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A strong trough digging ESE out of the Rockies with an associated ULJ will promulgate a disturbance across the Central Plains this evening into the Middle Mississippi River Valley and off into the Ohio River Valley towards the mid-Atlantic region by Friday morning.  A broad swath of heavier snowfall amounts from 4″ – 6″ is likely across the I-70 corridor from Kansas City, MO to near St. Louis, MO, with locally higher amounts of 7″ – 8″ possible in areas that experience the best frontogenetic forcing within the most favorable UVV/Omega zone.

Across the I-70 corridor overnight forecast QPF (total precipitation) values of 0.35″ – 0.43″ juxtaposed with the most favorable dendritic growth zone (ice crystal growth rate as a function of temperature) on the order of 100 mb (mean) and mean SLER (snow to liquid equivalent ratio) of 15:1.  As the system moves off into the mid-Atlantic region by FRI, snowfall amounts of 6″ to 12″ are not out of the question.

ChaserTV will have some live streams available for this event, so stay tuned to our live streams to watch the event as it unfolds across the region.

JLR

View 01/19 – 01/20/11 in a larger map

Current Surface Map per HPC

Forecast low pressure track ensembles through FRI

24 hr probability of snowfall 2"+ per HPC

SREP mean precip. / UVV/ Omega

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SYNOPSIS

Two major winter systems are taking shape over the continental United States, with the biggest weather maker being the gulf low that is currently sitting over far SE TX.  This low is forecast to move proximal to the Gulf Coast as it treks from near Corpus Christi, TX (1002 mb) this morning to near Jacksonville, FL by Monday evening, weakening slightly to near 1014 mb before moving into the Atlantic and strengthening (again) as it takes aim on the eastern seaboard.  The HPCs low track ensemble blend appears below (FIG 1).

This system will have ample gulf moisture to work with, with a warm layer aloft, along with ample cold air advection at the surface, meaning a wide variety of winter precipitation is in store for parts of the Gulf-Atlantic coastal plain region.  As you can see, freezing temperatures (FIG 2) currently extend all the way from NE TX to south of Atlanta, GA as a wedge of cold air interacts with ample warm, moister air sitting proximal to the Gulf of Mexico.

While snowfall amounts of 3-6+ inches will be possible across a broad section of the south, a wedge of warm air aloft will introduce the potential for ice as well, especially along and south of the I-20 corridor.  Though there is some dubiety as to where the actual rain/freezing rain/sleet and snow lines will fall, as winter systems are often difficult to forecast in this regard, this sounding below from near Montgomery, AL for tonight clearly shows the warm layer aloft atop surface temps just below the 0C (32 degrees F) freezing mark (see FIG 3 SKEW-T).

The system looks to be quite the prolific weather maker as it gyrates along the Gulf Coast and makes its way off into the Atlantic region overnight on Monday.  The HPC’s forecast snow (FIG 4) and ice accumulation (FIG 5) maps appear below, along with the SREF ensembles winter precipitation forecasts for the southern system (FIG 6).

Additionally, a longer duration snow event is on tap for parts of the Midwest, as a disturbance moves across that region, possibly dumping heaving snowfalls of 4″ – 8″ across a wide swath from NE into parts of MO and possibly SW IL through early Tuesday (see tomorrow’s FIG 7 HPC snowfall probabilistic graphic below for the Midwest).

As your go-to source for live and breaking severe weather video feeds, please tune back in to ChaserTV throughout the next 48 hours as we offer some streamer feeds from one or both of these regions as these potentially hazardous winter weather events unfold.  Folks across parts of the SE who will be impacted by the most adverse weather, including ice, should prepare for this event accordingly, staying tuned to local weather bulletins and preparing for the impossibility of travel, along with the potential for extended power outages where heavier ice accumulations are found.

JLR

Note: click on any of these images for a larger view.

FIG 1

FIG 2 - Current 2m Temps

FIG 3 - NAM Forecast SKEW-T

FIG 4 - Probability of snowfall >/= 4 inches through 7 AM MON

FIG 5 -Probability of ice accumulations >/= 0.25 inch through 7 AM MON

FIG 6- Preciptation Types (likely) per NCEP at 12z MON (Green - rain / Blue - snow / Red - freezing rain / Orange - sleet)

FIG 7 - MON Probabilistic Snowfall

A fairly potent winter system, dichotomous in its hazards yet tangential in its exact track (per trying to find consensus among the GFS, ECMWF and NAM), will move into the Plains and the Upper Midwest on New Year’s Eve, threatening to produce heavy snows (6” – 12”) with near blizzard conditions across parts of the Plains and possibly some severe weather across the Middle and Lower Mississippi Valley region.

A strong trough will dig equatorward, reaching into the Southern Plains by Friday. An associated surface cyclone will gravitate poleward from the northern TX panhandle (0z Friday) to southeast Minnesota (0z Saturday), deepening as it arrives in the western Great Lakes region early Saturday. Ample cold air and QPF on the back side of the system will support heavy snowfall of 6” -12” (see white shaded area on the map below) in parts of the Central and Northern Plains.

