US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 513 PM EDT Thu Jul 08 2021 Valid Sunday July 11 2021 - Thursday July 15 2021 Hazards: - Heavy rain across portions of the Mississippi Valley, the Great Lakes, the Ohio Valley, the Tennessee Valley, and the Southern Plains, Sun, Jul 11. - Heavy rain across portions of the Lower Mississippi and Tennessee Valleys, Mon, Jul 12. - Heavy rain across portions of the Middle and Upper Mississippi Valley, Wed, Jul 14. - Flooding possible across portions of the Southeast and the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Southeast, the Middle Mississippi Valley, and the Southern Plains. - Flooding likely across portions of the Southeast and the Southern Plains. - Excessive heat across portions of California, the Interior Pacific Northwest, and the Southwest, Sun-Tue, Jul 11-Jul 13. - Excessive heat across portions of the Southwest, Sun-Mon, Jul 11-Jul 12. - Excessive heat across portions of the Great Basin and the Interior Pacific Northwest, Sun-Thu, Jul 11-Jul 15. - Much above normal temperatures across portions of the Northern Rockies and Northern High Plains, Sun-Mon, Jul 11-Jul 12. - Heavy rain across portions of southern mainland Alaska, Sun, Jul 11. Detailed Summary: Heavy rain in the central and eastern U.S. as well as the continued hot weather in the western U.S. highlight the hazards outlook for the medium-range forecast period (Sunday, July 11 - Thursday, July 15). At the beginning of the period, a low pressure system over the Middle Mississippi Valley is forecast to move slowly northward towards the Great Lakes on Monday (July 12th) before accelerating northward into Canada by Tuesday (July 13th). A warm front will extend east from the low through the Ohio Valley into the Mid-Atlantic and move northward through the Midwest and the Northeast, while a cold front will extend to the southwest through the Southern Plains and slowly progress east-southeastward. An upper-level trough will also be in place over the central U.S. Storms are expected to be ongoing along and north of the warm front on Sunday (July 11th) from the northern Ohio Valley into the southern Great Lakes as the warm front advances northward. High boundary layer moisture in place along and south of the front (low 70s dew points) will lead to heavy rain of 1-2 inches with these storms. Additional storms are forecast to form along the slow moving cold front to the southwest across the Middle Mississippi Valley into the Southern Great Plains. Mean southwesterly flow associated with the upper-level trough roughly oriented along the cold front will lead to the chance for training storms, increasing the potential for heavy rain totals. Widespread heavy rain of 1-2 inches is expected given the high boundary layer moisture in place ahead of the front, with the potential for locally higher totals of 2-4 inches in any areas that see repeated rounds of storms. The heavy rain threat is expected to continue on Monday as the cold front moves into the Lower Mississippi Valley northward through the Tennessee Valley and Middle Ohio Valley. Another 1-2 inches of rain is possible. Some model guidance suggests the potential for heavy rain to continue west along the trailing cold front across North Texas as well. However, given the lack of consistency for this solution and the forecast for the cold front to begin to weaken with western extent, opted not to extend this outlook area to the west for now. There is also a chance for heavy rain further into the Northeast on Monday, Tuesday, and even Wednesday (July 14th) as the warm front progresses northward. However, there is not enough model consensus on the location and intensity of the rainfall to include an outlook area at this time. As this first system moves to the northeast and the low pressure center exits into Canada, another system and cold front will move southeastward into the north-central U.S. from Canada on Monday into Tuesday. Some storms may be possible during this time frame but heavy rain is not currently expected. However, on Wednesday, a mesoscale convective system is forecast to develop along the leading edge of moist return flow from the south. There is some uncertainty on the exact track of the system and the potential location for associated heavy rainfall. However, given the strong signal in model guidance for heavy rainfall of 1-2 inches and higher, opted to include a broad area across the Middle and Upper Mississippi Valley to account for this potential which can be further refined in future outlooks. In the west, the persistent upper-level ridge will remain in place over the western U.S. through at least the first half of the forecast period. Both excessive heat and much above normal temperatures are forecast across the region. Excessive heat is expected across portions of the western and northern Great Basin, the Wasatch Front of Utah, and the Snake River Valley of Idaho, with highs in the mid-100s, as well as across portions of eastern Oregon and Washington, with highs in the mid- to upper 90s, through the first part of the forecast period. High temperatures in the mid-90s to 100 degrees will continue Tuesday through Thursday. Excessive heat is also forecast for the Mojave Desert, the San Joaquin Valley/central California, the Sacramento Valley/northern California, and interior southwestern Oregon from Sunday into Tuesday. High temperatures in the 110s are likely for the Mojave Desert, San Joaquin valley, and southern Sacramento valley, with highs around 100 further north. Highs in the 110s are also forecast for the rest of the interior southern California deserts and southwestern Arizona Sunday and Monday. A weak shortwave trough may provide some relief from the hotter temperatures by Wednesday for northern California and southwestern Oregon. High temperatures will likely return closer to normal as well in southern California and Arizona as the ridge begins to shift to the northwest and weaken on Tuesday. Above normal temperatures in the mid- to upper 90s, and possibly as high as the low 100s, are forecast for the Northern Rockies and High Plains of Montana on Sunday as the upper-level ridge expands northeastward between passing troughs. These above normal temperatures will spread further southeastward across the Northern High Plains of northeastern Wyoming and the western Dakotas on Monday. Although not all locations will see their temperatures peak through the extent of the outlook periods, opted to keep areas a bit broad both location and time wise given the oppressive heat seen over the west the last few weeks. Even a moderate cool down will be of little relief to areas that have seen excessive heat and above normal highs for an extended time. Widespread extreme to exceptional drought across the west is also a concern with the continued hot weather. In Alaska, an occluding low pressure system will move east across the Gulf of Alaska on Sunday. Moist onshore flow ahead of the system will lead to heavy rain totals of around 2 inches from the Lower Cook Inlet east to the Kanai peninsula and Prince William Sound. This heavy rain may continue into Monday but some differences in the amounts and location in the model guidance preclude an extension of the outlook area at this time. Additionally, a low pressure system and associated cold front will move southward across the northern/eastern mainland Wednesday and become stationary in the eastern mainland by Thursday. Heavy rain is possible along and ahead of the system across the interior on Wednesday and into Southcentral on Thursday. Additionally, well below normal temperatures are also possible. Outlook areas for both heavy rain and below normal temperatures may be introduced if current model trends continue. Putnam