US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 341 PM EDT Fri Sep 17 2021 Valid Monday September 20 2021 - Friday September 24 2021 Hazards: - Heavy rain across portions of the Great Lakes and the Upper Mississippi Valley, Mon, Sep 20. - Heavy rain across portions of the Northeast, the Central Appalachians, the Mid-Atlantic, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Wed-Thu, Sep 22-Sep 23. - Heavy rain across portions of the Great Lakes, the Central Appalachians, and the Ohio Valley, Tue, Sep 21. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Much below normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Mon, Sep 20. - Much below normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, and the Northern Great Basin, Mon-Tue, Sep 20-Sep 21. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Sep 20-Sep 21 and Thu-Fri, Sep 23-Sep 24. - Heavy rain across portions of the Alaska Panhandle, Mon-Tue, Sep 20-Sep 21 and Thu-Fri, Sep 23-Sep 24. - Much below normal temperatures across portions of mainland Alaska, Mon-Fri, Sep 20-Sep 24. Detailed Summary: During this upcoming medium range period (Monday, September 20th - Friday, September 24rd) much of the activity both weather and temperature-wise over the CONUS will be driven by a strong, progressive upper-level trough. On Monday, its associated cold front will essentially be aligned north-south through the center of the lower 48. Behind this front, a cold, Canadian air mass will have settled over the Rockies and Intermountain Region/Great Basin, resulting in temperatures generally average from 10-15 degrees below normal. The most anomalously cool weather will be Monday into Tuesday, with potential first freezes across sections of the Great Basin and Central Rockies Tuesday morning. In addition to the cool temperatures, there will be some accumulating snow across the higher elevations of Wyoming and Colorado, including the Bighorn Range and the Tetons. Nothing heavy is expected, but it will be a first taste of winter weather for some. Along and ahead of the front, Gulf of Mexico moisture will stream northward around a sprawling high pressure area over New England. On Monday, the heaviest rainfall will be found over the upper Mississippi Valley associated with a low lifting along the front combined with lifting from short wave energy embedded in the upper trough. Rainfall totals up to and exceeding one inch may occur primarily in Minnesota and far northwestern Wisconsin. While soil across this area is relatively dry, heavier rainfall rates in convective precipitation may lead to isolated flash flooding. Another area of some concern Monday is the southern Blue Ridge Mountains where an upper level short wave and low-level convergence will support periods of moderate to heavy rainfall. The guidance has backed away from the heavier amounts they were forecasting yesterday, so did not have enough confidence to maintain the threat area. From Tuesday through the end of the week, the upper trough and surface cold front will march steadily eastward. There are some definite differences among model guidance regarding the speed of this front, but at this time, the general consensus suggests that the heaviest rainfall will be over the Ohio Valley Tuesday, mid-Atlantic region Wednesday, then into New England on Thursday. The heaviest activity with the greatest flooding potential is likely to be on Wednesday and Thursday. While the timing differences persist, the majority of guidance suggests the upper trough will amplify and pull in some Atlantic moisture. This will support higher precipitation rates, so introduced a broad area that covers the mid-Atlantic states and New England. Within this area, rainfall amounts of 1-2 inches will be common, with locally heavier amounts likely. In Alaska, it will be a pretty active pattern through the week for the south-central mainland and Panhandle. On Monday, a strong low and associated frontal system in the Gulf of Alaska will push into the Panhandle and south-central mainland. A tropical connection of moisture will result in some heavy rain totals over the Panhandle (2-3 inches and locally higher). Meanwhile, arctic air sinking southward will likely result in much of the precipitation over the Chugach Mountains to be snow, with rain or mixed precipitation along coastal sections into Prince William Sound. Onshore flow and a reinforcing shot of upper level energy will allow moderate to heavy precipitation to persist through Tuesday, though the bulk of the heaviest amounts will be across the Panhandle. After a relative break on Wednesday, heavy precipitation will return to the same regions for Thursday and Friday as the next storm system spins up just south of the Peninsula and moves through the Gulf of Alaska. With another feed of subtropical moisture, the Panhandle will see up to several inches of rainfall late next week, with more moderate to heavy snowfall over the Chugach Mountains. Meanwhile, Arctic air will be pretty well entrenched across much of the mainland, leading to temperatures averaging around 10-15 degrees below normal. Klein