Upcoming Major Snowstorm & Major Ice Storm, Plus Severe Weather

Confidence is increasing in a significant winter storm to impact parts of Texas, Oklahoma, Kansas, Missouri, Iowa, Illinois, Wisconsin, Indiana, and Michigan today through the day after New Years'. Snowfall amounts of 5-8" likely today across parts of SW Texas with some areas seeing 8-12" of snow accumulation. Snowfall amounts of 3-5" are likely across parts of northern Texas, Oklahoma, and Kansas as this low begins to track to the NE. Significate icing is likely to occur with freezing rain across parts of Oklahoma, southwestern Kansas, Missouri, Illinois, Indiana, Michigan, and Ohio. This may lead to travel disruptions and hazardous travel conditions on New Year's Eve and New Year's Day. Ice accretion of >0.1" is possible with areas that could see accretion of >0.25". Severe thunderstorms and tornadoes are forecasted to impact parts of the Gulf Coast and Lower Mississippi Valley tomorrow. These storms will then move off the east impacting parts of Northern Florida to southern North Carolina on New Year's Day.

An upper trough will continue to advance east across the Plains driving a cold front southeast across the southern Plains. By late today, an upper low closing off within the southern stream over northern Mexico will begin to advect moisture north of the

surface boundary and into the low-to-mid level front centered over Southwest Texas. Thermal profiles support snow accumulating snow, with WPC PWPF showing high probabilities for accumulations of 6-10 inches in the Davis Mountains. Precipitation type uncertainty and a likely mixture leads to lower snow amounts in surrounding valley areas.

The upper low will continue to deepen as it moves across northern Mexico from Wednesday night into Thursday morning as an MSWT and jet max moves into the base of the trough. The depth of the cold air is expected to remain sufficient for snow on the northwest side of the developing low-pressure system, with several inches of accumulation likely across the Stockton and Edwards plateaus into the Concho Valley. Moist air advection (MAA) to the east of the low, and isentropic ascent will enhance snowfall across southwestern Texas, along with low-level frontogenesis enhancing lift. As the low moves north towards Oklahoma, the precipitation will change from mixed types over to snow in western Oklahoma as cooling aloft occurs in tandem with the low. WPC PWPF shows probabilities for accumulations of 4 inches or more across portions of western Texas to western Oklahoma and adjacent south-central Kansas. Higher probabilities are closer to the low with the MAA overriding the warm front and the enhanced lift from the frontogenesis band, but less in Oklahoma and Kansas due to the mixed precipitation types. An icy mix is more likely farther to the east, with accumulating ice expected from the Texas Hill Country northward to central Oklahoma, southeast Kansas, and northern Missouri up into Illinois, Indiana, and southern Michigan. There is potential for a quarter-inch of icing in portions of northern Missouri to eastern Kansas with some areas that could see greater ice accumulation in Missouri. Icing will continue across Illinois, northern Indiana, and lower Michigan on New Year's Day. The axis/orientation of both snow and sleet/freezing rain depends on the low track, with east-west spread remaining that results in different models having differences in locations with heavier snow and freezing rain potential. However, the GFS, EURO, and NAM all seem to have an agreeance on the low-track.

Forecasted snowfall totals across southwestern Texas.

Current NWS Winter Watches, Warnings, and Advisories.

Day 1-3 Winter Storm Impacts: Heavy snow in southwest Texas will create extreme impacts on Days 1 and 2. On Days 2-3, the combination of snow and mostly ice will create minor to moderate impacts.

Be sure to continue to monitor road conditions throughout the day. Turn on the vehicle's low beams to improve visibility. Also, slow down. Increase your following distance from other cars. Do not use cruise control. If you feel it's too dangerous to continue, pull off in a safe area until the conditions improve.

The greatest threat for tornadoes will be along the Gulf Coast in parts of Louisiana, Mississippi, and western Alabama.

SPC DAY 2 (New Year's Eve) Outlook Forecast Discussion:

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. An upper-level low will move from northern Mexico into central Texas on Thursday. The associated upper-level trough will become negatively-tilted as a 70 to 90 kt mid-level jet rounds the base of the system. During the day, the moist sector will extend from the far southeast Texas eastward across Louisiana, southern Mississippi and southern Alabama. A surface low will move north-northeastward across the far western edge of the moist sector. Near the surface low, a large cluster of thunderstorms will be ongoing in the Texas Coastal Plain at the start of the period. This convection will be supported by large-scale ascent associated with the nose of the mid-level jet. The cluster of storms will move east-northeastward to the upper Texas Coast by afternoon and into the lower Mississippi Valley by early evening. Ahead of the cluster of storms, model forecasts increase MUCAPE into the 500 to 1500 J/kg range over much of the moist sector. In spite of the weak instability, the mid-level jet will create strong deep-layer shear profiles, which will be favorable for severe storms. A severe threat is forecast to first develop in far southeast Texas early Thursday afternoon as a 50 to 60 kt low-level jet moves inland across southwestern Louisiana. The severe threat should be the greatest along and ahead of a pre-frontal trough, which will progress eastward across southern Louisiana during the late afternoon and early evening. At that time, low-level shear is forecast to become maximized due to the presence of the 850 mb jet.

The jet will create 0-3 km storm relative helicities of 300 to 400 m2/s2, which will support a tornado threat with the more dominant rotating storms. The tornado threat may become maximized in southeastern Louisiana during the late afternoon and early evening, near a pocket of moderate instability. Wind damage will also be possible with the stronger thunderstorms embedded in the cluster. The severe threat is expected to continue across southern Mississippi and southern Alabama during the mid to late evening. The threat may persist into the early overnight period, especially if a line of strong thunderstorms can persist.

An upper-level trough over the western US, bringing cold air. Diffluence aloft over the Southeast/Gulf Coast Region will provide upper-level support and exhaust aloft for thunderstorms.

Strengthening low-level jet over Lousiana.

500mb, An upper-level Low and PVA (positive vorticity advection) will provide more upper-level support enhancing upward vertical motion and spin.

Moisture: Southeasterly winds ahead of the SFC low and the advancing front will advect warm, moist, unstable air across the region.

Moisture advecting an unstable airmass over the region. MUCAPE of 500-1500 j/kg will provide enough instability/energy for thunderstorms.

0-6KM shear is supportive of organized convection.

Storm Relative Helicity, a strengthening low-level jet will create 0-3 km storm-relative helicities of 300 to 400 m2/s2, which will support supercells and a tornado threat with the more dominant rotating storms.

Significant Tornado Parameter at 4PM EST/ 3PM CST.

Day 3 (New Year's Day)

Thunderstorms associated with marginally severe wind gusts will be possible on Friday into Friday night from northern Florida to southern North Carolina. An upper-level low will move northeastward into the mid Mississippi Valley on Friday as southwest mid-level flow remains from the Southeast into the Carolinas. By afternoon, a moist airmass will be located from eastern Alabama to South Carolina. This airmass is forecast to remain weakly unstable during the day. Thunderstorms are forecast to first develop in Alabama during the morning and then spread eastward across Georgia and South Carolina during the afternoon. Deep-layer shear of 40 to 50 kt across the moist sector will make conditions supportive of marginally severe storms. The best chance for a severe threat may develop in the early evening across parts of South Carolina as moisture advection increases surface dewpoints into the mid to upper 60s F. As a result, MLCAPE may peak in the 500 to 750 J/kg range across parts of southern and eastern South Carolina. The stronger storms could have an isolated wind damage threat. A marginal tornado threat will also be possible, associated with storms that remain discrete. The marginal severe threat may affect parts of southern North Carolina during the late evening and early overnight period.



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