Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Wed Aug 10 2022 Valid 12Z Sat Aug 13 2022 - 12Z Wed Aug 17 2022 ...Monsoonal moisture/rainfall threat continues for the Southwest and Intermountain West/Rockies... ...Excessive heat threat across the Plains, with warming temperatures in the Northwest next week... ...Overview... A fairly stagnant trough-ridge-trough pattern is forecast this weekend across the contiguous U.S., as shortwaves deepen and then maintain a trough in the East, an upper low off the West Coast lifts northeastward into British Columbia, and a warm upper upper high meanders near the central/southern High Plains. Upper ridging is forecast to expand into the West by Tuesday-Wednesday of next week, increasing temperatures to well above average particularly in the Northwest. The ridge aloft will also continue to support monsoonal moisture streaming into the Southwest and into the Intermountain West and Rockies through much of next week. A lingering frontal boundary over Florida will provide a focus for rainfall there, while possible shortwave energy rounding the ridge could lead to rounds of showers/thunderstorms across the Midwest/Upper Great Lakes this weekend, with rain chances expanding south and east early next week. ...Guidance Evaluation/Predictability Assessment... Model guidance remains steady in indicating the central U.S. upper high will separate the eastern trough and a compact upper low tracking across British Columbia this weekend (which has finally showed better agreement in track and timing). Less certain aspects of the forecast include small-scale shortwaves rounding the northern side of the ridge, which may not cause considerable differences on the larger scale, but do lead to variety in placement of storms/possible MCSs that develop in and around the Midwest in the northwest flow. There is also some variability with the details of energy distribution within the eastern trough, even as the period begins Saturday with the 12Z ECMWF showing a closed low within the trough. This initial energy looks to lift northeast while additional shortwaves dig to maintain the trough through the middle of next week, with perhaps better agreement for a closed upper low to form around Tuesday-Wednesday among deterministic models, but the placement of the upper low center varies. Among 12/18Z guidance the ECMWF was the farthest north into Canada and the 12Z GFS was farthest south over the Great Lakes, with the 12Z CMC leaning south as well while the 18Z GFS was in between. The newer 00Z guidance trended a bit toward the southern side. Despite the differences, no models seemed questionable enough to be called outliers. Thus the WPC forecast was based on an initial blend of the 18Z GFS and 12Z ECMWF/UKMET/CMC, with the blend serving to smooth out the small scale differences. Gradually incorporated the EC and GEFS ensemble means day 5-7, which were agreeable, to try to produce a middle ground forecast. The main change to the previous WPC forecast was regarding timing of a cold front tracking eastward through the Great Lakes to Northeast as the eastern trough seems to sharpen up, while the main change to the QPF from the National Blend of Models was to increase coverage of QPF in the east-central U.S. given shortwave energy could cause showers and storms there. ...Weather/Hazards Highlights... Parts of the West have seen persistent monsoonal moisture over the past couple of months, and the rest of this week and into next week will continue that pattern. In fact, notable moisture anomalies are forecast to stream into parts of the Great Basin and the Rockies and produce rain and thunderstorms. Rain totals may increase by Monday-Tuesday for the northern/central Rockies in particular. Meanwhile, a moist airmass and a lingering frontal boundary will cause showers and thunderstorms across the Florida Peninsula into early next week. Farther north, shortwave impulses rounding the ridge could lead to rain and storms across the Midwest/Upper Great Lakes into the Ohio Valley this weekend, but with low confidence on the specifics. Then rain chances could increase for the Ohio Valley to Appalachians and the East Coast early next week along a cold front, but the details remain uncertain there as well. Very warm to hot temperatures around 10-15F above average, though mainly not to record-breaking levels, are forecast across the Northern and especially the Central Plains over the weekend, with some gradual moderation and shift southward through the workweek, though highs could still reach 100F in the Southern Plains. As the upper ridge expands into the Northwest, this will warm up temperatures there and cause highs 10-20F above normal by Tuesday-Wednesday. Meanwhile, below normal highs are possible in the east-central U.S. behind the cold front, with the most persistent cooler than average weather in the Ohio and Tennessee Valleys toward the Mid-Atlantic. Given the rainfall and cloudiness over the Southwest and Great Basin into the Rockies, highs are likely to be several degrees below normal there through much of next week. 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