US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 409 PM EDT Mon Sep 06 2021 Valid Thursday September 09 2021 - Monday September 13 2021 Hazards: - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Mon, Sep 13. - Heavy rain across portions of the Northeast, Thu, Sep 9. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, and the Mid-Atlantic. - Much above normal temperatures across portions of the Central Rockies, the Central Plains, the Northern Plains, and the Northern Rockies, Thu-Fri, Sep 9-Sep 10. - Heavy rain across portions of mainland Alaska, Thu, Sep 9 and Sun-Mon, Sep 12-Sep 13. - 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 (Thursday, Sep 9 to Monday, Sep 13) 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 flow by early next week. The ridging out west supports daily temperature anomalies of 12-15F above normal both Thursday and Friday in the northern and central High Plains. This supports the need for a Much Above Normal temperature hazard areas to close out the work-week. Thursday features a sharp dip in the jet stream over the East while tapping into tropical moisture stemming from the Gulf of Mexico. Latest Thursday fronts forecast is for a cold front to be positioned somewhere near the New England coast. The key in potential heavy rain impacts is the speed and position of the front. Based on the latest 12Z deterministic guidance the GFS is the fastest, the Euro and UKMET are slower, and the Canadian is somewhere in between. Ensembles are a tad closer to the Canadian's idea with the 12Z GEFS mean a little farther west from the GFS Op. Chose to introduce a heavy rain area extending from eastern Massachusetts to Downeast Maine for Thursday. There is concern for flash flooding over eastern Massachusetts due to overly saturated soil conditions courtesy of Ida from last week. Lastly, the Upper Great Lakes may contend with passing upper level disturbances, most notably late Friday into Saturday. Should guidance trend wetter, a heavy rainfall area may be necessary in future forecasts. The other area to watch is the western Gulf of Mexico this weekend and into early next week. The subtropical northwest Atlantic ridge will restrengthen while the upper ridge over the Southwest remains anchored overhead. This will lead to a weakness in the steering pattern over the western Gulf of Mexico and a tropical wave exiting the Yucatan Peninsula has a chance to either strengthen or usher a surge tropical moisture towards the western Gulf coast. How strong the ridge in the Southwest is and any lingering troughing over the northeast Gulf will determine where the axis of rich tropical moisture goes. Should the tropical wave remain in tact, torrential rainfall with resulting inland flooding is a possibility to kickoff next week. Otherwise, the expansive area of ridging across the West and then the reemergence of the Bermuda High makes for a drier than normal pattern across most of the Lower 48. The state with the most active weather pattern in the medium range is Alaska. 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 on Thursday 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. The upper low tracking over the state late Sunday into Monday suggests freezing levels will also be dropping, supporting the potential for heavy snowfall in some of the higher peaks of the Alaska Range, the Talkeetnas, and Chugach mountains. Should guidance support a wetter setup for the mountain ranges farther inland from the coast, a heavy precipitation area may be necessary in future forecasts to account for heavier snowfall accumulations. Mullinax