Thursday, May 3, 2018

Kansas tornado season finally begins: The May 1, 2018 Tescott-Minneapolis tornado

Wow... this is the first year since 1980 that a tornado hasn't occurred in Kansas before May 1 (and the first year ever that the same has happened in Oklahoma!).  But May 1 really kicked off the Kansas tornado season in a big way with the EF3 tornado on Tuesday northwest of Salina, among other weaker tornadoes across northern Kansas (KS) and southern Nebraska.

With so many storm chasers out on Tuesday, there are more photos and video online of the Tescott-Minneapolis KS tornado than I've seen since the Dodge City tornado day back in 2016.   Here are some more, but we'll use the images to look at the storm's evolution and, later on down, the change toward an environment more favorable for tornadoes during the evening hours before dark.

My wife Shawna and I left Kansas CIty around 12:30 pm and approached one of the first storms near Russell around 4:00 pm CDT (2100 UTC).  The messy training of other storms into the cells north of Russell led us to move southward to intercept a more isolated storm northwest of Great Bend around 5:00 pm, which we followed for the next 3 hours or so until it became tornadic.

Here's an image of this initially non-tornadic supercell in "HP" (high-precipitation) mode north of Hoisington, KS around 5:30 pm (2230 UTC):

We left the storm for a period to navigate around an area lacking roads southeast of Wilson, and then re-intercepted the supercell near I-70 north of Ellsworth around 6:50 pm (2350 UTC). At this time, it was developing a new mesocyclone on its southwest flank (notice the inflow cloud bands converging into this new circulation):

For reference, here's a couple radar reflectivity images roughly an hour apart showing the supercell prior to and during the large tornado west and northwest of Salina:

Moving eastward to a hill north of I-70 and Glendale, KS (west-northwest of Salina), we watched the supercell evolve into a tornado-producer:

As the mesocyclone moved northwest of us, the wall cloud began to consolidate, and dirt began to rise off the ground amidst wrapping rain streaks:

Then, this circulation began to quickly "occlude" and weaken, while a new circulation formed immediately to its east:

As this new circulation moved across to our north, a "cone" tornado touched down just after 7:45 pm (0045 UTC) in reduced contrast to the north-northeast of our location:

A lightning flash behind the tornado made it a little easier to see, contrast-wise:

Shawna's photo image shows the whole structure of the storm at this point, looking toward the north-northeast:

The tornado then widened into a dusty wedge shape around 7:50 pm, and rapid horizontal motion became visible in the ragged condensation tags around the edge of the tornado:

A wider image shows the tornado becoming darker as dirt fills it, behind the widening rear-flank downdraft (RFD):

Shawna's striking image here shows the full storm structure again, with a striated "collar" cloud visible just above the wedge tornado to our northeast:

As often happens, as the large tornado matured, it began wrapping precipitation around it's back side (the radar "hook") as we drove north toward Tescott, making it less visible:

Around 8:00 pm (0100 UTC), the tornado became invisible to us as more curtains of rain wrapped around its backside to our northeast:

Although we could not see it from our position, the half-mile wide tornado was on the ground for at least 10-15 minutes more as it moved northeast to the south edge of Minneapolis, Kansas.  I've not heard of any injuries, although several farms/homes were damaged along its 14 mile path.

As a severe weather meteorologist, it was very informative being able to watch a storm go from an environment that was not very supportive of tornadoes into one that became quite supportive during the evening.  What follows is a brief analysis of the meteorological setting and its changes.

Here's the large-scale setting from the evening 500 mb forecast (roughly 18,000 ft MSL) and 7:00 pm CDT (0000 UTC) surface map on May 1:

The tornado threat area was, as is quite typical, between "splitting" jets, suggesting an area of upward forcing in the midlevels of the atmosphere east of a large trough or "dip" in the jet stream, and north of a "capped"/subsident environment over most of Oklahoma near and south of the southern midlevel wind branching.

