Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Tue Aug 09 2022 Valid 12Z Fri Aug 12 2022 - 12Z Tue Aug 16 2022 ...Monsoonal moisture/rainfall threat continues for the Southwest and Intermountain West/Rockies this week, with some localized heavy rain possible for the Gulf Coast states as well... ...Excessive heat threat from the interior Northwest to the north-central Plains... ...Overview... A fairly stagnant trough-ridge-trough pattern is forecast later this week into early next week 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 an upper high meanders near the central/southern High Plains. A cold front or two approaching the Gulf Coast and stalling will provide a focus for moisture and rain/thunderstorms there and lingering across much of Florida into early next week. Meanwhile, persistent monsoonal moisture looks to continue across the Southwest and expanding into the Intermountain West and Rockies this week into next week. ...Guidance Evaluation/Predictability Assessment... While model guidance is in reasonably good agreement with the synoptic pattern, differences remain on the smaller scale. Models continue to waffle regarding the timing and evolution of the upper low centered just offshore the Pacific Northwest as the period begins Friday. The GFS runs indicate a faster track with this low as it comes into southwestern Canada, opens into a shortwave, and moves eastward through south-central Canada early next week. Prefer a solution on the slower side of the full envelope of guidance given model trends and history. Meanwhile, the upper high in the central U.S. appears well handled in the models, though there is a fair amount of uncertainty with the energetic shortwave features rounding the ridge this weekend and beyond. These shortwaves may serve to suppress the ridge on the northeastern side in the Midwest early next week as well as produce some showers/storms/MCSs that have low confidence in the specifics at this point. Then there are some model differences with distribution of energy of the trough in the East, including some models indicating periods of a low closing off within it, but a composite blend of guidance averaged out these differences. The WPC medium range forecast was thus based on a blend favoring the ECMWF and CMC but still including the GFS and UKMET, and transitioning to a model/mean (particularly the EC mean) blend later in the period for the mass fields. For QPF, the main notable change to the NBM was to increase rain amounts around the Midwest into the Ohio Valley where multiple models indicate the potential for storms, albeit with timing and placement differences. ...Weather/Hazards Highlights... Moisture streaming ahead of a couple of cold fronts moving into the Southeast and toward the Gulf Coast late this week will produce rain and thunderstorms in those areas. This could cause localized flooding issues since storms should be slow-moving and may train over the same areas. Rain chances are likely to continue across much of Florida into early next week as the front stalls. Farther north, the cold front should clear out most rainfall from the Mid-Atlantic and likely the Northeast as well, but dependent on how close to the New England coast a low along the cold front sits. Shortwave impulses rounding the ridge could lead to rain and storms across the Midwest/Upper Great Lakes into the Ohio Valley this weekend into early next week, but with low confidence on the specifics. Meanwhile, monsoonal moisture continues to impact the Southwest into parts of the Great Basin and into the Rockies with rainfall through the medium range period. Some locally heavy totals are possible and a Slight Risk of flash flooding has been added to the experimental Day 4 ERO. Very warm to hot temperatures around 10-15F above average, though not quite to record-breaking levels, are forecast across interior portions of the Northwest and into the northern/central Plains through the latter part of the week, and lasting across the central Plains and parts of the Mid-Mississippi Valley this weekend into early next week. A slight southern shift is expected with time, leading to highs nearing/reaching 100F forecast early next week in the central/southern Plains. Farther east, 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. Given the rainfall and cloudiness over the Southwest and Great Basin, highs are likely to be several degrees below normal there through early 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