NW Florida Weather (4/1/2017) - Severe Weather Threat for Monday (4/3)

Though this post is being written on April 1st, the severe weather threat is not an April Fools' joke. We'll start with what we know as of right now.

The Storm Prediction Center (SPC) has place all of Northwest Florida in a "Slight Risk" of severe weather for Monday, which is a level 2 out of 5. All three modes of severe weather are possible: damaging straight-line winds, tornadoes, and large hail.

All categories illustrated with potential risks. Image courtesy: http://www.spc.noaa.gov/misc/about.html

However, it is possible that parts of the slight risk area could be upgraded to an "Enhanced Risk," which is a level 3 out of 5.

Now, for the forecast, a technical discussion follows. A summary of the forecast will be at the end. Just as a friendly heads up for those that don't want/need the technical stuff.

The Setup

300 mb Heights and Winds from the 18Z run of the NAM. Valid at 15Z (10 AM CDT) Monday. Image courtesy of pivotalweather.com. Annotations and analysis are my own.

At 300 mb (roughly 30,000 feet), I notice several things. This is where things get technical very quickly. If you follow the solid black lines, that helps to determine the wind flow and strength. The closer the lines of constant height (isohypses) are together, the stronger the wind flow, generally. The wind that flows parallel to these isohypses is called the "geostrophic wind," which always has lower heights to the left of the flow. Any motion that is not geostrophic is "ageostrophic" This usually results from a slowing down of the wind or a change in direction.

Without going into too much detail, when winds spread apart ("diffluence"), air is allowed to rise to fill in the space aloft. Flow is generally slower in the bases of troughs and faster in the crests of ridges. Thus, downstream of troughs but upstream of ridges, there is usually divergence, which is also a lifting mechanism if in the upper levels. Also, there is acceleration of winds to our northwest and deceleration to the southeast, which also helps create divergence.

In summary, the 300 mb setup is very favorable for any lifting to occur.

500 mb Heights and Vorticity from the 18Z run of the NAM. Valid at 15Z (10 AM CDT) Monday. Image courtesy of pivotalweather.com. Annotations and analysis are my own.

At 500 mb (about 17,000 ft.), we look at the heights and vorticity. Our trough is still present and is negatively tilted (like it was at 300 mb). Vorticity is a fancy term for spin, and if higher amounts of vorticity is moving toward lower amounts of vorticity, it is called positive vorticity advection (PVA). PVA is a lifting mechanism.

850 mb Heights, Temperatures, and Winds from the 18Z run of the NAM. Valid at 15Z (10 AM CDT) Monday. Image courtesy of pivotalweather.com. Annotations and analysis are my own.

At 850 mb (about 5,000 feet), warmer air is moving toward cooler ("not as warm") air. This is called warm air advection (WAA) which is a lifting mechanism. Also noted at 850 mb, there is a strong low level jet, which can enhance lift.

So, in summary of the setup, there is strong indications of lift, perhaps significant lift over our area, which is just one of the criteria needed for severe weather. But now, we're going to talk about specifics with the setup.

Precipitable water from the 18Z run of the NAM. Valid at 18Z (1 PM CDT) Monday. Image courtesy of pivotalweather.com.

This is the precipitable water (PW) product. Precipitable water is essentially the amount of water you'd get if you wrung out the atmosphere in a given column of air. In our case, we have PW over 1.5 inches over much of Northwest Florida, which is very moist. Thus, we have an abundance of moisture.

Now, we have the lift and the moisture. We need some shear.

Bulk shear from the surface to 500 mb from the 18Z run of the NAM. Valid at 18Z (1 PM CDT) Monday. Image courtesy of pivotalweather.com.

Shear is the change in wind speed and/or direction with height. About 40 knots of shear is sufficient for severe weather, but we have over 50 knots. So, shear is definitely supportive of severe weather. But there's one last ingredient we need to talk about - instability.

Surface-based CAPE from the 18Z run of the NAM. Valid at 18Z (1 PM CDT) Monday. Image courtesy of pivotalweather.com.

