National Weather Service United States Department of Commerce

Event Summary

A complex weather scenario occurred on April 5th, 2017, with much of northern Alabama under an enhanced or moderate outlook for severe weather. This occurred ahead of a strong area of low pressure over southern Missouri around 7 AM on the 5th and its associated dry-line (actually ahead of the cold front associated with the low pressure system) approaching Mississippi around daybreak on the 5th. To complicate the severe weather potential, additional strong to severe thunderstorms developed along a warm from and moved northward over southern and central Alabama around daybreak. These storms produced numerous reports of very large hail, some wind damage, and a few tornadoes just to the south of the Huntsville Forecast Area. The effects of cloud cover and some strong storms edging into locations east of I-65 made for a very uncertain severe weather forecast later that afternoon due to the potential for the atmosphere to remain stable. However, as the morning went forward, some signs in observational data hinted that the atmosphere might destabilize. Although we did not see any tornadoes in the Huntsville Forecast area (although enough ingredients were in place late that afternoon to have one form), large hail between quarter and golfball size were reported with storms during the afternoon and a tornado watch was issued for northern Alabama and Southern Middle Tennessee.


 
Day One Outlook issued at 3 PM and Verfication. Click on the image to see reports received in the Huntsville Forecast Area. Tornado Watch #127  issued at 150 PM.
 

Composite Radar Image at 630 AM. This morning convection produced quite a bit of severe weather just to our south. Click on the image above to see a radar loop of the morning convective activity.

Composite Radar Image at  3:30 PM, as the second round of storms begins to approach northern Alabama from the northwest. Click on the image above to see a radar loop of the afternoon convective activity.

AM Convection and Warm Temperatures Aloft Limited Severe Storm Development


Morning Storms

 

A warm front moved north into central Alabama overnight on April 4th and by daybreak on April 5th, 2017 it was positioned just south of the Huntsville Forecast Area (as seen in the image below on the left). Very moist air was near and south of this warm front, where dewpoints were in the upper 60s to lower 70s. Strong bulk shear, some low level helicity, and decent mid level lapse rates coupled with strong lift along and south of the front. Storms that developed within this region produced numerous reports of large hail, damaging winds, and a few tornadoes overnight into the early morning hours in central Alabama.
Surface Observations at 8 AM on 4/5/2017 with the warm front notated by the dashed red line. A weak surface low even developed along this boundary at that time.
Elevated instability from SPC Meso-Analysis Data. It shows just over 1000 J/KG of instability aloft for convective development to harness.

Closer to Cullman, Marshall, and DeKalb counties dewpoints dropped off into the 55 to 61 degree range. This kept storms from being surface based, as they were further south (thus keeping a low tornado threat a bay).  However, elevated CAPE was in place in northern Alabama between around 1000 J/KG.  This elevated instability did allow some strong storms producing small hail just below 1 inch to develop. Special Weather Statements highlighting these threats were issued as a result. Luckily the front did not move much further north (keeping the better instability and forcing to our south), but did edge to near the Birmingham area around 10 AM, before sinking south and dissipating again.
 
Although the TN Valley was not impacted with any severe storms, the early morning activity did have a very big impact on storm evolution as the day went on and led to two main questions regarding the potential for afternoon thunderstorms:

 

         1.  Will the airmass over northern Alabama/Southern Middle Tennessee recover ahead of the next round of convection
              enough for severe convection and be strong enough to overcome warm air aloft (weak cap)?

         2.  If it did, would we have a possibility of a strong tornado or two with the second round of activity due to enhanced
              low-level turning of winds left in the wake of the morning convection (assuming enough lows level
              instability (SBCAPE) develops)?
 
Although, this early morning convection did keep most of northern Alabama cloudy for several hours in the morning, water vapor imagery from our satellites did start to give us a clue that we might destabilize due to the thinning/breaking up of cloud cover. This feature seen on the imagery was quite a bit of dry air pushing into the convection from the southwest and can be seen in the image below.


However, the morning convection did keep instability very low through the mid-morning hours, so the airmass did have to recover significantly by later in the afternoon. The concern of just how much surface based instability would occur later that afternoon was based on not only insolation effects produced by decreasing cloud cover, but also how high the surface dewpoints could get. 

