Tuesday, March 22, 2011

Drought Relief on the Way...

Rain is on the way; it's not in the short term forecast, however. The simple problem over the last couple of weeks has been that the NAO (North Atlantic Oscillation) has been positive. The NAO basically represents pressures in and along the Atlantic Basin. When the NAO is positive there is usually a dominant subtropical high and an rather strong North Atlantic/Icelandic low. Because of the strength of both systems, they produce a strong gradient between pressures. Hence, the windy conditions we have experienced over the past couple of weeks.

Depiction of Positive NAO
Under a positive NAO setup, high pressure's influence typically reigns over the area. This equates to weak systems pushing through the Jacksonville area or bypassing the area completely. Examples of this include numerous squall lines this year, and others, falling apart before they reach the area.

Squall line quickly weakens as it enters North Florida.
An interesting statistic to look at it is the number of severe weather reports there are during positive and negative NAO events. The following numbers are for only the Florida peninsula, so far this year. (They do not include areas west of Tallahassee.) 

NAO (Negative): 43
NAO (Positive): 17

What this means is that as the NAO turns negative, we will have to watch for severe weather. Because of the extreme heat, as well as, converging jet streams the percent chance of severe weather will be the at the highest threat levels, so far, this year.

A negative NAO tends to have fewer storms over the North Atlantic and more over its mid sections. The dominate Atlantic high, which protrudes into the Southeast during positive NAO events, weakens and heads eastward. This leaves the Southeast usually vulnerable to an active pattern. This scenario is exactly what is in the pipeline for the last week of March and first week of April.

Negative Phase Setup of the NAO

NAO Turns Drastically Negative
The above picture is of esemble forecast models. They are unamious  that the NAO will turn negative over the next couple of weeks. This means two things: our constantly windy conditions will relax, and a lot of rainfall is on the way.

>GFS Forecast Model: 
Date: 3/28-4/05
Number of Signifcant Storms: 3
Precip.: 3.00 inches of rainfall

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