Major Winter Storm and Severe Weather

Updated: Dec 29, 2020

Confidence remains moderate in a significant winter storm on Tuesday and Wednesday. Heavy snow is most likely across parts of Nebraska, Iowa, South Dakota, Minnesota, and Wisconsin. Snowfall amounts of 4-8" are likely across parts of northern Nebraska, northern Iowa, southeastern South Dakota, southern Minnesota, and central Wisconsin with isolated 8"+ possible for northern Iowa and central Wisconsin. Snowfall rates may exceed 1"/h. This with the combination of gusty winds may lead to travel disruptions and hazardous travel conditions. Freezing rain will be possible from eastern Kansas and southeastern Nebraska through northern Missouri and into Iowa and Illinois. More than 0.1" of accretion is possible. Severe Thunderstorms likely across the southern Plains and into the lower Mississippi Valley on Thursday. The main threats would be for tornadoes and wind damage. This system will continue into the SE CONUS and Piedmont Valley region on Friday with the main threats being a few tornadoes and wind damage.

A longwave trough moving eastward across the Rockies into the northern and central Plains will bring deep-layer ascent for low development and strengthening of the SFC low expected to move northeast across the mid-Mississippi Valley Tuesday night to the upper Great Lakes. As the low moves northeastward, it will be accompanied by a band of low-mid level frontogenesis and warm air advection (WAA) bringing strong ascent and force. Within this band of WAA and strong forcing and ascent snow rates of >1"/h for a few hours is possible. Severe inches of snow is expected in northern

NE and southern SD to southern MN, northern IA, and southern WI. The higher amounts are expected in portions of northeast IA to southern WI within the longer duration low-level jet and enhancements from strong ascent and frontogenesis. The low-level warm advection causes a change from snow to freezing rain and eventually rain across most of KS and northern MO into parts of southern NE/southern IA, followed by northern IL. The existence of multiple precipitation types and uncertainty remains across the region regarding the duration of each one drives the slight forecast uncertainty. The possibility of ice accumulations as much as a quarter-inch exists in northwest MO to southern IA. WPC probabilities indicate

a moderate risk for 0.1" of accretion on Tuesday with isolated higher amounts near 0.25". With a more progressive solution and the continued northeast movement of the low-mid level front, probabilities for heavy snow have trended down just slightly. However, there is still a high risk for 4 inches from central NE into southeast SD, northern IA, southern MN, and much of southern to central WI. The heaviest

snow, which may reach 8", is most likely across NE IA into southern WI. Isolated areas could see close to 12" of snow.

An upper-level low is forecast to move northeastward across the southern Plains on Thursday as an associated cold front advances eastward into the lower Mississippi Valley. Moisture advection appears likely to continue ahead of the front across the central Gulf Coast states where surface dewpoints should be in the 60s F. This moist and unstable environment will interact with the advancing cold front and thunderstorms may develop during the day along the front with additional convection forming further east across the moist sector. Model forecasts suggest that a low-level jet will increase in strength, moving north-northeastward across Alabama during the late afternoon. Convective development may be favored on the nose of the low-level jet during the late afternoon and early evening as noted by the instability plumb in the video below. Additional storms may develop further to the west across the lower

Mississippi Valley. Low-level and deep-layer shear profiles should be favorable for severe storms across those two areas as favorable directional and speed shear suggest organized convection will develop. The main threats would be for tornadoes due to the backed SE SFC winds and the SW winds aloft and wind damage as those winds will mix down with any thunderstorms that develop. At this point, there is some uncertainty concerning how far the quality moisture can advect northward.

On Friday, the upper-level low is forecast to move north-northeastward into the mid-Mississippi Valley as the southwest mid-level flow remains entrenched across the Eastern Seaboard. Thunderstorm development will be possible along and ahead of a cold front as a low-level jet moves northward across the Carolinas. Low-level and deep-layer shear profiles are forecast to be favorable for severe storms with the main threats being tornado and wind damage.

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