However, extremely strong shear will be found to the right of the mid-level jet core as it rounds the basal side of the trough, with SFC-500 mb bulk shear profiles of 50-70 kts present across a broad section of the western Gulf Coast extending northward into the Lower and Mid-Mississippi Valley region. As typical with winter setups, there will be fairly high amounts of shear overlapping what will probably be mostly meager MLCAPE values of 250 – 750 J/KG, though some higher values (AOA 1000 J/KG) may be found over parts of E TX into LA. The plenteous shear profiles will be courtesy of a strong SW – NE Upper Level jet core of 130+ kts juxtaposed with a mid-level jet core of 110+ kts and a strong WSW LLJ jet of 40-50 kts across the region. With such strong forcing and deep layer flow/shear vectors forecast, per rawinsonde data, to be parallel to the attendant cold front, a more linear mode of convection appears probable in the greatest risk area (see red shaded area on the map below).

Upper-level divergence proximal to the main jet core should help to erode CINH by FRI afternoon, allowing for semi-steep lapse rates. In this warm sector, 0-30m Td values are progged to reach 55-60 (F) across much of the Lower and Middle Mississippi Valley, though instability concerns remain due to the models showing some ongoing precipitation across the best severe risk area early in the period. The extent of any instability that is able to materialize will be highly dependent upon what, if any, clearing is able to take place. However, given the strong forcing and shear profiles across the region, it won’t take much to produce at least a line on SVR t-storms capable of damaging winds and some hail, though an attendant TOR risk can’t be ruled out, either.

Since we’re still 36-48 hours away from the event, none of the mesoscale details will be refined until closer to the event. Also, it should be noted that some synoptic-scale details also remain in question, such as the exact track and strength of the surface cyclone, as well as the actual amount of QPF on the NW side of the cyclone, which will influence snowfall amounts across the Northern Plains.
Stay tuned to ChaserTV, as we’ll likely have live streaming from one or both parts of this system as adverse weather evolves on Friday.

Forecast written for ChaserTV by Convective Addiction’s Jesse Risley.

View New Year’s Eve Storm in a larger map

18z Surface map (GFS) - 20101231

MLCAPE - 18z 20101231

H5 profile - 18z 20101231

2m Theta-E 18z 20101231

At this juncture it appears that there is both good news and bad news to share with many.  There have been significant changes in the overall synoptic pattern since the original winter weather “admonition” forecast was written on Sunday concerning potential holiday travel headaches this coming weekend.  In essence, the models have changed quite drastically since that time, which isn’t at all unexpected, though the ominous snowstorm that was potentially being advertised 4-5 days ago simply doesn’t appear likely to come to fruition.  While I’m still awaiting the 12z GFS/GEM/ECMWF data, the upper-air network is getting a chance to fully sample the southern impulse, which is better refining the track of what would have been that ominous low pressure system tracking somewhere near the Ohio River valley.

The good news is that many areas will still likely see a white Christmas; while holiday travels in some areas will be molested by the onset of this system, the bad news is that winter storm aficionados will not likely be seeing a major 12+ inch snowfall event across parts of the country’s midsection.  I would expect further model changes with the final 12z suite of model outputs, but some basic forecast thoughts can be shared for our readers.

A split flow pattern will persist across the CONUS, with essentially two branches of energy: a northern and a southern stream.  The dome of cold air entrenched across the northern CONUS, coupled with the an emerging impulse across the northern plains, will be the major snowfall impetus for the immediate period.  Forecast QPF (precipitation amounts) with this impulse, as it produces plenteous forcing, appear to range from 0.10″ to 0.30″ (some aberrations in those amounts appear likely), which when coupled with ambient surface and tropospheric temperature profiles, appears to equate to a broad area of 2″ – 5″ snowfall totals (using a 15:1 average snowfall to liquid ratio).  In areas that see the maximum lift from this system, totals exceeding 5″ – 6″ are not at all implausible.  Some of the SKEW-Ts across the Ozarks region indicate a shallow layer of warm air aloft (i.e., 750 mb – 850 mb) juxtaposed with surface temperatures at the 0C (freezing) mark, so some minor icing is possible in those areas as the southern impulse moves northeastward.

The southern stream of this split flow pattern will begin gyrating out of the Southern Plains into the Gulf-Atlantic Coastal Plains region, and it will have the most bounteous moisture to work with, henceforth pilfering the better moisture from being engrossed by the northern branch as it peregrinates southeastward towards the Blue Ridge/Tennessee Valley regions.  There should be some phasing of both the northern and the southern stream heading into Christmas Day, which could spell trouble for the north-central Appalachian region, precipitation wise, and the evolution of that event should be better refined by both the rest of today’s 12z and Friday’s 00z model runs.

I’ve included a map below with some preliminary winter precipitation forecast types/amounts for FRI – early SAT, with the area highlighted in orange being that which is most likely to see snowfall amounts ranging between 2″ – 5″ (locally higher totals possible), whereas the light pink areas of the Ozarks correlating with favorable (forecast) atmospheric conditions for some light icing.

Please have a safe and joyous holiday season and, by all means, exercise due caution while driving if you will be traveling through any of the areas impacted by adverse weather this weekend!

Forecast written for ChaserTV by Jesse Risley of Convective Addiction

View Christmas 2010 Storm in a larger map

Forecast Sounding - NW AR Early FRI Morning

WRF Forecast H5 Vorticity - early FRI evening

WRF MSLP/Surface features - early FRI afternoon