Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Sat May 14 2022 Valid 12Z Tue May 17 2022 - 12Z Sat May 21 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... Upper troughing should be exiting the East early next week, with uncertain shortwave energy sliding through the Great Lakes and Northeast around Thursday-Friday. After this, models suggest the pattern over the CONUS may again amplify as troughing digs into the West and upper ridging expands into the East. Meanwhile to the south, strong and stubborn ridging will continue to promote unseasonably hot weather across the Southern U.S., with the most persistent and record-breaking heat centered in Texas and Louisiana. The best focus for showers and thunderstorms should be along a quasi-stationary boundary from the central Plains into the Ohio Valley, while the West sees 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 mostly good agreement with the overall pattern during the medium range period, but with plenty of uncertainties surrounding smaller scale features and evolution of the trough out West later in the period. Timing of the trough out of the East on Tuesday has improved, with models suggesting more separation between this and a deep closed low over Hudson Bay which looks to linger for several days next week. Even though the bulk of this feature is well north of the CONUS, it should play a role in the evolution of the pattern upstream over the West. Models are split on how strong a leading shortwave will be across the Northern tier Wednesday-Friday with the GFS and UKMET strongest, and ECMWF and CMC fairly weak. Guidance shows some varying run to run continuity suggesting low confidence on the specifics of this feature which has important implications on frontal positions and rainfall/convection. Despite the differences, it seems the best approach was a middle ground so the WPC forecast for days 3-4 was based on composite blend of the deterministic models, though with more emphasis towards the ECMWF which seemed most consistent across the board. Big uncertainties begin as early as day 5 with a closed low off the British Columbia coast which looks to dig into the Western U.S. late next week. The 12z/yesterday CMC was significantly weaker/less amplified with this trough through the Northwest with more eventual interaction/phasing with the low over Hudson Bay, but the newest 00z run (available after forecast production time) came in a bit more amplified and closer to the consensus. Recent runs of the GFS and ECMWF (along with the ensembles) show a more amplified pattern across the CONUS by next Saturday, but differ in the details of how to get there. WPC leaned on the ensemble means to help temper these smaller scale differences, but with at least half GFS/ECMWF to help with system definition. ...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. A warm front lifting through the Mid-Mississippi Valley and Ohio Valley will provide focus for multiple areas of showers and storms/convective complexes during the middle to latter part of next week. The ingredients are in place for heavy rainfall somewhere, but plenty of uncertainty remains in the guidance with respect to coverage and amounts, although it seems likely this should present at least a localized flood threat given recent heavy rains in some of these areas. 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 lower as the week goes on as the main upper trough arrives. A leading well defined front will press east across the region, with rainfall likely arriving into parts of the north-central Plains by next weekend. Santorelli/Tate 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