CAPE (Convective Available Potential Energy) is a measure of the instability. In this case, we are looking at surface-based CAPE, which is the amount of instability present if air was lifted from the surface. About 1,000 J/kg this time of year is sufficient for severe weather. In this model, we have about 1,500 to 2,000 J/kg. So, all of our ingredients are there, but let's take a look at one more thing, which will give a lot of specifics for our severe weather potential.

Forecast sounding from KVPS (Eglin AFB, FL) valid at 21Z (4 PM CDT) Monday. Image courtesy of pivotalweather.com. Annotations made by me.

I know this looks confusing, but there's only a few things you need to take away from this. The air near the surface (toward the bottom of the upper-left chart) is nearly saturated. However, a little higher in the atmosphere, there air is substantially drier (according to this model). This dry air can evaporate water vapor, which will help cool the air. This does two things. First, it cools the air aloft, meaning that air below can more readily rise. Second, with the cooler air, the air is more dense and can bring down strong winds in thunderstorms. The Storm Relative Helicity (SRH) is quite high. Helicity is a measure of rotation that storms can tap into. SRH values are over 300 according to this model when only about 150 is needed for rotating thunderstorms. So, the environment is supportive for damaging winds and tornadoes. Hail is not a substantial threat according to this model, but I wouldn't rule it out.

One thing to note: the "Psbl. Haz. Type" (Possible Hazard Type) should be taken as the worst-case scenario, meaning that the atmosphere predicted by the model is capable of producing, at worst, a tornado that is particularly dangerous. Not saying that is going to happen or not, but damaging winds, tornadoes, and hail are all possible.

Summary

So, whether you read through that lengthy explanation or skipped it, here are the takeaways about the severe weather:

1. Severe weather is possible on Monday. We are currently under a "Slight Risk" of severe weather as of this writing (4-1-2017). It is possible that parts of the slight risk area may be upgraded to an "Enhanced Risk."
2. Damaging winds, tornadoes, and large hail are all possible with severe thunderstorms.
3. Timing is a bit uncertain. However, all day, there will be a potential of severe thunderstorms. A squall line looks to enter Northwest Florida from the west sometime during the late morning or early afternoon. The line should move out of Northwest Florida during the evening hours.
4. Even ahead of the squall line, which could bring damaging winds and isolated tornadoes, isolated thunderstorms will likely develop and could be capable of producing tornadoes.
5. If we get any sunshine on Monday, it's a bad thing or severe weather. More heat = more instability.
6. About 2 inches of rainfall is expected with locally higher amounts. The National Weather Service in Mobile notes that they may need to issue a Flash Flood Watch. (NWS Mobile covers Escambia, Santa Rosa, and Okaloosa Counties. All other NW Florida counties are covered by NWS Tallahassee.)

Even outside of thunderstorms, there will be several hazards:

1. It will be rather windy, especially closer to the coast. Winds will be out of the south at 20-30 mph with gusts possibly as high as 40 mph. NWS Mobile notes that a Wind Advisory may become necessary.
2. The strong southerly winds will increase the rip current risk. I expect most area beaches to be flying red flags, if not double red flags.
3. The surf will be incredibly dangerous, with waves up to 8 feet in the surf zone. Waves this high may cause minor beach erosion.
4. The strong southerly winds when combined with the spring tidal cycle may also cause minor coastal flooding.

What should you do now to prepare?

1. Know your severe weather safety plan and practice it!
2. Check your batteries in your weather radio and flashlights.
3. Have your safe place planned out, not only at home but also at work! Interior room without windows on the lowest floor of a sturdy building.
4. Make sure you have multiple ways to receive warnings! These can include your Wireless Emergency Alerts on your cell phone, weather radio, internet, phone apps, television, radio, and friends/family.
5. DON'T RELY ON SIRENS! They are not sounded by the National Weather Service, nor are they meant to be heard inside!
6. Keep updated with the forecast!