US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 515 PM EDT Fri Sep 10 2021 Valid Monday September 13 2021 - Friday September 17 2021 Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Mon-Wed, Sep 13-Sep 15. - Heavy rain across portions of the Central Plains, the Northeast, the Central Appalachians, the Southern Plains, the Middle Mississippi Valley, the Mid-Atlantic, the Great Lakes, and the Ohio Valley, Tue, Sep 14. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Tue-Wed, Sep 14-Sep 15. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, and the Mid-Atlantic. - Flooding occurring or imminent across portions of the Middle Mississippi Valley and the Mid-Atlantic. - Flooding occurring or imminent across portions of the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Middle Mississippi Valley. - Excessive heat across portions of California and the Southwest, Mon, Sep 13. - Much above normal temperatures across portions of the Central Rockies, the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Mon, Sep 13. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Mon, Sep 13. - High winds across portions of mainland Alaska and the Aleutians, Mon, Sep 13. Detailed Summary: The weather pattern during the medium range forecast period (Monday, Sep 13 to Friday, Sep 17) over the CONUS is highlighted by zonal (West-East) flow with a series of shortwaves traversing along the Northern Tier. Above average warm temperatures will cover the southern half of the U.S. through midweek, with the most anomalous highs residing overhead the Desert Southwest and Central Plains. In the Southwest, high temperatures up to 10 degrees above average will support Excessive Heat on Monday, with scattered areas in the Sonoran Desert highlighted by a High-Very High Risk for heat impacts by NWS HeatRisk guidance as heat indices approach 110, although temperatures should begin a moderating trend beyond Monday. Warm temperatures will also linger over the Plains on Monday with temperature anomalies greater than 10 degrees anticipated with heat indices approaching 95. A cold front midweek will cool temperatures to around seasonal in the Plains, before a warming trend resumes from the Four Corners to the Northeast by the end of the work week as an upper-ridge builds over the Southwest. Over South Texas, the arrival of a moisture-laden airmass on Monday will crank heat indices up to a sweltering 105 through Friday, but remains below regional Excessive Heat hazard criteria. The heavy rainfall expected to begin late this weekend along the Texas and Louisiana coast ahead of a tropical disturbance will continue along the coast and further inland through at least midweek. Model guidance maintains support for a broad heavy rain risk area with localized significant rainfall accumulations possibly exceeding 10 inches, but uncertainty remains high as to where these higher rainfall totals may fall. The two big factors driving the uncertainty with the rainfall forecast include the eventual track and strength of the system, and it is possible model spread will remain fairly high on these key pieces until the disturbance in the Gulf is better organized/physically realized. For a technical discussion regarding individual model differences with the forecast, please refer to the latest WPC Medium Range Forecast Discussion. As the event approaches, users are encouraged to monitor the latest WPC and NHC forecasts for updates to the situation. Further north in the Midwest and Northeast, a broad Heavy Rain area was added for Tuesday as showers and thunderstorms form ahead of a sweeping cold front. A very moist pre-frontal airmass coupled with any instability that materializes will promote moderate to heavy rainfall with showers and thunderstorms that form, although the progressive nature of the system will limit the threat in the Central U.S. to the 14th. By Wednesday, the bulk of shower and thunderstorm activity will pivot ahead of the front into the Southeast while remaining over portions of the Northeast. The Heavy Rain area was maintained in the Northeast for the 15th given the similarly moist environment, appreciable instability, and vulnerable antecedent soil conditions. Further south along the front, moderate to locally heavy rainfall is possible, but a focused heavy rainfall threat is not apparent at this time. Meanwhile, Alaska continues the trend of active heavy rain and high wind. On Monday, a wave will lift northeast over the Gulf of Alaska and siphon a stream of Pacific moisture northward, setting up another bout of heavy rain on the 13th as moist southwest flow ascends the terrain, where 1-2 inches remain possible along the Alaska Panhandle. Moreover, a tight pressure gradient associated with the strong system will keep gusty winds approaching hazard criteria along the Alaska Panhandle for the 13th, so kept a portion of the High Winds area for one more day. The next best chance for rain and winds return to the southern quarter of Alaska from the 15th-16th as another system swings across the Gulf and taps into Pacific moisture. However, a fair amount of spread remains regarding the strength and timing of the system which precludes a Heavy Rain area. Asherman