Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Tue Aug 23 2022 Valid 12Z Fri Aug 26 2022 - 12Z Tue Aug 30 2022 ...Overview... Most guidance continues to advertise departure of the eastern North American mean trough aloft after Friday and then development of a mean trough over the central U.S. during the weekend/early next week, with this trough likely reaching the Mississippi Valley/Great Lakes by next Tuesday. The central U.S. trough will be supported by a quickly amplifying trough/ridge pattern over the eastern Pacific and western North America respectively. The western periphery of Atlantic upper ridging should build westward to cover an increasing portion of the East/South during the weekend and early next week. 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 Northeast/nearby areas (Sunday-Tuesday) while near or slightly below normal highs should prevail over the South, Rockies, and Northern Plains. ...Guidance Evaluation/Predictability Assessment... Models/ensembles are stable with the leading weak shortwave reaching the Northern Plains by Saturday. However they continue to vary with the details of incoming Northeast Pacific shortwave energy that should ultimately feed into the developing mean trough, and to some degree with the larger scale Pacific/western North America pattern early next week. One positive at this point is that the ensemble spread over the Northwest valid early Saturday has narrowed quite a bit from 24 hours ago, toward a moderate shortwave trough, but there is still meaningful spread for amplitude. Beyond then the spaghetti plots still become rather chaotic but with the greatest density of solutions gravitating toward a trough-ridge-trough configuration over the eastern Pacific through east-central North America by next Tuesday. Operational models differ significantly as well. For example, latest GFS runs are on the deep/south side with a compact closed low. The CMC has switched from a progressive open wave (faster than consensus) in the 12Z run to a slow closed low (weaker than the GFS) in the 00Z run. Later in the period there are some differences with the larger scale evolution that come into play. In particular the GFS has sporadically been shifting its pattern out of phase with consensus, such as in the 12Z run that had its Pacific/North America pattern farther west. 18Z and new 00Z runs are closer. Still, all of those GFS runs ultimately end up somewhat on the deep/slow side with the upper trough reaching the Mississippi Valley by next week. The updated forecast primarily focused on 12Z/18Z operational models during the first half of the period but already with a small ensemble component given the greater than usual detail spread. Then the blend trended toward half models/half means by day 7 Tuesday, with more 12Z ECMWF than 18Z GFS/12Z CMC for the model component and the 18Z GEFS/12Z ECens comprising the ensemble portion. This approach yielded good continuity overall. ...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 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. 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. 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. 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 through the Rockies and Northern Plains. 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 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