Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Fri May 13 2022 Valid 12Z Mon May 16 2022 - 12Z Fri May 20 2022 ...Heat threat for the Desert Southwest early next week while persistent southern Plains heat intensifies and expands eastward into the Southeast... ...Overview... Mean troughs over the East and West Coasts, with ridging in between, should make gradual progress eastward during the first half of next week. Upper ridging across the Southwest and south-central U.S. should spread into the Southeast the second half of the week in response to an amplifying trough over the West. This should continue to lead to a broad area of unseasonably hot weather across the southern U.S., with the most persistent and record-breaking heat centered in Texas and Louisiana. Meanwhile, expected 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 guidance continues to show fairly good agreement with the overall pattern during the extended range, but struggle with low-predictability shortwave details with the Northeastern trough, as well as energy moving through and ejecting from the Pacific to Western U.S. trough. Frontal positions and thus specifics of convective focus will be sensitive to these uncertain aspects which could take into the short range to resolve. The most notable differences seem to arrive around Wednesday with a leading shortwave ejecting from troughing over the northeast Pacific into the Northwest and eventually northern Plains. Up until the most recent 00z run (available after forecast generation time), the CMC had consistently been faster and more pronounced with this shortwave, while the GFS and ECMWF are weaker. Farther upstream, uncertainty surrounds the details of a closed low off the British Columbia coast which may move into the Northwest U.S. late next week. Likely in response to a stronger downstream shortwave, the 12z/May 12 CMC was significantly weaker and less amplified with this feature as it moves through the West but the 00z run is more consistent with the consensus. Ensembles and the ensemble means support a more amplified pattern across the U.S. by late next week as shown by both the GFS and ECMWF (which between themselves have some pretty significant timing and trough detail differences by day 6-7). The WPC blend for tonight used the deterministic models days 3-5, with increasing ensemble means late period to downplay the more uncertain details. Did continue some modest ECMWF and GFS just for some added definition to individual systems. This approach fit well with previous WPC continuity as well. ...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, with numerous daily record highs likely. Parts of the Southwest U.S. should see excessive heat early in the week, with a few records Monday-Tuesday, but temperatures should moderate as Pacific troughing moves in thereafter. Meanwhile by Tuesday or Wednesday the coverage of highs 10-15F above normal should extend eastward through the Lower Mississippi Valley and into the Southeast with again, 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 10-15F 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 lower 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. On Monday, the best focus for showers and thunderstorms will be along a cold front pushing through the East with the best chance 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 stalled boundary 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 are there for heavy rainfall, but plenty of uncertainty remains in the guidance with respect to coverage and amounts. 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