US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 407 PM EDT Wed Jul 07 2021 Valid Saturday July 10 2021 - Wednesday July 14 2021 Hazards: - Heavy rain across portions of the Central and Southern Plains, Sat, Jul 10. - Heavy rain across portions of the Southern Plains and Lower Mississippi Valley, Sun, Jul 11. - Heavy rain across portions of the Upper Missouri Valley, the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Sat-Sun, Jul 10-Jul 11. - Flooding possible across portions of the Southeast and the Southern Plains. - Flooding occurring or imminent across portions of the Middle Mississippi Valley and the Southern Plains. - Excessive heat across portions of northern California, southwestern Oregon, and the Desert Southwest, Sat-Mon, Jul 10-Jul 12. - Excessive heat across portions of southern California, the Southwest, the Great Basin, and the Interior Northwest, Sat-Wed, Jul 10-Jul 14. - Much above normal temperatures across portions of the Northern Rockies and Northern High Plains, Sun, Jul 11. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Sun, Jul 10-Jul 11. Detailed Summary: The threat for heavy rain in the central/eastern U.S. as well as the continued heat in the western U.S. highlight the hazards outlook for the medium-range forecast period (Saturday, July 10 - Wednesday, July 14). An upper-level trough will begin to develop in the lee of the Rockies on Saturday (July 10th) and progress to the southeast before beginning to break down Monday evening (July 12th). At the surface, a low pressure system over central Kansas will move northeastward towards the northern Great Lakes, with a warm front extending east into the Ohio Valley and a cold front to the southwest over the Central and Southern Plains. Heavy rain is expected with convection developing in the vicinity of the surface low and along the warm front given the very moist air (low 70s dew points) pooling along the front. This is most likely from the Upper Missouri Valley east across the Middle Mississippi Valley and into the Ohio Valley, as well as north into the southern Great Lakes region from Saturday through Sunday (July 11th) as the warm front moves slowly northward. This will likely also include the continuation of a mesoscale convective system developing Friday (July 9th) over the Upper Missouri Valley and progressing southeastward along the warm front into the Middle Mississippi Valley on Saturday. Widespread rainfall between 1-2 inches is expected, with locally higher amounts possible, particularly along the track of the forecast mesoscale convective system. Widespread convection is also expected to develop to the southwest given the lift ahead of the upper-level trough and forcing along the cold front as it moves to the southeast over the Central/Southern Plains on Saturday and Southern Plains/Lower Mississippi Valley on Sunday. Heavy rain of 1-2 inches is likely with these storms given the noted very moist air mass in place ahead of the front. There is a chance for heavy rain further east into the Mid-Atlantic and Northeast as the warm front progresses northward, but there is not enough model consensus on the location of the highest amounts for an outlook area at this time. The most likely time frame for heavy rain is on Sunday, with the potential for 1-2 inches of rain possible anywhere from the I-95 corridor in the Mid-Atlantic northward to Lake Ontario, the Adirondacks, and portions of northern New England. An outlook area may need to be introduced as confidence in the most likely location for the heavy rain increases. The heavy rain may also continue into Monday and Tuesday for New England. Additionally, a mesoscale convective system may develop in the Upper Mississippi Valley ahead of the next storm system on Wednesday (July 14th) at the end of the forecast period. Heavy rain of 1-2 inches with locally higher amounts is possible along the track of the system. However, the localized nature of the heavier rain totals with a mesoscale system this far out in the forecast period and differences in location between the model guidance precludes an outlook area at this time. In the west, an upper-level ridge will remain in place over the western U.S. throughout the period. Both excessive heat and much above normal temperatures are forecast across the region. Excessive heat is expected across the Mojave Desert, the San Joaquin Valley of California, portions of the central and northern Great Basin, the Wasatch Front of Utah, the Snake River Valley of Idaho, and portions of eastern Oregon and Washington throughout the period. Areas in the Mojave Desert and San Joaquin Valley can expect daily high temperatures of 110 and above. Further north, highs in the upper 90s and lower 100s are forecast. Excessive heat is also forecast for the rest of interior southern California and southwestern Arizona as well as the Sacramento Valley of northern California and interior southwestern Oregon from Saturday to Monday. A weak shortwave trough may provide some relief from the hotter temperatures earlier in the period on Tuesday and Wednesday for northern California and southwestern Oregon, while high temperatures will likely return closer to normal as well in southern California and Arizona as the ridge shifts to the northwest. Above normal temperatures in the middle to upper 90s are also forecast for the Northern Rockies and High Plains of Montana on Sunday as the upper-level ridge expands northward between passing troughs. In Alaska, an occluding low pressure system will move east across the Gulf of Alaska and into the Panhandle by Monday. Moist onshore flow ahead of the system on Saturday and Sunday will lead to daily heavy rain totals of 2 inches and higher for the extreme southeast Mainland and the Panhandle. Daily clouds and showers throughout the broader mainland will lead to temperatures 10 to 15 degrees below normal across the area, but these temperatures are not expected to be particularly hazardous at this time. Putnam