Extended Forecast Discussion NWS Weather Prediction Center College Park MD 349 PM EDT Mon Aug 22 2022 Valid 12Z Thu Aug 25 2022 - 12Z Mon Aug 29 2022 ...Southern tier heavy rainfall threat to become less pronounced after midweek... ...Heat over the Northwest may challenge some daily records on Thursday... ...Overview... Latest models and means still advertise a pattern transition during the period, with the eastern North America upper trough lifting out after Friday and a mean trough reaching the central U.S./southern Canada by Saturday-Monday. The latter trough should be composed of a leading trough/compact upper low emerging from the Northwest followed by upstream northeastern Pacific trough energy. Initial upper ridging covering the southern half of the West will trend weaker and more suppressed as Pacific flow arrives. Meanwhile 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. Expect some areas of rainfall to persist over the South and nearby areas during the period but with any heavy totals tending to be more localized. Rainfall focused over the northern High Plains on Thursday should expand and push eastward with time as the central U.S. upper trough takes shape while western U.S. monsoonal rainfall should become lighter and more scattered by the weekend as upper flow gains a more westerly component. Above normal temperatures will tend to be confined to the Northwest, especially Thursday and next Monday, as well as the Northeast. Below normal highs over the southern tier will moderate closer to normal. ...Guidance Evaluation/Predictability Assessment... The latest deterministic and ensemble guidance capture the general synoptic pattern relatively well. The main area of uncertainty is with the arrival of an upper-level trough over the Pacifc Northwest/British Columbia on day 5 continuing into day 6. A general model blend of 00z EC/UK/CMC and the 06z GFS were used on day 3 with the CMC and UK favored for their handling of a trough in the Southeast. A gmb was utilized again on day 4, except this time the 06z GFS was weighted less than the 00z suite due to small scale discrepancies in the upper trough over Montana. The 00z CMC is an outlier with respect to a potent shortwave that it has spinning through the Northern Plains. The 00z EC ensemble mean was incorporated into the gmb on day 5 to mitigate deep troughing across the Plains and coastal California by the 00z UK. 06z GFS introduces an anomalous trough in the Northwest as well which it moves into the Northern Rockies by day 6. The 00z GFS attempts to spin up a shortwave along the northern California coast on day 6 which would be more in line with what the other deterministics and ensembles are signaling which is why it was included in the blend for that day. An ensemble mean blend was used on day 7 with some 00z EC. ...Weather/Hazards Highlights... Expect some episodes of showers and thunderstorms to continue across the South and Florida Peninsula through the period, but with any areas of enhanced totals likely to be more localized and more uncertain in location/magnitude than in the shorter term. A weakening front reaching the Gulf/Southeast coasts may still provide some focus late this week into the first part of the weekend. Locations over the northern High Plains may see some enhanced rainfall Thursday and possibly into Friday with a couple fronts over the area and the upper trough/low approaching from the Northwest. This moisture should then progress eastward and expand with time as the central U.S. mean trough develops. Meanwhile monsoonal rain and thunderstorms should contine over and near the Four Corners states through late this week followed by a drier trend from west to east as the upper ridge initially over the region collapses. Some of the deep moisture may reach eastward and contribute to the expanding area of central U.S. rainfall during the weekend. A cold front dropping southward through the East during the period may produce areas of rainfall with varying intensity, though currently with no coherent signal for organized areas of significant totals. Warmest temperature anomalies should be over the Northwest on Thursday with some locations seeing highs 10-15F above normal and a few locations possibly challenging record highs. Morning lows will also be quite warm Thursday-Friday with some daily records possible. The Northwest should return to near normal for the weekend and then rebound to 5-10F above normal next Monday as an upper ridge builds in. Meanwhile parts of the Northeast should see highs up to 5-10F above normal late this week with the Great Lakes/Northeast seeing similar anomalies Sunday-Monday. Cool high temperatures across the southern tier, up to 5-10F below normal on Thursday, will likely moderate gradually to yield near to only slightly below normal readings by the weekend and early next week. Northern Plains areas should see a couple episodes of slightly below normal highs, one late this week and another Sunday-Monday. Rausch/Kebede 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