Weather Discussion for Saturday 6 August 2016

A cold front has stalled just to the northwest of us but it will still start to move toward the southeast during this afternoon. With the air heating up and the front moving through the region during this afternoon scattered showers and a few storms are possible. Greatest instability is to the S by SE of Albany with better shear and lift to the north. For now SPC has SE NY and Southern New England (mainly south of the MASS Pike) as having the better chance for severe storms.

This better chance is based on the assumption that this area will be "heating" for a longer time frame. BUT looking at the image below you can see that skies are mostly cloudy to the south and southeast of Albany, with a few breaks. The most sunshine and warming is occurring to the north and northwest but instability in this area is not too strong.

So I have to say that it's a low confidence forecast on today's severe threat. While I am not thinking that there is going to be a widespread severe threat, I feel that most of the #518wx area does have a chance for a strong or severe storm. Storms will be widely scattered if they do occur. For the weather geeks, the morning upper air sounding for Albany (modified for expected conditions at 1 PM this afternoon) is below. Analysis of this sounding shows moderate to strong CAPE (Convective Available Potential Energy) but the profile of this CAPE is somewhat "skinny". Instability indices are mostly weak ( one or two moderate); these mostly weak indices are probably due to a weak temperature lapse rate (change of temperature with height) between about 5 thousand feet and 12 thousand feet above the ground.

(8 AM EDT 6 Aug 2016  RAOB for Albany NY. Modified for expected afternoon conditions)

IF SEVERE storms happen, the main threat: strong wind gusts (wet downbursts) of 50-60 mph and very heavy rainfall. The chance for large hail is LOW. Best chance for storms/showers (40% probability) is between 1PM and 6PM today. Once again activity will be scattered, so not all of us will get wet. Also note that some storms could produce a quick 1" to 1.5" of rain in a short time, with an isolated 2-2.5 inches even a possibility. This heavy rainfall could cause local flooding in areas of poor drainage, even isolated flash flooding, as well as ponding fo water on the roads.

Earl rather low central pressure but still not a hurricane

Usually when tropical systems in the Atlantic basin have a central pressure around  992 millibars (29.29" Hg) the maximum wind speed is at least minimal hurricane force. Earl's latest central pressure has been just below 992 mbs for the past 12-15 hours and the wind still remain in the 65-70 mph range.

Earl is still expected to become a minimal (Category 1) hurricane today. However, it is interesting to note that it has yet to become one.Two possible reasons why it has not done so (and MAY NOT do so) are: 1) It now has the Isthmus of Central America to its south. This could be disrupting inflow into the lower level of the storm along with "relatively" drier air being drawn into the storm's circulation from off the isthmus (as depicted in image below).

2) Dropsonde data (plot of temperature and dew point from a specified height above ground downward to the surface) from recon aircraft indicates dry air between about the 900 millibar surface up to approximately 760 millibars (purple circle on image below).

Dropsonde plot courtesy of: http://aircraft.myfoxhurricane.com/

On the favorable side for intensification are very warm SST (Sea Surface Temperatures) and little shear along with a small area of high pressure aloft over the cyclone providing some favorable outflow.

Tropical Storm EARL

 Reconnaissance aircraft reports indicate that the strong tropical disturbance moving rapidly west across the Caribbean Sea has developed a closed (low pressure) circulation. In addition strongest observed winds are 45 mph or tropical storm intensity thus the system is given the name EARL, the fifth named tropical cyclone of 2016 for the Atlantic Basin

EARL will make landfall on Belize late Wednesday night (3 Aug) or early morning of the 4th. It will weaken to below tropical storm intensity. By Friday it is forecast to move back over the warm waters across the Bay of Campeche, where some re-strengthening is possible before it makes a second landfall on Mexico's east coast.

In addition to strong tropical storm force winds (perhaps even localized hurricane force wind gusts), EARL will also produce storm surge flooding of 2 to 4 feet along the Belize coast at and to the north of where the center makes landfall.

EARL will also likely be a prolific rainmaker. An additional 2-4 inches of rain is expected across Jamaica. Total rainfall accumulations of 8-12 inches are forecast for Belize, Honduras, Guatemala and the Yucatan Peninsula with some 16 inch amounts possible. These rains could result in life-threatening flash floods and mud slides.

Hello Earl?

Earlier today, I posted ( Atlantic Tropics) about a tropical disturbance (97L)  moving quickly across the Caribbean Sea. Since then based on satellite and synoptic data the system has shown signs of much better organization.


The satellite "signature" itself is quite impressive. The system is taking on a much more circular appearance almost like that of a buzzsaw blade.















This is a sign that indicates that the system is developing outflow. Here is a schematic diagram of the air flow in a tropical cyclone's circulation

Image from: (From NOAA, Hurricane, Washington, DC: Superintendent of Documents, 1977)

Outflow is like a chimney, it ventilates the storm:  air currents move in at surface toward the center of the system, where they converge and rise up. As the air rises the moisture in it condenses releasing latent heat which lowers the air pressure. If this rising air wasn't ventilated from the top of the storm, the rising air would eventually cool and begin to sink. But with an outflow mechanism higher up over the storm this rising air is "removed" and allows the system to develop. The better the outflow aloft the greater the chance for the system to intensify and develop. There is a parodox of tropical system's and their intensification: the more air that is removed from the system aloft, the better the chance for the system to strengthen.

Based on the past few hours of satellite imagery I would not be surprised if 97L were at least a tropical depression come Monday. Time will tell. As of now the NHC is planning a possible aircraft reconnaissance mission for Tuesday afternoon but if recent satellite imagery continues to show better organization I wouldn't be surprised if an invest of it occurred sooner.

Atlantic Tropics

Two tropical disturbances have been working their way across the Atlantic Ocean during the past few days. One out over the far Eastern Atlantic, a couple of hundred miles to the west-southwest of the Cape Verde Islands remains poorly organized and is not expected to develop much as it continues its westward trek over the Atlantic, moving into a hostile environment of strong wind shear and dry air.

On the other hand, a strong tropical disturbance dubbed 97L, is zipping quickly west across the Caribbean Sea.

Satellite imagery of tropical disturbance 97L

While surface data indicates no "closed" circulation  center (of low pressure) with this feature at the present time, the associated area of convection is increasing in both coverage and strength. The fast forward motion of the system is currently a detriment to its development but the forward speed is expected to decrease over the next 48 hours.

As 97L moves into the Western Caribbean Sea (west of 72°W longitude) by late Tuesday (2 Aug 2016), it will begin to move into an area that from a statistical climatology perspective is very favorable for tropical systems to intensify.

Also from a climatology perspective, systems that enter the Caribbean Sea near or south of approximately 15° N latitude on a general westerly heading (both of which 97L has done), tend to "keep" this heading. Forecast model tracks support the "climatology motion", with  97L likely passing close to or more likely to the south of Jamaica.

Forecast model tracks for 97L

As for the models that predict intensity, most show 97L to become a depression once it moves west of longitude 72° W, a few indicate it becoming a tropical storm and a couple have it intensifying into a hurricane.

Very warm Sea Surface Temperatures (SSTs) of near 80° F or warmer and the forecast of favorable

environmental conditions: very moist air and little wind shear would likely favor intensification of this system into a tropical storm. If this were to happen it would be christened with the name EARL.