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Major Winter Storm/Snowstorm/Ice Storm & Severe Weather

Our confidence is high on an expansive Major Winter Storm to spread freezing rain and snow across a large portion of the Southern and Central Plains, the Upper Mississippi Valley, the Midwest, and Ohio Valley New Year's Eve and into New Year's Day. Freezing rain will be likely falling across portions of Oklahoma, Kansas, Missouri as will ring out the New Year before spreading its way to the northeast. Impactful freezing rain across this area will lead to hazardous travel, with power outages possible due to downed tree limbs and wire from the ice. Severe thunderstorms capable of damaging winds and tornadoes are expected across the Gulf Coast and the lower Mississippi Valley.


A deep upper-level trough is digging its way south across the western CONUS. The upper-level over northern Mexico will continue to deepen as it moves across northern Mexico today as an MSWT and jet max moves through the base of the upper-level trough. This upper-level low will gradually begin to fill as those upper-level features move downstream as it lifts northeast across the Central Plains and towards the Great Lakes Friday night, and then into New England Saturday. This deep upper-level trough is bringing in a depth of cold air that is expected to remain sufficient enough for snow on the northwest side of the developing low-pressure system, with several inches of snow accumulation likely. Beneath this mid-level feature, a surface low pressure is likely to lift northeast from TX to MO today, and then more rapidly to the northeast to be off the coast of Maine by Saturday afternoon. This low will be accompanied by a leading warm front, and robust antecedent WAA spreading significant and widespread precipitation across the eastern half of the CONUS. 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. Cool surface high pressure ahead of this low will be reinforced by mid-level confluence ahead of the upper feature, and as WAA spreads precipitation northward, an expansive area of freezing rain is likely. The WAA is likely to be quite strong so many areas from eastern OK through the Ohio Valley and into parts of the Northeast will start as freezing rain but then change to rain. Further north, a band of significant freezing rain is likely, and WPC probabilities for 0.25" are greater than 50% today in eastern KS and much of central MO, lifting into northern MO, IL, and IN/MI tomorrow, as well as spreading into the higher terrain of

PA ahead of the expansive warm front where a Cold Air Damming setup is likely. Isolated amounts to 0.5" are not out of the question, but heavy rates should somewhat limit accretion efficiency and the antecedent surface airmass is not extremely cold. In addition to the freezing rain, areas of heavy snow are also likely on the northern and western edge of this system. Today heavy snow is likely in West Texas beneath the upper low where lapse rates and cold air aloft combine to drive instability and heavy snow rates. The strong height falls, intense upper diffluence, and steep lapse rates will likely combine with mid-level frontogenesis to drive a corridor of heavy snowfall, and WPC probabilities are high for 4 inches, with locally as much as 8" possible with some areas seeing 10-12" near Alpine.


Tomorrow, New Yea's Day, as the low shifts northeast, a trough of warm air aloft is likely to develop throwing rich theta-e air NW combining with an axis of deformation as northern stream energy interacts with the primary trough. This should lead to a band of heavy snow NW of the center from north-central OK into eastern KS

and far NW MO. WPC probabilities for this area have increased as the HREF snow band probabilities indicate a high chance for 1"/hr snow rates as the low pulls off to the northeast. There is a moderate risk for 4 inches across this area tomorrow. As the low

moves further northeast and begins to spread precipitation across New England on New Year's Day and the second, heavy snow is likely to overspread much of upstate NY and northern New England which will be cold enough for primary snow. Eventually, the WAA may still cause a changeover to freezing rain in some places, but WPC probabilities for 4" of snow are high in the Adirondacks, Greens, Northeast Vermont, and much of northern NH and ME.



Current Winter Weather Watches, Warnings and Advisories

Forecasted Snowfall


Freezing Rain Accumulation


Day 1-3 Winter Storm Impacts: Heavy snow in southwest Texas will create extreme impacts today. New Year's Day and January 2nd, 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.


Severe thunderstorms with tornadoes and damaging winds as the primary hazards will initially be possible across the Upper Texas coast and southwest Louisiana through this afternoon, before expanding across the rest of Louisiana into Mississippi and Alabama this evening and persisting through tonight.



A mid-level closed low over western Coahuila will amplify as it advances north into OK by early Friday, in response to an upstream shortwave trough digging south-southeast from southern CA to Sonora.

At the surface, a 1004 mb cyclone should track north from the

Middle/Upper TX coast to northeast TX. By 06Z, this cyclone will

become increasingly occluded, yielding a T-shaped surface pattern

with the apex of the warm sector across the Ark-La-Miss as a cold

front sweeps east across the western Gulf and a west/east-oriented

warm front advances north across central portions of MS/AL. The warm

sector characterized by boundary-layer dewpoints in the mid-upper

60s will spread inland to the east of the cyclone and contribute to

MLCAPE of 500-1000 J/kg across extreme southeast TX/southern LA

through this afternoon, and into tonight across central/southern

portions of MS and AL.


