Northwest Florida Weather (11/29/2016) - Another Severe Weather Threat

Before I begin this blog post, I'd like to extend my sincerest thoughts and prayers to those in Gatlinburg, Pigeon Forge, and surrounding areas that were affected by the wildfire. Extremely sad situation with loss of life. I'm thankful, as I'm sure many are, for the first responders' and emergency personnel's acting quickly as it could have been a lot worse.

Now, back to Northwest Florida weather information.

Well, frankly, last night ended up being a lack of rain for most areas. The good news is that we didn't have any severe weather in our area, but the bad news is that most areas didn't get much rain, if any. Why was that?

Well, last night, we were lacking instability, which some models were not picking up on. The line of storms was forecast to weaken as it moved further east, but the actual line weakened sooner than what models were thinking. Most of the better lift and instability was to our north and away from us in Northwest Florida.

So, what about tonight and tomorrow?

The Convective Outlook issued by the Storm Prediction Center (SPC). This map is valid through 6 AM CST Wednesday morning.

Parts of Northwest Florida are in the "Marginal Risk" of severe weather. This is a Level 1 out of 5, meaning we could get some isolated severe thunderstorms. But, what severe weather we might get would be closer to 6 AM Wednesday.

SPC's Convective Outlook for 6 AM Wednesday through 6 AM Thursday.

SPC has place all of Northwest Florida under a "Marginal Risk" for Wednesday. This again is a Level 1 out of 5. Isolated severe thunderstorms will be possible, with damaging winds being the main threat with any severe thunderstorms. Let's talk some meteorology!

Model sounding from the HRRR (High-Resolution Rapid Refresh) Model valid at 11 AM CST Wednesday at KVPS (Eglin AFB, FL). Focusing mostly on the left half of the image. The solid red line is the temperature profile through the atmosphere, the solid green line is the dew point profile. Pressure is labeled in millibars on the left, decreasing with height. Temperatures are labeled on the bottom in degrees Celsius. Multiple parameter are located at the bottom of the image. Image courtesy of pivotalweather.com and SHARPpy.

I personally think these model soundings are a meteorologist's best friend. There's just an abundance of information you can get from these. So, let's see what there is to see.

• Generally, there is atmosphere is abundantly moist, especially at/below 700 mb. There are two ways we can see this. First, the temperature and dew point lines are pretty close together. Second, the precipitable water (labeled PW in the parameters table) is around 1.67 inches, meaning the atmosphere is sufficiently moist. Note about the precipitable water: Basically, it's how much water would accumulate if all the moisture in the atmosphere above a point was wrung out. The moisture allows for cloud and precipitation development. Also, the more moist the atmosphere is, the higher possibility of heavy rainfall, and the rainfall can help transfer some higher winds aloft to the surface ("waterloading")
• Warm air advection through the lower portion of the atmosphere. This is a lifting mechanism. Warm air is less dense and warm air can rise.
• CAPE is high, meaning plentiful instability, allowing air to rise easily.
• Decent shear, which is favorable for thunderstorm development.
• Dry adiabatic layer from about 900 mb to the surface. (Dry adiabatic layer means that the temperature profile is parallel to the dry adiabatic lapse rate, denoted by the thin diagonal white lines going up and to the left. The dry adiabatic lapse rate is the rate at which the atmosphere cools as you proceed upward in the atmosphere if the atmosphere is not saturated.) This will allow winds to be transported downward more easily.

One note, the sounding algorithms do spit out a "possible hazard type" of tornado. I use this only as a worst case scenario. This should not be taken as there gospel truth.

So, the signals are good for at least thunderstorms, and maybe some severe storms. But there are a couple of caveats.

• Helicity is not all that impressive, meaning that there is not as much spin in the atmosphere. Thus, it does not lend itself to an atmosphere supportive of tornadoes. That's not to say it can't happen, but it's less likely.
• This model assumes that the temperature will make it to near 80 degrees at the surface. I'm not fully confident in that solution. I do think it will be warm, but I'm not sure about it being that warm. This affects CAPE, low level moisture, and that dry adiabatic layer.

Here is what the HRRR model (the same one the sounding is from) predicts the radar will look like:

HRRR predicted 1 km reflectivity valid from 8 PM CST Tuesday through 11 AM CST Wednesday.

The HRRR only goes out 18 hours. It shows a line of thunderstorms (potentially strong to severe) entering Northwest Florida around 9 AM (+/- 2 hours). But the general trend looks to be slow weakening as it moves further east.

So, again, this is just one run of one model. Just one tool in our toolbox. Let's take a look at one other model.

This is a model sounding from the NAM valid at 12 PM CST Wednesday. Same properties as the sounding above.

So, just like the HRRR, the NAM has abundant moisture, warm air advection, decent CAPE (for this time of year), and decent shear. Now, there are a few differences. It's not as warm at the surface, thus nearly eliminating the dry adiabatic layer. Speaking of which, it looks like the dry adiabatic layer is only from 975 mb to the surface, not nearly enough for easily transporting winds from aloft to the surface. But, there is more helicity than there was in the HRRR sounding, meaning more possibility for rotation. So, what does this mean?

With lower instability, we may not see as much thunderstorm development. With fewer thunderstorms to tap into that helicity, the threat of tornadoes is lower. There could still be damaging winds with stronger storms. But what does the predicted radar look like with the NAM?

NAM predicted radar valid from 9 PM CST Tuesday through 9 PM Wednesday.

The NAM does not have as much in the way of intense convection (meaning that it may not be predicting as much thunderstorm activity). The NAM also brings in the convection later than the HRRR, closer to 12 PM. But the general trend is the same, weakening as it moves further east.

So what does this all mean for your forecast? We will probably see more scattered showers around Northwest Florida tonight, not really expecting much in the way of thunderstorm activity tonight, but can't rule out an isolated storm. Severe weather is not expected tonight. Wednesday is our better chance of rain. Showers and thunderstorms look to move into Northwest Florida during the mid to late morning and move eastward. The general trend is for the line to weaken as it moves further east. An isolated severe thunderstorm or two cannot be ruled out with damaging winds being the main threats. Widespread severe weather is not expected in our area.