Possible interesting scenario over the Bahamas

[MiamiensisWx]

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I know the main attention has been diverted to Gabrielle, but I want to show an area that may gain some support for favorable cyclogenesis parameters. Currently, there is a sfc trough extending SW from Gabrielle into the Bahamas. It does not receive model support, but a closer look at the synoptics reveals some interesting details. A persistent area of diurnal-induced convective activity has encompassed this area, especially over the central and southern Bahamas. Mid to upper-level divergence has been strong, but some signs of change may occur over the next 48 hours. Decent 850 hPa (low-level) vorticity has been lingering over the area. Additionally, this area (in terms of the upper air environment) strongly resembles some developing pre-TC systems (i.e. Katrina, Ophelia, and others).

Firstly, shear has been decreasing in recent hours. If Gabrielle moves out of the area at a quicker pace, an adjacent upper low (near 22N and 71W) could provide some substantial low-level convergence. Additionally, the recent sounding from Miami indicates some decent CAPE values and convective parameters over SE Florida and the adjacent areas. Low-level moisture is sufficient, and WV imagery shows increasing moisture prevalence in the middle and upper levels. Some model guidance suggests the mid-level flow could relax over the next several days, too. Recent radar imagery from Nassau indicates persistent thunderstorm activity and heavy precipitation to the SE:



Nassau, Bahamas long range radar

It's a hypothesis, but I think I can see some vague signs of a mid-level rotation. If we see more convective bursts over the next several days, I would not be shocked if a low-level feature becomes established in the vicinity. Additionally, interaction with the Gulf system could provide the "fuel" for a "surprise event". This scenario could unfold if Gabrielle does not deepen considerably and the Gulf system (associated with a trough) remains disorganized. Look at the latest shortwave loop:

http://www.ssd.noaa.gov/goes/east/tatl/loop-ir2.html

GOES IR:

http://www.ssd.noaa.gov/goes/east/tatl/loop-avn.html

I'm not hyping this area, but I thought it deserved some in-depth discussion. I could be wrong, but it has been persistent over the past 24 hours.

[MiamiensisWx]

Here is the specific area:

[Ptarmigan]

Hmmmmmm, it's in the same area where Katrina formed. This is something that would really warrant watching. I don't think any model has anything for this cloud yet. If it persists for another couple days, I would not be surprised if it develops. Maybe you should let Jeff Masters know about it.

[Stormcenter]

I think the same system that will eventually sweep away Gabby from the East coast toward the NE will do the same with whatever is in the Bahamas. IMO

Also there is a lot of cool dry air to the north that if this were to develop would eventually be drawn into the system as is being currently done by Gabby.

[wzrgirl1]

I am very interested in this area as I think you are correct as to the possibilities that may arise in the next couple of days.........definitely keeping an eagle eye on this one

[MiamiensisWx]

I still see some energy here. Evidently, instability is sufficient for convection:

http://www.ssd.noaa.gov/goes/east/tatl/loop-ir2.html

Shear isn't too inhibitive over the area, too. I know these maps can be inaccurate, but satellite obs appear to support this data:

http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/wg8sht.html

If the tropical wave east of the Leeward Islands (discussed in another thread) eventually interacts with this sfc trough in the Bahamas, things could become more interesting with Gabrielle moving out of the area. What do you think? Obviously, the islands will deal with both areas, too - I want to keep them in my mind. This area seems too persistent; if it lingers throughout the day, we may see another convective burst over the Bahamas.

Edit: Sanibel, this sfc trough appears to be slowly detatching from Gabrielle, too. Convection is numerous over the area and SE Florida:

Miami radar loop

Additionally, there's an anticyclone over the area.

http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/wg8shr.GIF

This area may look dead, but I think the next several days could be interesting. The upper-air environment is more favorable over this area than the Gulf system.

[Sanibel]

I think you are looking at energy associated with the previous frontal boundary connected to Gabrielle that is being enhanced by Gabrielle.

[MiamiensisWx]

It may appear to be tranquil, but I think the area still deserves some close attention. The upper low near FL and flow over the Gulf disturbance initially produced strong upper shear, but the sfc trough has remained in its place over the Bahamas. Low to mid-level shear has remained less hostile over this region, and 850 hPa vorticity (plus instability) aided convective activity. Additionally, this persistent "isolated" area of lower shear has existed throughout the past 36 hours. Currently, low-level convergence has decreased, but the other parameters favor more intermittent thunderstorm activity. The diurnal maximum will provide some interesting insighrs over the next few days. I would appreciate more thoughts on this area.

The area is much more "juiced" than yesterday:

RAMSDIS WV loop

It could become more noteworthy when the upper low moves away from the vicinity, too. The next passing wave axis could enhance vorticity within the next five days.

http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/wg8vor4.html

700 - 850 mbar steering:

http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/wg8dlm1.html

I'm not -removed-, but it certainly appears to rank among the more interesting candidates in the Atlantic (over the next several days). The wave east of the Antilles could eventually interact with this area, too. On another note, conditions are slowly becoming more conducive (shear and convergence wise) for cyclogenesis in the western Caribbean Sea. Ultimately, something may get going in the Bahamas or western Caribbean Sea.

[MiamiensisWx]

Here's an excerpt from the 5:30 p.m. EDT TWO. I think the Bahamian convection is partially associated with the upper low and sfc trough.