As was detailed in the morning Storm Prediction Center (SPC) outlook discussions, the prime environment for tornadoes was expected to be in the evening before or near dark when low-level winds just above ground increased, and before nighttime cooling lessened surface-based instability.  This can be seen in the wind profiles (hodographs) below.  The first hodograph is a short-term model forecast for Great Bend KS valid at 5:00 pm CDT (2200 UTC) just south of the supercell in its earlier non-tornadic stage:

Notice how the low-level wind shear (storm-relative helicity or SRH; see yellow areas above) changed dramatically by the time of the model forecast hodograph for Salina (2nd hodograph) during the large tornado at 8:00 pm CDT (0100 UTC).  The SRH values actually quadrupled (!) as the low-level jet intensified diurnally, approaching dusk.  Given that instability (CAPE) was about the same from afternoon to evening (not shown), it was this low-level wind shear that made the big difference in rotational support for the supercell as it moved northwest of Salina to produce the large EF3 tornado.  The lack of any interfering storms to the south of the Salina supercell (see earlier radar images) also helped this storm utilize the increasing low-level wind shear flowing into it from the south. 

Accordingly, the RAP forecast of the energy-helicity index (EHI; a composite of instability and low-level wind shear helpful in identifying areas potentially more supportive of supercell tornadoes) suggested that, compared to the afternoon environment, the evening setting would be more supportive (larger EHI values, in red) for tornadoes over central Kansas:

This was confirmed by the large significant tornado parameter and EHI values south of the Salina supercell on the SPC mesoanalysis at 8:00 pm (0100 UTC) during the large tornado:

Another factor was probably cloud base heights (lifting condensation levels, or LCL heights).  During the afternoon with initial storms westward closer to the dryline, LCL heights were much higher (mixed-layer LCL heights > 1500 m AGL, not shown) than during the evening in the more deeply-mixed moisture enviroment near Salina (LCL heights < 1000 m AGL, not shown).  This would make a difference in reducing low-level evaporative cooling that could be detrimental to tornado formation. 

Unlike some recent posts where I've talked about the role of boundaries in helping to produce tornadoes, the Tescott-Minneapolis KS tornado formed from a supercell in the warm sector away from any pre-existing boundaries.  It appears that the unobstructed evening combinations of wind shear and instability over central Kansas flowing into this supercell were simply large enough to support a strong tornado without the help of any detectable boundaries. 

The end result was one of only three EF3 tornadoes in the United States so far in 2018, and Kansas' first tornado day of the year.

- Jon Davies - 5/3/18

Monday, April 23, 2018

Stunning Images from Ft. Walton Beach tornado -- April 22, 2018

It has not been a particularly active tornado year so far in 2018, largely due to repeated intrusions of cold air over the eastern United States.  But Sunday, April 22nd (yesterday) yielded some striking images from tornadoes in the western Florida panhandle and southern Alabama.

This was especially true in Ft. Walton Beach, Florida, where a supercell tornado came onshore over Okaloosa Island just after 4 p.m. CDT (2100 UTC), and then crossed into Fort Walton Beach proper.  Preliminary indications are that the tornado was only EF1 in intensity, and there were no major injuries reported, but many people caught the tornado on video, and it was visually impressive.

The sequence below (looking west) shows the tornado crossing from Okaloosa Island over into Fort Walton Beach, with debris visible from a roof being torn off a building.  Confirming the storm's supercell structure, a rear flank downdraft (RFD -- area of sinking air on the south side of the tornado; see last couple images in the sequence) is visible as the tornado passes just east of a tall condo building and onto the mainland:
A closer video shot from this tall condo building looking south at the tornado also shows the same large piece of roof debris moving from east to west across the front of the tornado and splashing into the water:

It's always interesting how different a tornado can look depending on which side a photographer is located.  An earlier video after the tornado had moved into Okaloosa Island as a waterspout from the Gulf of Mexico shows how "white" it appeared from behind, illuminated by light from the RFD against the darker background to the north.  Some smaller vortices (suction vortices) are also visible within the tornado:
Other tornadoes occurred on Sunday, including an EF0 near Foley, Alabama (west of Fort Walton Beach) where 3 people were injured in overturned RVs, another EF1 in Escambia and Crenshaw Counties of southern Alabama, and an EF0 near Fort Rucker, Alabama.