Early in the day, very moist air was seen southwest (upstream) of northern Alabama originating from a region over the Gulf of Mexico that had dewpoints in the lower to mid 70s. This can be seen in the image below. It was pooling in front of the dry-line, which at 8 AM was west of Mississippi and could provide the forcing needed to produce severe convection later that afternoon. This gave another clue that we could still destabilize later in the day. Between 10 am and noon most of the shower and thunderstorm activity and cloud cover moved east into Georgia, allowing temperatures to warm up.

Severe Afternoon Convection

 

By 1245 PM the dry slot (outlined in yellow in the image below) had moved over most of northern Alabama and abundant sunshine had broken out, except in Marshall and DeKalb counties, as seen in the images below.  
Water Vapor Imagery at 1245 PM on 4/5/2017. 
Visible Imagery at 1245 PM.
 

This sunshine continued into the early afternoon hours (visible later at 207 PM). By then, temperatures had risen into the mid to upper 70s and dewpoints into the mid 60 due to the effects of sunshine and the advection of more moist air ahead of the dryline into northern Alabama (especially east of I-65). As a result instability increased to between 1000 J/KG and 2000 J/KG.  The images below show the progression these higher dewpoints into the area and increased instability.

Surface Analysis at 1 PM on 4/5/2017 
Surface Based Instability (CAPE) Values at 2 PM on 4/5/2017 
 

Good shear was in place ahead of the dryline even in the early afternoon hours and this shear increased as the dryline moved east later in the afternoon. Area soundings provided by the VORTEX Southeast research group did show a cap in place in the late morning/early afternoon hours. This cap was evident further east as can be seen below on the left. Meanwhile, drier air was undercutting the dryline and cut off some of the instability immediately ahead of it as it moved into the northwestern Alabama (it was uncertain from the start of this event if the dryline would begin to develop convection that far west). This likely kept showers/storm activity very minimal to non-existent over the Huntsville Forecast Area, despite developing unstable conditions. 

However, strong shear and better instability combined with forcing ahead of the dryline near and east of I-65 to break the cap and initiate deep convection between 3 and 4 pm, as seen on the image on the right above. Isolated severe storms developed by 3 pm producing large hail and increased in coverage by 6 pm as it moved east. 

Earlier convection had modified low-level winds near the surface from the predominant synoptic wind flow pattern aloft ahead of the dryline. This can be seen in surface analysis shown above at 1 PM east of I-65. This increased low level directional shear in the atmosphere as dryline moved towards the I-65 corridor during the mid-afternoon hours. This continued into the late afternoon hours and increased low-level helicity values to between 300 m2/s2 and 400 m2/s2 east of I-65. Given instability in place, at 150 PM the National Weather Service in Huntsville issued Tornado Watch #127 for all of our northern Alabama and Southern Middle Tennessee counties, as can be seen above on the right. Although no tornadoes developed, storms did exhibit some rotation as they moved into northeastern Alabama.  Numerous reports of large hail from quarter to golfball size were confirmed near and east of I-65.

0-1 KM Helicity (turning of winds) at 3 PM on 4/5/2017. Click on the image to see an image of these values at 5 PM.
Tornado Watch Issued at 150 PM on 4/5/2017

 


Why was the tornado/wind threat not realized and severe weather not more widespread on April 5th, 2017?

More stringent research will need to be done to find out. However, below are some possible factors that limited how
widespread the severe weather was on April 5th, 2017 and why no tornadoes occurred in the Huntsville
Forecast Area.

 

1. The dry slot - Area soundings showed mid to upper level drying in the atmosphere which likely
    inhibited storm growth in some areas. Instability ultimately did not increase enough to overcome
    this west of I-65.

2. A cap (warm temperatures aloft above 800 mb) was in place - This helped to inhibit storm initiation
    ahead of the dryline. It kept any storms from forming in northwestern Alabama.

