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Pre-season subtropical storm

1030AM EDT UPDATE: Have a look at Adam's latest post at http://www.thestormtrack.com/2007/05/a_difficult_forecast.php

10AM EDT UPDATE:
As of 10:00AM EDT, the subtropical system has yet to be named by NHC. It is slowly but steadily transitioning to a tropical system as convection continues to build around its center of circulation. Tropical transition can be a long process (often in excess of 48 hours) in a dry environment and in this case the subtropical system probably won't have time to fully transition before landfall, but should get close. The more recent satellite imagery is posted and discussion continues below. Notice that the more recent water vapor image is showing deeper blues and more convective cells wrapping around the center, indicating more moisture and a more tropical storm.

Latest GOES visible image
GOES water vapor image. (7:45am EDT Tuesday)

The North American Model (NAM) continues to forecast the storm to transition to a shallow warm-core system before landfall near Jacksonville, FL. Below is the latest Cyclone Phase Space estimate and forecast track from the NAM.

Latest NAM forecast.

The National Hurricane Center has also issued a Special Tropical Disturbance Statement:

WONT41 KNHC 081346
SPECIAL TROPICAL DISTURBANCE STATEMENT
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
950 AM EDT TUE MAY 8 2007

A NON-TROPICAL LOW PRESSURE SYSTEM...CENTERED ABOUT 230 MILES
EAST-SOUTHEAST OF THE GEORGIA AND SOUTH CAROLINA COASTS...HAS BEEN
MOVING SLOWLY WESTWARD AT 5 TO 10 MPH. THIS SYSTEM IS PRODUCING
GALE-FORCE WINDS AND HEAVY SURF ALONG THE COASTS OF NORTH
CAROLINA...SOUTH CAROLINA...AND GEORGIA...WITH STRONGER WINDS
OFFSHORE. ASSOCIATED SHOWER ACTIVITY HAS INCREASED SINCE
YESTERDAY...BUT NO SIGNIFICANT STRENGTHENING OF THIS SYSTEM IS
EXPECTED. THE LOW IS BEING MONITORED FOR SIGNS OF TROPICAL OR
SUBTROPICAL CYCLONE DEVELOPMENT...AND AN AIR FORCE RESERVE
RECONNAISSANCE AIRCRAFT WILL BE AVAILABLE TO INVESTIGATE THE SYSTEM
TOMORROW MORNING...IF NECESSARY.

INTERESTS ALONG THE COAST OF THE SOUTHEASTERN UNITED STATES SHOULD
MONITOR PRODUCTS ISSUED BY LOCAL NATIONAL WEATHER SERVICE FORECAST
OFFICES. ADDITIONAL INFORMATION ON THIS SYSTEM CAN ALSO BE FOUND
IN HIGH SEAS FORECASTS ISSUED BY THE NATIONAL WEATHER
SERVICE...UNDER AWIPS HEADER NFDHSFAT1 AND WMO HEADER FZNT01 KWBC.

Previous discussion:
As a meteorologist today is one of those days where I am glad that I am not a decision maker at the National Hurricane Center. The subtropical storm that is drifting off the coast of the Carolinas is truly a tough call for those at NHC. If NHC names this system they will instantly be faced with a pre-season hurricane only a couple hundred miles off the U.S. coast and many people will panic and be angry about a perceived lack of warning. Due to this societal constraint, it seems unlikely that NHC will name this system. If it were October right now, I have no doubt that their decision would be different. Instead the National Weather Service is taking a more diplomatic approach and has issued Hurricane Force Wind Warnings. However, in my professional opinion we are absolutely dealing with at least a subtropical storm at this time.

A quick look at the water vapor satellite image clearly shows an increasingly tropical form. This image looks like a classic subtropical system undergoing "tropical transition" into a purely tropical system. In the words of NHC forecaster Dr. Knabb is his Discussion 2 from Vince in 2005: "If it looks like a hurricane... it probably is... despite its environment and unusual location."

Figure 1. GOES water vapor. (10:15PM Monday)

Regardless of whether or not this system is considered tropical, it is certainly packing very damaging winds. The latest QuickSCAT satellite pass detected surface sustained winds over a large area in excess of 60 knots.

Figure 2. Latest QuickSCAT pass.

Looking at the latest analysis and forecast from the U.S. Navy shows that the system is currently located right on the theoretical border between an extratropical and tropical storm, right in the middle of the range that we typically classify as subtropical.


Figure 3. Latest NOGAPS analysis on a Cyclone Phase Space chart. Note that the "C" denotes the latest analysis on the border between a symmetric cold-core and symmetric warm core system. This position represents classic tropically transitioning storm.

Today we have heard criticism from people who think that we don't know what we are talking about and are just being alarmist. This is why I try to explain all of my thoughts going into my forecasts and also explain that all StormTrack forecasters are degreed meteorologists with at least Master's degrees. With that said, here is a little lesson into why we are calling this storm a case of tropical transition.

Traditional meteorological canon has for years taught that tropical and extratropical storms are two completely different types of storms. In truth, this can often be the case, however, there can frequently be transition between classes (see references at the end of the post). In that context, here is a visible image from earlier in the day. Note the large occluded front shown by the cloud mass to the northwest of the center of circulation. This is a classic representation of a system about to undergo tropical transition.

Vis_7May07_18Z.jpg

IR_7May07_18Z.jpg
Figure 4. Visible (top) and infrared (bottom) satellite images of the area of interest off the East Coast (Courtesy NHC).

Now I would like to refer you to an image from the scientific literature on tropical transition. The image below is a sample of four satellite images taken from systems that were about to undergo tropical transition from extratropical storms and ended up as hurricanes. Can you see the similarities between our cases today and those in the literature?

Figure 5. Example of tropically transition precursors taken from David and Bosart (2004)

For those of you with an interest in tropical transition, please contact us for more information. Adam and I have a great personal interest in the topic. First and foremost we consider ourselves scientists and would love to point you are papers and references on the topic.

Davis CA, Bosart LF (2004) The TT Problem: Forecasting the Tropical Transition of Cyclones. Bulletin of the American Meteorological Society: Vol. 85, No. 11 pp. 1657-1662

Davis CA, Bosart LF (2003) Baroclinically Induced Tropical Cyclogenesis. Monthly Weather Review: Vol. 131, No. 11 pp. 2730-2747


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