Extended Forecast Discussion NWS Weather Prediction Center College Park MD 355 AM EDT Sat Aug 20 2022 Valid 12Z Tue Aug 23 2022 - 12Z Sat Aug 27 2022 ...Significant excessive rainfall/flash flooding threat likely to settle over the Southern Plains/Lower Mississippi Valley next week... ...Heat over the Northwest mid-late week may challenge some daily records... ...Overview... Mean troughing over the East will relax somewhat after midweek with the departure of initial energy, but still persist enough to support a mean frontal boundary that ultimately settles near the Southeast/Gulf coasts. Meanwhile flow around the southern periphery of a deeper eastern Canada mean trough should push one or more fronts into the northern U.S. Upper ridging over the southern half of the West should persist into Friday before becoming more suppressed. Energy from a Northwest upper low/trough should ultimately make its way into the Northern Plains toward the end of the week. Continue to expect the best focus for heavy rainfall during the period to be over the Southern Plains and vicinity with the interaction of southern tier shortwave energy, a stationary front, and abundant moisture. The Northeast should trend drier after Tuesday but southeastern areas will likely continue to see episodes of rainfall through the week. The combination of upper trough/low drifting from the Northwest into the Plains and a front setting up over the northern tier may produce areas of rainfall from the northern Rockies eastward. Persistent clouds and rainfall will keep southern tier high temperatures well below normal for most of the week while the main area of above normal highs will be over the Northwest Wednesday-Friday. ...Guidance Evaluation/Predictability Assessment... Within the large scale pattern evolution, there are a number of embedded uncertainties. The most significant one from the heavy rainfall perspective is the handling of fairly small scale (and thus having low predictability) shortwave energy across the southern tier. Every model shows different specifics while multiple UKMET runs have been on the extreme side of the spread with much faster eastward progression (and farther north over the East). Guidance has also varied with details of the eastern Canada mean trough, leading to variability in details of a front that may push into the northern tier by late in the week. There are typical differences for how Northwest trough/upper low energy evolves but at least there is decent clustering in principle for it to reach the Northern Plains late in the period. Typically more sensitive specifics of the surface reflection are a greater unknown though. Finally, toward Friday-Saturday the operational model runs and individual ensemble members wildly diverge for flow details over the eastern Pacific into western North America. An amplified offshore trough in 12Z/18Z GFS runs, a ridge in the new 00Z run and the CMC, and a progressive shortwave in the 12Z ECMWF are some of the potential options. Prefer the flat flow seen in the ensemble means until some degree of clustering and continuity emerge. Based on guidance comparisons, the early part of the forecast emphasized a composite of the 12Z/18Z operational models but still included a small 18Z GEFS/12Z ECMWF mean component in light of some detail questions. Issues with eastern Pacific into western North America flow late in the period as well as other detail uncertainties led to trending the total ensemble weight in the forecast up to 70 percent by day 7 Saturday. ...Weather/Hazards Highlights... The expected Southern Plains heavy rainfall event should be in progress at the start of the period early Tuesday and continue through a decent portion of week, possibly extending as far east as the Lower Mississippi Valley. Contributing moisture should originate from the Southwest, as well as what may be left over from Potential Tropical Cyclone Four (forecast to be dissipated by the start of the medium range period, check latest National Hurricane Center products). Multiple shortwave features aloft and a wavy stationary surface front will be added ingredients to promote the heavy rainfall potential. Portions of Texas should see the highest rainfall totals in this event. WPC experimental days 4-5 outlooks highlight a Slight Risk for excessive rainfall over parts of Texas/Louisiana from Tuesday into early Thursday. Persistent guidance spread suggests there is still at least a moderate degree of uncertainty in specifics, with the relatively 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. Monsoon activity over the West should persist but in less extreme fashion than in the short term. Still, there may be a westward push of moisture for a time around midweek. The Northeast should see organized rainfall of varying intensity Tuesday into Wednesday with one or more frontal waves, followed by a drier trend. The trailing front will be more persistent over southern parts of the East, maintaining a daily focus for diurnally favored showers and thunderstorms. Florida will see multiple episodes of showers/storms as well. Expect rainfall to expand between the northern Rockies and Great Lakes, eventually spreading into the Northeast, as a couple fronts settle over this area. The combination of the upper trough/low emerging from the Northwest and some low level upslope flow may enhance rainfall over and near the northern High Plains. Other areas of locally heavy rainfall could be possible within the broader moisture shield but with low confidence in specifics at this time. The primary focus for anomalous heat next week will be over the Northwest during Wednesday-Friday when highs may reach 10-20F above normal, with morning lows also well above normal. Some locations could see temperatures challenge daily records for highs and/or warm lows. Clouds and rainfall will keep high temperatures below normal over the southern states through the period. Some locations may see one or more days with highs 10-15F below normal especially during Tuesday-Wednesday. Slightly below normal highs should settle into northern parts of the Plains later in the week after the region sees above normal readings early in the week. The Northeast may see highs reach up to 5-10F above normal after midweek. 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