Thunderstorm development has been focused near and offshore of the

Upper TX coast this morning, with multiple supercells, noted

offshore. Convection will spread northeast through tonight along and

just ahead of the surging cold front. An additional swath of

convection will likely develop separately downstream across the

central Gulf coast by early evening and persist north over eastern

MS and AL through tonight within a robust low-level warm advection

regime.


Greater low-level shear and enlarged hodograph curvature will

initially, be confined to the TX/LA portion of the outlook region

this afternoon before spatially expanding this evening as flow

fields strengthen downstream of the amplifying low. The combination

of linear forcing for ascent along the cold front and substantial

cross-boundary flow of deep-layer shear vectors will support broken

bands of supercells and line segments, capable of producing

tornadoes and damaging winds. However, the magnitude of the tornado

threat should be tempered some by rather modest low-level lapse

rates inland, and less-than-ideal phasing with the ejecting

mid-level trough and surface cyclone glancing the northwest side of

the warm sector. 12Z HREF and recent HRRR guidance remain subdued

with the intensity and coverage of rotation potential within

simulated cells, lowering confidence for identifying a corridor of

10 percent or greater probability of significant tornadoes.


SPC Day 1 Tornado Probability- 10% driving the Enhanced Risk for severe thunderstorms across the Gulf Coast and Lower Mississippi Valley region.



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

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.

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

Strengthening low-level jet over western Louisiana.

Our moisture is advecting an unstable airmass over the region. MUCAPE of 500-1500 j/kg will provide enough instability/energy for thunderstorms. CAPE will be limited due to cloud cover, and not enough mixing to steeping low-level lapse rates.


Storm Relative Helicity, a strengthening low-level jet will create 0-3 km storm-relative helicities of 300 to 500 m2/s2, which will support supercells and a tornado threat with the more dominant rotating storms.

0-6KM shear is supportive of organized convection and supercells

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


Thunderstorms associated with isolated severe wind gusts and a marginal tornado threat will be possible on Friday from the Florida Panhandle northward to the Ohio Valley and northeastward into South Carolina.


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

Split flow aloft will persist across the U.S. Friday. An upper low

in the southern stream -- initially forecast over Oklahoma -- will

shift quickly northeastward into the Midwest with time, gradually

becoming re-absorbed into the northern stream of westerlies through

the end of the period. Meanwhile, a second low in the southern

stream -- initially over northwestern Mexico -- will progress

steadily east-northeastward into the southern Plains through the

period.


At the surface, a low over the eastern Oklahoma/Arkansas vicinity

Friday morning will move northeastward across the Ozarks through the

day, and then continue northeastward through the evening and

overnight, reaching the Lower Great Lakes Saturday morning. A cold

front trailing from this low is forecast to advance slowly eastward

across the central Gulf Coast states/Mid South region through the

first half of the period, and then approach/reach the central and

southern Appalachian crest by the end of the period. East of the

mountains, a cold air dam/wedge front lying across northern Georgia

and South Carolina will linger in place through the day, before

weakening overnight as low-level southerly flow evolves across the

East Coast states ahead of the advancing cold front.


...Kentucky/Tennessee south to the Florida Panhandle...

Showers and isolated thunderstorms are forecast to be ongoing at the

start of the period, from near the Missouri/Illinois border

south-southeast across western Tennessee and into eastern

Mississippi/western Alabama, in the vicinity of the cold front.

Ahead of the front, weak lapse rates and widespread cloud cover --

plus scattered, warm-advection-induced convection will substantially

limit warm-sector instability. Still, favorably strong deep-layer

flow, and veering in the low-level wind field -- particularly with

northward extent -- will support potential for a few

organized/rotating storms. While severe potential should remain

subdued given the thermodynamic deficiencies, a couple of locally

damaging gusts, or a tornado or two, cannot be ruled out.


...Georgia and South Carolina vicinity...

Showers and scattered thunderstorms are forecast to be ongoing over

portions of the area at the start of the period, and will persist

through the afternoon and into the evening, within a persistent zone

of warm advection ahead of the cold front. Given weak lapse rates

and widespread precipitation and associated cloud cover, minimal

destabilization potential is noted. Still, with strong deep-layer

flow across the region, a few stronger storms -- capable of gusty

winds possibly nearing or reaching severe levels, may occur. In

addition, limited potential for a tornado or two may also evolve,

perhaps greatest near the remnant wedge front where low-level

vorticity would be maximized. Severe risk will eventually diminish

into the overnight hours, as weak short-wave ridging -- and

associated large-scale subsidence/warming aloft evolves across the

Southeast through the end of the period.



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