Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 most contentious part of the forecast continues to involve the details of northeastern Pacific into North American flow from about day 4 Friday onward. Highlighting the uncertainty, the full suite of 12Z ensemble members showed anything from a trough to a ridge as possibilities over the West Coast states and southwestern Canada as of early day 5 Saturday. Members remain divergent through the rest of the period but do eventually show a majority settling into a Pacific trough/West Coast-western Canada ridge and central North America trough by day 7 Monday. Operational models leaned more to a shortwave reaching the Northwest on Saturday but with the 12Z/18Z GFS and 12Z UKMET flatter than the 12Z CMC and especially 12Z ECMWF. The new 00Z GFS has trended much deeper with this shortwave but other models remain more moderate. By late in the period the 18Z GFS conflicted with other guidance, developing an eastern Pacific ridge where consensus had a trough. The new 00Z GFS brings a broad shortwave through the consensus ridge building into British Columbia Sunday-Monday but at least it better fits the trough to the west. The one other item of note is that recent trends have become more subdued with any possible weak feature over the Gulf of Mexico. The 12Z CMC had a weak surface wave lifting through the western Gulf but the 00Z run has adjusted weaker/southwestward. Guidance comparisons led to starting with a 12Z/18Z operational model composite during the first half of the period, yielding an intermediate depiction for the shortwave expected to reach the Northwest by early Saturday (and now supported by the new 00Z ECMWF). Increasing detail uncertainties thereafter, along with increasingly suspect 18Z GFS handling of the eastern Pacific pattern, led to a rapid increase of 18Z GEFS/00Z ECMWF mean weight and eventual replacement of the 18Z GFS component with the 12Z run. Total ensemble weight reached 60 percent by day 7 Monday. ...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 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