Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sun Jun 19 2022 Valid 12Z Wed Jun 22 2022 - 12Z Sun Jun 26 2022 ...Expansive area of excessive heat likely to persist across the South into next weekend... ...Overview... A broad upper level ridge centered over the Lower Mississippi Valley midweek and drifting westward into the southern Plains for the latter half of the week will be the most prominent feature during the medium range period and will cause widespread excessive heat across much of the South. To the north, expect weak and progressive systems to ride the top of the ridge through the Northwest to Midwest, while troughing is likely for both the East and West Coasts. Monsoonal moisture will likely continue through much of the week across the Four Corners region, and some showers and storms are also possible in the north-central U.S. and the East Coast. ...Guidance/Predictability Evaluation... Good model agreement persists with the ridge/upper high anchored across the South, but models show generally more variability with troughing features surrounding the ridge. The most uncertain part of the forecast continues to be the possibility for energy to cut off and create a closed low over the western Atlantic, which could then retrograde toward the East Coast during the latter part of the week. There is still considerable spread with recent model guidance regarding placement of the low possibly embedded in broad troughing--GFS runs remain the fastest/farthest north with the upper low getting absorbed back into the mean flow over the Northeast before Friday, while most other guidance showed the low lingering farther south into next weekend. At the time of forecast creation, the 00Z ECMWF, CMC, and UKMET along with many EC ensemble members and a few GEFS ensemble members all agreed upon a farther south position of the low (though the CMC was considered too far south, with other guidance near the Carolinas Fri-Sat while the CMC was near Florida). The WPC forecast was closer to this cluster of guidance, though tempered by favoring the EC and GEFS ensemble means especially later in the period, which were weaker due to increased variability in individual member solutions. However, after three full model cycles indicating the closed low potential, the 12Z model suite that has arrived so far has notably backed off on the possibility of an upper low that far south--the 12Z ECMWF, UKMET, and CMC look closer to the Northeast solution that the GFS has been showing, though maintaining troughing longer there (especially the CMC). The feature has very low predictability given it is reliant on energy distribution within a narrow trough, and some energy could stem from features as small as MCSs and MCVs, so future refinements in the forecast are likely. Other systems such as a compact upper low near California Wed-Thu with lingering weaker energy moving eastward somewhat, as well as a northern stream low moving across southern Canada with troughing into the northwestern to north-central U.S., fortunately show better agreement that could be handled with a multi-model blend of guidance. ...Sensible Weather/Threat Highlights... Anomalously high monsoonal moisture for this time of year, likely tied at least in part to eastern Pacific tropical systems, will continue streaming into the Four Corners states through midweek and beyond. Locally heavy to excessive rainfall is possible, particularly over sensitive burn scar areas and considering the multiple day repeating nature of this activity. A showery/stormy pattern could also set up through Friday in central parts of the Plains and Mississippi Valley as a front becomes aligned west-east, with some potential for rain and storms to train in some areas. Rain activity is then expected to focus farther north across the northern Plains to Midwest by late in the week in the vicinity of a cold front. Farther east, moisture is forecast to stream ahead of a frontal system across much of the East Coast Wednesday-Thursday. Then how much rain lingers across the East Coast will be dependent on the aforementioned upper low/trough feature's existence and track as well as a possible frontal boundary at the surface. Likely the biggest threat of the medium range period will be a broad area of potentially record-breaking heat across parts of the Plains eastward lasting through the week. The southeastern quadrant of the country is most likely to see record max or min temperatures be set, with highs exceeding 100F and lows well into the 70s meaning little relief overnight in many areas. Temperatures 10-15F above normal could stretch farther north into much of the Plains and Mississippi and Ohio Valleys at times as well. But temperatures along the East Coast show the most forecast spread, and will be dependent on the upper pattern. Relatively cooler temperatures with well below normal highs are forecast particularly for New Mexico given the monsoonal clouds and rain coverage, while near normal temperatures stretch through the Rockies, and the West Coast states could be a few degrees above average. 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