Friday, February 17, 2012

Update on Major Storm System for This Weekend: Severe Weather & Snow...

A strong storm system in the middle and upper atmosphere is spinning around to the South of the Arizona border this morning (as shown by the yellow circled area on the above water vapor satellite image).  Pieces of energy and stronger winds aloft are already spreading well out ahead of the system as indicated by the yellow arrows.

This system will bring a variety of weather, from heavy rain and severe storms in the South, to heavy snow in portions of the mid-Atlantic and/or Northeast as we progress into the weekend.

For later today and especially tonight, widespread shower and thunderstorm activity will break out ahead of the system over much of Texas and adjacent portions of Louisiana.  Rainfall amounts of 1-2 inches will be widespread, with locally heavier amounts of 2-4+ inches in association with some of the stronger and more persistent activity:

Some of the thunderstorms across Deep South Texas and the lower Rio Grande Valley may become strong to locally severe, especially tonight after sunset and into the pre-dawn hours of Saturday as the upper-level storm system approaches.  Large hail and strong wind gusts will be the primary severe weather threats in this region (in addition to the aforementioned heavy rainfall):

Some of the thunderstorms tonight across southcentral and southeast Texas may become strong, with hail just under severe limits the primary threat.  This includes the Austin/San Antonio area and the Houston area.  Widespread severe weather is not expected in this region, with the greatest hazard overall being heavy rainfall that could cause flash flooding in some areas.

On Saturday, the system is forecast to progress Eastward into the lower Mississippi Valley, Gulf Coast and southeast U.S.  As we've been talking about for the past couple of days, the threat for a round of significant severe weather will exist in this region on Saturday afternoon into Saturday night.  While severe thunderstorms are possible anywhere within the brown and yellow shaded areas on the image below, the greatest risk will take place within the red shaded area:

Large hail, damaging wind gusts and tornadoes will all be possible with severe activity in this region.  At this time, it appears that severe storms will develop as early as midday near the western portions of the outlook area, then increase in coverage and intensity as they move East/Northeastward into the afternoon and evening hours.  An overnight severe weather threat, into the pre-dawn hours of Sunday, will exist near the Eastern portions of the outlook area.

The potential will exist for a strong tornado or two, especially across the Louisiana, Mississippi, Alabama and Florida panhandle portions of the red shaded area on the above image.  The wind profiles from the lower through middle levels of the atmosphere will be very supportive of this threat.  The only real question right now is just how unstable the lower-levels of the atmosphere will become during the day Saturday (which will depend on the amount of cloud cover, rain ahead of the storms, etc.).  Depending upon how the instability element develops, this could either increase or decrease the strong tornado stay tuned for updates on this aspect of the situation.

Very heavy rainfall can also be expected across the South Saturday and Saturday Night, which could lead to flash flooding in many areas.  Widespread rainfall of 2-4 inches can be expected, with locally higher amounts possible:

The focus will shift to winter weather as the system moves up into the mid-Atlantic and Northeast region on Sunday.  While there is still some uncertainty as to the exact track that the system will take (which will ultimately affect where the band(s) of heaviest snow will set-up), the forecast models are starting to reach a fair level of consistency.

The consensus of the last few model runs have suggested that the primary "significant" snow band will likely set-up from eastern Kentucky into the Delmarva region (including Washington, DC and Richmond, VA).  Within this band, widespread 3-6 inch accumulations can be expected, with locally heavier amounts of 6-12 inches possible: 

The above image is the forecast probability of snowfall exceeding 4 inches from the HPC.  It is valid for the period 7am EST Sunday through 7am EST Monday.  At this time, it appears that the most likely time period for the heaviest snow would take place from midday through the evening hours on Sunday, spreading from West to East.

Stay tuned for updates on the snowfall forecast as it becomes even more clear how the system is going to track out over the next 24-36 hours...

If you live in the severe weather threat areas outlined up toward the top and middle of the post, please be alert this weekend.  I would suggest that you review severe weather safety and preparedness tips now, that way you have a plan of action in place before severe weather threatens your area.

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Anthill_Goddess said...

As I've watched you cover Saturday's storm potential, I'm also seeing updates from friends in NOLA talking about the parties that are being planned for Saturday...

Hard to get the Mardi Gras crowd to pay even a *little* bit of attention!

Happy Friday, anyway!

Rob White said...

Thanks Anthill, good point. Hopefully those folks will pay attention to the weather as well as the other festivities...