US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 456 PM EDT Tue Sep 07 2021 Valid Friday September 10 2021 - Tuesday September 14 2021 Hazards: - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Mon-Tue, Sep 13-Sep 14. - 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. - Much above normal temperatures across portions of the Central Rockies, the Central Plains, and the Southern Plains, Fri-Sat, Sep 10-Sep 11. - Much above normal temperatures across portions of the Central Plains, the Northern Plains, and the Northern Rockies, Fri, Sep 10. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Fri, Sep 10 and Sun-Mon, Sep 12-Sep 13. Detailed Summary: The weather pattern during the medium range forecast period (Friday, Sep 10 to Tuesday, Sep 14) is highlighted by a strong ridge of high pressure over the Four Corners region while upper level troughing over much of the East to start the period gives way to a more zonal (West-East) flow by early next week. The ridging out west supports daily temperature anomalies of 12-15F above normal both Friday and Saturday in the Northern and Central Plains. This supports the need for a Much Above Normal temperature hazard areas to close out the work-week as high temperatures approach 100 degrees within the Much Above Normal Area. Afterwards, relief with slightly cooler-than-average temperatures should arrive to the Central U.S. beginning Saturday as cool air flows in behind a cold front, alongside some cloud cover and rain. It is a different story in the Desert Southwest throughout the medium range, as very warm temperatures 5-10 degrees above average approaching 110 are possible in and around the Sonoran Desert. Some localized areas (particularly cities) may see higher impacts from the very warm temperatures, although widespread high impacts for an Excessive Heat area are not anticipated at this time. Temperatures aside, the medium range looks fairly dry over a majority of the CONUS, and the main concern for widespread heavy rain falls along the western Gulf of Mexico beginning Monday ahead of a moisture-rich tropical disturbance emanating from the Yucatan Peninsula. Uncertainty remains high with the timing and evolution of this system; the GFS is stronger, faster, and maintains a westerly track toward South Texas while the ECMWF is slower and to the east of the GFS. Therefore, have expanded the Heavy Rain area further east into Louisiana to account for a possible track closer to the East-Central Texas and Louisiana coastline. Users are encouraged to monitor the latest NHC outlooks for any developments to the forecast. Tropical moisture from this system will eventually lift north and interact with a front sweeping across the Upper Midwest and Great Lakes early next week to produce moderate rainfall, although timing differences with the front and how it interacts with the incoming moisture precludes a Heavy Rain area at this time. Otherwise, shower and thunderstorm chances will persist over Florida through next week as a front stalls and moderates in the Southeast/Gulf of Mexico. A shot of moderate to locally heavy rainfall is also likely on Friday over the northern Intermountain West ahead of a frontal system, although dry antecedent soils and the progressive nature of the system should modulate any widespread heavy rain concerns. Meanwhile, Alaska will remain very wet throughout the medium range. The period starts with a potent upper trough spawning an anomalously strong low pressure system in the Gulf of Alaska. A steady barrage of Pacific moisture will stream from Kodiak Island and the Kenai Peninsula all the way to the Panhandle by Friday. Rainfall totals of 1-3 inches are possible with locally higher amounts possible in the Upper Panhandle where orographic lift will be maximized by SW flow oriented perpendicular to the region's higher elevations. Locally brisk winds are also possible along coastal areas from the Kenai Peninsula to the Panhandle coast. Once this storm system fully occludes and dissipates by the start of the weekend, an intense upper low over the Bering Sea will approach late Saturday into Sunday. This storm system taps into rich subtropical moisture stemming as far back as the West Pacific and brings yet another round of heavy rainfall to the same areas Sunday and into Monday. Another 1-3 inches are possible with locally higher amounts in favorable upslope areas. Portions of the Alaska Range adjacent to the Cook Inlet were added to the Heavy Rain area given good consensus for topographically enhanced rainfall this Friday and early next week. Asherman/Mullinax