Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Aug 21 2022 Valid 12Z Wed Aug 24 2022 - 12Z Sun Aug 28 2022 ...Southern tier heavy rainfall threat likely to continue through at least Wednesday... ...Heat over the Northwest mid-late week may challenge some daily records... 19Z Update: The 12Z deterministic guidance agrees well on the synoptic scale pattern through the end of the week, although the past few runs of the UKMET have been more progressive with the surface low and heavy QPF associated with it across the Deep South for the middle of the week. Going forward into next weekend, the most uncertain aspect of the forecast revolves around the eventual break down of the upper ridge and embedded closed low across the northwestern U.S., but an improvement compared to this time yesterday. In terms of the excessive rainfall outlook, the existing Slight Risk area was adjusted some to the east and the western portion across parts of Texas trimmed back to account for a slightly faster trend in the NBM 4.1 guidance. The WPC forecast was primarily derived from a GFS/ECMWF/CMC/previous WPC blend through much of the forecast period, with some inclusion of the ensemble means by the Friday-Sunday time period. /Hamrick --------------- ...Overview... Most guidance is advertising a transition in the pattern from weak eastern U.S. mean troughing with a stronger trough over the eastern half of Canada Wednesday-Friday toward a modest mean trough over the central U.S./southern Canada by next weekend. The details of this evolution involve lifting out of eastern North America trough energy while a modest trough initially anchored by a compact Northwest upper low heads into the Northern Plains while upstream North Pacific trough energy feeds into the overall trough. The western periphery of Atlantic upper ridging may also push a little westward over the eastern U.S. next weekend. Mean ridging over the southern half of the West should weaken and/or drop southward by the weekend but with uncertain details. Initial southern tier heavy rainfall should trend lighter and eastward with time as the supporting front settles toward the southeastern coast. The combination of the upper trough/low drifting from the Northwest into the Plains and a couple northern tier fronts may focus rainfall over the northern High Plains with some rain also extending east along the one front that reaches the Northeast. The most prominent area of above normal temperatures will be in the Northwest Wednesday-Friday while less extreme anomalies are likely over the Northeast late this week. Southern tier high temperatures will be below normal for most of the period though with some moderation after midweek. ...Guidance Evaluation/Predictability Assessment... Guidance comparisons during the first half of the period favored incorporation of the 12Z/18Z GFS and 12Z ECMWF/CMC. The 12Z UKMET was not usable due to having a more pronounced and faster/northward wave tracking out of the South. The new 00Z UKMET has adjusted back some but remains rather extreme compared to other solutions. Models continue to vary with specifics of the southern periphery of the eastern Canada upper trough that pushes a cold front into Great Lakes/Northeast, with current consensus showing a faster adjustment versus prior forecast (continuing through the end of the period). Also, after early Thursday there is quicker progression/weakening of the western part of the southern tier surface front. A blend of the operational models and 18Z GEFS/12Z ECMWF means provided a reasonable intermediate/most likely scenario by the latter half of the period. There is still significant model and ensemble member spread for eastern Pacific into western North America flow from late week through the weekend, originating from differences in how flow separates within an upper trough to the south of Alaska. The 00Z GFS is starting to come into better agreement with the majority cluster over the Pacific, trimming the model's differences versus other guidance in some respects. There is still considerable spread for amplitude/timing of shortwave energy reaching western North America though. The Gulf of Mexico provides another area of uncertainty by the weekend. Some GFS runs have been reflecting a weak surface feature over the eastern Gulf (dampened in the new 00Z run) while the CMC has been favoring the far southwestern Gulf. 12Z GEFS/ECMWF ensemble members for low level vorticity centers are dispersed across the Gulf in no defined pattern though at the same time ensembles show a little greater probability for higher moisture/low shear over the eastern half of the Gulf. The manual forecast valid on day 6 Saturday maintains the eastern gulf surface trough from yesterday's NHC/WPC coordination while day 7 Sunday extrapolates this trough northwestward. ...Weather/Hazards Highlights... Expect areas of heavy rainfall over the southern tier to continue at least into Wednesday, with lingering moisture and a wavy front contributing to enhanced totals. The WPC experimental day 4 outlook valid Wednesday into early Thursday highlights a Slight Risk for excessive rainfall over some areas from Texas through the Lower Mississippi Valley. As has been the case in previous cycles, guidance spread still suggests at least a moderate degree of uncertainty in specifics. Low predictability of important small-scale features aloft as well as dependence on mesoscale convective aspects that may not become apparent until close to the event complicate the medium range forecast. Recent trends have been toward a gradually more progressive/suppressed evolution for rainfall, leading to less confidence in where any organized areas of significant rain may remain by late in the week. The Southeast should continue to see periods of showers and thunderstorms through the weekend with a front lingering near the southeastern coast, while Florida sees periods of showers/storms as well. Monsoonal moisture over the West should push westward somewhat around midweek, with some degree of rainfall enhancement possible. Then the moisture will likely return back to the east as the upper ridge over the southern half of the West weakens. The forecast is consistent in showing potential for some enhanced rainfall over the northern High Plains mid-late week with the combination of a couple surface fronts and the approaching Northwest upper trough/low, plus some low level upslope flow. Areas farther east all the way to the East Coast may also see some rainfall with the main front expected to reach the Northeast/Mid-Atlantic. A broader area of rainfall may spread over the central U.S. next weekend as Pacific energy aloft approaches. The primary focus for anomalous heat will be over the Northwest during Wednesday-Friday when highs may reach 10-15F above normal, with morning lows also well above normal and covering a larger portion of the West. Some locations could see temperatures challenge daily records for highs and/or warm lows. The region should remain above normal next weekend but with lower anomalies. High temperatures should be below normal over the southern states through the period but with most areas moderating to single-digit anomalies. Slightly below normal highs should settle into northern parts of the Plains later in the week. The Northeast may see highs reach up to 5-10F above normal Thursday-Friday. 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