US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 321 PM EDT Thu Sep 16 2021 Valid Sunday September 19 2021 - Thursday September 23 2021 Hazards: - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Heavy rain across portions of the Northern Rockies and the Northern Great Basin, Sun, Sep 19. - Heavy rain across portions of the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley, Tue, Sep 21. - Heavy rain across portions of the Upper Mississippi Valley, Tue-Wed, Sep 21-Sep 22. - High winds across portions of the Northern/Central Great Basin and the Northern/Central Rockies, Sun, Sep 19. - Much below normal temperatures across portions of California and the Pacific Northwest, Mon-Tue, Sep 20-Sep 21. - Much below normal temperatures across portions of the Northern/Central Great Basin and the Northern/Central Rockies, Tue-Wed, Sep 21-Sep 22. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Sep 20-Sep 21 and Thu, Sep 23. - Heavy rain across portions of the Alaska Panhandle, Mon-Tue, Sep 20-Sep 21 and Thu, Sep 23. - Much below normal temperatures across portions of mainland Alaska, Mon-Thu, Sep 20-Sep 23. Detailed Summary: During the medium range period (Sunday, September 19th - Thursday, September 23rd) much of the activity both weather and temperature-wise over the CONUS will be driven by a strong, progressive upper-level trough. On Sunday, remnant moisture from an atmospheric river that is expected to make landfall over the Pacific Northwest later this week will settle over northern Idaho, interacting with shortwave energy at the surface to bring moderate rain to the region with the potential for heavier accumulations of over one inch. Though this rain will be generally beneficial to help abate the extreme drought in the region, isolated flash flooding may be possible. Furthermore, there is a chance that some of the first snow of the season could fall at higher elevations (winter is coming!). To the southeast of this activity a cold front is forecast to simultaneously be passing through the Central/Northern Rockies and Central Great Basin generating high winds with gusts that could reach or exceed 50 knots, especially over the Northern Rockies and in the Snake River Valley. In addition to producing gusty winds, the cold front will also drop daily high temperatures in the Pacific Northwest and Northern Great Basin 10 to 15 degrees below normal. As the upper-level trough over the west coast begins digging south on Monday the cold front at the surface will also push further south, bringing moderate winds and colder air to the Central Great Basin, Rockies and High Plains through Tuesday. In each of the aforementioned regions the lower daily high temperatures will also translate to lower overnight temperatures. At high elevations, temperatures are expected to drop to or below freezing, providing some areas with their first freeze of the 2021-2022 winter season. As such, much below normal temperature hazard contours were included in today's graphic to highlight where these first freezes will be possible. With the cold front having been pushed to the south, high pressure is anticipated to begin building in the Pacific Northwest on Monday. Simultaneously, a low pressure system is expected to form and deepen over the Dakotas. By Tuesday this system and its trailing cold front is forecast to pass through the Great Plains, pushed ahead by both the eastward propagating trough aloft and high pressure which has moved into the Rockies and Great Basin at the surface. The heaviest rain associated with this system is expected to fall over the Upper Mississippi Valley where both the cold front and persistent shortwave energy behind the low pressure center will allow for an extended period of convective precipitation. Rainfall totals up to and exceeding one inch may occur primarily in Minnesota and western Iowa. While these areas have generally low soil moisture, isolated instances of flash flooding may be possible. Further southeast, upper-level shortwave energy is expected to produce heavy rainfall over a small area around the southern Blue Ridge Mountains. Antecedent wet soils and the potential for rainfall accumulations between one and two inches will make flash flooding a concern. As the low pressure system and its trailing cold front continue moving eastward, moderate rainfall will be likely over the Great Lakes/Ohio Valley and the eastern seaboard on Wednesday and Thursday, respectively. Though it is possible this rain could become heavy at times, current model guidance is not in agreement with regard to the timing of this heavy rain or the location where this heavy rain may occur. As such, no heavy rain area has been included for these regions on today's graphic, however, this may change over the coming days as the evolution of this system comes into better focus. In Alaska, a duo of deep low pressure systems are expected to impact the southern half of the state over the course of the medium range period. On Sunday, the first of these systems is expected to make its way up the Alaska Peninsula, bringing gusty winds to the southern and central parts of the Peninsula. After gathering a large amount of subtropical moisture as it passes through the Gulf of Alaska, the system is forecast to bring heavy rain to the Alaska Panhandle and snow/mixed precipitation to the Chugach Mountains where temperatures are colder on Tuesday and Wednesday. While the first system is tapering off on Wednesday, the second system (which includes the absorbed remnants of T.S. Chanthu) is expected to be moving in on the Alaska Peninsula. Following in its predecessor's footsteps, this second system is forecast to move up the Alaska Peninsula on Wednesday and Thursday, once again bringing gusty winds to the region. Similarly to the first storm, this system is expected to gather subtropical moisture along its leading warm front, which will then bring a second round of heavy rain to the Alaska Panhandle and snow/mixed precipitation to the Chugach Mountains on Thursday. Temperature-wise, much of mainland Alaska will experience below normal low temperatures throughout a majority of the period due to a cold front that passes through the state on Sunday and Monday. Though temperatures will likely only sit 10 to 15 degrees below normal, which is slightly below the hazardous threshold, the low temperatures combined with the duration of this cold event support the inclusion of a much below normal temperature area. Zavadoff