Tropical Update

Sad news over the weekend involved the death of a 5-year old boy at the Huntsville Airshow. A localized microburst occurred at the event and blew over several tents and injured 12. The storm was produced by the deep trough and strong cold front that swept through the region. My thoughts and prayers are with the family and friends of the young boy.

The tropics are beginning to get heat up. The Eastern Pacific has been active for the last couple of weeks with Tropical Storms Boris and Douglas currently churning. Both storms only pose a threat to shipping at least point. The Atlantic basin looks get in on the action this week with two tropical disturbances showing some potential for further organization.

The first disturbance moving through the Lesser Antilles is producing heavy rains and gusty winds. Hewanorra, St. Lucia measured a little over 0.50" of rain with sustained winds up to 28 mph out of the E and ENE. Thunderstorm activity increased today, but the system is currently experiencing 20-30 knots of southwesterly wind shear due to an upper-level trough to its west. This is a bit too high for significant development, but if the upper trough dissipates or moves further west, this would allow the shear to relax and strengthening would be more likely. Water temperatures are supportive of tropical development with SSTs in the lower 80s in the eastern Caribbean. In fact, the SHIPS model calls for slow strengthening of this system over the next 24 hours, and it even develops the wave into a named storm within 48 hours. I think that may be a bit optimistic given the current wind shear in place, but a lessening of the shear would allow for development. Regardless, this system bears watching as it moves to the WNW. If the system does develop, the forecast upper-level pattern would more than likely keep the main threat to Mexico.

A larger and much healthier looking tropical wave is centered about 220 miles southeast of the Cape Verde Islands. It's climatologically early to be looking this far east for tropical development, but this system appears impressive on satellite with deep convection clustered the center. SSTs near and to the west of the wave are marginal (~79 deg F) for development, but some slow strengthening is possible as the system moves quickly to the west along the southern periphery of the subtropical high. The marginal SSTs may be compensated by weak/moderate wind shear (10-15 knots) and an absence of Saharan dust/dry air. The Saharan air is well to the north of the system, and wind shear will become very weak as the system moves to the west. These two factors argue for continued strengthening, but the marginal SSTs should prevent significant development for the next several days. However, SHIPS calls for the wave to be a named system by 48 hours and up to hurricane intensity within 72 hours. I think this is overly aggressive, but I wouldn't be surprised to see a tropical depression and possibly a weak tropical storm form within 48 hours. Computer models are unanimous at this point in recurving the system in about 5 days.

Weekend Storms?

It's been an active week and a half for much of the Northern Plains, and the same song and dance continues this evening with several severe weather watches in effect. Fortunately, the tornado outbreaks have been limited by a fairly weak zonal flow aloft. Unfortunately, large hail storms have been the rule, and I'm sure the insurance companies are staying quite busy.

Turning my attention closer to home, models are in good agreement on digging an abnormally strong trough into the Southeast U.S. this weekend. After reading area forecast discussions around the area, I'm a bit surprised that the severe weather threat is not being hit harder. There should be plenty of instability in advance of the cold front. Combine that with unseasonably strong wind fields, and this set-up should support a much higher than normal chance of severe weather in states like Alabama, Mississippi, Tennessee, and Arkansas. It will be interesting to see if this gets more hype tomorrow. Regardless, the cold front should push well into the Deep South by Monday and will cool temperatures off significantly for most areas -- at least for a couple of days.

Very quiet weather continues to dominate the tropics, but I get the feeling that won't last much longer. Any storms that do form later in the year will probably intense with the undisturbed ocean waters. However, it will be interesting to see if a weak El Nino develops this season, which may help to limit the number of named storms. The 90 day SOI is fairly neutral and doesn't point to any strong signals of a La Nina or El Nino event.

Wednesday Storm Surveys

Just a few links to select storm surveys from this past Wednesday's tornado outbreak across the Central Plains.
Manhattan, KS tornado -- EF4
Chapman, KS tornado -- EF3
Little Sioux, IA tornado -- EF3

A northwest flow aloft should develop across the Plains states this week and will likely prevent large tornado outbreaks for the foreseeable future. The chances for MCSs will increase and with that the likelihood of more flooding rains across the Missouri and Mississippi Valleys. I like this pattern in the summer because it means chances for surprise nocturnal thunderstorm complexes and lightning photography opportunities in Mississippi!

