Thu Jul 20 2017 05:34 PM
Re: 2017: 'Active Era' + Busy Season?

The image below is a screenshot of the Tropical Cyclone Formation Probability Product, developed by the Regional and Mesoscale Meteorology Branch at CIRA.

This product, widely used by tropical cyclone forecasters, assigns a tropical cyclone genesis probability within 0-48 hours for a given area. Input parameters include 850 hPa Relative Vorticity (pre-existing low to mid level spin), Percent Pixels Colder than -40°C (indicative of deep convection), 850-200 hPa Vertical Shear (less deep layer shear is usually more conducive for TC development), Vertical Instability (is convection suppressed or unrestrained), Cloud-cleared Water Vapor Brightness Temperature (a measure of mid to upper level water vapor), 850 hPa Horizontal Divergence and Mean Sea Level Pressure, among a few others.

Even though this is a product used to aid in forecasts out to 48 hours, it can also serve as a bit of a clue as to the state of various basins and their sub-regions, when looked at over time. For the following exercise, I have looked at each main input parameter, save a few that would not apply so much here, for this season. Specifically, in the image below, we can look at the state of each Atlantic region during the first two months so far of the 2017 hurricane season, and try to ferret out any possible trends. And there have been some, which I will explain below.

First, it is important to point out those input parameters that are largely considered most useful in determining the potential for very near-term TC genesis. The Atlantic basin's top three non-climatology 48-Hour Genesis predictors are: RVOR (Relative vorticity), MSLP (Mean sea level pressure), PCCD (Percent pixels colder than -40°C). How often have these been favorable (positive), this season so far?

It is also instructive to look at the wider view inputs, such as Shear, Water Vapor, Sea Surface Temps, and so on.

For the input parameters in the screen shot below, if they have averaged above climatology since June 1, I have marked them with a plus (+) sign. For those that have averaged below climatology since June 1, I have marked them with a minus (-).

So, on to the discernible possible trends, relative to climatology. The regions and their respective inputs break down as follows:

Atlantic basin top three non-climatology 48-Hour Genesis predictors relative to climo:
GOM: One negative, two neutral
Caribbean: One positive, one neutral, one negative
East Coast: All three negative
Tropical Atlantic: One positive, two neutral
Sub-tropical Atlantic: Two negative, one neutral

Eight NESDIS Atlantic basin non-climatology 48-Hour Genesis predictors relative to climo:
GOM: One positive, three negative, four neutral
Caribbean: Three positive, three negative, two neutral
East Coast: One positive, four negative, three neutral
Tropical Atlantic: Four positive, two negative, two neutral
Sub-tropical Atlantic: One positive, five negative, two neutral

Keep in mind that this only helps understand where TCs have been more likely to form, and not necessarily where they will be likely to travel, survive, intensify, weaken, or dissipate. With that not insignificant caveat in mind, it is probably safe to conclude the following

A) The Tropical Atlantic has been the most favored region for TC genesis this season relative to climatology.
B) The Caribbean has been the second most favored region for TC genesis this season relative to climatology.
C) The Gulf of Mexico has generally been unfavorable for genesis so far relative to climo (although we did have one formation there, Cindy).
D) Both the East Coast and Sub-Tropical Atlantic have been hostile to development so far in the official season, overall, relative to climatology.

Should things remain stead-state for a while, then we might expect to continue this season to look to the Main Development Region in the Tropical Atlantic first and foremost heading into the peak Aug-Sep-Oct for the best and above-climo chances of formation, with respectable attention paid to the Caribbean - having been roughly in line with climatology, and still certainly the GOM, given that it is landlocked and anything forming there is very likely to affect land. Development off the East Coast and in the Sub-Tropical Atlantic might be less than average, save changes going forward.

As always, all opinions here are my own, and are not meant in any way, shape, or form to be used for any planning, critical or otherwise. Here at Flhurricane we usually suggest paying close attention to official forecasts from official agencies for that.

Note: This is NOT an official page. It is run by weather hobbyists and should not be used as a replacement for official sources. 
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