US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 354 PM EDT Fri Sep 11 2020 Valid Monday September 14 2020 - Friday September 18 2020 Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Wed-Fri, Sep 16-Sep 18. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Mon-Tue, Sep 14-Sep 15 and Thu-Fri, Sep 17-Sep 18. - Heavy rain across portions of the Southeast and the Lower Mississippi Valley, Mon-Fri, Sep 14-Sep 18. - Heavy rain across portions of the Southeast, Mon-Tue, Sep 14-Sep 15. - Much above normal temperatures across portions of the Central Plains and the Northern Plains, Mon-Tue, Sep 14-Sep 15. - Much above normal temperatures across portions of the Central Plains, the Upper Mississippi Valley, and the Northern Plains, Tue-Wed, Sep 15-Sep 16. - Heavy rain across portions of mainland Alaska and the Aleutians, Mon-Tue, Sep 14-Sep 15. - Heavy rain across portions of mainland Alaska, Tue-Wed, Sep 15-Sep 16. Detailed Summary: The spotlight during the medium range (Mon Sept. 14 - Fri Sept. 18) will be on the Gulf Coast as a tropical wave (currently near the Bahamas) is expected to track into the northeast Gulf of Mexico. As of this afternoon, the NHC currently gives this tropical disturbance a 80% chance of development over the next five days. Environmental conditions are favorable (abnormally warm sea surface temperatures/upper level anticyclone overhead) for this disturbance to organize and strengthen before it reaches the central Gulf Coast. While impacts such as winds and surf are unclear given the uncertainty of development and intensity, an abundance of tropical moisture will be available with this feature. The concerning aspect of this tropical wave is it becomes caught in a weak steering flow pattern as a ridge develops over the west-central U.S. and a weak ridge sits over the Southeast. This could lead to several days of showers and thunderstorms containing torrential downpours and the potential for the training of convection from the Florida Panhandle on west to southern Louisiana. Copious amounts of rainfall are possible as a result with totals in excess of 10" possible in areas where training convection occurs. Eventually, an upper trough passing through the Great Lakes pushes a cold front south and finally allows a path for the storm to head north into the Tennessee Valley late week. The axis of heavy precipitation by late next week will be dependent upon the exact timing of the front, but it should force the tropical moisture north and lead to the opportunity for soaking rains into the Mid-South and perhaps the southern Appalachians as well. Elsewhere, temperatures will remain hot for a couple days across the northern and central High Plains due to upper level high pressure over the Rockies and the Great Plains. The heat does not last long, however, as a second injection (in as many weeks) of below normal temperatures returns to these regions. Compared to the most recent cold blast, this will seem more like a pleasant shot of October-like temperatures rather than the frigid and snowy regime from earlier this week. In addition, soaking rainfall may develop in parts of South Texas both at the start period as a weak mid-level circulation center over the western Gulf of Mexico funnels tropical moisture ahead of an approaching cold front. The front should then become stationary and trigger additional showers and thunderstorms the second half of the week. In the West, guidance continues to indicate an upper-level trough ushers in beneficial moisture and precipitation to the wildfire-ravaged portions of the Northwest early next week. The amounts will need to be closely monitored, because while precipitation is certainty welcomed, too much in a short time span could give way to some localized debris flows in burn scarred areas. In Alaska, the model trends continue to support a more amplified and slower pattern, especially through the first half of the week. The result is guidance holding the large upper low back in the Aleutians through early week and a strong ridge forming along the southern coast. The storm system is likely to produce plenty of precipitation, rough seas, and gusty winds most notably throughout the Aleutians and into far western Alaska. Precipitable water levels are likely to be roughly 2-3 standard deviations above normal as the front swings through early next week and rainfall rates could be excessive in spots. There is still plenty of spread in ensemble guidance making for a low confidence forecast beyond day 5, but guidance does suggest the potential for a new frontal system to swing south of the Aleutians and could introduce a wetter pattern along the Gulf Coast the second half of the week. Ensemble guidance spread remains high however, and thus no hazards have been posted at this time. Mullinax