US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 356 PM EDT Thu Sep 10 2020 Valid Sunday September 13 2020 - Thursday September 17 2020 Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, the Southern Plains, and the Tennessee Valley, Sun-Mon, Sep 13-Sep 14. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Sun-Thu, Sep 13-Sep 17. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Wed-Thu, Sep 16-Sep 17. - Flooding likely across portions of the Southern Plains. - Much above normal temperatures across portions of the Central Great Basin, the Northern Rockies, and the Northern Great Basin, Sun, Sep 13. - Much above normal temperatures across portions of the Central Plains, the Northern Plains, and the Northern Rockies, Mon-Wed, Sep 14-Sep 16. - Much above normal temperatures across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Tue-Wed, Sep 15-Sep 16. - Heavy rain across portions of mainland Alaska, Mon-Wed, Sep 14-Sep 16. - Heavy rain across portions of the Aleutians, Sun-Mon, Sep 13-Sep 14. Detailed Summary: The medium range period (Sun Sept. 12 - Tues Sept. 16) features the return of broad upper-level ridging across the West and a separate area of ridging over the Southeast. The hottest temperatures versus normal are to develop across the northern Great Basin, Rockies, and Plains. The Southeast can also expect above normal temperature anomalies that are mostly due to abnormally warm minimum temperatures. High temps in the region will hover close to, or slightly below normal due to cloud cover and precipitation. Speaking of precipitation, the Gulf Coast contains the best odds for heavy rain through the mid-September. To start the period, guidance is keying in on a tropical wave emerging into the eastern Gulf of Mexico on Sunday. The NHC currently gives this disturbance a 40% chance of development over the next five days. Regardless of whether this disturbance becomes a named storm or not, there will be ample tropical moisture available to generate areas of rain along the central Gulf Coast next week. This disturbance may become caught in weak steering flow between the strengthening ridge over the west-central U.S. and a weak area of upper level high pressure over the Southeast, thus keeping the threat for heavy rainfall along the Gulf Coast into the middle of next week. A near by frontal boundary may also act as a trigger for additional showers and thunderstorms along the Gulf Coast into mid-week. Southeasterly flow still makes for a good up-sloping flow setup in the southern Appalachians Sunday and Monday. Eventually, as an upper trough digs into the Great Lakes late in the period, moisture associated with this tropical disturbance may be funneled north into the Mid-South and Ohio Valley. This could in turn allow for excessive rainfall to manifest itself over these regions next Wednesday and Thursday. Farther west, a weak upper trough in the western Gulf of Mexico and a feed tropical moisture will take aim at the Rio Grande Valley where heavy rainfall could lead to flash flooding concerns both Sunday and Monday. In Alaska, the last 24 hours worth of model guidance has been telling with the pattern becoming more amplified and slower. 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. Given recent trends, the heavy rain area from Kodiak Island to southern coast and Upper Peninsula was taken off the chart. The storm system is still likely to produce plenty of precipitation, rough seas, and gusty winds, most notably throughout the Aleutians and into far southwestern Alaska. Precipitable water levels are likely to be 2-3 STDs 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. Mullinax