Tropics Trying to Come Alive - 6-16-2017

DISCLAIMER: The views expressed in the blog post are strictly my own and do not necessarily express the views of the National Hurricane Center, National Weather Service, or other official sources unless explicitly stated. The ideas expressed here should not be the sole source of your information. Always consult official sources before making any decisions.

Tropical Weather Outlook from the National Hurricane Center showing our two areas of interest. Valid as of 7 PM CDT Friday (June 16, 2017).

We are in Hurricane Season, yes. But having 2 systems that bear watching in the tropics is a bit unusual for June, especially Invest 92L in the central Atlantic. Climatologically speaking, Invest 92L is in an area uncommon for development this time of year.

The climatologically-favored areas of tropical cyclone formation during June. Image from the National Hurricane Center. Source: http://www.nhc.noaa.gov/climo

Invest 92L

Infrared satellite imagery of Invest 92L. Source: http://www.ssd.noaa.gov/PS/TROP/floaters.html

Let's start with the system that has a higher chance of development in the short term--Invest 92L. Since this is one of the first times we're talking about "invests" this season, remember that an "invest" is an area of investigation. These are called 90L to 99L ("L" for Atlantic), and the numbers are reused through the season.

92L isn't looking too shabby, especially for June. This tropical wave came off the coast of Africa a couple days ago, and has maintained its organization fairly well. Here's the National Hurricane Center's outlook for 92L:

Satellite images indicate that an area of disturbed weather
associated with a tropical wave is located about 1800 miles east of
the southern Windward Islands. This disturbance has become
better organized today, and additional development is possible
during the next day or two before conditions become less favorable
for tropical cyclone formation. This system is expected to continue
moving toward the west at 15-20 mph over the tropical Atlantic
during the next several days.
* Formation chance through 48 hours...medium...40 percent.
* Formation chance through 5 days...medium...60 percent.

Conditions are expected to become less favorable for development in a few days as it approaches the Windward Islands. But it's still several days away from land--plenty of time to watch this. Just so you can see them, here are the spaghetti plots:

Spaghetti models for Invest 92L (as of 7 PM CDT June 16, 2017). Source: http://www.tropicaltidbits.com/

 It's important to note a couple things about spaghetti models:

• These are just one run of these models, and only give a general idea of it's potential movement. Thus they can change from run to run.
• These don't tell you about the possible intensity of whatever system.
• These don't tell you about any of the impacts, only about the center of the system.

Generally, the system is expected to move toward the southern Windward Islands, and maybe brush the northern coasts of Guyana and Venezuela, bringing some squally weather for those areas.

The Mess in the Caribbean

GOES-16 Infrared satellite imagery showing the Gulf of Mexico and the NW Caribbean Sea. Source: http://weather.cod.edu/satrad/exper/. NOTE: GOES-16 data is undergoing testing as is considered experimental and non-operational.

Yes, I'm using "mess" as a technical term. This view doesn't even give the full view of the Caribbean Sea, but I used it because the GOES-16 imagery is meteorological awesomeness. Very high resolution and rapid data. But, let's give you a complete view of the Caribbean.

Infrared satellite imagery of the Caribbean Sea. Source: http://www.nhc.noaa.gov/satellite.php

Here's the official word from the National Hurricane Season (as of 7 PM CDT, June 16, 2017):

Surface pressures are falling in the western Caribbean Sea and
the areal coverage of the disturbed weather has continued to
increase. Conditions appear to be favorable for gradual development
of this system while it moves slowly northwestward across the
Yucatan Peninsula this weekend and into the south central Gulf of
Mexico early next week. Regardless of development, heavy rains are
likely to spread over portions of Central America, the Yucatan
Peninsula and western Cuba during the next several days.
* Formation chance through 48 hours...low...20 percent.
* Formation chance through 5 days...medium...60 percent.

There's nothing organized about it at this time, and that's causing a lot of problems for forecasting. How? Allow me to explain.

Without anything defined in the Caribbean, the models can't lock on to anything, but they will try to make some sort of sense of what's going on with what data they have. Unfortunately, there isn't a whole lot of data in this area, so the model uses its own forecasts. This can lead to the model consistently performing poorly. Also, as of this writing, this area has not been designated an "invest" (again, due to the lack of an organized low pressure center). This means that we do not have spaghetti models to show you. Let's break down some models.

GFS

GFS model valid Monday morning. Model image source: tropicaltidbits.com. Annotations made by me.

Okay, let's break down the image. The different color shadings represent the flow at 500 millibars (about 17,000 feet). I tried to illustrate the flow using the black and white arrows. Our surface low is the red and white "L."

The GFS predicts that there will be a ridge from Texas westward (part of what will bring record heat to the Desert Southwest) and another ridge from the East Coast into the Atlantic (ridges = high pressure = "H"). However, there is a bit of a break, or "weakness" in the ridge. This may help draw the low pressure system further north.

GFS model valid Wednesday morning. Model image source: tropicaltidbits.com. Annotations made by me.

By Wednesday morning, a trough dips down, according to the GFS, and starts to pick up the low pressure system and starts lifting it northeastward while it weakens further. The low at this point is relatively weak, but the main threat would not be wind, but would be heavy rain according to the GFS.