The synoptic weather setting (see image below with insets) showed a midlevel trough/disturbance moving eastward in the southern jet stream.  The afternoon tornadoes occurred ahead of this trough in an area where there was branching/spreading of the jet winds (see white arrows).  The surface map (first inset) at mid-afternoon also showed a warm front across the western Florida panhandle.  In previous posts this year, I've mentioned how tornadoes like boundaries due to the increased wind shear along them, and in the case of warm fronts, increasing warmth, moisture, and resulting instability.  This case was no exception (see yellow arrow on inset):

Although CAPE, a measure of instability, was not particularly large in this case (around 1200 J/kg; not shown), there was plenty of low-level wind shear or storm-relative helicity (SRH, greater than 300 m2/s2; not shown).  Combinations of CAPE and SRH included in such parameters as the significant tornado parameter (STP) and the energy-helicity index (EHI) help forecasters in predicting where storms may rotate in low-levels and tornadoes may be possible.  

In this case, an "enhanced' version of the EHI that I've been working on highlighted the area along the western Florida panhandle at mid-afternoon near the warm front, and STP from the Storm Prediction Center mesoanalysis page did the same:

This "enhanced" EHI (which also factors in deep-layer shear and low-level CAPE) needs some work on my part to improve it, as the values often tend to be too large compared to the STP (see above).  But both parameters definitely suggested an environment supportive of supercell tornadoes over the western Florida panhandle and southern Alabama yesterday afternoon.

- Jon Davies 4/23/18  

Tuesday, April 17, 2018

A beautiful funnel aloft in Iowa, and the April 13-15, 2018 tornadoes

Last Friday through Sunday (April 13-15, 2018) were active severe weather days across the central, southern, and mid-Atlantic states, and sadly, there were two deaths related to tornadoes.  I'll touch on the setting for these severe weather days later in this blog.  But first, I'll briefly discuss an interesting storm chase to southwest Iowa on Friday, April 13 (see image above).

Near the Kansas City area where I live, Friday's storm environment didn't look very supportive of tornadoes. There wasn't much low-level wind shear (storm-relative helicity or SRH), and cloud bases looked to be too high with large temperature-dew point spreads around 20 degrees F or more.  The best tornado potential on 4/13/18 was farther south in Arkansas, northern Louisiana, and northeast Texas. But in southwest Iowa, east of a surface low near a warm front/stationary front, things looked at least a little interesting. 

So, my wife Shawna and I decided to take a quick trip north to the Iowa front to see if anything of note would happen there.  We were rewarded with a tornado-warned storm, mesocyclone, and lowering that briefly tried to produce a tornado just north of the front, north-northeast of Hastings, Iowa:

A second storm then produced a fascinating high-based horizontal funnel right over our heads (!) that lasted around 10 minutes near Emerson, Iowa as this newer storm crossed the same stationary front:

The surface map below shows the east-west front at about 3:00 pm CDT (2000 UTC), and also lowest elevation radar reflectivity (inset with storms labeled) about 45 minutes later (at 2044 UTC) in the Red Oak (RDK)-Atlantic (AIO), Iowa area:

We found that the temperature contrast when driving back and forth across the frontal boundary just north of Emerson was quite sharp, a nearly 15 degree F drop across only 2 miles!  That probably explains why neither storm produced a tornado with all that near-surface cool air, even though low-level shear and SRH increased significantly north of the front.  Notice on the SPC mesoanalysis panels below that the large values of SRH were north of the front, while surface-based CAPE (from surface lifted parcels) was restricted to the area south of the front:

With little or no overlap between these two environments, there was little chance for tornadoes as storms crossed the boundary, moving rapidly into the inhibiting cool surface air.  However, the storms north of the front still had enough "elevated" CAPE (from lifted parcels above the cool surface air) to become supercells and produce sizable hail and even some wind damage near Atlantic, Iowa.

And, of course, there was that cool funnel cloud as the second storm crossed the boundary :-).

On a much broader scale, the graphics below show the upper wind flow pattern and forecast environmental CAPE-SRH combinations (via the energy-helicity index, or EHI) for Friday evening April 13 (0000 UTC 4/14/18), and Sunday afternoon April 15 (2100 UTC 4/15/18).  SPC storm reports for those two days are also shown (for brevity, Saturday April 14 is omitted):

Notice how the large upper trough (sharp "dip" in the jet stream) moved eastward, and how tornadoes, in a broad sense, were most numerous between the jet stream branches (large white arrows) spreading out east of the upper trough.  These areas also had sizable EHI values (CAPE-SRH combinations) that were more supportive of rotating storms.

