Confidence is high in a significant winter storm Today and on 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.
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.
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 with isolated higher amounts near 0.25".
Winter Weather Advisories and Winter Storm Warning stretch across the Central Plains into the Midwest and Great Lakes Region.
Winter Storm Warning from the NWS Forecast Office Quad Cities, IA/IL
.WINTER STORM WARNING NOW IN EFFECT UNTIL 9 AM CST WEDNESDAY...
* WHAT...Heavy snow expected. A period of mixed precipitation is
also expected for counties south of highway 20. Total snow and
sleet accumulations of 6 to 10 inches and ice accumulations of
a light glaze.
* WHERE...Portions of east central, northeast and southeast Iowa
and northwest Illinois.
* WHEN...Until 9 AM CST Wednesday.
* IMPACTS...Travel will be very difficult to impossible. The
hazardous conditions will impact the evening commute and could
impact the Wednesday morning commute.
* ADDITIONAL DETAILS...Peak snowfall rates up to 2 inches per
hour, combined with southeast winds gusting up to 30 mph, may
drop visibilities below a quarter mile in the heaviest bands
of snow this afternoon and evening.
Mesoscale Discussion 1884
NWS Storm Prediction Center Norman OK
1002 AM CST Tue Dec 29 2020
Areas affected...Northeast KS...Eastern NE...Southwest/central
Concerning...Winter mixed precipitation
Valid 291602Z - 292100Z
SUMMARY...Moderate to occasionally heavy winter precipitation will
continue into this afternoon. The snow/freezing rain and freezing
rain/rain lines should gradually shift northward with time, with
some sleet possible within the snow/freezing rain transition zone.
Severe thunderstorms associated with a tornado threat and wind damage will be possible Thursday into Thursday night from far southeast Texas into the lower Mississippi Valley and across the central Gulf Coast States. Discrete thunderstorms that develop ahead of the cold front across the moist sector could have a wind damage and tornado potential. During the late afternoon, the most favorable area would be in far southeast Louisiana, where the models maximize instability
at the northern end of the moisture axis. This corridor of maximized low-level moisture is forecast to shift eastward across southern Mississippi and southern Alabama during the evening and overnight period. Models suggest the possibility of a convective line located along the moisture axis as the low-level jet increases over the region. This line would be associated with a continued wind damage with those increased low-level winds with the increased low-level jet and tornado threat during the overnight period.
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.