US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 435 PM EDT Thu Jul 15 2021 Valid Sunday July 18 2021 - Thursday July 22 2021 Hazards: - Heavy rain across portions of the central Plains to the lower Mississippi Valley, as well as portions of the Mid-Atlantic, Sun-Mon, Jul 18-Jul 19. - Heavy rain across portions of the southwestern Texas, Tue-Thu, Jul 20-Jul 22. - Heavy rain across portions of the Mogollon Rim to the Gila Mountains in Arizona, Sun-Thu, Jul 18-Jul 22. - Flooding possible across portions of the mid-Mississippi Valley, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of Florida and the mid-Mississippi Valley. - Excessive heat across portions of the Snake River valley and northern Rockies, Sun-Mon, Jul 18-Jul 19. - Excessive heat across portions of the northern High Plains, Sun-Tue, Jul 18-Jul 20. - Much above normal temperatures across portions of the central and northern Great Basin, the northern to central Rockies, across the northern Plains, and into the upper Midwest, Sun-Thu, Jul 18-Jul 22. - Much above normal temperatures across portions of the central Rockies, northern Plains, and into the upper Midwest, Wed-Thu, Jul 21-Jul 22. - Heavy rain across portions of northwestern Alaska, Tue-Wed, Jul 20-Jul 21. Detailed Summary: During the medium range forecast period (Sunday, July 18th to Wednesday, July 22nd), an upper-level trough settling across the Great Lakes and the Northeast will extend a cold front southwestward into the central Plains. The front is forecast to gradually push through the central U.S., then into the East Coast and across the Deep South by the middle of next week. Model guidance continues to indicate the likelihood of moderate rainfall ahead of this slowly advancing front but with a fair amount of uncertainty regarding if and where low pressure waves will form and develop along the front. The development of these frontal waves will be important in determining if and where heavier rainfall and thunderstorms will be triggered. The highest chance for marginally heavy rain appears to be across portions of the Mid-Atlantic as well as from the central Plains to the lower Mississippi Valley on Sunday and Monday. Other areas of heavy rainfall are possible especially near where low pressure waves form near the front. By Tuesday and into midweek, models are indicating a good chance of heavy rain lingering across southwestern Texas as the front stalls across the area. Over the southern Rockies into the Desert Southwest, monsoonal moisture will keep a good chance of showers and afternoon/early evening thunderstorms through the medium-range period. Most areas should see rainfall below hazard levels but the upslope portion of the Mogollon Rim to the Gila Mountains in Arizona appears to be under the highest threat of heavy rain through the middle of next week when models indicate an increasing influx of the moisture from the south. Meanwhile, the heatwave is expected to continue from the central and northern Great Basin eastward across the northern Plains and into the upper Midwest under a persistent upper ridge through much of the medium range period. The highest temperature anomalies are expected over the northern High Plains, where high temperatures into the triple digits are forecast through midweek next week. Excessive heat is thus indicated for this area. Excessive heat is also indicted along the Snake River valley and parts of the northern Rockies Sunday and Monday when the heat is forecast to be most intense. In addition, much of the Dakotas, northeast Wyoming, and the western half of Minnesota are experiencing severe to exceptional drought that will likely worsen due to the persistent heat and lack of rain. Alaska will likely enter an extended period of wet weather toward the middle of next week as a series of occluded cyclones is forecast to steadily move across the Bering Sea toward Mainland. Latest model guidance has generally backed off regarding the predicted rainfall amounts across western Alaska with the associated upper trough sliding farther southeast to develop a low over the Gulf of Alaska. Only a small area of heavy rain is indicated over portions of northwestern Alaska Tuesday to Wednesday ahead of the arrival of the first cyclone. However, widespread moderate rainfall can be expected to make steady progress across much of Alaska through the middle of next week. Kong