Posts Tagged ‘MODEL’

Invest 93L – Thursday Lunchtime Update

Published on Thursday, 01 September 2011 11:28

David Reimer

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[caption id="attachment_3993" align="aligncenter" width="550" caption="Visible Satellite - 10:15 AM CDT"][/caption]

Every few years we seem to get one system that will spin up in the Gulf of Mexico, whether it be caused by a frontal boundary or a large convective complex. This system is actually produced from a more tropical origin. If you reference the blog post a few days ago, we were talking about a disturbance in the Caribbean that was forecasted to slowly move into the Gulf of Mexico. This is the same disturbance, now located in the central Gulf of Mexico. Taking a look at Invest 93L this morning, we see a large area of disorganized convection across the Central Gulf of Mexico, roughly along and east of 80W. So far, there is no signs of a defined low level circulation which would classify this as a Tropical Depression or Tropical Storm.

[caption id="attachment_3994" align="aligncenter" width="550" caption="Wind Shear - 10 AM CDT"][/caption]

One reason why Invest 93L remains disorganized can be attributed to the wind shear present over the Gulf of Mexico. Right now, wind shear is analyzed around 30 knots across the central and eastern sections of the Gulf, with lesser amounts of wind shear across the western section. Invest 93L is located in the region of 30 knots of wind shear, which is pretty strong and will keep the development of this system ongoing at a slow pace, which is good for a few reasons which I’ll outline below.

[caption id="attachment_3995" align="aligncenter" width="550" caption="Invest 93L - 12Z Model Tracks"][/caption]

Before I dive into the model guidance, I thought it would be benifitial to explain what models we’re looking at. A forecaster at the National Weather Service in Fort Worth published this explanation this morning and I figured it would help explain tropical models a bit.

 

THERE ARE FOUR TYPES OF MODELS USED...STATISTICAL...STATISTICAL-
DYNAMICAL...DYNAMICAL...AND TRAJECTORY. ENSEMBLE AND CONSENSUS
FORECAST METHODS ARE ALSO USED BUT THESE METHODS USE A COMBINATION
FOUR TYPES OF MODELS LISTED ABOVE.

STATISTICAL MODELS USE FACTORS SUCH AS LOCATION OF THE
STORM...TIME OF YEAR...AND PAST STORM TRACK AND INTENSITY TO MAKE
THEIR FORECASTS. AS SUCH...THEY PROVIDE A HISTORICAL BASELINE FOR
A STORMS TRACK. THE AVERAGE TRACK ERROR FOR THE CLIPER5...THE MOST
WIDELY USED STATISTICAL MODEL...WAS 200 N MI AT 96 HRS. THESE MODELS HAVE
RECENTLY BEEN SURPASSED IN ACCURACY OF TRACK BY DYNAMICAL MODELS.

DYNAMICAL-STATISTICAL MODELS...SUCH AS THE SHIPS AND LGEM...ARE
BASED ON STATISTICAL RELATIONSHIPS BETWEEN CLIMATOLOGY...THE
DYNAMICAL OUTPUT FROM THE GLOBAL MODELS...SUCH AS SEA SURFACE
TEMPERATURES AND VERTICAL WIND SHEAR...AND STORM MOTION AND
INTENSITY. THE SHIPS MODEL IS ONE OF THE MOST RELIABLE TROPICAL
CYCLONE MODELS.

DYNAMICAL MODELS...SUCH AS THE GFS...ECMWF...GEM...AND CMC...ARE
THE MODELS TYPICALLY USED IN EVERYDAY FORECASTING. THERE IS A LONG
LIST OF THESE TYPE OF MODELS AND THEY VARY IN DOMAIN FROM GLOBAL
TO MESOSCALE. THE BEST PERFORMING MODELS IN 2010 WERE THE
ECMWF...GFS..AND UKMET..IN THAT ORDER. ONE THING THAT SHOULD BE
NOTED ABOUT DYNAMICAL MODELS IS THE DIFFERENCE BETWEEN EARLY-TRACK
VERSUS LATE-TRACK MODEL GUIDANCE. LATE-TRACK GUIDANCE IS TYPICALLY
WHAT WE THINK OF WITH COMPUTER MODELS. LATE-TRACK GUIDANCE
INCLUDES THE ENTIRE FORECAST OF A PARTICULAR MODEL RUN. THE EARLY-
TRACK GUIDANCE IS THE FORECAST DATA FROM THE PREVIOUS RUN OF MODEL
IS INTERPOLATED SO THAT THE 6 HOUR FORECAST MATCHES THE OBSERVED
DATA. EARLY-TRACK MODEL RUNS ARE NOT NEW MODEL RUNS. DYNAMICAL
MODEL RUNS USUALLY TAKE THREE TO FOUR HOURS TO COMPLETE. FOR
EXAMPLE...THE EARLY-TRACK 12Z IS NOT A NEW MODEL RUN. THE 12Z RUNS
OF THE GLOBAL MODELS ARE USUALLY AVAILABLE AROUND 16Z WHILE THE
REGIONAL AND MESOSCALE MODELS AROUND 15Z.

