Tropical Storm Andrea forms in the Gulf of Mexico

The first named storm of the 2013 Atlantic Hurricane season has arrived! The area of disturbed weather in the Eastern Gulf of Mexico we have been following has acquired enough organization to be classified as a tropical storm. An aircraft reconnaissance mission was flown this afternoon and they determined that Andrea had formed a better defined low-level circulation center. Peak flight level winds were around 47 knots, which would normally mean about 40 knot surface winds, but the National Hurricane Center chose 35 knots at the time of the first advisory due to SFMR readings. At the time of the most recent advisory (10:00pm CDT) here is what the NHC had Andrea listed at:

SUMMARY OF 1000 PM CDT...0300 UTC...INFORMATION
-----------------------------------------------
LOCATION...26.0N 86.3W
ABOUT 270 MI...430 KM WSW OF TAMPA FLORIDA
ABOUT 270 MI...430 KM SSW OF APALACHICOLA FLORIDA
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...N OR 10 DEGREES AT 6 MPH...9 KM/H
MINIMUM CENTRAL PRESSURE...1002 MB...29.59 INCHES

Tropical Storm watches and warnings have been issued for parts of Florida, Georgia, South Carolina and North Carolina:

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE WEST COAST OF FLORIDA FROM BOCA GRANDE TO OCHLOCKNEE RIVER

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* FLAGLER BEACH FLORIDA TO SURF CITY NORTH CAROLINA

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

Currently another hurricane hunter mission is investigating Andrea and should provide a more reliable reading of the storm’s intensity. On satellite imagery, Andrea exemplifies the typical June Gulf of Mexico tropical storm:

An enhanced infrared satellite image of Tropical Storm Andrea taken at 4:45Z. The center of the storm is along the western edge of the deep burst of convection (purples and white colors) west of Tampa, Florida.
An enhanced infrared satellite image of Tropical Storm Andrea taken at 4:45Z. The center of the storm is along the western edge of the deep burst of convection (purples and white colors) west-southwest of Tampa, Florida.

Andrea is currently experiencing about 20-30 knots of westerly shear which is keeping all of the heavy thunderstorm activity on the eastern half of the circulation. This sheared pattern is expected to continue until landfall on Thursday Night. As a result, Andrea is not expected to intensify significantly before landfall and should come ashore in the Big Bend region of the Florida peninsula with maximum sustained winds in the 45-50 mph range.

However, the impacts extend well away from the center. Heavy rains (3-6″ with isolated amounts of over 8″) and gusty winds will affect Florida over the next 24-36 hours. Furthermore, there will be a threat of tornadoes embedded within the bands of Andrea. Peak storm surge heights are expected to be between 2-4 feet from Tampa Bay northward to Apalachicola.

As Andrea approaches Florida she will begin to accelerate and move up the Eastern Seaboard. By 7am Friday, the storm is expected to be around coastal South Carolina according the latest NHC forecast track. By this time, the storm is expected to begin/have already lost tropical characteristics, but still will be capable of bringing heavy rains and gusty winds.

A couple of things to keep track of tonight:

  • The hurricane hunter mission currently investigating TS Andrea and has just found maximum flight level winds of 52 knots, so it appears Andrea is slowly intensifying.
  • This buoy, which has reported sustained winds of 31 knots, and a pressure of 1001.7mb. This means it is probable that the central pressure of TS Andrea is around 999mb or so.
  • I have read that satellite imagery provided by SSD is temporarily unavailable, and I can recommend this website, which has rapidly updating images over the Gulf of Mexico in the midst of this shortage. EDIT: it appears the SSD imagery has just returned.

Most importantly, stay tuned to the latest advisories provided by the National Hurricane Center and your local Weather Forecast Office.

I will update again once conditions warrant.

-Mike

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

Possible Development in the Gulf of Mexico This Week

EDIT: Quick Update. The disturbance I describe in this article has just been designated Invest 91L.

The official start of the Atlantic hurricane season began yesterday and we are already looking at a suspect to become the season’s first tropical cyclone. A large area of disturbed weather  resides across the NW Caribbean Sea, Gulf of Mexico and Bay of Campeche. Part of this disturbance is the remains of former Hurricane Barbara which crossed over Mexico via the East Pacific.

The 8:00pm Tropical Weather Outlook (TWO) had this to say about the disturbance:

CLOUDINESS AND SHOWERS...ASSOCIATED WITH A BROAD TROUGH OF LOW
PRESSURE EXTENDING FROM THE YUCATAN PENINSULA TO THE STRAITS OF
FLORIDA...HAVE BECOME SLIGHTLY BETTER ORGANIZED THIS AFTERNOON. 
ANY DEVELOPMENT OF THIS SYSTEM IS LIKELY TO BE SLOW TO OCCUR DUE TO
MARGINALLY FAVORABLE ENVIRONMENTAL CONDITIONS.  THIS SYSTEM HAS A
LOW CHANCE...20 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS AS IT DRIFTS GENERALLY NORTHWARD.
Visible satellite image of disturbance
Visible satellite image of the disturbance as of 22:45Z

The forecast for this area of disturbed weather is very complex. Most of the reliable computer models show that an area of low pressure will attempt to intensify in the Eastern Gulf of Mexico over the next 72 hours. Complicating this process is a region of high wind shear (about 20-40 knots) from the SW. The disturbance will be sandwiched between an upper-level trough to the North and an upper-level anticyclone (providing lighter and more favorable upper-level winds) to its SE.

