US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 324 PM EST Thu Jan 28 2021 Valid Sunday January 31 2021 - Thursday February 04 2021 Hazards: - Heavy precipitation across portions of the Northeast, Mon-Tue, Feb 1-Feb 2. - Heavy precipitation across portions of the Mid-Atlantic, Sun-Mon, Jan 31-Feb 1. - Heavy rain across portions of the Southeast, California, the Mid-Atlantic, and the Pacific Northwest, Sun-Mon, Jan 31-Feb 1. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Thu, Feb 4. - Heavy snow across portions of California, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sun-Mon, Jan 31-Feb 1. - Heavy snow across portions of the Northeast, the Central Appalachians, the Pacific Northwest, the Mid-Atlantic, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Sun-Mon, Jan 31-Feb 1. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Sun-Tue, Jan 31-Feb 2. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Mon-Tue, Feb 1-Feb 2. - Heavy snow across portions of the Central Rockies, the Central Great Basin, the Northern Rockies, and the Northern Great Basin, Tue, Feb 2. - Heavy snow across portions of the Central Great Basin, Tue-Wed, Feb 2-Feb 3. - Heavy snow across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Northern Plains, the Great Lakes, the Southern Rockies, the Upper Mississippi Valley, the Southern Plains, and the Southwest, Wed-Thu, Feb 3-Feb 4. - Heavy snow across portions of the Great Lakes, the Central Appalachians, and the Ohio Valley, Sun, Jan 31. - Heavy snow across portions of the Mid-Atlantic and the Northeast, Mon-Wed, Feb 1-Feb 3. - Flooding possible across portions of the Middle Mississippi Valley. Detailed Summary: The medium range period (Jan 31 - Feb 4) contains a plethora of hazards from coast to coast. Starting in the East, guidance is trending towards a better consensus on an amplifying upper trough that sets the stage for a major winter storm in the Mid-Atlantic and Northeast early this week. The latest 500mb forecast chart 12Z Sunday morning indicates an upper low over northern Illinois digs south and east underneath a strong ridge of high pressure east of Hudson Bay. A building ridge over the Intermountain West and lingering confluent flow over the northwest Atlantic allows the upper trough to slow down and deepen as it approaches the East Coast. At the surface, an occluded low pressure system tracking through the Ohio Valley heads east while a coastal front strengthens over the eastern Carolinas. Heavy snow falling north of a warm front draped over the Ohio Valley leads to heavy snowfall accumulations across parts of the Lower Great Lakes. Warm air advection and isentropic lift stemming from the Southeast into the Mid-Atlantic will lead to blossoming areas of precipitation throughout the day on Sunday. Precipitation looks to fall in the form of snow from the Smokey and Blue Ridge mountains to the DC/Baltimore metro areas while a wintry mix leads to potentially treacherous ice accretion in the Piedmont region of southern VA and west-central North Carolina. How much accumulates in these areas will be dependent upon the moisture flux, best frontogentical forcing and placement, and depth of cold air damming at low levels. Hazardous travel is likely in these regions on Sunday with conditions worsening across the northern Mid-Atlantic into the evening hours. What happens next with the upper trough progression and intensification still remains somewhat unclear. While ensemble guidance does agree across the board on an amplifying trough and eventual mid-upper level low positioned near the coast, the "how, when, and where" are vital to the positioning of a couple key features: the areal extent, position, and duration of a dry slot aloft, and the cold conveyor belt and resulting deformation zone that forms as the coastal low quickly intensifies. These features of interest remain hard to nail down over 72 hours out. Recent 12Z ensembles did come into better alignment on where the upper low may become positioned off the coast, but timing of the primary-to-coastal transition and the coastal low's duration/position near the coast still vary across many ensemble members. Regardless, the coastal low tucked in just off the Northeast coast and cold Canadian high pressure to the north is a prime setup for heavy snowfall from the northern Mid-Atlantic to New England Monday and into Tuesday. Depending on how long the low stays in place, snow may fall in some locations for 24-48 hours. Resulting hazards: significant snowfall appears to be on tap from northern Virginia and Maryland up the Appalachians and along the I-95 corridor Sunday night, through Monday, and continuing into Tuesday (especially in southern New England). While exact totals remain subject to change, latest WPC snow probabilities show the axis for heaviest snowfall resides from the central Appalachians on northeast through east-central Pennsylvania and into interior portions of southern New England. Expect near whiteout conditions under the heaviest mesoscale bands with snowfall rates over 1" per hour. A "heavy precipitation" area denotes uncertainty with precipitation types; south-central Virginia and north-central North Carolina could witness disruptive ice accretion on Sunday while parts of eastern Virginia, the central and lower DelMarVa, and the Massachusetts Capes could mix with, or at times be all, rain for parts of the event. Along the coast, strong onshore winds may lead to battering surf that could promote areas of coastal flooding and beach erosion. Not to be out done, the West and the Plains also have their own storm system to contend with. A potent frontal system ushers in Pacific moisture to the West Coast. Heavy rainfall along the coast and in the valleys is likely from western Washington on south into central California. Meanwhile, heavy mountain snow will blanket the Cascades and Sierra Nevada. The upper trough responsible for the development of the aforementioned frontal system tracks through the Intermountain West the first couple days of February and looks to produce areas of heavy mountain snow throughout the region. By mid-week, the trough deepens as it enters the Plains and leads to the an influx of moisture within the warm sector of a strengthening surface low. This should lead to heavy showers and thunderstorms in parts of the Mid-South in days 6-7. On the storm system's northern flank, periods of heavy snow are possible along with the potential for gusty winds. The axis of heaviest snowfall may still shift in the coming forecast cycles due to high spread among ensemble guidance. Hazardous travel is possible across portions of the northern Plains and Upper Midwest where the heaviest snow totals preside. In Alaska, a passing cold front delivers a shot of below normal temperatures to most of the western regions, but departures appear to fall short of "much below normal" criteria. The next potential weather maker comes in the form of a strong cyclone west of the Aleutians. Gusty winds are likely Sunday and into Monday but do not appear to reach excessive levels. Ensemble guidance does suggest a plume of rich Pacific moisture streams north towards the Gulf shores by early next week, but high model variance and the amplification of an upper ridge over the Gulf of Alaska has resulted in precipitation totals falling short of heavy thresholds. Will revisit this again tomorrow to see if confidence and totals rise enough to add a "heavy precipitation" area on Friday. Mullinax