US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 352 PM EDT Mon Aug 09 2021 Valid Thursday August 12 2021 - Monday August 16 2021 Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, the Central Appalachians, and the Southwest, Sat-Mon, Aug 14-Aug 16. - Heavy rain across portions of the Southern Florida, Sat-Sun, Aug 14-Aug 15. - Heavy rain across portions of the Southeast, Sun-Mon, Aug 15-Aug 16. - Flooding possible across portions of the Central Plains and the Southeast. - Flooding occurring or imminent across portions of the Southeast. - Flooding likely across portions of the Southeast. - Excessive heat across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Thu-Sun, Aug 12-Aug 15. - Excessive heat across portions of the Mid-Atlantic, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Northeast, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Thu-Fri, Aug 12-Aug 13. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Thu-Fri, Aug 12-Aug 13. Detailed Summary: The medium range forecast period enters mid-August, more specifically August 12th to 16th, and includes the potential for dangerous heat and heavy rain across parts of the contiguous United States. The overall weather pattern is expected to feature upper-level ridging across the eastern U.S. and Pacific Northwest at the beginning of the period, with an upper-level trough swinging across the north-central U.S. and Great Lakes. By early next week, the upper-level ridging is forecast to break down slightly and lead to weak troughing over western and central sections of the country. At the surface, the main features will be a refreshing cold front forecast to push from the Midwest and Central Plains towards the East Coast before stalling over the Southeast and Deep South, as well as a potential tropical disturbance near Florida. Starting with the potential heavy rain threats, much of the activity across the Mid-Atlantic and Southeast is reliant on a possible tropical cyclone forecast to approach southern Florida by Saturday. The current forecast track would suggest heavy rain possibly impacting southern Florida on Saturday and Sunday, before moving up the Florida Peninsula and into southern Georgia by Monday. Uncertainty remains very high with both strength and track of this feature and therefore its potential impacts. Be sure stay up to date with the latest forecasts issued by the National Hurricane Center. Farther north, as the aforementioned cold front enters the Mid-Atlantic on Saturday, it will run into a very warm and moist airmass conducive of producing thunderstorms capable of packing intense rainfall rates. Additionally, if a tropical system enters the Southeast near or slightly faster than currently forecast, the position of an upper-level trough over the Ohio Valley could funnel tropical moisture into the Carolinas on Sunday and Monday. Adjustments to the southern portion of this highlighted area may be needed as it is contingent upon the timing and location of a tropical disturbance. Meanwhile, an uptick in monsoon activity is expected across parts of Arizona and southwest New Mexico this weekend into early next week. Total precipitable water amounts near 2 inches are possible and would produce a ripe environment for scattered thunderstorms and heavy rainfall. Another hazard to contend with for certain parts of the country will be excessive heat. Once again, an anomalous upper-level ridge over the Pacific Northwest will produce a scorching heat wave between Thursday and Sunday. Highs into the upper 90s and low triple digits will impact highly populated regions east of the Cascade mountains through Saturday, as well as the interior Northwest through the end of the weekend. The mid-August heat will also be felt farther south into northern California and Nevada. Temperatures are expected to reach upwards of 20 degrees above average and break several daily high temperature records. A few all-time records would have been at risk, but most were shattered back in the June heat wave experienced earlier this summer. The good news is that this heat wave is expected to relax by early next week as above average temperatures shift into the northern High Plains. For the Mid/Deep South, hot weather and oppressive humidity is forecast on Thursday and Friday before a cold front brings much-needed relief. Heat indices should exceed 100 degrees both days and approach 105-110 during the peak heating hours of the afternoon. These high heat indices may continue across the Gulf Coast through Saturday, but temperatures are currently forecast to remain close to average. Last, but certainty not least, excessive heat is also a concern throughout the densely populated Mid-Atlantic and Northeast between Richmond, VA and Portland, ME on Thursday and Friday. Heat indices are expected to reach near 105 degrees, with low temperatures only dipping into the mid-70s. For Alaska, heavy rain associated with a potent atmospheric river event is anticipated to end the week over parts of the southeast mainland and northern panhandle. An impressive surge of moisture that reaches southwest toward ongoing typhoon activity in the west-central Pacific could lead to multiple inches of rain across both coastal and mountainous regions. Upwards of 8 to 10 inches of rain is a possibility near and surrounding Yakutat, Alaska. Snell