US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 354 PM EDT Wed Jun 30 2021 Valid Saturday July 03 2021 - Wednesday July 07 2021 Hazards: - Heavy rain across portions of the Southeast and the Mid-Atlantic, Sat, Jul 3. - Heavy rain across portions of the Southern Plains and the Lower Mississippi Valley, Mon-Wed, Jul 5-Jul 7. - Flooding possible across portions of the Central Plains, the Middle Mississippi Valley, the Ohio Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Central/Southern Plains, the Middle Mississippi Valley, and the Great Lakes. - Flooding likely across portions of the Central/Southern Plains and the Middle Mississippi Valley. - Excessive heat across portions of the Pacific Northwest and the Northern Great Basin, Sat-Mon, Jul 3-Jul 5. - Excessive heat across portions of the Northern Plains, Sat-Sun, Jul 3-Jul 4. - Much above normal temperatures across portions of California, the Pacific Northwest, the Northern Great Basin, and the Northern Rockies, Sat-Mon, Jul 3-Jul 5. - Much above normal temperatures across portions of the Northern Rockies, the Northern Plains, the Upper Mississippi Valley, and the Great Lakes, Sat-Sun, Jul 3-Jul 4. Detailed Summary: During the medium range period (Saturday, July 3rd - Wednesday, July 7th) the anomalous ridging that has dominated the upper-level pattern over the western half of the United States will dissipate making way for a more progressive and zonal flow, but it won't go down without a fight. Daily high temperatures from the Northwest to the Upper Midwest will hover 15 to 20 degrees above average through early next week, with some areas consistently reaching the upper 90s or low 100s. Lucky for the regions east of the Rockies the upper-level ridge is expected to quickly deteriorate by Monday, allowing temperatures to quickly drop down closer to normal. Due to the ongoing and likely prolonged exposure to oppressively high daytime temperatures in parts of the Columbia Basin, an excessive heat area has been maintained for portions of eastern Washington, northern Oregon, and western Idaho for the first half medium range period. Dangerous heat is also forecast to affect eastern Montana Saturday before moving into the Dakotas on Sunday, where highs are anticipated to approach or reach triple digits. In each of these outlined areas residents are urged to take proper precautions to safeguard their health and prevent the onset of heat-related illness. These precautions include: avoiding long periods of time outdoors in the sun, staying hydrated, remaining in a cool or air conditioned location, and canceling outdoor activities during the heat of the day. In stark juxtaposition to conditions out West, the South and East are forecast to experience cooler and wetter weather throughout the course of the medium range due to a deep cut-off low aloft and a slow-moving frontal system extending from the Mid-Atlantic through the South to the Southern Plains at the surface. Fueled by moisture from the Gulf of Mexico pooling ahead of the system, this frontal boundary will serve as the primary focus for the development of showers and thunderstorms from the Carolinas to the Southern Rockies throughout the weekend. The slow progression of the system combined with high rainfall rates from convective precipitation could lead to accumulations of 1 to 2 inches in 24 hours along the East Coast on Saturday. Though the affected coastal areas currently have relatively dry soils and normal streamflow levels, heavy rainfall from the previous day could erode these conditions enough to facilitate isolated flash flooding. Pushed ahead by high pressure settling over the Midwest, the frontal system is expected to creep towards the Gulf Coast on Sunday before dissipating on Monday. Early next week, an upper-low sitting over Texas is forecast to work in tandem with a low pressure wave over the Sierra Madre to drive very moist air in the western Gulf of Mexico onshore. As this moist air interacts with shortwave energy at the surface, showers and thunderstorms will likely develop over western Louisiana and southern Texas, where heavy rainfall could lead to two-day accumulations of 3 to 4 inches. Taking into account the highly saturated soils and much above normal streamflow in these areas from recent anomalously high amounts of rain, the risk of widespread flash flooding will be a significant concern. Elsewhere, a long frontal boundary is forecast to drape over the north-central part of the country throughout much of the medium range period, extending from the Pacific Northwest to the Great Lakes. Though precipitation along the boundary is expected to remain relatively light, models suggest that heavy rain could develop in the Upper Midwest on Wednesday. At present, the models disagree as to where the heaviest rain will fall and how much, but the timing is consistent. Given the long lead time and the potential for the forecast to change over the course of the week no hazard area has been drawn, however, this region will be watched in the coming days for further development. No hazards have been posted for Alaska, however, it is worth noting that multiple frontal systems are forecast to pass over the mainland throughout the period that may produce localized heavy rainfall. While individual models suggest that accumulations in some areas could approach or even slightly exceed the hazardous threshold of 1.5 inches in 24 hours, there is no general consensus as to where, how much, and if any rain will fall. Due to the high degree of uncertainty present in the guidance no area was highlighted on today's graphic. Zavadoff