Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Jul 25 2022 Valid 12Z Thu Jul 28 2022 - 12Z Mon Aug 01 2022 ...Dangerous heat wave over the south-central U.S. to moderate by medium range, while heat builds across the Northwest into next weekend... ...Excessive rainfall likely for parts of the central Appalachians into Thursday, with heavy rainfall possible also extending as far west as the central/southern Rockies... ...Overview... A west-east oriented ridge will persist across the southern and western U.S. through next weekend as somewhat amplified troughing slides across the Midwest/Ohio Valley and into the Northeast. This energy/flow pattern as well as a surface front meandering across the eastern/central U.S. should finally bring some modest relief to the heat across the south-central Plains, but excessive heat across the Northwest could continue through the end of the week. A multi-day period of heavy to excessive rainfall remains likely in places along this boundary, particularly the central Appalachians, but also farther west into the Tennessee/lower Ohio Valleys and central Plains. Additionally, enhanced monsoonal moisture lasting from the short to medium range period will continue to promote daily risks of locally heavy showers and storms that may produce instances of flash flooding in parts of the Southwest and southern/central Rockies and High Plains. ...Guidance Evaluation/Predictability Assessment... The latest model and ensemble guidance remains in good agreement with the large scale pattern of expansive and persistent ridging from the Northwest to the southern half of the U.S., with a more consolidated trough (stemming from a southern Canada upper low) digging across the Upper Midwest/Great Lakes around midweek and shifting into the Northeast by next Friday-Saturday. There are some model differences regarding the distribution and timing of energy within the trough, but most differences are well within reason for a medium range forecast. The first half of the WPC forecast was able to utilize a blend of the latest deterministic guidance. Beyond day 5/Saturday, models agree reasonably well that troughing may linger across the Northeast into next weekend, just in a much less amplified fashion, as various shortwaves act to reinforce the trough. The main upper ridge axis across the Northwest should shift east in favor of a couple of weak shortwaves entering the Pacific Northwest by around Saturday-Sunday with varying differences in the details. A stronger shortwave may move into the Northwest again early next week. Given the increased uncertainties, the WPC blend transitioned towards majority ensemble means to help mitigate the differences. This approach maintains good continuity with the previous forecast as well. ...Weather/Hazards Highlights... The south-central U.S. has seen prolonged excessive heat in recent weeks, and this is forecast to finally moderate back towards normal temperatures by the start of the medium range period as a cold front settles across the south-central U.S.. Meanwhile, a building upper ridge over the Northwest this week will bring multiple days of excessive heat, with some locations likely to see highs as much as 10-20F above normal. This equates to temperatures in the low to mid 100s for the Columbia Basin, with 90s expected to reach as far west as Portland and Seattle. Some daily records for highs/warm lows could be set across interior portions of the Northwest. There is some forecast uncertainty with the timing of when the temperatures will moderate again, but current forecasts show some lowering of the temperatures, though still above normal, by next weekend, while temperatures of about 10F above normal shift into the northern Plains. The latest guidance continues to suggest increasing potential for a multiple day period of heavy to possibly excessive rainfall initially along a front dropping into the Mid-Atlantic and extending back into the middle Mississippi Valley and central Plains. Sufficient moisture and instability pooled along this boundary as it becomes quasi-stationary should support rounds of heavy rainfall and convection across some of the same regions over multiple days. The days 4 and 5 experimental EROs have expansive slight risks depicted along this boundary from the central Appalachians to the central Plains. A moderate risk was introduced across portions of West Virginia and far eastern Kentucky on the day 4 ERO which covers the potential for continued heavy rainfall over an already soggy area likely to receive heavy rainfall in the short range as well. Activity along this boundary may sink slightly southward with time, continuing through next weekend for some regions in especially the Tennessee Valley. Given several days of rainfall over the same area, this likely will raise concerns for flooding as flash flooding. Rainfall should slide across portions of the Northeast ahead of a more progressive cold front on Thursday, though linger across portions of the southern mid-Atlantic for a couple of days. Persistent monsoonal moisture will continue to support diurnally favored showers/storms with some locally heavy rainfall over the Four Corners states through this week. Across parts of Arizona in particular, anomalous moisture lingering over likely wet soil from activity in the short range period could lead to additional flash flooding there, with another uptick expected on Friday and prompting the introduction of a slight risk on the day 5 ERO. The moisture plume is also expected to interact with the central Plains front, enhancing rainfall totals farther north and east into New Mexico and Colorado midweek, with increasing potential for flooding and flash flooding. Slight risks remain in place for the days 4 and 5 EROs in this area. 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