Extended Forecast Discussion NWS Weather Prediction Center College Park MD 443 PM EDT Fri May 13 2022 Valid 12Z Mon May 16 2022 - 12Z Fri May 20 2022 ...Persistent record-breaking heat is likely in the south-central U.S. next week with periods of anomalous heat for the rest of the southern tier as well.. ...Overview... The medium range period will feature upper troughing in the east-central U.S. moving eastward and lifting with time, while a series of shortwaves and eventually deeper troughing late next week come into the Northwest. Between and south of these trough features, ridging will promote unseasonably hot weather across the southern U.S., with the most persistent and record-breaking heat centered in Texas and Louisiana. Meanwhile, expect showers and thunderstorms to accompany an initial front across the East, with increasing showers also expected along stalling fronts in 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 model guidance remains in good agreement with the overall pattern described above through much of the medium range period, but with continued discrepancies in the smaller scale details like shortwaves embedded in the Northeast trough and those moving through the Northwest. Guidance has been waffling but has shown a general trend toward a bit more separation with the Hudson Bay upper low and troughing/a potentially closed low farther southeast closer to New England around Tuesday, with some separation lasting into Wednesday. In the Northwest, the 00Z UKMET appeared too aggressive/displaced with a shortwave in the northwestern to north-central U.S. by Wednesday. Issues like both of these can affect frontal positions and thus specifics of convective focus will be sensitive to these uncertain aspects which could take into the short range to resolve. But the overall pattern was agreeable enough that the first half of the WPC forecast used a blend of the 00Z/06Z deterministic guidance, though eliminating the UKMET by Wednesday. By Thursday-Friday, there are some relatively larger differences with the pattern, though still within reason for a day 6-7 forecast. Models vary on the process of dislodging the upper low just west of British Columbia to produce deeper troughing across the western U.S., with questions of maintaining a closed low or opening up a trough as the energy digs and the depth to which it digs, but at least there is agreement the trough will amplify in the West. Meanwhile, the Hudson Bay upper low shows some significant differences in its track/position, with the 00Z ECMWF and now the 12Z CMC indicating it could retrograde west while other models maintain its position or slowly move it east. While this is north of the CONUS, it affects the north-central U.S. with potential for troughing or ridging and frontal position, as well as the possibility it could interact with the western trough/low. Did not favor the models with the most interaction (like the 00Z ECMWF) at this time, but this will continue to be monitored. The latter part of the WPC forecast phased in some GEFS and EC ensemble means to temper individual model differences but still maintained about 60 percent deterministic models to keep sufficient strength of systems. ...Weather/Threat Highlights... Initial ridging across the Southwest and southern Plains and then eventually rising heights farther east over the Deep South will 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, with numerous daily record highs likely. Parts of the Southwest into the central Great Basin and southern/central Rockies can expect highs around 10-15F above average for the first half of the week before a cooling trend as the deeper troughing presses inland. However, warm to hot temperatures could increase for the southeastern quadrant of the U.S. later next week, with potential for daily record highs. 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 10-20F below normal over the Northwest/northern Rockies by next Thursday-Friday. Snow levels should be quite high early next week and then begin to lower as the main upper trough arrives. It could 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 initial and supporting shortwaves. Areas to the east of the Rockies should generally see two rainfall regimes during the period. On Monday, the best focus for showers and thunderstorms (some of which could be strong to severe) will be along a cold front pushing through the East with the best chances for locally moderate to heavy rainfall across New England. Localized flooding impacts over New England are possible given a suitable environment for high rainfall rates, but dry soils and the front being progressive tempers confidence. Into the middle part of the week, a couple of frontal boundaries from the Plains to the Ohio Valley 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. Ingredients will be in place for heavy rainfall, but plenty of uncertainty remains in the guidance with respect to coverage and amounts. Tate/Santorelli 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 Ohio Valley, and the Tennessee Valley, Wed, May 18. - 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. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Mon-Tue, May 16-May 17. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Mon-Fri, May 16-May 20. - Excessive heat across portions of the Southeast, the Lower Mississippi Valley, and the Tennessee Valley, Tue-Fri, May 17-May 20. - High winds across portions of the Southern Rockies, the Central Great Basin, the Southwest, California, and the Central Rockies, Thu, 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