Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Sun Aug 07 2022 Valid 12Z Wed Aug 10 2022 - 12Z Sun Aug 14 2022 ...Potentially heavy rainfall and flash flooding are possible in the Ohio/Tennessee Valleys Wednesday while monsoonal moisture continues in the West... ...Overview... The overall weather pattern across the contiguous U.S. through the latter half of this week is forecast to amplify into a trough-ridge-trough pattern, as shortwaves combine to deepen a trough in the East, an upper low off the West Coast lifts northeastward toward the Pacific Northwest, and an upper high nudges eastward from the central Rockies into the central/southern Plains. Moisture lifted ahead of the eastern trough and associated cold front will likely lead to areas of heavy rain and thunderstorms mainly across the Ohio and Tennessee Valleys into the Mid-Atlantic and Northeast Wednesday, with scattered convection possible farther south as well. Meanwhile, persistent monsoonal moisture looks to continue across portions of the West, leading to additional rainfall with a gradual northward expansion of the rain toward the northern Rockies as the week progresses. ...Guidance Evaluation/Predictability Assessment... Recent model guidance remains quite agreeable with the upper high/ridge persisting and meandering a bit across the central U.S. through the week into next weekend, and also agrees upon the trough deepening across the East late week, with detail differences well within normal spread for a medium range forecast. The biggest model differences continue to be related to the upper low centered just off the West Coast, where models have been waffling considerably with track and timing. In terms of the 12/18Z model cycle that could be utilized for the WPC forecast, the ECMWF and UKMET indicated a faster solution, taking the low/trough inland across the Pacific Northwest Thursday while GFS runs showed the low spinning offshore and eventually moving inland by the weekend. The CMC appeared to be a middle ground solution in between the two, though with low confidence as ensemble members from the EC/GEFS/CMCE showed considerable spread, smoothing out the ensemble means. However, newer 00Z guidance is almost uniformly preferring a slow solution with the upper low quasi-stationary centered offshore through late week, like the older GFS runs and also like an EC run from a day or so ago. Hopefully this agreement persists through future model cycles but given the past back and forth in the guidance, confidence is still somewhat low. The WPC forecast was based on a multi-model blend early on, but with increasing weights of the ensemble means perhaps earlier than normal in the medium range period given the differences especially with the western low/trough. In general, continuity with the previous forecast was fairly good, but with some changes to frontal timing differences in the northwestern/north-central U.S., which will likely have to be edited in future forecasts as well. ...Weather/Hazards Highlights... As the medium range period begins Wednesday, an axis of locally heavy rain and thunderstorms ahead of a cold front is expected to continue pushing southeastward across the Ohio Valley to Northeast into the Tennessee Valley and Mid-Atlantic region. The relatively slow movement of the front could lead to training storms that could cause flash flooding. Flooding concerns are particularly enhanced across portions of Kentucky/Tennessee/West Virginia, as they have received abundant rainfall recently and are sensitive to additional rain amounts, so a Slight Risk remains in the experimental Day 4 ERO. The Northeast/New England also may receive heavy rain that could cause flash flooding given a sharpening trough aloft providing upper level support and above average moisture, but model spread in the exact placement of the heaviest rainfall precluded adding a Slight Risk there at this time--specifically with recent GFS runs showing rainfall offshore instead. The front continuing to push southeastward into the latter half of the week should clear rainfall out of the northeastern quadrant of the country, but scattered convective activity is forecast to continue in the Southeast. Meanwhile, monsoonal moisture (that could be enhanced due to T.D. Nine-E) continues to impact the Southwest with rainfall, with a trend toward increasing chances for rain farther north as the moisture plume expands into the Intermountain West/Great Basin and eastward into the northern/central Rockies, with some locally heavy totals possible. Very warm to hot temperatures around 10-15F above average, though not quite to record-breaking levels, are forecast across the northern/central Plains through the latter part of the week, with a slight eastward trend as the upper high moves that direction. Farther east, slightly below normal highs are possible in the Ohio and Tennessee Valleys behind the cold front, while temperatures along the Eastern Seaboard will flip from slightly above to slightly below average. Given the rainfall and cloudiness over the Southwest and Great Basin, highs are likely to be several degrees below normal there through next weekend. Tate 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