Here's a couple photos of tornadoes from this 3-day tornado-producing storm system; one in southwest Arkansas near Umpire on Friday evening 4/13/18 (very large and ominous), and one near Greensboro, North Carolina on Sunday afternoon 4/15/18:

Unfortunately, a toddler was killed on Friday night in an EF1 tornado near Shreveport, Louisiana, and a man was killed near Greensboro, North Carolina from winds located just west of the EF2 tornado shown in the image above. Both deaths were from falling trees, a significant problem regarding both wind and tornadoes in the eastern half of the U.S., where trees are more numerous.

-  Jon Davies  4/17/18

Wednesday, April 11, 2018

Another "Downtown" tornado - Ft. Lauderdale April 10, 2018

Yesterday's tornado (from the supercell pictured above) in downtown Ft. Lauderdale, Florida was just another reminder that the urban legend saying tornadoes don't strike in the heart of cities is very false.   Thankfully, the downtown tornado was only EF0 in intensity, and there were no injuries. Another EF0 tornado occurred about an hour later at the Ft. Lauderdale-Hollywood International Airport. Although both were weak, it's interesting to take a quick look at the setting that helped generate the tornadoes.

As many researchers have shown, tornadoes like boundaries because they are a source of vorticity ("spin"), among other factors.  The surface map and radar below about 30 minutes before the Ft. Lauderdale downtown tornado suggests that a boundary laid down by an ongoing storm may have helped the second storm trailing along behind it to become tornadic:

As this second storm became dominant and moved along the boundary toward the east-northeast (see radar and satellite below), it produced a weak tornado shortly after 1930 UTC (around 3:35 pm EDT) that was visible as a dust cloud passing through downtown Ft. Lauderdale:

The setting wasn't conducive to strong tornadoes, but in addition to the boundary, the SPC mesoanalysis at 1900 UTC (3:00 pm EDT) showed an environment that was at least marginally supportive of tornadoes.  The first graphic below shows that deep layer wind shear (0-6 km) was around 30 kt, which can support supercells, even though storm-relative helicity (SRH; a source of "spin" for supercell tornadoes) was meager (only around 50 m2/s2):
However, 0-3 km lapse rates were 7.0 to 8.0 deg C/km (see below), a rather steep temperature decrease in the lowest few kilometers that could facilitate low-level stretching.  This is more akin to the High Plains of the U.S. rather than near sea level in Florida, and instability (CAPE) in lowest levels was also plentiful.
The resulting non-supercell tornado parameter showed a maximum near Ft. Lauderdale, and the  energy-helicity index (a supercell tornado forecast parameter) was also marginally above 1.0 in the same area:
While none of these parameters are particularly impressive, the combination of this marginal environment containing moderate to strong instability (>2000 J/kg of total CAPE, not shown) with the boundary noted earlier was apparently enough to generate the Ft. Lauderdale downtown tornado.

The weak tornado at Ft. Lauderdale's major airport around 4:25 PM EDT (2025 UTC) may have also been influenced by another boundary (a broad outflow boundary moving S, see satellite image below).  This boundary could have interacted briefly with yet another storm moving into Ft. Lauderdale from the west:
By any stretch (pun intended), these tornadoes weren't really predictable in advance.  But an alert forecaster might see ingredients coming together in real-time to heighten situational awareness for monitoring of radar and public reports.   

The brief analysis above also suggests that, although the tornadic storms were supercells, some non-supercell/non-mesocyclone processes (related to steep low-level lapse rates along boundaries) may have also been at work for this event.

- Jon Davies  4/11/18

Friday, March 23, 2018

Alabama tornado outbreak March 19, 2018 -- A look at the meteorological setting

My wife Shawna has been bugging me to start up my severe weather blog again 😊, so I thought I'd do a post about the setting for last Monday's episode of tornadoes in northern Alabama (March 19, 2018). With the exception of the February 24 outbreak from northeast Arkansas to south-central Tennessee, this was the first tornado outbreak heading into spring after a rather quiet winter.

Here is my preliminary mapping of the tornadoes in northern and eastern Alabama on 3/19/18, from online NWS surveys:

The tornadoes were produced mainly by three different supercells moving across the northern third of Alabama, as is suggested by the way the tornado damage paths line up from west to east or east-southeast along three different paths. Later in this post, I'll pay particular attention to the evening tornadic supercell that struck Jacksonville, Alabama with an EF3 tornado.

