Invest 91L – A Large Rainmaker

Almost immediately after I wrote my last blog post, the area of disturbed weather in the Gulf of Mexico was declared Invest 91L. Since that time, the structure of the storm has only slightly improved and the NHC is still giving the system a 20% chance to develop into a tropical cyclone over the next 48 hours. An aircraft reconnaissance mission is scheduled to fly into 91L tomorrow afternoon, if necessary.

Visible satellite image of Invest 91L taken today at 20:45Z
Visible satellite image of Invest 91L taken today at 20:45Z.

The satellite image above reveals a storm that is still struggling with moderate wind shear (20-30 knots) out of the west, according to the latest CIMSS analysis. Throughout the day, a large and very broad cyclonic turning has been evident in the low level cloud field. Embedded in this large area of cyclonic turning as has been smaller vortices. ASCAT confirms the borad cyclonic turning:

An ASCAT pass caught the "center" of Invest 91L at 16:14Z today.
An ASCAT pass caught the “center” of Invest 91L at 16:14Z today. Light winds prevail around the center of the broad low with east winds to the north of the center and west winds to the south.

In terms of future track, the models have come into slightly better agreement in today’s 12Z runs. The broad area of low pressure will slowly slide around and attempt to consolidate in the Eastern Gulf of Mexico over the next two days. The system will then begin to move NE towards the Big Bend area of the Florida Peninsula as an upper-level trough approaches the Eastern United States and scoops up the low. The models are trending toward a possible landfall sometime on Thursday Night/Friday Morning. The majority of the computer models keep 91L around low-end tropical storm strength at landfall with the exception of the GEM, which can be considered an outlier at this time. The GEM has a reputation for over-doing things.

Until Invest 91L can develop a better defined a low-level circulation center, the intensification process will be put on hold. If a better defined center does form, computer models indicate it should still remain fairly broad and elongated, courtesy of the marginal environmental conditions and the large size of the disturbance. These marginal environmental conditions are expected to continue over the next 3-4 days as Invest 91L moves NE towards Florida.

As 91L moves NE, it may enter the right entrance region of a jetstreak, which would provide large-scale rising motion (an environment conducive for convection). This may lead to an intensification process that is not truly tropical in nature. Nonetheless the intensity forecast remains unchanged.

The computer models also depict that the worst weather associated with this potential tropical cyclone will be to the North and East of the center of circulation, which is currently the case now:

Water vapor satellite image taken of the Gulf of Mexico on 21:15Z. Notice the areas of dry air (brown and black colors) being injected towards Invest 91L.
Water vapor satellite image taken of the Gulf of Mexico on 21:15Z. Notice the areas of dry air (brown and black colors) being injected towards Invest 91L from the west.

Since 91L is forecast to remain a tropical storm at most before it slides into Florida, rainfall and the potential for tornadoes will be the system’s biggest threats. HPC’s latest forecast calls for 4+ inches of rain to almost the entire peninsula over the next 5 days. The SW part of the peninsula could receive 5-7″ or even more in that same time frame. Residents in Florida should keep up to date with the National Weather Service’s and National Hurricane Center’s latest forecasts.

I will update again when things change.

-Mike

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