Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Wed Aug 24 2022 Valid 12Z Sat Aug 27 2022 - 12Z Wed Aug 31 2022 ...Overview... This weekend into early next week, reasonably flat upper flow but with multiple embedded shortwave impulses over the lower 48 is forecast, but the flow will amplify through next week in response to the sharp digging of an amplified Gulf of Alaska upper trough. This should produce a ridge overall in western North America, but perhaps with an embedded shortwave or two, and broad troughing over eastern portions of the continent. ...Guidance Evaluation/Predictability Assessment... Model guidance has continued to waffle considerably with the details of the upper-level pattern especially early in the period. The small scale of shortwaves dominating the flow and their origins from the high latitude Pacific/Alaska leads to lower than normal confidence with the details. Most models show one such shortwave entering the Pacific Northwest around Saturday, though the 00Z CMC was weaker than consensus, and the 00Z UKMET maintained phased shortwave energy across southwestern Canada to cause a different pattern and seemed to be an outlier. By early next week, there were already considerable differences in the position of this shortwave (possibly compact closed low) in the 00Z/06Z model guidance. The 12Z guidance has emerged looking most similar to the 00Z ECMWF, with this shortwave tracking slowly through the northern portions of the Great Basin, Rockies, and High Plains through Wednesday, which could disrupt the overall ridge pattern across the West. Earlier runs of the GFS for example tracked the feature farther east and becoming embedded in the central-eastern U.S. flow. Confidence remains low with the shortwave track given the run-to-run differences and the various solutions shown by the GEFS/EC/CMCE ensemble members. By Tuesday-Wednesday, at least models are more agreeable with amplifying flow ahead of the digging Pacific trough, leading to general ridging in the western or perhaps central U.S. (depending on the aforementioned shortwave placement) and troughing in the East. Given the shortwave differences, the WPC forecast used a higher proportion of ensemble means compared to normal early in the period, starting around 30 percent while the 06Z GFS, 00Z ECMWF, and 00Z CMC comprised the rest of the blend. Gradually increased the proportion of ensemble means to over half day 6-7. This maintained reasonable continuity with the mass fields. The QPF did generally trend down compared to the previous forecast given an overall model trend downward but also the lack of confidence in the specifics of any heavy totals. ...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 Southwest/Four Corners states into the Intermountain West. 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 Oklahoma/Missouri/Arkansas where the front may be less progressive than the northern part of the front and as enhanced Gulf moisture returns inland across the southern Plains and Lower Mississippi Valley. Farther east, 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. Temperatures around 5-10 degrees warmer than average are forecast to shift from the northern Plains this weekend into the Great Lakes/Ohio Valley and into the Northeast early next week. Farther south, temperatures should be seasonable or even a bit below average for late summer but still may feel warm in the upper 80s-90s. With the building ridge in the Northwest, there is a possibility for anomalous heat there with temperatures 10-15F above normal, but with uncertainty at this time given differences with an embedded shortwave. Tate/Schichtel 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