Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Sun Jul 24 2022 Valid 12Z Wed Jul 27 2022 - 12Z Sun Jul 31 2022 ...Dangerous heat wave over the south-central U.S. to moderate some by midweek, while heat builds across the Northwest through the week... ...Heavy to excessive rainfall likely in parts of the central/southern Rockies and High Plains as well as the Ohio/Tennessee Valleys and central Appalachians... ...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 into the Tennessee/lower Ohio Valleys. 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 of energy within the trough, especially once an additional shortwave or two drops into the western side behind the main upper low around Friday, but most differences are well within reason for a medium range forecast. The first half of the WPC forecast used a blend of the 00/06Z deterministic guidance since there was good agreement overall. Models (particularly the ECMWF and GFS runs) agree reasonably well that troughing across the northeastern quadrant of the U.S. should linger but deamplify into next weekend as the upper low shifts northeastward well into Canada, with some minor variations from model to model. However, somewhat more impactful differences arise late this week into the weekend upstream with flow approaching and moving through the Northwest, which has implications for the heat wave there. GFS runs have persistently shown a shortwave tracking through the Pacific Northwest toward the northern Rockies over the weekend, while ECMWF runs take that Pacific energy and split off a small upper low lingering over the Pacific, leading to ridging remaining over the Northwest. CMC runs are perhaps more like the GFS but with an upper high extending much farther west into the Pacific south of the main flow with the embedded shortwaves. Though these differences are relatively small scale for a day 6-7 forecast, they could have implications for the temperature forecasts in the Northwest given they affect if the ridge persists. The WPC forecast attempts a middle ground approach with the shortwave not moving through as fast as the GFS runs, but not lingering it offshore like the ECMWF either--favored the ensemble means for the timing/track of this feature. Thus the WPC forecast used half deterministic models/half ensemble means by day 7. This maintained good continuity with the previous forecast. ...Weather/Hazards Highlights... The south-central U.S. has seen prolonged excessive heat in recent weeks, and this is forecast to last into the beginning of the medium range period for the southern half of the Plains and Mississippi Valley. Temperatures near or above 100 degrees (with higher heat indices) will continue in this region through at least Wednesday. The next front that reaches the northern Plains early next week looks to bring moderately below normal highs to the northern-central Plains Wednesday-Thursday, and should have a strong enough southward push to finally bring some moderating temperatures back towards normal for the southern Plains as well for the latter half of the week. Meanwhile, a building upper ridge over the Northwest this week will bring multiple days of hot weather, with some locations likely to see highs as much as 10-20F above normal. This equates to highs in the low to mid 100s for the Columbia Basin, and 90s are 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 hot temperatures there will moderate, 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 warm front draped from the Mid-Atlantic to the middle Mississippi Valley, but then again as the next front settles into the mid-South 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 expanded the Slight Risks across the central Appalachians and back into the Ohio and Tennessee Valleys and parts of the middle Mississippi Valley. Heavy rainfall will expand into the southern Mid-Atlantic and the central Plains later in the week as well, while additional rounds of convection continue to affect the Tennessee Valley. Persistent monsoonal moisture will 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, as indicated by the day 4 ERO. The moisture plume is also expected to stream into the central Plains front, enhancing rainfall totals farther north and east into New Mexico and Colorado midweek, with potential for flash flooding as well (and the presence of a slight risk on both the days 4 and 5 ERO). Tate/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