INVEST 90 Update

[Anonymous]

From our discussion...

Caribbean Sea: As we've stated over the past few days, much of the model guidance forecasting a northeast track into southern Florida is unreliable in THIS situation. The A98E, BAMD and LBAR have been gradually shifting westward over the past 24 hours. Now we also mentioned that more stock should be put into the BAMM model more than those listed above. The BAMM originally took the disturbance into the central Gulf. The model is even farther west this evening, with an area of low pressure moving inland just north of Tampico, Mexico. The GFDL has finally begun to pick up on the disturbance and takes it into the Yucatan. This is the same scenario the GEM, GFS, NOGAPS and UKMET have been showing for days. Based on trends and consistency, it is becoming more apparent that the upper level trough to the north may NOT force the disturbance straight into southeast Florida. Please remember that we are dealing with a very weak system and a lot of things can change when it's sometimes difficult to even find a center. But based on the latest data, we may see a more westerly track into the Bay of Campeche or southwest Gulf of Mexico. IF the low were to move into the BOC, it could sit down there for a while.

Enough speculation about where this disturbance could go. Will we see development? This is still up in the air. If the low does move west into the BOC, it may stand a better chance than simply going northeast into the southeast Gulf. The same trough that would steer the disturbance northeast is producing 40-50 knots of vertical shear in the central and northern Gulf. No system can stand a chance in upper level winds that strong. There is more in the way of upper level ridging farther south. This is why we still have a low to watch. In order for development to occur, the low must gain and sustain convection while moving towards the southern GOM/BOC (assuming this is the path it will take). We also need enough ridging to prevent the trough to the north from shearing the system apart. Model guidance shows marginal upper level winds in the BOC through six days. To sum things up, slow development is possible as the disturbance moves WNW over the next several days. A very tricky forecast indeed...

http://www.independentwx.com/atl_discussion1.html

[Stormcenter]

Good post.

[DelrayMorris]

My understanding is that the LBAR is absolutely horrid at predicting the path of tropical systems. So why do all the model maps seem to use it anyway? Am I misinformed or what?

[Stormsfury]

My understanding is that the LBAR is absolutely horrid at predicting the path of tropical systems. So why do all the model maps seem to use it anyway? Am I misinformed or what?

The LBAR is a limited area barotropic model and is generally terrible outside of the deep tropical regions. It doesn't pick up on temperate zone north of 20ºN very well.

LBAR (Limited area sine transform BARotropic) is a two-dimensional track prediction model which solves the shallow-water equations initialized with vertically averaged (850-200 hPa) winds and heights from the Aviation Run of the MRF global model (Horsfall et al 1997). An idealized symmetric vortex and a constant vector (equal to the initial storm motion vector) are added to the global model analysis to represent the storm circulation. The boundary conditions are obtained from the global model forecast, and the model equations are solved using the spectral sine transform technique described by Chen et al (1997). To make LBAR timely, initial and boundary conditions are obtained from the 6 hr old MRF forecast. LBAR was developed as an operational version of the experimental VICBAR model (Aberson and DeMaria 1994).

[wxman57]

Actually, the models have been showing somewhat of a split of 90L in the past few days, with part of the moisture zipping up the front across south Florida and part of the moisture continuing west into the northern BOC/southwest Gulf. But it doesn't look like either system has too much chance of significant developoment. There is some very cool and dry air heading into the western Gulf in a few days as the second front moves offshore. This cool/dry air will accelerate south down the coast of Mexico and into the BOC. Such a front will form a natural wave ESE of Brownsville as the cold front is oriented ENE-WSW across the northern Gulf and N-S across the western Gulf. This is how winter-time west Gulf lows get set up. It's possible that the energy from the wave could interact with the front in the western Gulf and form a low on the front. With high pressure to the north, the most likely track would be ENE, as with any west Gulf frontal low.

Now there's another possibility. The low could form so far south that it doesn't get picked up and moved ENE. It could sit around for days and eventually develop into a tropical storm as the air behind the cold front modifies. If it does that, then it may be able to go just about anywhere, as by next weekend the high center is way off to the east.

Just something to keep an eye on.

[Anonymous]

I agree with all of you comments wxman. Pretty much all of the globals show a broad area of low pressure sitting in the southern GOM/BOC for days. However, no model shows much in the way of development. One interesting thing is the latest Canadian run which shows the low stalling in the BOC and then heading NE as a 996MB low. Sounds like both the local mets and the NHC missed this scenario based on their recon coords the other day.

[Anonymous]

My understanding is that the LBAR is absolutely horrid at predicting the path of tropical systems. So why do all the model maps seem to use it anyway? Am I misinformed or what?

If I were to include the globals right now the map would really be messy and hard to read. If we get some development and the global tracks become easier to see, I will begin to eliminate the models that shouldn't be used.

[Anonymous]

Models beginning to pick up on the GOM development..


Canadian:
http://www.sca.uqam.ca/models/glb00_192.html

NOGAPS:
https://www.fnmoc.navy.mil/CGI/PUBLIC/w ... sf&tau=144

[BEER980]

Hmmm what's that at 13N and 47W on the NOGAPS?