GFS model precipitation through Thursday afternoon. Model image source: tropicaltidbits.com.

Wherever this system goes, the eastern and northern sides will see quite a bit of rain. The GFS paints well over 6" of rain over the Big Bend of Florida through Thursday afternoon.

But, if you read my posts, you know that I am quick to point out to never use one run of one model. So, let's take a look at what the past runs have done with this system.

GFS model trend using the last 10 runs, all valid for Wednesday morning. Model image source: tropicaltidbits.com. 

The last 10 runs of the GFS haven't been exactly consistent, ranging from the low being in the southwestern Gulf to northwest Florida in just 10 runs. However, it is somewhat consistent in strength, keeping it weak. But, again, we don't have an organized system to pinpoint yet, so we don't know if this model (or any of its runs) will be correct.

Let's take a look at another reliable model...

ECMWF (European)

ECMWF model valid Monday morning. Model image source: tropicaltidbits.com

The ECMWF, another reliable model, shows that the ridging is a bit stronger than the GFS. The result? Any northward motion gets blocked since there's little to no weakness. It gets shoved to the west. So what happens further down the line?

ECMWF model valid Wednesday morning. Model image source: tropicaltidbits.com

It moves toward Mexico instead of the northern Gulf Coast. The mid to upper level flow blocks any northward progression.

However, a lot of moisture will still be drawn up from the Caribbean, and very heavy rain would still happen to the east and north of the center. I can't show the precipitation data from the ECMWF due to proprietary issues. But how consistent has the ECMWF been?

ECMWF model trend of last 6 runs, all valid Wednesday morning. Model image source: tropicaltidbits.com

Actually, the ECMWF has been relatively consistent in placement and strength. But does that mean that it's more accurate? Not necessarily, and only time will tell what actually happens.

One last model (or set of models) to show you.

GFS Ensembles (GEFS)

GFS Ensembles showing different low positions from Sunday afternoon through Wednesday afternoon. Model image source: tropicaltidbits.com

You've actually seen this model before, sort of. This is the GFS model run several times, each time being tweaked slightly. This results in different outcomes. Each red number you see is a low pressure center that each run produced. The more clustered they are, the more confident the model is in that solution. The more spread out they are, the more uncertain the forecast. The GFS Ensembles are actually fairly consistent in the central Gulf of Mexico, but it quickly become uncertain, and it has to do with the ridge and/or weakness we've seen in the other discussions.

Summary

So, you've read this far, what are the takeaways?

• Invest 92L is still really far away from land, and won't bother anyone for several days. But it does have a 60% chance of formation during the next 5 days.
• The mess in the Caribbean is just that right now, a mess.
• There is much uncertainty in the forecast with the mess in the Caribbean. Part of this is due to nothing being organized right now. Another factor is its large size. It takes longer for larger systems to become organized.
• The models are very split as to what will actually happen.

So, what should you do?

• Don't panic! There's still time to watch what will happen.
• Keep up with forecasts this weekend.
• Consult official sources when making any decisions.
• Now is a good time to review your emergency plans and make sure your kit is stocked. You can never be over-prepared!

The Dangers of Storms that Haven't Even Formed

Yeah, maybe the title was a little "clickbaity" (if I may make up a word), but it certainly got your attention. Every Hurricane Season, we seem to go through the same thing: someone posts a picture of the GFS out 384 hours (16 days) that shows a hurricane hitting somewhere popular. Then the same page posts the lone image with a caption reading something like:

"Major Hurricane will Strike this U.S. City! Catastrophic Destruction is CERTAIN!"

And every meteorologist collectively rolls their eyes. Well, let's talk about some ways that you can be sure you don't fall prey to these social media storms.

Don't Believe Everything You Read

Social media is a very useful tool for dispersing information, but that doesn't mean that it's always trustworthy. Sometimes you end up with posts like this:

Hyped-up hurricane "forecast." Image source: http://media.nola.com/hurricane_impact/photo/bogus-hurricane-path-d2cf288128cb5eb0.png

 Notice a few things on this single image:

• Have you ever heard of WeatherAlertCentral.com? Research before believing. Confirm that what they post is accurate and corresponds with official sources (National Weather Service, National Hurricane Center, your favorite TV meteorologist).
• The title automatically goes to the worst case scenario, and it's in ALL CAPS! Hyped titles like this should cast some doubt about its credibility.
• The map. Notice how nearly every major Gulf Coast city is in the "path" of this "hurricane," including the entire state of Florida. That should also be another red flag. Again, pay attention to official sources.

Pages like this are posting content like this simply to get more likes and shares. Look at how many people shared the post: 77,159. Yikes! This spread of misinformation makes social media potentially dangerous.

