US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 502 PM EDT Thu Sep 09 2021 Valid Sunday September 12 2021 - Thursday September 16 2021 Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Mon-Wed, Sep 13-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. - 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, Sun-Mon, Sep 12-Sep 13. - Heavy rain across portions of mainland Alaska, Sun, Sep 12. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Mon, Sep 13. - High winds across portions of mainland Alaska and the Aleutians, Sun, Sep 12. Detailed Summary: The weather pattern at the beginning of medium range period (Sunday, Sep 12 to Thursday, Sep 16) is highlighted by a quasi-zonal (West-East) flow pattern as high pressure migrates eastward out of the Midwest toward the Mid-Atlantic early next week. Slight height rises with the eastward propagating ridge will maintain above average warm temperatures for the lower half of the CONUS, with the most anomalous high temperatures residing overhead the Central Plains within the Much Above Normal Temperatures area through Monday. The majority of above average temperatures for the CONUS abate by midweek as a series of shortwave troughs traverse the Northern Tier, but a return to anomalous warmth is possible later next week over the Northern Plains as warm advection streams in ahead of an approaching wave. Over South Texas, the arrival of a moisture-laden airmass beginning Monday will crank heat indices up to 100-105 through at least the 16th, but remains below regional Excessive Heat hazard criteria. Past this weekend, a more active rainfall pattern returns to the central and northern U.S. with the arrival of the aforementioned shortwaves and associated frontal systems. Moderate to locally heavy rainfall is possible Sunday-Monday over the Northern Plains as a frontal system interacts with a plume of deep Pacific moisture, before the best rain chances shift east into the Central Plains and Midwest ahead of a sweeping cold front. Heavier rainfall is possible on the 14th-15th from the Plains into the Great Lakes/Upper Midwest as a plume of Gulf Moisture lifts north ahead of the front, although questions remain as to the position of the front and moisture return trajectories. The faster deterministic guidance has backed off on the speed of the front over the last few runs, so will evaluate if enough clustering exists to introduce a possible Heavy Rain area for midweek in subsequent Hazards updates. The main heavy rain threat still falls along the Texas and Louisiana coast beginning Monday ahead of a moisture-rich tropical disturbance propagating northward out of the Gulf of Mexico. Guidance supports the idea of a broad heavy rain threat along the coast with the arrival of this feature, but a fair amount of uncertainty remains as to where the highest rainfall amounts will fall. Thus, have maintained continuity for the Heavy Rain area along the coast for now. Meanwhile, Alaska will remain very wet throughout the medium range. On Sunday, a strong mean trough with embedded shortwaves will pivot southeast across the Aleutians and Alaska Peninsula, propagating a series of frontal systems across the southern Alaska. As the system moves east late this weekend it will siphon a stream of Pacific moisture northward through the Gulf of Alaska, setting up another bout of heavy rain from the 12th-13th. Rainfall totals of 1-2 inches are initially possible on the 12th from the Chugach Mountains westward, including Kodiak Island, East Kenai Peninsula, northern Alaska Peninsula, and areas adjacent to Bristol Bay. By the 13th, the Heavy Rain threat will shift south toward the Alaska Panhandle as the low tracks eastward with another 1-2 inches possible. Moreover, the deep system will produce strong gusty as it swings through, with gusts exceeding 50 mph within the High Winds area on the 12th. Gusty winds will remain on the 13th, before the frontal system begins to weaken after Monday. Another chance of heavy rain may return to southern Alaska on the later end of the medium range period as another shortwave swings across the Alaska Peninsula and taps into Pacific moisture, but will wait for better agreement on the location of higher amounts before introducing a Heavy Rain area. Asherman