The Storm Prediction Center (SPC) did a great job of forecasting this outbreak. Also, local awareness and timely NWS warnings resulted in no deaths and only a few injuries. That's always great to see!

Forecast-wise, it wasn't too difficult to pick out the potential for tornadoes. The model forecast below showed strong west to east winds at roughly 18,000 ft MSL (a "southern-stream" jet, see thick white line and arrow) into the southeastern U.S., with an embedded short-wave disturbance approaching Alabama at 7 pm CDT on 3/19/18. The model forecast of 0-1 km energy-helicity index (EHI; see inset), which combines instability and wind shear important for supporting tornadoes, was also maximized over northern Alabama ahead of the short wave where thunderstorms were expected:

A side note about west to east southern-stream jet wind axes: Tornadoes seem to be more common along and just north of the southern-stream jet, rather than further south. This has to do with jet stream dynamics and related issues I won't get into here. But it's worth noting that the day before (3/18/18, not shown), tornado potential that was forecast for eastern Texas but did not verify was in an area south of this southern-stream jet axis. Yet on 3/19/18, when supportive tornado environment parameters were maximized just north of this same southern jet stream core, a number of tornadoes occurred.

The 3-panel radar graphic below, from NWS BMX radar, is after 7 pm CDT (00 UTC), and shows the three main tornado-producing supercells (marked 'A', 'B', and 'C'):

Supercell A near the AL/TN border had been producing tornadoes during the prior hour and was weakening, at the end of its tornado-production phase. Supercell B was the largest, beginning in Mississippi before 5 pm CDT, and produced a number of weak short-track tornadoes across Alabama. However, the tornadic supercell of the day was the last one to form (supercell C), which developed rapidly after 7 pm CDT, and produced the long-track EF3 tornado that hit Jacksonville; this tornadic supercell was responsible for at least 7 injuries.

Why was cell C the strongest tornado producer? It's hard to say, but there are some possible clues. First, supercell B moving across Alabama during the late afternoon was large and long-lived, generating a traveling meso-low over northern Alabama (see the southernmost red "L" on the surface analysis below) that helped to focus and converge the low-level wind environment:

This wind stream out of the south and southeast can be seen on the streamline map of surface winds below at 7 pm CDT (00 UTC), flowing and converging into the area where supercell C formed rapidly between 00 UTC and 01 UTC (see the earlier 3-panel radar graphic):

This area of increased low-level wind flow (also helped by the tendency for low-level winds ahead of storm systems to increase near and after dark) would certainly increase low-level wind shear (storm-relative helicity, SRH) that supports supercell tornadoes. The significant tornado parameter (STP) incorporates low-level wind shear/SRH as one of its key components, and, in the 8 pm CDT (01 UTC) graphic below from the SPC mesoanalysis page, STP was quite large and very focused just east of Birmingham:

Another factor may have been the location of the stationary front (refer back to the surface map shown earlier) over east central Alabama. There's been plenty of research to indicate that tornadoes favor surface boundaries, such as warm fronts and stationary fronts, which increase wind shear through backing of surface winds. Supercell C from our earlier radar graphic would have interacted with this stationary front as it moved east and east-southeastward, improving its chances of producing tornadoes. However, as this supercell moved over into Georgia, it encountered increasingly cooler surface air northeast of the stationary front, and only produced an additional brief tornado or two.

So, with unobstructed wind flow and SRH to the south of newly formed supercell C northeast of Birmingham, it only took about 45 minutes from the storm's initiation for it to begin generating tornadoes, which is an impressively short time. The increasing southerly low-level flow, due both to supercell B's inflow and the diurnal increase of SRH near dark, in addition to the cell's interaction with the northwest-southeast stationary front, may have all contributed to this storm's efficient tornado production. The third tornado from supercell C (see photo below) hit Jacksonville shortly after 8:30 pm CDT (0130 UTC):

This tornado continued onward to the Georgia border for a track of over 30 miles, and was over a mile wide at one point. It is truly fortunate that this specific tornado only caused 4 injuries.

I'll try to do more posts as severe weather warrants and time permits this spring. Thanks for reading!

- Jon Davies 3/22/18

PS:  For anyone interested, my detailed peer-reviewed study of the meteorological setting for the devastating Joplin tornado back in 2011, published last December, is online here.