Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM 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 anomalous heat expanding into the East.. ...Overview... Upper troughing should shift out of the East Tuesday-Wednesday early in the medium range period, with broad ridging to zonal flow behind. The pattern is expected to amplify later in the week as upper troughing digs in the West, yielding cooler temperatures and lowering snow levels in a round of precipitation spreading across the Northwest. Stubborn ridging across the South will continue to promote unseasonably hot weather, with the most persistent and record-breaking heat centered in Texas and Louisiana. Warmer than normal temperatures are forecast to spread eastward as the ridge amplifies ahead of the trough in the West. The best focus for showers and thunderstorms should be along a quasi-stationary boundary from the central Plains into the Ohio Valley, while some rounds of rain are also possible in the north-central U.S. and shower/thunderstorm chances will increase in Florida late next week. ...Guidance Evaluation/Preferences... Recent model guidance remains in mostly good agreement with the overall pattern at least through the middle of next week, though uncertainty in the details lingers. Despite some small scale shortwave differences within the eastern trough, guidance continues to show it lifting out of the U.S. on Wednesday with some separation from a deep closed low over the Hudson Bay that looks to linger for several days next week. Models still vary with the potential for another shortwave traversing near the U.S./Canada border on Wednesday into Thursday, with the 00Z GFS the strongest/deepest and the 00Z UKMET on the deeper side as well. The 06Z and 12Z GFS runs backed off on this feature though. Though these shortwaves are small scale, they impact positions of the fronts 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 led by the 00Z ECMWF, which seemed most consistent across the board. By around day 5/Thursday, overall models agree that the upper low spinning off the British Columbia coast through the short to early medium range period should shift southeast into the Pacific Northwest, or at least the energy from it will--models are quite variable with the details of embedded upper lows within the trough being created by this digging energy, which is impacted by potential interaction with the Hudson Bay low as well. CMC runs have shown a more shallow trough that is faster to move eastward compared to GFS and ECMWF runs. The northern Plains is a particularly uncertain region late week as far as a ridge or trough in the upper flow. The WPC forecast for the latter part of the period phased out the CMC in favor of the GEFS and EC means, turning into about an even blend of the 00Z ECMWF/EC mean/06Z GFS/GEFS mean by day 7, which maintained good continuity from the previous WPC forecast. ...Weather/Threat Highlights... Initial ridging across the 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 10F above average into midweek 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, and warmer than average temperatures by 10-20F should expand in coverage across the Ohio Valley to Eastern Seaboard by the end of the week. A warm front lifting through the Mid-Mississippi Valley and Ohio Valley will provide a 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, though the specifics of coverage and amounts are uncertain, but at least a localized flood threat is likely especially if heavy rain falls in areas that are sensitive to additional rainfall. Rounds of likely lighter rainfall are possible across the north-central U.S. through the latter half of the week as well. After a couple of initial fronts crossing the Northwest, the main well-defined front ahead of the deepening trough will bring unseasonably cool temperatures to the Northwest and expanding into the northern Rockies and High Plains by late week. This will also provide support for a notable round of precipitation, with lowering snow levels in the Cascades to Northern Rockies as the week progresses, with some chance of snow even into northern parts of the High Plains, though with very low confidence at this point. 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 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