-Tuesday of next week. Ridging is forecast to build across the interior West behind that trough and ahead of amplifying troughing in the eastern Pacific. The brief period of more Pacific influence on mean flow aloft over the West into the weekend should lead to a somewhat drier trend over the Four Corners states while potentially adding some moisture to an expanding area of Plains into Mississippi Valley/Great Lakes rainfall ahead of the developing central U.S. mean trough and leading surface system. Some areas of rainfall should persist over the South and nearby areas during the period but with any heavy totals tending to be fairly localized. Above normal temperatures will tend to be confined to the Northwest (Friday and Monday-Tuesday) and northeastern U.S. (Sunday-Tuesday) while near or slightly below normal highs should prevail over the Rockies into the Plains and Southeast. ...Guidance Evaluation/Predictability Assessment... Model guidance remains stable with the larger scale trough exiting out of the East as well as with the initial shortwave tracking across the Northern Plains late this week. But models show more variability with a second notable shortwave that feeds into the developing mean trough and to some degree the larger scale Pacific/western North America pattern early next week. While its track and timing entering the Pacific Northwest likely Saturday have shown better agreement in the past few model cycles, there are considerable differences in timing after that, as well as with its amplitude. 00/06Z GFS runs remained on the stronger side of the spread with a compact closed low moving through the northwestern U.S. around Sunday, while the CMC is also strong but slower than consensus and the ECMWF (for the 00Z and incoming 12Z run) is weaker and more progressive. The 12Z GFS is still stronger than the ECMWF but weaker than its previous runs. Though some differences are on the small scale initially, they have implications for the larger scale pattern downstream into Monday-Tuesday. Preferred the ensemble means rather than any particular model at that point given ample differences, with the GFS runs perhaps closest to the EC/GEFS ensemble means. The newer 12Z guidance appears at least to converge somewhat with the larger scale flow by the end of the period with more agreeable ridge and trough axes compared to the previous 00/06Z cycle that was available for this medium range forecast. Thus the WPC forecast began with a blend of mainly deterministic 00/06Z models, but a small ensemble component given the greater than usual detail spread. Quickly increased the proportion of the GEFS/EC ensemble means in the blend to over half by day 6-7 as the deterministic differences became larger scale, which led to good consistency with the previous forecast. ...Weather/Hazards Highlights... The brief transition to more progressive flow aloft over the West during the weekend should promote a lighter and more scattered trend for monsoon rainfall over the Four Corners states after Friday. Some of the western U.S. deep moisture that is pushed eastward may contribute to the expanding area of rainfall forecast to spread across the Plains and eventually into the Mississippi Valley/Great Lakes. A leading warm front followed by a surface trough and cold front, as well as northern tier upper dynamics, will all provide varying degrees of focus for what could be multiple areas of locally moderate to heavy rainfall within the broad moisture shield. Current forecasts show one area of rainfall focus around Kansas-Missouri where the front could move a little more slowly than the more progressive north part of the front, but it will take additional time to resolve where the most favored areas for highest totals will be. Showers/thunderstorms will likely continue over the South and Florida Peninsula through the period but with any areas of enhanced totals likely to be more localized and uncertain on a day-to-day basis, including some model variability with whether the bulk of rainfall will be offshore or onshore in any given period. A weakening front reaching the Gulf/Southeast coasts may still provide some focus late this week into the first part of the weekend. A cold front dropping southward through the East from Friday through the weekend may produce areas of rainfall with varying intensity. Waviness along the front could enhance totals over northern New England on Friday. The primary areas of above normal temperatures during the period will be over the Northwest as well as the Northeast and vicinity. The Northwest will be modestly above normal Friday, followed by a cooler weekend and then rebound to plus 5-10F or so anomalies next Monday-Tuesday as the upper ridge develops and strengthens. Meanwhile parts of the Northeast/northern Mid-Atlantic will see moderately above normal readings on Friday. Then a larger area of warm temperatures should spread across the Great Lakes through Northeast/northern Mid-Atlantic Sunday-Tuesday. Highs could locally reach 10-15F above normal early next week. Near to somewhat below normal high temperatures (single-digit anomalies) will prevail across much of the southern tier and north through the Rockies and Plains. Tate/Rausch 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 Central/Southern Plains, the Middle Mississippi Valley, and the Lower Mississippi Valley, Sun, Aug 28. - Flooding occurring or imminent across portions of the Southern Plains. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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