Little Sioux, IA Tornado

The big news from yesterday's tornado outbreak was the killer tornado just north of Little Sioux, IA. Four scout troops died at the Little Sioux Scout Ranch when the tornado tore through the campground. The storm that spawned the tornado was clustered in a line of storms -- certainly not an ideal setup for tornadoes. The yellow triangle (Tornado Detection Algorithm) indicates the approximate location of the tornado in the messy base reflectivity scan.


However, the storm relative velocity clears up the picture and certainly shows strong rotation within the storm. The Omaha, NE radar sampled the storm at about 2,500 feet above the surface, and the SRV product shows a 41 knot inbound velocity and a 56 knot outbound velocity. The gate-to-gate rotation was 97 knots or 112 mph.


The same storm system produced a rash of tornadoes in north-central Kansas last evening as well (see the post below). In fact, heavy damage was reported around Kansas State University, including a building that houses a nuclear research reactor! The same supercell earlier wiped out the town of Chapman and later killed a person near Soldier, KS.

Manhattan, KS Tornado

A long-lived supercell plowed across north-central Kansas and produced several tornadoes along its track. One of the supercell's tornadoes struck Manhattan, home of Kansas State University, around 11:00 p.m. this evening. The base reflectivity and storm relative velocity images I saved show a classic tornado signature. In fact, the base reflectivity (scanned at 0.5 degrees) shows a possible debris signature associated with the tornado. The high reflectivity (66 dBZ) that I circled is a significant energy return, but the VIL product at the same time only returned 16 kg m^-2. Furthermore, the echo was only 1,400 feet above the ground, and there was no hail core suspended above this signature. Therefore, it's quite possible this could be some type of debris that was sampled by the radar.


The storm relative velocity sampled at the same time also shows a very tight, cyclonic circulation at the approximate location of the high reflectivity return. The green colors indicate motion toward the radar, and the red/pink colors indicate motion away from the radar; the Topeka radar is southeast of this image. The inner green color is a 33 knot inbound velocity, and the inner pink is a 58 knot outbound velocity; added together, that is a 91 knot (105 mph) total circulation -- quite impressive for about 1,400 feet above the ground.

Iowa Supercell

A pristine environment for supercells developed across Iowa this evening with large instability (MLCAPE >3,000 J kg-1) balanced by strong deep layer wind shear (55 kts). One large, discrete and cyclical supercell has persisted for the last several hours over southwest Iowa, and has occasionally produced large tornadoes. This is a classic example of a supercell riding along a boundary and ingesting large amounts of storm relative helicity, which helps aid in tornadogenesis. For more information about this, I highly recommend the Markowski et al. (1998) paper.

Tomorrow's severe weather environment setup looks great for a large portion of the Central and Southern Plains. The western U.S. trough swings out into the Plains tomorrow and should provide the lift necessary to generate a severe weather outbreak. It appears that storms should initially develop across Nebraska as discrete supercells, but they should congeal into a linear complex somewhat quickly given the parallel orientation of the deep layer shear vectors to the front. Further south across Kansas and Oklahoma, it appears as though the winds may be slightly more perpendicular to the front to allow for a longer period of discrete storms. Therefore, this should increase the risk for a tornado outbreak across these two states, especially given that 0-1 km SRH values are forecast to range between 150-300 m^2 s^-2. Sadly, I'll be arm chairing this event.

Monday Thoughts

A derecho swept across northern Kansas to southwest Missouri today. Derechoes are notoriously known for the wind damage they produce, but this particular derecho produced significant hail in addition to damaging winds. In fact, 4.25" hail was reported near Manhattan and Waverly, KS this morning. 4.25" hail is just slightly smaller than a softball!

I was fortunate enough to capture the image below at sunset last evening. After we got back to Starkville, a long-lived MCS moved through and produced gusty winds and minor tree damage in town. After its exit, it made for a picturesque sunset.


The deepening western trough kicks out into the Plains on Thursday, and pattern recognition certainly suggests the potential for a significant severe weather outbreak. Copious moisture will surge northward Wednesday evening and Thursday. Meanwhile, an energetic upper-air system will spread across the warm sector, and will create an environment highly supportive of supercells. More specific details tomorrow.