Damaging Ice Storm Likely Across Georgia, South Carolina

A zone of of prolonged freezing rain will extended the I-20 corridor from Atlanta to Augusta to Columbia, SC.  Model freezing rain liquid equivalent totals in this area exceed 2 inches. While drip loss will be relatively high with accretion efficiencies likely only being in the 30-40% range due to the high precipitation rates and the temperatures likely remaining close to the freezing point, this still yields  radial ice accretion amounts of .50-75″ which would cause widespread electrical distribution disruptions and tree damage.

Here are the latest Freezing Rain Graphics.

Major Winter Storm To Impact Southeast Then East Coast.

A strong winter storm will bring a variety of winter weather to the Southeastern U.S. as an upper level low develops across the mid-south, in combination with a cold air damming event.  A strong coastal low will strengthen and move up along and just off the Eastern seaboard and bring heavy precipitation to the Mid-Atlantic and into the Northeast

WRF Hourly Winter Radar through Wednesday Evening

NAM 4km 60hr Winter Radar

New SREF Accumulated Winter Precipitation Products

New Short Range Ensemble Forecast (SREF)  Mean Accumulated Winter Precipitation have been added.  They are:

Snow change from previous run (dProg/dt)
Ice Pellets (Sleet)
Freezing Rain

They can be found in the drop down menu on the SREF model page under “Forecast Parameter (Mean)” 


Here is an example.  Freezing Rain Accumulation across the Southeast. 



Significant amounts of freezing rain likely across the New York City area tonight and early Wednesday

2/5/14 1:45 Z

Freezing rain will moving into the populated region of New Jersey and New York City around midnight (2/5/14) and will persist until around noon Wednesday.  Temperatures in the New York city metro area are expected to remain near or slightly below freezing the duration of the event. Just northwest and west of the downtown area, slightly colder surface temperatures are likely.

Total precipitation amounts of 1.00-1.25″ is expected. A significant amount of drip loss will occur due to the heavy precipitation rate and the small air temperature delta to freezing point.  Where temperatures are likely closer to 29 or 30 degrees just west and northwest of the city, ice accretion amounts of 1/4″ to as much as 1/2″ are possible and could lead to a significant amount of power outages, if ice accretion amounts surpass 1/3″.


Severe weather outbreak possible Saturday for E-TX/AR/MS/AL

Quite a bit of uncertainty, but the latest data suggest the potential for severe thunderstorms and possible tornadoes across Eastern Texas and the Deep South Saturday as a strong, negatively tilted, shortwave ejects across Texas.  Stay tuned over the next couple of days if you live in this area.


Update: 12/20/13 12:40AM EST
After reviewing the 00Z model guidance, it appears that the instability will be somewhat limited on Saturday, due to clouds and marginal lapse rates.  Severe thunderstorms with damaging winds are likely to develop across East Texas during the morning and spread quickly eastward during the day. 0-6Km shear vectors are large and 0-1km helicity values will be locally high,  so some low-top supercells are likely, including the risk of a few tornadoes. But given the expected limited instability, SFC CAPE <1000, a widespread outbreak of tornadic supercells seems unlikely.  The threat of damaging winds is high however, as the low level jet increase to over 70kts during morning and early afternoon.

850 Wind Late Saturday morning near 80 Knots


Updated: ATL Tornado on 88D?

Did the 88D at FFC capture a Tornado ?  Dual Pole data may have picked up some debris.  Survey may tell later today.  Here is a screen shot I took last night while the event was unfolding. The velocity data was not very compelling, but in the previous 3 volume scans there were much better indications of a circulation.

Click images for much larger view

Update: I pulled the archive data from last night. Here are previous volume scans. You can see the progression of the development of the CC min & other radar data.

Updated: Severe Weather Threat Ohio Valley Wednesday-Wednesday Night

Complicated forecast for the Ohio-Valley into West Virginia and Western Pennsylvania, Wednesday into Thursday Morning.  As indicated in last nights discussion, synoptic forcing is very strong and will create favorable conditions for a large area of severe weather as an intense low pressure for this time of year, moves across the Ohio Valley.  The primary forecast problems are the convective mode that the storms take once they develop and whether the downstream ambient environment is contaminated by earlier convection that will disrupt the inflow and stabilize the boundary layer.  This is exactly what happens with the WRF-NMM and WRF-ARW 4KM models, each a little bit differently.  The WRF-4KM NAM CONUS Nest does not, and creates very little convective contamination prior to the main system arriving and develops a very dangerous environment across IL/IN/OH/ Western PA that would support a widespread severe weather event with supercells. While not explicitly, it would likely yield some tornadoes given the intense parameters it is generating.