Errors Grow Quickly

Let's get a little geeky here. Every computer model is based on math and physics that attempt to describe the atmosphere. There are also some things that have to be calculated using workaround methods (parameterization). Small differences in the math/physics from the real atmosphere can lead to enormous errors rather quickly. Not only that, but the model tries to make a guess at what the atmosphere looks like using its own data and some real observations. While the models do adjust somewhat for the observations, it can also reject actual observations if it's too different from what the model thinks. If there is no data available (a big problem over oceans), then there is (next to) nothing that can correct the model. Thus, the model keeps any errors, and it can maintain those errors for a long time. Remember, all of these errors quickly multiply the further out you run the model.

For example, let's take a simple math problem:

We know that 2+2 is equal to 4, but what if we made a slight mistake and said it was equal to 5. An error of 1 doesn't seem so bad, but those errors can grow very quickly. If we use the error and multiply it by itself once, we're now off by 9. Keep multiplying, and you see the error grow substantially. Once we multiplied the wrong answer by itself for the fifth time, we are now off by over 2,100. Yikes! While the errors likely don't accumulate that fast in the computer models, this gives you the idea that errors can grow quickly. This is why you shouldn't trust the models too far out. They are just guidance, not gospel!

Also, don't just look at one run of a model or just one model. "If you live by one model, you will die by that model." No one model is perfect all the time. Also, things change from run to run as we get new data put in. You want to look for some sort of consistency among the models and the runs of the models.

Stay Calm, but Be Prepared!

If you do see something from an official source that could be threatening to you, stay calm. I know its easier said than done, but one thing that can help you remain calm is being prepared.

• Have your emergency kit fully stocked at all times (including food, water, batteries, etc.)
• Know your evacuation zone and route.
• Keep up with forecasts and the latest information from official sources.
• Don't read the hyped posts. Those may cause you to panic more.

Remember, we are in Hurricane Season right now, so we do have to keep our eyes out for anything that may form.

TL;DR

• Know your sources, and don't believe everything you read.
• Do some research before believing or sharing something.
• Don't trust models that show something more than a week out. Errors can grow rapidly.
• Consistency in the models is better, but models can be consistently wrong.
• No one model is always accurate.
• It only takes one storm. You should always be prepared.

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!

NW Florida Weather - Severe Weather Recap

After 3 rounds of severe weather within 2 days here in Northwest Florida, we definitely need a break from the severe weather, and it looks like we will get that. Sunshine and near-normal temperatures can be expected for the next several days (except Wednesday when we could see some showers and warm temperatures).

Below is a recap of the severe weather reports received through the weekend:

Green = Hail; Blue = Wind Damage/Gusts; Red = Tornado/Funnel Cloud/Waterspout

Saturday, January 21, 2017

Time	Location	County	Observation	Description
10:31 AM	4 mi. ENE of Dean Church Road	Walton	Unknown	Tree down on power line on Spring Lake Road.
10:40 AM	6 mi. W of Destin	Okaloosa & GMZ655	Waterspout	Report of two waterspouts off the coast of Okaloosa Island.
7:49 PM	2 mi. NNW of Pace	Santa Rosa	1.25” Hail	One to two inch hail reported near Pace. Time estimated via radar.
8:00 PM	Walnut Hill	Escambia	2.00” Hail	Reports of measured 2 inch hail near Walnut Hill.

Sunday, January 22, 2017

8:45 AM	1 mi. S of Navarre	Santa Rosa	1.00” Hail	Quarter sized hail in Navarre, FL
9:25 AM	Fort Walton Beach	Okaloosa	1.00” Hail	Quarter sized hail. Pictures from social media.
9:32 AM	Cantonment	Escambia	1.00” Hail	Quarter sized hail in Cantonment.
9:34 AM	4 mi. SSW of Fort Walton Beach	Okaloosa & GMZ655	Waterspout	Waterspout reported from Hurlburt Field ASOS.
10:10 AM	DeFuniak Springs	Walton	1.00” Hail	Photo received via email.
10:30 AM	4 mi. E of Brent	Escambia	Funnel Cloud	Funnel cloud seen near Spanish Trail Rd. via video on social media.
10:30 AM	2 mi. SW of Eglin AFB	Okaloosa	Unknown	Large tree limb snapped in Shalimar, FL.
10:48 AM	4 mi. W of Fort Walton Beach	Okaloosa	1.00” Hail	Hail on Hurlburt Field. Pictures via social media.
12:00 PM	DeFuniak Springs Airport	Walton	Unknown	Multiple trees and power lines down in DeFuniak Springs and Freeport. Time based on radar and operator logs.
1:00 PM	Panama City Airport	Bay	Unknown	Multiple trees and power lines down in greater Panama City area.
1:00 PM	Vernon	Washington	Unknown	Multiple reports of trees down countywide. Time estimated using radar and operator logs.
1:00 PM	Bonifay	Holmes	Unknown	Multiple trees down countywide. Time estimated using radar and operator logs.
1:04 PM	Tyndall AFB	Bay	68 mph	68 mph wind gust reported at the drone runway.
1:30 PM	Clarksville	Calhoun	Unknown	Multiple trees and power lines down countywide
1:34 PM	Marianna	Jackson	58 mph	58 mph wind gust reported at Marianna Municipal Airport
1:34 PM	Marianna	Jackson	Unknown	Several trees down countywide. Time estimated using radar, operator logs, and KMAI wind data.

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.