Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Tue Jun 21 2022 Valid 12Z Fri Jun 24 2022 - 12Z Tue Jun 28 2022 ...Expansive area of excessive heat likely to persist across the South through Saturday... ...Overview... A broad upper level ridge parked over the Southern U.S. when the period begins Friday, will continue to bring an extended period of excessive heat and record temperatures into the weekend, but should finally begin to weaken and break down early next week. To the north, a modest trough 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 acting to erode 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... Models and ensembles continue to show good overall agreement on the pattern the first half of the period, when the focus will be on a persistent and expansive upper ridge parked over the Southern U.S.. The most uncertain part of the forecast continues to be with details of the trough rounding the top of the ridge and individual pieces of energy contained within acting to eventually amplify the trough as it moves through the Great Lakes and the Northeast. These kinds of small scale uncertainties play a role in the timing of the leading frontal system through the northern tier states. Other areas to monitor include the details of initial closed low/troughing off the Northeast coast late this week (though models seem to be coming to a consensus on this compared to previous days), as well as a large closed upper low approaching the Northwest coast late in the period. The WPC progs relied on a blend of the deterministic guidance for the first half of the period, which seemed to mitigate any small scale detail differences which make take until the short range to resolve. By days 6 and 7, the preference is towards the ensemble means and the CMC which are most middle ground between the faster ECMWF and slower GFS with the main trough through the Great Lakes and Northeast. ...Sensible Weather/Threat Highlights... Anomalously high monsoonal moisture for this time of year, likely tied at least in part to an eastern Pacific tropical system, will continue streaming into the Four Corners states into early next week. 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. A front settling across the southeast Friday-Saturday should also bring a period of enhanced rainfall to parts of the northern Florida peninsula. 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 Friday 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. 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