3. The bulk shear was 70-80kts - This was possibly too much shear.

4. The warm sector and associated morning convection didn’t make it as far north as originally
    predicted in the early morning hours. (This mainly kept the initially morning round from becoming severe)

5. Directional shear was unidirectional even though the speed shear was substantial west of I-65. Further
    east, enough directional shear was present in the mid/late afternoon hours for tornadoes, but
    other factors kept the threat from materializing. 

Gradient Wind Damage

 

Near and west of the dry-line, with a well mixed atmospheric profile, strong winds were able to be brought down to the surface outside of thunderstorms.  

A few different things were noted that could have contributed to the increased wind speed. 

1  - The pressure gradient was tightening quickly due to the strong surface high pressure departing to the west and the surface low was rapidly deepening over SE Missouri. This increased 700 mb winds from around 35 knots at 7 AM  to between 50 and 65 knots at 7 PM near and behind the dryline. This can be seen in top row of images below.

2  -  The heating produced by the dissipation of cloud cover helped to produce steep low level lapse rates, which allowed winds between 40 and 50 knots just above 850mb to be brought down to the surface in the form of gusty winds. This can be seen in the bottom row of images below.

700 mb Analysis at 7 AM on April 5th, 2017
700 mb Analysis at 7 PM on April 5th, 2017
The main takeaway is that winds highlighted by the horizontal lines depict a height in the atmosphere where winds around 45 knot or higher were situated. Since the atmosphere was well mixed up to that level (due to point two mentioned above), it could take ~ 80% of this wind speed down to the surface. This resulted in winds gusts between 35 and 45 mph near and just behind the dry-line. In fact, the Huntsville International Airport gusted to 46 mph around 7 PM. Later that night, as the cold front pushed through a very dry atmosphere, winds just above 850 mb remained nearly as strong and a Wind Advisory remained in effect until 7 AM. Below is a chart of wind gusts April 5-6, 2017.
 
County Location Time Wind Gust
Lauderdale, AL 4 E Waterloo 2:21 PM 39 MPH
Lauderdale, AL 5 West of Underwood/Petersville 3:58 PM 36 MPH
Madison, AL 2 SSW Redstone Arsenal 4:53 PM 48 MPH
Madison, AL 2 NW Huntsville 6:00 PM 41 MPH
Lincoln, TN Fayetteville 6:15 PM 46 MPH
Lauderdale, AL 1 SE of Underwood/Petersville 6:15 PM 39 MPH
Colbert, AL 3 NNW Littleville 6:19 PM 39 MPH
Morgan, AL 2 NNE Trinity 6:20 PM 43 MPH
Morgan, AL Pryor Field Airport 6:28 PM 45 MPH
Jackson, AL 3 WSW Hollywood 6:35 PM 37 MPH
Lauderdale, AL 1 NNE Rogersville 6:35 PM 36 MPH
Madison, AL 3 SSW Redstone Arsenal 6:43 PM 36 MPH
Madison, AL Huntsville International Airport 6:50 PM 48 MPH
Franklin, TN Winchester 6:55 PM 38 MPH
Madison, AL 2 NW Huntsville 7:00 PM 35 MPH
Morgan, AL 1 NNW Flint City 7:09 PM 35 MPH
Madison, AL 2 E Moores Mill 7:10 PM 43 MPH
Madison, AL 3 W Gurley 7:24 PM 35 MPH
Madison, AL 4 N Grayson 7:27 PM 35 MPH
Madison, AL 3 SW Meridianville 7;40 PM 40 MPH
Madison, AL 1 N Madison 7:50 PM 37 MPH
Madison, AL 2 W Redstone Arsenal 7;55 PM 38 MPH
Madison, AL Madison County Executive Airport 7;55 PM 38 MPH
Marshall, AL Albertville 8:35 PM 40 MPH
Madison, AL 2 SSW Redstone Arsenal 8:48 PM 37 MPH
Colbert, AL North West Alabama Regional Airport 10:34 PM 46 MPH
Madison, AL 1 SSW Redstone Arsenal 12:30 AM  36 MPH
Madison, AL 4 NW Huntsville 1:33 AM 41 MPH
Lawrence, AL Moulton 1:38 AM 37 MPH
Madison, AL 3 S Redstone Arsenal 2:03 AM 35 MPH