Below are some animated GIF’s from the WRF-NMM to illustrate the forecast issue of convective contamination of the environment prior to the main forcing/convection related to the surface low.  If this AM convection does not develop across WV/VA as indicated, the stable outflow may not be near as strong and may not provide the cool theta-e values and therefore not weaken the main convective system as much or at all when it arrives late Wednesday night.

WRF-NMM Radar Simulation. Note Convection that develops across WV/VA AM Hours Wed Morning.

WRF-NMM  Note the Theta-e minimum associated with the AM convection across WV/VA mountains that works westward and helps weakens the approaching convective system late Wednesday night as it arrives in OH.

Even as it is,  with some possible convective contamination. The mesocale models are generating some strong indications of a wide spread severe weather event.

Across the Ohio-Valley Wednesday into Wednesday night, the  0-1km helicity and the 0-6km shear will increase as the surface low deepens and the upper level shortwave approaches.  Kinematic forcing will favor the formation of supercells and possibly tornadoes over a fairly large area, but the most favorable location will be just south of the stationary front/warm front which will maximize the 0-1km helicity.

Here is just one of the Updraft Helicity products from the various mesoscale models. This is from the WRF-NMM. It’s developing a long lived Supercell from just west of Chicago and tracking it southeastward. Other supercells develop across Indiana and Ohio. This is one of the longest and most intense updraft helicity values I have seen since observing these parameters.


Forecast Sounding 40 Miles South of Chicago, IL  01Z Wednesday Evening. WRF-4KM NAM Nest

Forecast Sounding near Wooster, OH this evening at 00Z. Very Unstable atmosphere in place ahead of main forcing. Supercells and tornadoes are possible if convection can develop  ahead of main convective system. 

While the threat from widespread damaging wind is certainly high and linear mode convection may very well be the primary mode of convection for the duration of this event. I feel that there very well may be a period of time where there are either discrete supercells or embedded supercells with in a line. Given all the parameters, I believe the tornado risk is quite high along and just south of the stationary front from Central IL through IN into OH.  This is a highly conditional forecast, since convective contamination from earlier storms may affect the environment or if the system becomes a large bow-echo or derecho very early in the evolution, then the tornado risk is greatly reduced.

Here is a map outlining the risk area of tornadoes based on all the available guidance through 05Z.


Severe Weather Outbreak Likely Friday/Friday Night Across Southern Plains / Midwest

Severe Weather Outbreak Likely Friday Afternoon/Night Midwest – Southern Plains

As an upper level low begins to move eastward, the strong core of the mid and upper level winds will rotate around the upper low. An increasingly favorable kinematic environment for supercells and potentially strong tornadoes will develop from Central Illinois/Central Indiana through Southern Missouri into Eastern/Central Oklahoma and  Northern/Western Arkansas.

The area at greatest risk for strong tornadoes is from Central/Eastern Oklahoma through Southern Missouri into Central Illinois.  Although there will likely be AM storms, some severe, the storms that will develop Friday afternoon and move into Ohio-Valley during the overnight hours are the ones that pose the greatest risk for that area. Boundaries from earlier convection in the day will likely focus localized helicities values over 400 m^2^s which will enhance the tornado potential with storms that develop in non-linear modes. Bulk Richardson Shear values favor supercell storm modes across these areas, at least initially.

Other storms will likely form further south across Oklahoma where the instability will become extreme with surface capes over 5000 and shear values in excess of 40kts. CIN will drop below 25 j/kg by afternoon and supercells should form with the aid of an approaching shortwave. With LCL’s AOB 1000 meters, tornadoes are quite likely, especially across Northeastern OK.

The Maximum Updraft Helicity product from the 4km WRF/ARW has been consistently generating strong supercells across OK, MO, IL Friday Afternoon & Evening.

1 Hour Max Updraft Helicity. Indicate Rotating storms develop late.

Max 1 Hour Updraft Helicity from WRF-ARW

WRF-4KM Simulated Radar 23Z Friday

0-6 KM Shear Increase as Mid-Level Jet Works into the Region

High Surface Dewpoints work northward into IL/IN feeding storms.

Surface CAPE is quite high.

Surface Low over MN and Secondary Low over SW OK will help back Low Level  Winds.

Supercell Composite Parameter indicates likelihood of rotating storms from OK northeastward into IL.

Significant Tornado Parameter indicates risk of strong tornadoes.

0-1Km Helicity 00Z

0-1km Helicity 03Z

250mb RRQ of Jet will enhance Vertical Motion

Upper Level Low will move Eastward with cold pool and wind max

St. Louis, MO Forecast Sounding for Friday Evening.  Small Cap, but supportive of Supercells and possible tornadoes

GFS Forecast Sounding for Springfield, IL for 00Z. Again kinematics and thermodynamics supportive of supercells and tornadoes.

Extreme Instability at Oklahoma City per the GFS Forecast Sounding

-Mike Dross