Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Mon Jun 20 2022 Valid 12Z Thu Jun 23 2022 - 12Z Mon Jun 27 2022 ...Expansive area of excessive heat likely to persist across the South into the weekend... ...Overview... A broad upper level ridge parked over the Southern U.S. through the weekend will bring an extended period of excessive heat and record temperatures to many locations across the region. To the north, a modest trough initially anchored by a southern British Columbia upper low will ride around the top of the ridge from the Northwest to the Midwest, eventually amplifying as it reaches the Great Lakes/Northeast later in the period. This amplification should finally help to break down the massive Southern heat dome. Monsoonal moisture will likely persist over the Four Corners region through much of the period and could contribute to heavier rainfall along the southern half of the Rockies by the weekend with the arrival of a surface front and trailing low level upslope flow. ...Guidance/Predictability Evaluation... There is decent agreement on the overall pattern in the latest model guidance, especially regarding a persistent and expansive upper ridge over the Southern U.S. The most uncertain parts of the forecast continue to be with the overall trough rounding the top of the upper ridge (leading to timing differences with the leading frontal system) and details within the area of mean troughing from New England and the Canadian Maritimes through the western Atlantic. For the northern stream trough, guidance through the 00Z/06Z cycles was split between the slower GFS/CMC and faster ECMWF/UKMET. The GEFS mean has trended somewhat faster over the past day or so, closer to the ECMWF mean, so that hints toward nudging a little more toward the ECMWF cluster. Among the new 12Z runs, the CMC has adjusted a little faster to leave the GFS more in the minority. However the 12Z ECMWF has adjusted a bit slower. Meanwhile the models already diverge significantly over the Canadian Maritimes/East Coast/western Atlantic area in the short range, with considerable spread and run-to-run variability for where one or more embedded upper lows may be from the start of the period (early Thursday) onward. Needless to say, predictability is quite low over this region and confidence in any specific solution is likewise very low, thus favoring a conservative blend approach. Finally, a model/ensemble mean compromise appears best for the upper low that may approach the Pacific Northwest and southern British Columbia by early next week. The upper low moves along rather slowly for most of the period under Alaska/northwestern Canada ridging, so the latest GFS runs and 00Z ECMWF could become a tad fast by the end of the period. CMC/GEFS mean runs through the 12Z cycle and the 00Z ECMWF mean favor somewhat slower progression. Considerations for various features led to starting the first half of the forecast with a mostly 00Z/06Z operational model blend incorporating the ECMWF/GFS/UKMET/CMC from highest to lowest weight and a little 06Z GEFS/00Z ECens input. Then the starting blend quickly transitioned to half models/half means by days 6-7 Sunday-Monday. ...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 into the weekend. Locally heavy to excessive rainfall is possible, particularly over sensitive burn scar areas and considering the multiple day repeating nature of this activity. Rainfall may trend heavier over parts of Colorado/New Mexico by the weekend as a front dropping south through the region and trailing low level upslope flow could interact with the moisture already in place. A showery/stormy pattern could also set up through Friday in parts of the northern half of the Plains/Mississippi Valley with the combination of a cold front pushing into the area from the west and leading warm front lifting northeastward. Rainfall totals will depend in part on still uncertain frontal timing. Progression of the Plains cold front should increase coverage of rain/thunderstorms over the East by the start of next week. Ahead of this feature, the East may also see episodes of rain/storms of varying intensity late this week with a leading wavy front as well as a surface trough to its south. East Coast rainfall details will depend on uncertain specifics of features within the nearby mean trough aloft. Likely the biggest threat of the medium range period will be a broad area of potentially record-breaking heat across southern parts of the Plains into the Southeast. Expect daily record max or min temperatures be set within this area, including 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, mainly Thursday into Saturday. The South should finally see a moderating trend by the weekend and especially early next week as a leading front dips into the Southeast and then the late week Plains front reaches into the South and East. 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 likely to be most persistent over New Mexico given the monsoonal clouds and rain coverage. The northern High Plains may also see cool highs from Friday into the weekend, dropping into Colorado from the weekend into early next week with southward cold front progression. The West Coast states should see highs 5-15F above average. Rausch/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