Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 PM EDT Fri Aug 19 2022 Valid 12Z Mon Aug 22 2022 - 12Z Fri Aug 26 2022 ...Significant excessive rainfall/flash flooding threat likely to settle over the Southern Plains/Lower Mississippi Valley next week while Southwest and south-central Great Basin/Rockies monsoonal rainfall trends lighter... 19Z Update: The 12Z deterministic guidance has good overall agreement on mass fields across the continental U.S. for the beginning of next week, with mesoscale differences with individual shortwaves becoming apparent as early as Tuesday. The UKMET is considerably more progressive with the disturbance across the south-central U.S. and therefore its QPF is well east of the model consensus, which is mainly across central Texas where the Slight Risk for excessive rainfall is maintained for the Day 4 and 5 time periods. The potential for 4 to 8 inch totals from the ArkLaTex to the greater Austin-San Antonio area are still expected through Tuesday. The antecedent dry conditions mitigates the potential for flooding some, so no Moderate Risk areas are warranted at this time. By the end of the forecast period, the ECMWF becomes more progressive with the trough crossing the northern Rockies and northern Plains. The WPC forecast was primarily derived from a GFS/ECMWF/CMC consensus with a little of the UKMET (away from Texas and the Gulf Coast) and previous WPC continuity through Tuesday night, and then gradually increasing use of the GEFS and ECENS for the end of the week. The previous discussion follows below for reference. /Hamrick ...Overview... Expect mean troughing over the East to trend a bit weaker by the latter half of next week as initial energy in its core lifts northeastward, while deeper troughing prevails over the eastern half of Canada. Over the West, southern areas should see mean ridging aloft for most of the period as a trough with embedded upper low tracks over the Northwest. The heaviest rainfall during the period should be over the Southern Plains and vicinity as Rockies/southern tier shortwave energy interacts with a stationary front. One or more fronts on the leading side of the eastern upper trough will focus showers/thunderstorms at times, and the combination of upper trough/low crossing the Northwest 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 should be over the Northwest mid-late week. ...Guidance Evaluation/Predictability Assessment... Based on guidance from the 12Z/18Z cycles, a composite of operational model runs provided a reasonable depiction of the pattern evolution during the first half of the period and adding in some 18Z GEFS/12Z ECMWF means with the models accounted for decreasing confidence in detail specifics later in the period. The resulting forecast offered good continuity, especially aloft. Shortwave details from the Rockies into the southern tier have lower than desired predictability given their medium to smaller scale, but will be very important for resolving specifics of the potential heavy rainfall over the region. Some details of the shortwave energy crossing the Northeast become somewhat murky after early Tuesday. There has been ongoing timing spread for the Northwest trough/upper low, with the GFS leaning a bit on the faster side of the envelope. The ECMWF has trended moderately faster over the past couple days so a multi-model compromise seems reasonable. One last item of note is that by day 7 Friday the new 00Z GFS develops much more upper troughing offshore the Pacific Northwest than other guidance including the GEFS mean. ...Weather/Hazards Highlights... The widespread heavy to extreme rainfall across the Southwest and south-central Great Basin/Rockies during the short range period through the weekend should trend lighter for next week. Lingering moisture and instability over the region could still allow for some locally enhanced rainfall but overall the threat for excessive rainfall will decrease. Meanwhile the focus for substantial heavy rain will shift to the Southern Plains into the Lower Mississippi Valley for several days as moisture and instability plus multiple shortwave features aloft interact with a wavy stationary surface front. Heaviest rainfall should align over Texas and southern Oklahoma, with meaningful totals extending eastward to some degree. WPC experimental days 4-5 outlooks highlight a Slight Risk for excessive rainfall over this area from Monday into early Wednesday. Some adjustments in the rainfall forecast are likely over coming days, given 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. Elsewhere, rainfall may gradually expand between the northern Rockies and the Upper Great Lakes as a front settles over this area. The combination of the upper trough/low crossing the Northwest and potential for low level upslope flow could enhance rainfall over and near the northern High Plains. Meanwhile one or more wavy fronts over the East will help to focus episodes of showers and thunderstorms, some of which may be locally heavy and lead to runoff issues. Coverage should be greatest during the first half of the week as a defined shortwave lifts from the Great Lakes/Ohio Valley through the Northeast. Diurnal convection should persist over the South through the end of the week. Clouds and rainfall will likely keep southern tier high temperatures below normal through the period, with some locations seeing one or more days with highs 10-15F below normal especially during the first half of the week. Moderately below normal highs should settle into northern parts of the Plains later in the week after the region sees above normal readings on Monday. Temperatures over the Northwest should trend warmer mid-late week after the upper trough passes through, with highs reaching up to 10-15F or so above normal. A broader portion of the West will see above normal lows through the week. 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 Hazards: - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Mon-Thu, Aug 22-Aug 25. - Flooding occurring or imminent across portions of the Southern Plains. - Excessive heat across portions of California, the Pacific Northwest, and the Northern Great Basin, Thu-Fri, Aug 25-Aug 26. 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