Extended Forecast Discussion NWS Weather Prediction Center College Park MD 402 PM EDT Thu May 12 2022 Valid 12Z Sun May 15 2022 - 12Z Thu May 19 2022 ...Heat threat for the Desert Southwest early next week while persistent southern Plains heat intensifies and expands eastward into the Southeast... ...Overview... A trough-ridge-trough pattern aloft across the U.S. northern tier is forecast to make gradual progress eastward next week, such that the mean troughs should reach near the East and West Coasts around Wednesday. Ridging early in the week across the southwestern to south-central U.S. followed by rising heights over the Southeast should lead to a broad area of unseasonably hot weather across the southern U.S., with the most persistent record-breaking heat centered in Texas and Louisiana. Meanwhile showers and thunderstorms of varying intensity are possible along fronts pushed by the mean trough crossing the eastern U.S. and with stalling fronts across the Plains. The West should see cooling temperatures in response to upper troughing as the week progresses, along with lowering snow levels by midweek within rounds of precipitation. ...Guidance Evaluation/Preferences... Recent guidance continues to be agreeable with the mean pattern, but with lingering differences with low-predictability shortwave details within the eastern U.S. trough as well as for shortwaves moving through and ejecting from the Pacific trough eventually pushing into the West. Day-to-day frontal position and thus specifics of convective focus will be sensitive to these uncertain medium to smaller scale aspects of the forecast that may take until the short range to resolve. Over the past few days, CMC runs have been generally slow and deep with the eastern trough around Mon-Tue. This persisted in the 00Z run but to a lesser extent, while the newer 12Z run appears even closer to consensus fortunately. CMC runs have also been more amplified than most other guidance with the Pacific to western U.S. troughing later Tuesday into Wednesday-Thursday--though this feature begins showing more spread in general by midweek onward among deterministic models and GEFS/EC/CMC ensemble members as not only uncertain shortwaves pivot, but as stronger energy stemming from an initial upper low spinning just west of British Columbia could start spilling southward. Models vary with whether energy will consolidate into one upper low/trough feature or possibly two upper lows/troughs, with the latter shown by the 00Z CMC and 12Z GFS for example. For the updated WPC forecast, began with a blend of the 00Z/06Z deterministic guidance led by the ECMWF and GFS runs, then phased out the CMC (and the UKMET as it ends) in favor of gradually increasing proportions of the GEFS and EC ensemble means that were reasonably agreeable to downplay the more uncertain details. ...Weather/Threat Highlights... Initial ridging across the Southwest and southern Plains and then eventually rising heights farther east over the Deep South will likely promote a broad area of heat across much of the southern U.S. during the period. The greatest and most lasting anomalies through the period should be over Texas (persisting from the short range time frame) where some locations could see highs up to 15-25F above normal. The southern half or so of the West will tend to see highs up to 10-15F above normal for the early part of the week, followed by a slight cooling trend that reaches California initially and extends farther eastward by next Thursday as Pacific upper troughing arrives. Meanwhile by Tuesday or Wednesday through Thursday the coverage of highs 10-15F above normal should extend eastward through the Lower Mississippi Valley and into the Southeast. Potential for daily record highs will be most persistent through the period over parts of the southern Plains into Louisiana, with numerous records likely to be set. In contrast, the series of surface waves/fronts ahead of the approaching/arriving eastern Pacific upper trough will keep the Pacific Northwest on the cool side with frequent episodes of precipitation. The trough should reach far enough eastward to bring highs down to 10-15F below normal over the Northwest/northern Rockies by next Wednesday-Thursday. Snow levels should be quite high into early next week and then begin to decline as the upper trough arrives. It will likely take into the short range time frame to resolve timing/intensity aspects of each precipitation episode due to the medium to smaller scale of the supporting shortwaves. Areas to the east of the Rockies should generally see two rainfall regimes during the period. From Sunday into Monday the best focus for showers and thunderstorms will be along a cold front pushing through the East. There may be one area of emphasis over and near the south-central Mississippi Valley on Sunday while additional areas of locally moderate to heavy rainfall may occur from the Great Lakes/Ohio Valley into New England around Monday. There could be some localized flooding impacts over New England given a suitable environment for high rainfall rates, but dry soils and the front being progressive may temper effects. Then from later Monday through Thursday, the stalling of the leading front and another one behind it over the Plains along with shortwaves aloft will provide focus for multiple areas of showers and thunderstorms/convective complexes across the northern half of the Plains and Mississippi Valley into the Ohio Valley and central Appalachians. Some rainfall may be heavy, with some added time required to refine the details of coverage and timing of highest totals. Tate/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 Southern Appalachians, the Central Appalachians, and the Ohio Valley, Wed-Thu, May 18-May 19. - Flooding possible across portions of the Upper Mississippi Valley and the Northern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Upper Mississippi Valley and the Northern Plains. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Sun-Tue, May 15-May 17. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Sun-Thu, May 15-May 19. - Excessive heat across portions of the Southeast, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Tue-Thu, May 17-May 19. 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