Over the past few runs, the computer models have slightly backed off on the magnitude of the wind shear in the Gulf, and thus more ensemble members are forecasting tropical cyclone formation. Further complicating the forecast is the large size of the disturbance.  Over the next few days there will probably be many circulation centers attempting to establish themselves, but which one, if any, become the dominant center remains to be seen. This partially explains why one of the most reliable computer models (the GFS) has been so wishy washy with the details of the development of this system. Tropical cyclone genesis is tricky even in favorable conditions, so it is not surprising the models have been having a hard time getting the details right in this situation. After about 72 hours, the computer models forecast that the low pressure area will begin to eject NE out of the Gulf and towards Florida.

If a tropical cyclone does form, it appears very unlikely it will strengthen too much and will probably remain a tropical depression or a tropical storm at most. The reason for this is the aforementioned wind shear and marginal sea surface temperatures (SST’s). Regardless of development, the main threat this system poses is extreme amounts of rainfall. Here is the latest HPC 5-day total rainfall accumulation forecast:

Latest HPC 5-day total rainfall forecast
Latest HPC 5-day total rainfall forecast. Some parts of South Florida are forecast to receive 5-7 inches of rain with locally higher amounts.

To sum things up, we have a slight/decent chance of seeing Tropical Storm Andrea this week. Even if a tropical cyclone does not form, Florida can expect heavy rainfall over the next five days. The exact hypothetical landfall location is not too important at this time since the system is large and rains will extend far from whatever center establishes itself.

I will try to post updates periodically as conditions warrant.

-Mike

What computer model is the best?

It’s a question everyone asks in regards to hurricane season, and weather in general: What computer model is the best?

Well, what sounds like a simple question actually has a complex answer. First of all, what are we asking about? Track forecast or the intensity forecast? Before I dive into this answer I would recommend you read Dr. Jeff Masters’ explanation on the different kinds computer models and some model jargon. Also, for a more detailed explanation, the National Hurricane Center (NHC) just released the verification of their 2012 forecasts as well as the forecasts of the computer models they use. It is important to note that for the remainder of this piece I will simply focus on the forecasts for tropical cyclones in the North Atlantic Basin.

I will start off with the track forecast.

Over the past few decades the NHC, as well as the computer models, have gotten pretty good at forecasting the track of the storm. The way this is measured can be done in two ways: Either by 1) total forecast error compared to the actual storm track or 2) the forecast skill relative to what we know from climatology. The NHC uses CLIPER5 as a climatology baseline.

2010_2012_Track_Forecast_S

Over the last 3 years, the European Model known as the ECMWF, but above listed as the EMXI, has performed better than any other model. In fact, it has virtually the same amount of skill as the NHC official (OFCL above) forecasts. Also to note is that the GFS (GFSI above) and the two main consensus models (TVCN and the FSSE) performed very well. Consensus models are not “true models” but rather some sort of combination of multiple models.

However, the world of modeling is very complex and each year has its ups and downs. Furthermore, models are continually being tinkered with in hopes of improving their forecasts. In fact, last year the GFS received a large upgrade to the way it operates and yielded very promising results:

2012_Track_Forecast_S

The NHC stated that the GFS was the top performing model last year, and that is particularly clear looking at the 72 hour forecasts above. In the medium range of 120 hours, the Euro did slightly outperform the GFS. Also of interest is that last year the FSSE consistently outperformed the NHC forecasts and every other model. Even simple statistical models like the BAMM can perform very well in the medium range of 120 hours.

It will be interesting to see what computer models perform well for the upcoming hurricane season. The Canadian model (CMC) just received a large upgrade and has been performing very well for winter weather. Perhaps it will be able to see a large leap in skill this year. Also, will the FSSE continue its dominance over the other models? Will the GFS continue the skill it showed last season? We will see.

I will be updating again with an analysis of computer model intensity forecasts. These forecasts have shown relatively far less skill than track forecasts.

-Mike

Active Atlantic

Active Hurricane Season on the way?

I will come out with my full annual May Atlantic hurricane season forecast later this spring. However, many other agencies are coming out with their preseason forecasts. Not surprisingly, many are expecting an above average season. Without getting into too much detail, one of the primary reasons is the very warm sea surface temperature (SST) anomalies in the Atlantic this spring. A large contributor for the strong anomalies has been the negative North Atlantic Oscillation (NAO) this winter and spring. A negative NAO means the subtropical high pressure system over the Atlantic has been weaker than normal, which in turn causes weaker trades winds and less evaporational cooling.

Check out the large warm anomaly in the eastern Atlantic today:

Atlantic SST anomalies as of March 28, 2013

In particular, and probably most concerning, SST anomalies are very warm in the Main Development Region of the Atlantic (about 10-20 degrees North). Here is what we are currently running at:

Running SST anomalies for the MDR

As of today the Atlantic MDR is a full degree Celsius above normal! If these conditions persist into the peak hurricane season, we could have a very busy year. As of now, it is simply something to keep an eye on. Both of the plots above were taken from tropicaltidbits.com , a great place to keep up to date with the tropics as well.

-Mike