US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 434 PM EDT Fri Jul 09 2021 Valid Monday July 12 2021 - Friday July 16 2021 Hazards: - Heavy rain across portions of the Lower Mississippi Valley and Tennessee Valley, Mon, Jul 12. - Heavy rain across portions of the Middle Missouri Valley and the Middle and Upper Mississippi Valley, Wed, Jul 14. - Heavy rain across portions of the Central Plains, Middle Mississippi Valley, Ohio Valley, and Great Lakes, Thu-Fri, Jul 15-Jul 16. - Flooding possible across portions of the Northeast, the Middle Mississippi Valley, the Mid-Atlantic, the Upper Mississippi Valley, the Southeast, the Great Lakes, and the Ohio 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 the Interior Pacific Northwest, Mon-Wed, Jul 12-Jul 14. - Excessive heat across portions of California, the Southwest, the Central and Southern Great Basin, and the Central Rockies, Mon-Tue, Jul 12-Jul 13. - Excessive heat across portions of the Central and Northern Great Basin, Mon-Fri, Jul 12-Jul 16. - Much above normal temperatures across portions of the Northern High Plains, Thu-Fri, Jul 15-Jul 16. Detailed Summary: Heavy rain in the central and eastern U.S. as well as the prolonged heatwave in the western U.S. highlight the hazards outlook for the medium-range forecast period (Monday, July 12 - Friday, July 16). At the beginning of the period, a low pressure system over the Middle Mississippi Valley is forecast to move northeastward into Canada by Wednesday (July 14th). A warm front will extend east from the low across the Lower Great Lakes into the Mid-Atlantic and move northward through the Upper Great Lakes and Northeast, while a cold front will extend to the southwest through the Middle and Lower Mississippi Valley into Southern Plains and slowly progress eastward. An upper-level trough will also be in place over the central U.S. Storms are expected to be ongoing along the cold front across the Lower Mississippi and Tennessee Valleys at the start of the forecast period on Monday (July 12th). High boundary layer moisture in place along the front (dew points in the low to mid-70s) as well as mean southwesterly flow roughly oriented along the cold front associated with the upper-level trough will lead to heavy rain of 1-2 inches with these storms. There is also a chance for heavy rain along the warm front in New England. However, it appears any higher rain totals will remain relatively isolated 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 (July 13th). Some storms may be possible during this time frame but heavy rain is not currently expected. However, on Wednesday, more organized convection is expected to develop along the front, aided by a shortwave trough moving across the Northern Tier. There is some uncertainty on the exact track of the storms and the location for associated heavy rainfall. However, given the high boundary layer moisture once again in place (dewpoints in the mid-70s) as well as the strong signal in model guidance for heavy rainfall of 1-2 inches, with locally higher amounts above 3 inches, a broad area from the Middle Missouri Valley into the Middle and Upper Mississippi Valley has been maintained to account for this potential. Convection is expected to continue and increase in coverage as the shortwave continues east and the cold front progresses eastward across the Great Lakes into the Northeast as well as southward into the Central Plains, Middle Mississippi Valley, and Ohio Valley on Thursday (July 15th) and Friday (July 16th). Daily heavy rain totals of 1-2 inches with locally higher amounts are expected along the length of the cold front as high the high boundary layer moisture remains in place. In the west, the persistent upper-level ridge that has been in place over the western U.S. will finally begin to weaken some through the forecast period. However, excessive heat and above normal temperatures are still expected across a broad area of the west at the beginning of the period and will continue through the end of the period where subtle ridging remains in place. This excessive heat throughout the period is expected across portions of the western and northern Great Basin, the Wasatch Front of Utah, and the Snake River Valley of Idaho. Highs in the upper 90s to mid-100s are forecast on Monday and Tuesday and will remain in the upper 90s through Friday, providing little relief. Excessive heat is also forecast for the Mojave Desert, the San Joaquin Valley/central California, the Sacramento Valley/northern California, portions of the southern Great Basin, and western portions of Central Rockies on Monday and Tuesday. High temperatures in the 110s are likely for the Mojave Desert and San Joaquin valley, with highs in the upper 90s further north. Highs in the upper 90s to low 100s are expected further east across the Great Basin and western Colorado. Excessive heat will also continue for the Interior Northwest through Wednesday, with highs in the mid to upper 90s forecast. High temperatures will likely return closer to normal throughout California, the Mojave Desert, and northeast into the Great Basin and western Colorado on Wednesday as the ridge begins to break down. A weak shortwave trough moving east from the Pacific will provide some relief from the hotter temperatures by Thursday for the Northwest as well. Above normal temperatures in the mid- to upper 90s are also forecast for the Northern High Plains of Montana on Thursday and Friday as subtle ridging increases ahead of the shortwave trough to the west. 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. Widespread extreme to exceptional drought across the west is also a concern with the continued hot weather. Besides the heat, there is a signal in model guidance for an increase in storm chances southeast of the ridge, particularly across eastern Arizona. Heavy rain chances look to remain isolated at this time, but could be a concern for flash flooding locally. In Alaska, an occluding low pressure system currently over the Gulf of Alaska will begin to dissipate by Tuesday. Moist onshore flow ahead of the system may lead to some heavy rain along the southern Mainland on Monday, but the localized nature of any higher amounts and their location in the model guidance preclude an outlook area. Additionally, a low pressure system and associated cold front will move southward across the Far North and eastern Interior Tuesday into Wednesday, with the front becoming stationary across the Interior. Some rain is possible along and ahead of the system across the Interior on Wednesday and Thursday, but model guidance differs as to how heavy the rain is expected to be. Additionally, below normal temperatures are also possible, particularly across the Far North, but do not look to be particularly hazardous at this time. Putnam