Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue Jul 20 2021 Valid 12Z Fri Jul 23 2021 - 12Z Tue Jul 27 2021 ...Locally heavy rainfall threat in an active Southwest/Four Corners Monsoon pattern... ...Heat will continue over the northern/central Plains into the Upper Midwest... ...Overview... During the Friday-Tuesday period expect only modest variations of a similar theme for the mean pattern. Upper ridging should remain centered over the central Rockies/Great Basin into the weekend. A separate Gulf of Mexico ridge drifting northwestward may ultimately merge with the Rockies ridge to yield a high center over or near the central Plains by next week. Mean troughing will likely prevail over the Northeast Pacific and eastern North America. Features ejecting from the Pacific trough and rounding the northern periphery of the ridge will bring a couple fronts into the northern tier states but with only modest impact on the persistent heat over the region. To the south of the initial upper high, a weak upper low over the southern High Plains/Rockies along with other impulses should provide occasional enhancement of monsoonal rainfall over the Four Corners/Southwest and nearby areas. ...Guidance/Predictability Evaluation... Confidence remains above average for the expected large scale pattern while there are ongoing uncertainties/trends for some of the medium to smaller scale details that have lower predictability. An operational model blend of 12Z/18Z solutions provided a good representation of consensus Friday into the weekend. Guidance has finally converged better for details of the moderately deep trough over the Northeast on Friday and then lifting away--ultimately leading to greater progression of the main surface front into the Atlantic over the past couple days of runs. Meanwhile there is an ongoing trend for better defined low pressure near the southern Mid-Atlantic and/or Southeast coast but for now there is wide north-south spread (intermediate position favored at this time). Consensus has been more stable for the system crossing central Canada and the trailing front extending into the northern tier of the U.S. During the first half of next week there is decent model signal for an ejecting Pacific shortwave to support a southern Canada wave that brings another front into the northern Plains. The weak/progressive nature of the feature leads to a fair degree of model spread and run-to-run variability thus far, favoring an average of 12Z/18Z GFS and 12Z/old 00Z ECMWF runs along with the 18Z GEFS/12Z ECMWF means. The 12Z ECMWF strayed to the fast side of the envelope (new 00Z run slower like consensus) while the 12Z CMC was on the slow side. Meanwhile energy rounding the initial Rockies ridge will reach the eastern North America mean trough next week, bringing the associated cold front into the eastern states while a leading wave/front cross the northeastern quadrant of the lower 48. After a lot of spread/variability for the leading wave in recent days, guidance has lately stabilized upon a fairly modest depiction but differences persist for specifics. A GFS/ECMWF model/mean combination provides reasonable continuity for the main cold front. ...Weather Highlights/Hazards... Some areas of heavy rainfall will be possible on one or more days within the Southwest/Four Corners Monsoon. Currently expect the best potential for highest totals to be over central Arizona late this week. There is still uncertainty over day-to-day specifics given the low predictability of small scale impulses that will be important for resolving the location and timing of significant rainfall. Some areas will be very sensitive to additional rain given already wet soil from prior rainfall. Overall expect the core of greatest deep moisture and best rainfall potential to lift northwestward early next week. Elsewhere, an organized area of rain and thunderstorms should progress from the Upper Midwest through the Great Lakes and into the East, ahead of a Canadian/northern tier front and leading wave/frontal system. Some rainfall could be locally moderate to heavy. Areas of rain may accompany a wavy frontal system over the southern Mid-Atlantic/Southeast coast late this week into the weekend while Florida should see multiple episodes of showers/thunderstorms during the period. Northern/central portions of the Plains into the Upper Midwest will likely see persistent heat through the period with temperatures 5-15F above normal, including at least isolated Plains locations reaching 100F on one or more days. Fronts reaching the northern tier will provide a little variability from day to day but without any meaningful cooling trend. On the other hand the clouds and rainfall over the Southwest and vicinity will tend to keep highs there 5-15F below normal, with a slight northwestward progression of coolest anomalies possible by early next week. The upper trough over the Northeast late this week into the weekend may keep highs 5F or more below normal. Otherwise expect temperatures over most the East to be within a few degrees on either side of normal. Rausch 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml