US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 419 PM EDT Thu Oct 28 2021 Valid Sunday October 31 2021 - Thursday November 04 2021 Hazards: - Heavy precipitation across portions of the Central Great Basin and California, Thu, Nov 4. - Heavy rain across portions of California and the Pacific Northwest, Mon, Nov 1 and Wed-Thu, Nov 3-Nov 4. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Tue-Wed, Nov 2-Nov 3. - Flooding possible across portions of the Mid-Atlantic. - Flooding occurring or imminent across portions of the Northeast and the Great Lakes. - Flooding likely across portions of the Middle Mississippi Valley, the Pacific Northwest, and the Northern Great Basin. - Much below normal temperatures across portions of the Central Plains, the Lower Mississippi Valley, the Southern Plains, the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Tue-Thurs, Nov 2-Nov 4. - Heavy precipitation across portions of mainland Alaska, Sun-Tue, Oct 31-Nov 2. - High winds across portions of mainland Alaska, Sun, Oct 31. Detailed Summary: As October ends and the calendar flips to November, the weather pattern over most of North America can be summarized with a massive upper level ridge over western Canada aiding in the development of downstream troughing over the Upper Midwest and Great Lakes. Meanwhile, an active storm track in the North Pacific will occasionally send disturbances into the West Coast that undercuts the ridge over western Canada. The resulting temperature pattern is for cooler than normal temperatures from the High Plains to the Ohio Valley and Great Lakes. Temperatures in the Mid-South and the Northeast, after starting out more seasonal, should also trend cooler in the latter half of the period. The coldest temperature anomalies are forecast to set up from the Central Plains to the lower Great Lakes. It is there where some locations may also witness their first freeze of the season. This combined with temperature anomalies approaching 10-12 degrees below normal for multiple days has resulted in the issuance of a Much Below Normal temperature outlook area for these regions. In terms of precipitation hazards, an elongated frontal boundary stretching from the Four Corners region and Southern Plains to the Mid-South becomes the focal point for active weather the first half of next week. As an upper level disturbance exits the Intermountain West, precipitation will blossom along the front, leading to periods of heavy rain and even thunderstorms on Tuesday (11/2) across the Southern Plains. By Wednesday (11/3), the heaviest precipitation advances east into the Lower Mississippi Valley with trailing convection possible into eastern Texas. Given the long-wave trough positioned over the Great Lakes, the aforementioned disturbance ejecting out of the Intermountain West may head for the East Coast and bring wet weather to the Mid-Atlantic and Northeast. Some guidance has even suggested the potential for wintry weather in the northern and central Appalachians. This potential scenario will be monitored in the coming days, but for this forecast cycle, confidence in totals, timing, and position of heaviest amounts were low confidence. Additionally, the West Coast will continue to be susceptible to stretches of wet weather thanks to the continuously active North Pacific jet stream pattern. The first wave of Pacific moisture arrives on Monday (11/1) as a frontal system associated with an occluded low near the Gulf of Alaska approaches. Rain will be the primary precipitation type, although some accumulating snowfall in the highest elevations of the Cascades, Shastas, and northern Sierra Nevada cannot is likely. The heaviest bouts of precipitation for the period arrives Wednesday and Thursday (11/3-11/4). This time, the trough will be deeper and be more supportive of generating heavy mountain snow in the northern and central Sierra Nevada with heavy rain along the lower elevated slopes. This is likely shaping up to be another atmospheric river setup with no shortage of moisture (12Z GEFS shows PWs of +2 STDs out to day 6 near the California coast). This continues to be a welcome sight for the drought-stricken West, but should excessive rainfall rates take place, flash flooding near burn scar areas would be possible. In Alaska, a steady barrage of wet, wintry, and windy weather is expected from the Alaskan Peninsula and Kodiak Island through the south-central mainland this weekend into early next week as a strong and slow moving low pressure system works into the Gulf of Alaska. Increased moisture streaming inland ahead of the system will fuel several days of heavy rain and snow at lower elevations and significant mountain snowfall. This includes the Alaska Range, Chugach Mountains and the Kenai Mountains. Multiple feet of snow is likely through the period as one dissipating occluded low over southern Alaska gives way to another one tracking into the Gulf of Alaska early next week. In addition, strong mountain pass and Chinook winds to the lee side are possible. On the southern periphery of this storm, a Pacific cyclone, carrying the remnant moisture of Typhoon Malou, will also track towards the Panhandle by mid-week. There is still enough uncertainty and spread in ensemble guidance that exact impacts and where they occur are too low of confidence to add threat areas at this time, but it could mean soaking rainfall and gusty winds for the Panhandle towards the middle of the week. Mullinax