US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 421 PM EDT Wed Nov 03 2021 Valid Saturday November 06 2021 - Wednesday November 10 2021 Hazards: - Heavy rain across portions of California and the Pacific Northwest, Mon-Wed, Nov 8-Nov 10. - Heavy snow across portions of California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Mon-Wed, Nov 8-Nov 10. - Heavy snow across portions of the Central Rockies, the Central Great Basin, the Northern Rockies, the Northern Great Basin, and the Northern Plains, Tue-Wed, Nov 9-Nov 10. - Flooding occurring or imminent across portions of the Northeast and the Great Lakes. - Flooding occurring or imminent across portions of the Northeast. - Flooding likely across portions of the Middle Mississippi Valley. - Flooding likely across portions of the Middle Mississippi Valley. - High winds across portions of the Mid-Atlantic, Sat-Sun, Nov 6-Nov 7. Detailed Summary: During this upcoming medium range period (Saturday, November 6th - Wednesday, November 10th), much of the active weather will focus across the West Coast. A series of storms pivoting through eastern Pacific upper troughing will produce rain and elevation snow beginning Saturday over the Pacific Northwest, northern California, and eastward across the northern Intermountain Region. Typical medium range shortwave timing issues remain with the forecast, but reasonable agreement exists that the influx of Pacific moisture should stay weak over the weekend, which will limit hazardous precipitation potential through Sunday. Daily precipitation amounts in the Pacific Northwest should remain on the moderate side, with accumulating snow at higher elevations this weekend. The most impactful storm system is expected to arrive over the Pacific Northwest early next week, this time with a healthy feed of Pacific moisture and more robust onshore flow to support moderate to heavy rainfall and heavy mountain snows. Beyond Sunday, uncertainty increases regarding the timing, placement, and strength of the storm as it arrives inland, with ramifications for where the highest precipitation totals fall. However, the general idea of decreasing snow levels, strong upslope flow, and moist approaching airmass (corroborated by ensemble and analogue guidance) prompted the introduction of a Heavy Snow area over the Sierra Nevada and Cascade Range beginning Monday. As the system lifts east toward midweek, accumulating moderate to locally heavy snows should spread atop the Bitterroot and Absaroka Range, where a Heavy Snow area was added for a similar rationale as above. Moreover, deepening low pressure as the system ejects into the Plains Wednesday may produce gusty winds along and east of the Rockies, which will be monitored in subsequent Hazards updates. Farther east, guidance continues to highlight the potential for low development off the Southeast coast this weekend as a Midwest shortwave phases with an amplified southern stream upper-trough and interacts with a stationary front. The heavy rainfall threat associated with the resulting low pressure system is closely tied to the low track as it moves east out of Florida on Saturday. An outlier track (i.e. the ECMWF) progresses the low closer to the Coastal Carolinas, which would support higher inland rainfall amounts. However, the WPC medium range forecast continues to favor a system track more out to sea based on all other guidance, which would keep the highest rain totals offshore the Mid-Atlantic. Rain aside, the tight gradient between high pressure over New England and the developing cyclone will result in strong winds along coastal sections of South Carolina and the Outer Banks of North Carolina. The highest winds are likely to be on Saturday, but may persist into Sunday depending on the exact track and strength of the low. Temperature wise, some locations in the Ohio Valley could see first freeze potential on Saturday as overnight low temperatures dip slightly below 32 beneath cool high pressure. Afterwards, a warming trend will spread eastward out of the Plains toward the Mid-Atlantic as an extension of subtropical high pressure over Mexico migrates east through Day 7. Overnight minimum temperatures will remain well above average beyond Saturday, owing to rising heights in the east, and gradually increasing cloudiness in the western and central CONUS. Moreover, maximum temperature anomalies 15-20 degrees above average will translate out of the Plains on Sunday with the ridge before moderating, leaving a broad swath of anomalies 5-10 degrees above average over the central and eastern CONUS through midweek. Alaska remains fairly quiet through the medium range. The forecast upper level flow will suppress the Pacific storm track south of a large part of the state, so significant precipitation is not likely. Some gusty winds are possible across the Aleutians Monday and Tuesday as low pressure skirts to the south, but are not expected to reach hazardous levels for now. Temperatures will remain above normal for the central/eastern interior sections into the weekend under southerly flow, but an overall cooling trend is expected as upper ridging builds north of the state and shuts off the southerly inflow. Asherman/Klein