Wednesday, August 28, 2013

23 Years Ago: The Only F-5 Tornado to Occur in August: Plainfield, IL

On this date in 1990, an F-5 tornado struck the community of Plainfield, IL, causing extensive damage.  The tornado killed 29 people and injured over 300 more.  To this day it is still the only F-5 (now called EF-5) tornado known to occur anytime during the month of August in the United States.

Steve L., the pilot of a Cessna 172, took the aerial photos above and below along the damage path in Plainfield a few days after the tornado took place:

The path of destruction was 16 miles long and 600 yards wide at one point as the tornado tracked from Northwest to Southeast across the area during the late afternoon hours:

The National Weather Service (NWS) was heavily criticized for failing to warn of the tornado in this particular event.  Back at the time, the area that is now covered solely by the NWS Office in Chicago was split between that office and one in Rockford, IL.  The Rockford office performed quite well, giving advanced warning of a tornado that was produced by the same thunderstorm over that part of the region.  As the storm moved into NWS Chicago's area of responsibility, the warning performance degraded significantly.

The NWS's own "service assessment" report following the event was rightfully hard on the Chicago office, as you can see in the excerpt below:

At the time that was the harshest criticism I had ever read in a NWS service assessment (of itself), and I don't think it's been surpassed to this date (if anything, the NWS has become "easier" on itself in subsequent service assessments).

In its defense, the NWS in Chicago pointed out that the Storm Prediction Center (which was then called the National Severe Storms Forecast Center) had issued a Severe Thunderstorm Watch instead of a Tornado Watch for the region that afternoon.

While "only" one supercell storm produced tornadoes within the watch area on that day, the Plainfield EF-5 tornado was one of them.  Hence the reason that I always caution folks not to let their guard down, particularly when dealing with an isolated, supercell storm.

You can be that residents of the area will never forget that day, nor are they likely to let their guard down "just because" there isn't a Tornado Watch in effect - and who can blame them, especially after seeing images like these:

This was also one of the last tornadoes to be examined by the late Dr. Ted Fujita, the preeminent tornado researcher and damage survey pioneer of our lifetime (creator of the Fujita "F-Scale").

You may find it surprising that the F-5 rating was arrived upon by Dr. Fujita's examination of damage to a corn field, rather than by any structural damage within the city itself:

Dr. Fujita gave a presentation on the Plainfield tornado as shown in the following YouTube video.  This was one of his last formal presentations which was held at the 3rd Tornado Symposium in Norman, OK back in 1991.  I had the great pleasure of attending that meeting and seeing his presentation in person.  If you have the time and interest, I highly recommend viewing it at some point as it gives a lot of great information on this event:

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Tuesday, August 27, 2013

Spectacular Photos of Haboob in Phoenix Yesterday Evening...

A dense, widespread haboob (fancy name for dust storm) took place in the Phoenix area yesterday evening.  Numerous (and many spectacular) photos of the phenomenon circulated throughout social media:

Jerry Ferguson (via twitter)

Rob Schumacher (via twitter)

Brian Pivar (via twitter)

Jeff Piotrowski (via twitter)

If you look closely at the last 2 photos, you can also see a slight "curl" in the flow along the left side of the dust plume.  That suggests downward air motion associated with a microburst or downburst at that particular location.

On the 2nd photo (from the top) you can see an airplane that had just taken off from Sky Harbor Airport ahead of the oncoming deluge (it was reportedly the last flight to depart prior to the arrival of the dust storm).

The sudden reduction of visibility and strong winds resulted in a 1 hour or more "holding pattern" above the airport for a time yesterday evening:

...during which time several flights decided to divert to Tucson or other nearby airports.

Strong wind gusts of 45-50 mph pushed the wall of dust from South to North through the region.  The following chart from the Phoenix Airport shows a peak wind gust near 50 mph just before 7:00 PM local time yesterday: 

Haboobs/dust storms are not necessarily uncommon in the area, especially with dry weather and strong, gusty winds there is plenty of "fuel" for the fire once things get going on a hot summer afternoon.

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Sunday, August 25, 2013

Tropics Looking to "Heat Up" - Right on Schedule...

After a relatively quiet start to the season (in the Atlantic), the tropics are starting to show signs of "waking up" as we head toward the traditionally busy month of September...

A disturbance over the southwestern Gulf of Mexico has become well organized during the past 24 hours, and is likely to become a Tropical Depression or perhaps even a weak Tropical Storm before making landfall in Mexico tonight into early Monday.  I've circled the area in red on the latest visible satellite image:

Fortunately, with the system so close to land, there is little opportunity for significant strengthening, and this will mainly be a heavy rain and potential flooding/mudslide event for the impacted areas.

Meanwhile, a string of tropical waves are about to emerge from Africa into the Atlantic basin, the first two circled in red and yellow on the satellite image below: 

There is still quite a bit of African dust that could inhibit vigorous development early in the week, as shown by this computer model dust plume forecast from NASA valid 7pm CDT Monday:

...but the mass is forecast to thin out somewhat by 7pm CST on Wednesday:

So, as the week progresses, it appears as though conditions will gradually become more favorable for one or more of these disturbances to hold together and become better organized as they move Westward.

Conditions are likely to become even more favorable for development in the Atlantic as we head into at least the early part of September, so please don't let your guard down just because we've had a "slow start" this season.  There's still plenty of hurricane season yet to come, and as I always say, it only takes 1 system to hit the wrong place at the wrong time for us to have a very bad result.

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Friday, August 9, 2013

Saharan Dust Keeping a Lid on the Tropical Atlantic So far...

Above is an animation from the NASA GEOS-5 model, showing the spread of dust from the Saharan Desert of Africa, Westward across the tropical Atlantic (model data via Dr. Ryan Maue of WeatherBell).

The plume of dust is associated with a hot, dry layer of air that continues to spread West into the tropics, eroding the deep moist layer that is needed for significant tropical development.  It will be interesting to monitor this resource for signs of a breakdown in the "dust train", which could signal an uptick in tropical development shortly thereafter.

I'd also like to take this opportunity to remind everyone not to be lured into a false sense of security due to the seemingly "slow start" to the tropical weather season.  As I always point out, whether we have 15 hurricanes or 1 hurricane in a given season, it only takes 1 hitting the wrong place at the wrong time to have a potentially disastrous result.

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