NW Florida Weather - Saturday & Sunday's Severe Setup

I know that you've been enjoying highs in the 70s over the past week or so, but, unfortunately in winter, that usually comes with a price. The price? Severe weather. We have two rounds of severe weather as we move forward into the weekend. Let's talk about timing and threats.

ROUND 1 - Saturday Morning into early Saturday Afternoon

Storm Prediction Center (SPC) Convective Outlooks for Friday (top) and Saturday (bottom). Top image is valid until 6 AM CST Saturday; bottom image is valid from 6 AM CST Saturday until 6 AM CST Sunday.

The severe threat starts to increase from the west late tonight into early Saturday morning. It appears for the majority of Northwest Florida, the first round of severe weather will likely occur between 6 AM and 3 PM as a line of thunderstorms moves through the region. The Storm Prediction Center has outlined the western half of our area in a Marginal or Slight Risk of severe weather (levels 1 and 2 out of 5, respectively) until 6 AM Saturday morning. Then, for Saturday, the risk of severe weather increases to an Enhanced Risk (level 3 out of 5).

Simulated radar from the NAM 3km model. Valid from 6 AM CST Saturday through 3 PM CST Saturday. Animation courtesy of TropicalTidbits.com.

The model forecast above shows a broken line of showers and thunderstorms advancing across Northwest Florida during the morning and early afternoon hours. Now, this is one run of one model, but most model guidance does agree, and I tend to agree with this solution. You should never take any one run of any one model as gospel! They are just guidance!

One concern that I have is that as the line approaches, the storms that sweep through Northwest Florida become more individualized. This means that these storms could become supercellular (rotating storms). This may increase our tornado threat initially in the morning hours. The ingredients will be in place for severe storms. Let's take a look at some tools:

NAM model forecast of MLCAPE (mixed-layer CAPE) from 6 AM CST to 3 PM CST Saturday. Animation courtesy of pivotalweather.com.

The NAM forecasts a sufficient amount of instability in our area as we head into the late morning and early afternoon. Generally in the winter, MLCAPE values of 500 or more are sufficient for severe weather. Values will be increasing to nearly 2,000 in some places, which is very impressive for January.

NAM model forecast of the Bulk Wind Shear from the surface up to 500 mb (about 18,000 feet) valid from 6 AM CST to 3 PM CST Saturday. Animation courtesy of pivotalweather.com.

Shear is another requirement for severe weather. Generally, 35 to 40 knots of shear is sufficient for severe thunderstorm development. The NAM forecasts roughly 45-55 knots of shear through this time period, which favors severe storms and supercells.

NAM model forecast of 0-3 km Storm Relative Helicity valid from 6 AM CST to 3 PM CST Saturday. Animation courtesy of pivotalweather.com.

Storm relative helicity takes into account the change of wind speed and direction with height. This is for the lowest 3 km of the atmosphere. Values of 250 or greater are supportive of supercells, but the NAM is pushing values of 250-300 in some areas.

The atmosphere becomes more supportive of severe weather as we head through the morning and into the early afternoon hours. Large hail, damaging winds, and tornadoes are possible with the first round. All thunderstorms will be capable of heavy rain and dangerous lightning.

ROUND 2 - Saturday Evening through Sunday Morning

NAM model forecast of the radar valid from 6 PM CST Saturday through Noon CST Sunday. Animation courtesy of TropicalTidbits.com.

CAVEAT: I don't fully agree with the end of this animation. I do not know if that low that pops up at the end of the animation, and that can greatly affect the forecast. I would like to see a little more consistency in the model runs and also among the models before I adjust the forecast.

There may be a break in the activity during the late afternoon. However, any storms that develop during the afternoon will still be capable of producing severe weather.

SPC outlooks for Saturday (top) and Sunday (bottom). Saturday's outlook is valid from 6 AM CST Saturday until 6 AM CST Sunday. Sunday's outlook is valid from 6 AM CST Sunday until 6 AM CST Monday.

Even through the evening and overnight hours, Northwest Florida remains in an Enhanced Risk (level 3 out of 5) of severe weather. By Sunday, the risk starts shifting eastward. Calhoun and Gulf Counties remain in an Enhanced Risk, while the risk starts dropping back to the west. The threat for large hail, damaging winds, and tornadoes will continue into the overnight hours.

NAM model forecast MLCAPE valid from 6 PM CST Saturday until Noon CST Sunday. Animation courtesy of pivotalweather.com.

Instability will be highest in the evening hours, but will decrease during the overnight hours as temperatures cool off and storms move off to the east. Even though instability will decrease, it will still be sufficient for severe thunderstorm development. Plus, shear will complement the instability.

NAM model forecast surface to 500 mb (about 18,000 feet) wind shear valid from 6 PM CST Saturday until Noon CST Sunday. Animation courtesy of pivotalweather.com.

Shear will greatly increase as we head into the overnight hours to 60 to 80 knots, well above what's needed for severe weather development. Even with the decreased instability, the shear will compensate and will help contribute to severe weather development.

NAM model forecast for 0-3 km Storm Relative Helicity valid from 6 PM CST Saturday until Noon CST Sunday. Animation courtesy of pivotalweather.com.

With shear increasing, the storm relative helicity will also increase to over 250 m^2/s^2. This increases the threat of supercells. With all of these ingredients in place, the severe weather threat will continue into the night. Large hail, damaging winds, and tornadoes will continue to be possible.

OVERVIEW

NAM model forecast for Significant Tornado Parameter valid from 6 AM CST Saturday until Noon CST Sunday. Animation courtesy of pivotalweather.com.

This animation shows the Significant Tornado Parameter. A "significant tornado" is defined as a tornado of EF-2 intensity or greater (winds of 111 mph or more). Now, high values on the significant tornado parameter do not mean significant tornadoes will occur. I use this as a worst-case scenario, essentially the worst that the atmosphere could produce given the other conditions and given that thunderstorms actually form. Large hail, damaging winds, and tornadoes are all possible. I will not rule out the potential of one or two significant and/or long-track tornadoes. There will be two rounds, Saturday morning/early afternoon and then Saturday evening/Sunday morning.

SAFETY

• Know your safe place - it should be an interior room without windows on the lowest floor of a well-built building. If you live in a mobile home or manufactured home, you need to have a plan to evacuate and get to a well-built building if a tornado warning is issued. Have things to protect your head such as helmets, pots/pans, etc.
• Have multiple ways to get warnings - TV, internet, radio, smartphone apps, wireless emergency alerts, weather radio. Make sure your phone has wireless emergency alerts enabled. They will alert you if a tornado warning or flash flood warning is issued by pinging your phone off the nearest cell tower. If you use weather apps, make sure you have location enabled so your phone's GPS can locate you and the app can alert you based on your location. Program your weather radio so that it alerts you to warnings issued for your county. If you don't have a weather radio, you should get one. It's a $20-30 investment that could save your life.
• Be sure you can get woken up at night if a warning is issued - this is where the wireless emergency alerts and weather radio come in handy. Turn the volume up on your phone so that you can hear the wireless emergency alert being issued. Turn your weather radio on alert mode. If a warning is issued for your county, it will sound a very loud tone that will likely wake you up. Trust me, it's a loud tone.