Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 AM 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 next weekend... ...Overview... A broad upper level ridge parked over the Southern U.S. through next weekend will bring an extended period of excessive heat and record temps to many across the region. To the north, weak and progressive systems will ride the top of the ridge through the Northwest to the Midwest, eventually amplifying as it reaches the Great Lakes/Northeast later in the period, which should help to finally break down the massive Southern heat dome. Monsoonal moisture will also likely continue through much of the period across the Four Corners region. ...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 continues to be with individual systems rounding the top of the ridge, most of which are detail differences and typical run to run variability. On the East Coast, models have trended away from a closed retrograding low towards the East Coast, rather favoring the low being drawn northward and pushed eastward into the Canadian Maritimes next weekend. There's still quite a bit of uncertainty with this feature and variability leading to a very low confidence forecast. Regardless, models do agree on some degree of elongated troughing along or off the East Coast through the period. Timing has improved with a shortwave into the Northwest U.S. and skirting the U.S./Canadian border Friday into Saturday. The CMC is slower than the ECMWF/GFS which becomes most apparent by Sunday as the feature tries to amplify some over the Great Lakes. Good enough agreement the first half of the period allowed for a multi-model blend which helped mitigate some of the differences across the northern tier and East Coast. By Sunday, incorporated more of the ensemble means along with the ECMWF and GFS which were closest to the ensemble means. ...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 next weekend. 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 on 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. Tempeartures should finally moderate across the South by next weekend and especially early next week as the upper pattern begins to amplify again. 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. 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