TRAJECTORY MODELS...SUCH AS THE BAM...ARE VERY SIMPLE AND MOVE A
TROPICAL CYCLONE BASED ON THE PREVAILING FLOW OUTPUT FROM
DYNAMICAL MODELS. THEY INCLUDE A CORRECTION TERM FOR THE SPIN OF
THE EARTH...KNOWN AS THE VARIABLE BETA...AND ARE SOMETIMES CALLED
BETA-ADVECTION MODELS. IF THE FLOW IS WEAK AND THERE ARE NO MAJOR
SYNOPTIC CHANCES...TROPICAL CYCLONE WILL FOLLOW THESE MODELS
CLOSELY.

ENSEMBLE AND CONSENSUS MODELS USUALLY PROVIDE THE BEST FORECAST.
CONSENSUS MODELS FOR THE TRACK FORECAST BEGIN WITH THE LETTER
T...SUCH AS THE TCON AND TVCN WHILE THE CONSENSUS MODELS FOR THE
INTENSITY BEGIN WITH THE LETTER I...SUCH AS THE ICON AND IVCN.

HOPEFULLY THIS INFORMATION WILL HELP SHED SOME LIGHT ON THE
COMPLEX SPAGHETTI PLOTS. MOST OF THIS INFORMATION WAS FOUND IN TWO
DOCUMENTS PRODUCED BY THE NHC. THESE DOCUMENTS CAN BE FOUND AT
WWW.NHC.NOAA.GOV/VERIFICATION/PDFS/VERIFICATION_2010.PDF AND
WWW.NHC.NOAA.GOV/PDFS/MODEL_SUMMARY_20090724.PDF

Hopefully that helps explain tropical models and our terminolgy. Now with that out of the way, lets dive into the latest tracks. Tracks for Invest 93L can be described best as a ‘bowl of spaghetti’. There isn’t much to steer or move Invest 93L along, so movement is going to be very slow to at times, stationary. This could result in a very interesting and potentially dangerous flooding event wherever this system ends up. Much of the deep south is experiencing drought conditions, and this is the type of system that can break a drought. However, remember that droughts are often ended in major rainfall and flooding events. Just because we’re in a drought, that does not make us immune to flooding. In fact, it increases that potential slightly as the ground is unable to absorb heavy rainfall quickly because it’s so dry. As we continue to track Invest 93L, keep in mind that this system has a high potential of causing flooding.

Until a defined low level circulation forms, it’s likely that the models will continue to shift around in their solutions. Last night’s 0Z models had this system sitting on top of New Orleans, dropping upwards of 20 inches of rain over the parishes in southeast Louisiana. This morning’s models have shifted a bit west, which would allow portions of Texas to get in on the rain. I’m going to be honest, it’s nearly impossible to say where this system is going to end up until we get a defined low level circulation, but the effects of this system will be fairly widespread and not confined to the center of the storm. We’ll keep continous updates on the track updates with this system.

[caption id="attachment_3996" align="aligncenter" width="550" caption="Invest 93L - 12Z Intensity Guidance"][/caption]

As I said above, moderate to strong wind shear should keep Invest 93L in check from getting too out of hand, however it still appears likely that it should be able to attain Tropical Storm status. Beyond that, it will all depend on several factors such as the organization of the low level circulation, strength of the wind shear in the coming days, and how close this system ends up to the coastline. Right now, this system does not look like it will become anything more then a Tropical Storm, maybe a minimal hurricane. This should not become a major hurricane. This is exactly what we need, a tropical storm with lots of rain, but without the major damage a huge hurricane can bring.

[caption id="attachment_3997" align="aligncenter" width="550" caption="Rainfall through Tuesday Morning"][/caption]

The rainfall forecast depends on where Invest 93L ends up, which as I said above is still uncertain, but this is the latest rainfall forecast from the Hydrometeorological Prediction Center. Currently, they have the heaviest rains just east of Texas with a astoundingly high 19 inches of rain falling over the southern parishes of Lousiana. If this were to play out as shown above, I would be extremely concerned about major to catastrophic flooding in that area, including New Orleans. Lets hope the levee system can support that much water. Luckily, we’re still a few days out and its quite possible that the heavy rainfall will shift west. If the 12Z models are right, then that heavy rainfall threat will extend into Texas. Based off this forecast, extreme southeast Texas would receive a few inches of rain, but 99 percent of the state would remain fairly dry.

Another consideration is the northerly flow aloft that would help bring in some milder air across Texas. Because we would be on the western side of the circulation, the northerly flow would allow milder air to flow into the state. We’ll see how that plays out, but it looks like we should be getting a cool off by next week. For the latest information on all Texas weather, follow us on Facebook and Twitter. Links are available on the right side of the sidebar and on the top of the page. We’ll have another blog post by 11 PM central tonight. Thanks for stopping by!

- David