Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Thu Jul 28 2022 Valid 12Z Sun Jul 31 2022 - 12Z Thu Aug 04 2022 ...Dangerous heat wave over the Northwest/Great Basin persists through Sunday before the heat shifts east across the northern tier states next week... ...Excessive rain threat from the Tennessee Valley into the southern Mid-Atlantic into Monday, with continued monsoonal flooding in the Southwest... ...Overview... The upper level pattern by late Sunday will be a ridge centered over the Intermountain West, an upper low off Oregon, and broad troughing over the Great Lakes south of a large gyre over the northern Hudson Bay. This pattern will slowly shift east through next week (with the PacNW low eventually opening and ejecting east) as the Hudson Bay gyre remains in place. Heat under the upper ridge axis will shift east (though timing is uncertain) ending the dangerous heat wave in the Northwest, but result in hot days across the northern states through next week. Ongoing excessive rain threats from the Tennessee Valley to the southern Mid-Atlantic will persist into Monday along a warm front before it lifts north. The pivoting upper ridge out west will maintain the Southwest monsoon through the middle of next week with daily localized flooding concerns. ...Guidance Evaluation/Predictability Assessment... While the overall flow pattern is in good agreement, uncertainty remains with the movement of an upper low that develops well off the PacNW coast west tomorrow. The GFS remains the most progressive, bringing the low to the Oregon coast Sunday night. This was in somewhat agreement with the 00Z UKMET. However, the particularly strong ridge over the north-central Pacific (600dm heights on Day 4) and the ridge over the Intermountain West favor keeping the low offshore which has been featured by the EC/CMC for a few days. The 12Z GFS/UKMET continue a slower trend, offering credence to a model preference toward the EC/CMC camp with this feature. The key with the timing of this trough is the timing of the heat wave cessation over the Inland Northwest (and the timing of the heat max farther east). However, the 00Z (and now 12Z) EC keeps the low offshore into Wednesday which becomes a slow outlier by Day 5. Overall, troughing is likely to persist along the PacNW coast into the middle of next week, even if it is a reinforcing trough, not just one upper low. A general blend of the deterministic guidance favoring the EC/CMC was used for Days 3/4, then more of a general model blend with increasing incorporation of the 06Z GEFS/00Z ECENS means was used for Days 5-7. ...Weather/Hazards Highlights... The heat wave in the Northwest/Great Basin continues through Sunday night with the heat axis shifting east ahead of an upper trough approaching the Pacific Northwest Monday. However, this relief may be delayed as latest indications are that the upper trough may be delayed. Eventually, the anomalous heat shifts east east across the northern and central Plains through the first half of next week where daytime highs could once again approach or exceed 100F. Temperatures should also be on the rise from the Ohio Valley into the Northeast as heights begin to build through the middle of next week. Meanwhile, slightly above normal heat and humidity continues south of a frontal system over the mid-section, continuing the record hot summer for much of the southern Plains and Southeast through the next week. A quasi-stationary boundary remaining over the Mid-South to the southern Mid-Atlantic by Sunday morning before lifting north as a warm front later Sunday. However, heavy rain remains a threat from parts of the Tennessee Valley into the southern Mid-Atlantic where Slight Risks of excessive rain remain for Day 4. The threat for continued rain over the greatly impacted Tennessee Valley and western slopes of the southern Appalachians into Monday warrants another Slight Risk for Day 5. There is potential for an upgrade to the flash flood risk especially for parts of the Tennessee Valley and south-central Appalachians particularly for Sunday after great impacts already and continued heavy rain threats in the short range. Persistent monsoonal moisture will continue to support diurnally favored heavy showers/storms with locally excessive rainfall over an already very soggy Southwest through the middle of next week. A progressive cold front through the Great Lakes on Sunday may bring some moderate to locally heavy showers as well. Jackson Hazards: - Heavy rain across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Southern Rockies, the Southern Plains, and the Southwest, Sun-Thu, Jul 31-Aug 4. - Heavy rain across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Sun, Jul 31. - Heavy rain across portions of the Southern Appalachians, the Central Appalachians, the Ohio Valley, and the Tennessee Valley, Mon, Aug 1. - Flooding possible across portions of the Southern Appalachians, the Mid-Atlantic, the Central Appalachians, the Ohio Valley, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Northern Plains. - Excessive heat across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sun, Jul 31. - Excessive heat across portions of the Southeast, the Mid-Atlantic, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Sun-Thu, Jul 31-Aug 4. - Excessive heat across portions of the Central Plains, the Lower Mississippi Valley, the Northern Plains, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley, Mon-Thu, Aug 1-Aug 4. - Excessive heat across portions of the Central Plains, the Upper Mississippi Valley, the Middle Mississippi Valley, the Northern Plains, and the Northern Rockies, Mon-Wed, Aug 1-Aug 3. 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