US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 520 PM EDT Thu Jul 22 2021 Valid Sunday July 25 2021 - Thursday July 29 2021 Hazards: - Heavy rain across portions of the Southern Rockies, the Central Great Basin, and the Southwest, Sun-Mon, Jul 25-Jul 26. - Heavy rain across portions of the Southeast, Sun, Jul 25. - Heavy rain across portions of the Mid-Atlantic, Mon-Tue, Jul 26-Jul 27. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, and the Southern Plains. - Flooding occurring or imminent across portions of the Southeast and the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Southeast and the Middle Mississippi Valley. - Flooding likely across portions of the Central Plains and the Middle Mississippi Valley. - Excessive heat across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Sun-Wed, Jul 25-Jul 28. - Excessive heat across portions of the Central Plains, the Upper Mississippi Valley, and the Northern Plains, Mon-Wed, Jul 26-Jul 28. - Excessive heat across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Tue-Thu, Jul 27-Jul 29. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sun-Mon, Jul 25-Jul 26. - Much above normal temperatures across portions of the Central Plains, the Northern Plains, the Southern Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Great Lakes, Sun-Thu, Jul 25-Jul 29. Detailed Summary: The medium range hazards forecast during the latter part of July (Sunday, July 25th - Thursday, July 29th) features the migration of lingering hot weather atop the interior Northwest to east of the Rockies and the Midwest. Meanwhile, the ongoing Southwest Monsoon remains active to close out the weekend, before rainfall coverage decreases in the Southwest early next week. Several rounds of thunderstorms press southeast from the Northeast and the Southeast Missouri Valley ahead of several fronts. Heavy rain chances increase over Florida as a surface trough in the Gulf of Mexico focuses thunderstorm development to begin next week. No changes have occurred regarding the Heavy Rain area over the Desert Southwest; Sunday falls toward the tail end of an active heavy rain period in the region. Higher rainfall amounts over an inch are likely towards the Arizona Mojave Desert as a retrograding upper low phases with deep Pacific moisture embedded in the area. Rainfall amounts begin to wane by Monday as the upper low propagates off the California coast and ridging consolidates over the Colorado Rockies, ultimately pulling the monsoon moisture north into the Great Basin. Areas containing inundated soils or burn scars will be especially sensitive to the heavy rainfall. It is worth noting that locations north of the Heavy Rain area could experience very localized burn scar flooding even with sub-hazard rainfall amounts midweek. Users residing in and close to sensitive areas are encouraged to monitor the local forecast for the latest developments. Otherwise, clouds and rain will provide relief from the usual desert heat, with afternoon temperatures 10-15 degrees below average possible through Wednesday over the Southwest and South-Central Great Basin. Rainfall also should assist with the prolonged drought conditions that blanket the majority of the region. North of the monsoon activity, hot weather will linger over the interior Northwest until Monday with high temperatures anomalies greater than 10 degrees forecast. By Tuesday, temperatures in interior Northwest will moderate to around seasonal as the upper ridge strengthens over the Central Plains, bringing hot temperatures east of the Rockies. Triple digit high temperatures resume within the Excessive Heat over eastern Montana into the Dakotas through Wednesday. A weak frontal system is forecast to pass through on Tuesday, but will do little to quell the heat overall. Given the recent impacts from extreme heat in the region, synoptic footprint of a strengthening upper ridge, and supporting ensemble probabilities of exceeding 100 degrees, the Excessive Heat area was extended until the 28th. Over the Central Plains the Excessive Heat area was expanded north to match increasing probabilities of exceeding 100 degrees, and trimmed in the south where the hot temperatures are less anomalous. Thunderstorms initiating along a front in the Northeast Missouri Valley could throw a wrench in the the heat that materializes, but specifics are unclear at the medium range. Elsewhere, the Excessive Heat area in the Ozarks/Southern Plains was remains until the 28th, where sweltering maximum heat indices of 110 are possible. Similar to the Central Plains, thunderstorm development may impact the heat that materializes but confidence in specifics is low. On the eastern periphery of the upper ridge, a broad area of thunderstorms is likely from the Northeast into the Ohio Valley ahead of a series of fronts Sunday into Monday, with uncertainty as to where higher totals set up. From then on, the bulk of the medium range thunderstorm activity is expected to shift along and east of the Mississippi River. Moderate to locally heavy rainfall totals are possible across the Mid-Atlantic into the Mississippi Delta Monday/Tuesday as thunderstorms develop ahead of a cold front. A Heavy Rain area was added across southern Virginia into eastern North Carolina amongst reasonable deterministic/ensemble agreement over an inch ahead of the front, corresponding with above average streamflows along the coast. The activity will push toward the coast later next week, but hazardous heavy rain is not anticipated. To the south of the frontal activity, Heavy Rain is possible across Florida on Sunday associated with a surface low/trough in the Gulf of Mexico. The National Hurricane Center is monitoring this feature for potential tropical development, noting a marginally conductive environment for development into early next week. Users are encouraged to monitor the National Hurricane Center for the latest forecast information regarding this feature. Looking into Thursday-Friday next week, a stalled front over the Upper Midwest focuses showers and thunderstorms across Wisconsin into the Great Lakes, but uncertainty regarding the amounts and placement of rainfall precludes a Heavy Rain area. For Alaska, rain showers are expected for the the Sunday ahead of a system over the Gulf of Alaska. Some higher amounts are possible along the West Coast of Alaska as a system approaches from the Aleutians toward the later half of the forecast period, but confidence is too low to introduce a Heavy Rain area. Asherman