October surprise

Don't worry, this entry is about weather, not politics: the atmosphere pulled a fast one -- literally, as in Omar's winds.

HURRICANE OMAR - OCTOBER 2008

[Source of all images unless otherwise indicated: NOAA]

HURRICANE CHARLEY - AUGUST 2004

[Image source: University of Wisconsin / CIMSS]

Omar was a Category 1 hurricane early Wednesday evening, and by the wee hours Thursday was a high-end Category 3 and may have been a Category 4.

After Omar's closest approach to St. Croix, its central pressure continued dropping and the aircraft recon found some exceptionally strong winds. The average velocity throughout the lowest 150 meters of the atmosphere was measured to be nearly 150 mph.

In the National Hurricane Center advisories, the maximum sustained surface winds listed were 110 knots, which equates to 126.5 mph and rounds to 125 mph.

However, there was this line in the early morning discussion from NHC:

IT IS ESTIMATED THAT OMAR REACHED A PEAK INTENSITY OF 115 KNOTS EARLIER.

115 knots equates to 132.25 mph. Since NHC operates with increments of 5 mph, "2.25" would ordinarily round down, but 132.25 is complicated by the fact that Category 4 is defined as 131-155 mph. Though not always, historically NHC has therefore usually categorized 115 knot hurricanes as 135 mph Category 4s.

Likewise, when they put a line like the one above in a discussion, they usually adopt it in their postseason report.

*IF* that happens once a complete post-storm analysis is done, the NOAA historical hurricane site indicates there have only been six Category 4 or higher hurricanes later than this in the Atlantic hurricane season since 1960. (And Hattie, Mitch, and Wilma all reached Category 5 intensity.)

Oct 21 - Wilma 2005
Oct 22 - Joan 1988
Oct 28 - Mitch 1998
Oct 31 - Hattie 1961
Nov 4 - Michelle 2001
Nov 18 - Lenny 1999

Omar was yet another fascinating tropical cyclone, and fortunately this time the most severe winds missed land, occurring over the Anegada Passage between the Virgin Islands and the islands to the east of there, thanks to a fortuitous track and compact size of the hurricane. Nevertheless, there was damage in some places, especially St. Croix.

Why did it strengthen so much so rapidly?

Can you say "outflow channel"?

It was 50 years ago that a Banner Miller, a meteorologist with what was then called the U.S. Weather Bureau, published a paper in the Journal of the Atmospheric Sciences identifying that a key factor in intensification of hurricanes can be a trough to the northwest which produces upper-level outflow from the cyclone. This was assessed with limited data and prior to the existence of satellite imagery! (There's now an annual American Meteorological Society award named after him.)

A trough in this context is a southward dip in the jet stream, or at least the overall wind flow aloft, ahead of which are brisk winds from a southerly or southwesterly (or sometimes SSE) direction.

In cases such as this it's a cat-and-mouse game between shear and outflow. If those strong winds aloft blast right over a tropical cyclone, it can create sufficient wind shear (winds at different altitudes blowing at different speeds/directions) to literally blow the top off the storm and cause it to weaken.

If however the strongest upper-level winds are not quite overhead but are nearby and blowing from the cyclone toward a direction away from it, that can cause it to strengthen by "evacuating mass." (Yes, humans aren't all that evacuates from hurricanes!) Way back in '58, the year of my birth, Banner Miller wrote that in tandem with other atmospheric features, "the trough to the northwest furnishes an efficient outflow mechanism for the evacuation of the vast quantities of air that have been lifted from the surface."

As the air gets sucked away from the top of the cyclone while the surface flow converges inward, the central pressure drops and the winds increase.

That's precisely what happened with Omar, whose pressure plunged from 982 millibars at midday Wednesday to 971 mb at around 7pm and 959 mb (28.32") by 1:45 am Thursday.

This water vapor satellite image will help visualize the setup I discussed above. The circle I've drawn is Omar when the center was near St. Croix, the dashed line is the trough axis, and the arrows represent strong upper-level winds blowing away from the top of the hurricane.

Here's an analysis of the same stuff courtesy of CIMSS. The bright colors show that there's a lot of shear near -- but not over -- the hurricane.

By midday Thursday the 16th, though, while there's still outflow, there's now more shear overhead and that was too much for the hurricane, whose head got shaved. The low-level circulation center (circled) is exposed and the hurricane had gone back down from a borderline Category 3/4 to a Category 1 even more quickly than the reverse had occurred. Omar had shrunk in size; that also was a factor as small circulations can be very prone to large and rapid intensity changes up or down.

[Image source: Mauna Kea Weather Center]

Here are two other classic examples of outflow channels and intensifying tropical cyclones, in this case in southwest Florida. Note the similarity of the satellite signatures below.

The first one is Hurricane Charley in 2004 (whose satellite photo accompanies Omar's at the beginning of this entry), which before it made landfall pulled a similar stunt: quickly strengthening a lot (and more than expected).

The second is Tropical Storm Fay, which earlier this season actually intensified over land, and the outflow channel was undoubtedly a primary reason why.

Both circulations were also small in size at the time, Charley's being exceptionally so.

CHARLEY

FAY

Now here's Isabel over the central Atlantic in September 2003 as it was quickly changing from a Category 1 hurricane to a Category 4. (It would later undergo another round of strengthening and become a Category 5 for awhile before eventually slamming the mid-Atlantic region as a Category 2.)

There's just basically an upper-level anticyclone (high pressure system) overhead with the air flowing out in all directions along with very healthy, deep convection (rain clouds & thunderstorms) helping to provide energy ... illustrating that while in general outflow is important for tropical cyclone intensification, a vivid single outflow channel like those highlighted above isn't absolutely necessary.