DISORGANIZED SHOWERS AND THUNDERSTORMS EXTENDING FROM CUBA AND
ADJACENT WATERS WESTWARD INTO THE SOUTHERN GULF OF MEXICO ARE
ASSOCIATED WITH A WEAK SURFACE TROUGH AND AN UPPER-LEVEL LOW.
UPPER-LEVEL WINDS ARE NOT CURRENTLY FAVORABLE FOR DEVELOPMENT.

Latest 5:30 p.m. EDT TWO

[MiamiensisWx]

000
FXUS62 KMFL 082016
AFDMFL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MIAMI FL
416 PM EDT SAT SEP 8 2007

...LOCALLY HEAVY RAINFALL POSSIBLE SOUTHEAST COAST LATE TONIGHT
AND AREA WIDE ON SUNDAY...

.DISCUSSION...A TROUGH EXTENDING FROM THE SURFACE TO THE MID
LEVELS CURRENTLY STRETCHES FROM SOUTH OF GABRIELLE INTO THE
FLORIDA STRAITS. THE AXIS OF THIS TROUGH IS FORECAST TO MOVE INTO
OUR FAR SOUTHERN AREAS LATE TONIGHT THEN SLOWLY MOVE NW ACROSS THE
LOCAL AREA ON SUNDAY...PULLING AMPLE DEEP TROPICAL MOISTURE INTO
SOUTH FLORIDA. GFS SHOWS PWATS CLIMBING TO OVER TWO AND A QUARTER
INCHES...AND GPS MET ALREADY SHOWS PWATS ON THE RISE (NEAR 1.8").
MEANWHILE...THE UPPER LOW JUST OFF THE EAST CENTRAL FLORIDA COAST
IS EXPECTED TO MOVE WESTWARD. AS THIS HAPPENS...THE LOCAL AREA
WILL SET UP IN THE FAVORABLE SECTOR FOR GOOD UPPER DIVERGENCE/DIFFLUENCE.
SATELLITE IMAGERY SHOWS THAT CONVECTION CONTINUES IN AREAS OF THIS
DIV/DIF ZONE. THUS...AS THE DEEP TROPICAL MOISTURE POURS INTO THE
AREA ALONG WITH A MORE FAVORABLE UPPER LEVEL PATTERN FOR DEEP
CONVECTION...AM EXPECTING WIDESPREAD SHOWERS ALONG WITH SCATTERED
THUNDERSTORMS TO AFFECT THE AREA ON SUNDAY. THIS UNSETTLED WEATHER
WILL BEGIN TO AFFECT THE SOUTHEAST COAST LATE TONIGHT AS THE
TROUGH MOVES IN AND THE ENTIRE AREA ON SUNDAY. GIVEN CURRENT
VISIBLE SAT PICS AND WATER VAPOR...ALONG WITH THE TROUGH
POSITION...I LIKED THE GFS DEPICTION OF THINGS MUCH BETTER THAN THAT
OF THE NAM. NAM POPS SEEM FAR TOO LOW GIVEN THE AFOREMENTIONED
FACTORS. THUS...FOLLOWED THE GFS VERY CLOSELY.

THERE IS A HEAVY RAINFALL POTENTIAL GIVEN THIS EXPECTED SET UP.
COORDINATED WITH HPC AS FAR AS QPF IS CONCERNED. RAINFALL IS
EXPECTED TO BE A MAXIMUM ON THE ATLANTIC COAST...GIVEN THAT THIS
IS ACTUALLY WHERE THE BEST CONVERGENCE LOOKS TO SET UP. IT SHOULD
BE NOTED THAT GFS SHOWS EXCELLENT LOW LEVEL CONVERGENCE ACROSS THE
SOUTHEAST COAST ESPECIALLY OVERNIGHT/EARLY SUNDAY MORNING. WILL UPDATE
THE HAZARDOUS WEATHER OUTLOOK TO HIGHLIGHT THIS POTENTIAL. HAVE
GONE WITH HPC IN DEPICTING AREAL AVG QPF BETWEEN TONIGHT AND
SUNDAY OF AROUND 1.5 INCHES ATLANTIC COAST TO JUST UNDER AN INCH
GULF COAST. LOCALLY HIGHER AMOUNTS ARE LIKELY...POSSIBLY 3+
INCHES. RFC 6 HR FLASH FLOOD GUIDANCE IS IN THE 4-6 INCH
RANGE...SO A FLOOD WATCH IS NOT WARRANTED BUT WILL HIGHLIGHT IN
THE HWO.

BEYOND SUNDAY...THE TROUGH WILL LIFT OUT OF THE AREA AND THE
ATLANTIC RIDGE WILL BUILD IN ACROSS CENTRAL AND NORTH FLORIDA.
THERE WILL BE SOME DRYING AS WELL...SO AM EXPECTING A RETURN TO A
CLIMO EASTERLY REGIME IN THE LATTER PERIODS.

NWS Miami discussion

[Tampa Bay Hurricane]

Convection has really exploded
in this area!
http://www.goes.noaa.gov/browsat.html

[MiamiensisWx]

I'm slowly becoming more interested in this very persistent broad area (sfc trough/UL-induced convection). It looks like the upper low off eastern FL is gradually filling and moving W. This trend could precede weakening shear and a building H8 (low-level) ridge. Look at this RAMSDIS WV loop. Additionally, some model guidance supports a building sfc ridge and better upper-air support - look at the operational 18Z GFS suite (72 hours) and Euro (similar time frame). The former model suite builds the 850 hPa (HGT) ridge westward, which brings a sfc low (tropical wave E of Antilles) WNW toward the southern Bahamas and Greater Antilles. Both models show some signs of a conducive low-level environment (sfc convergence and divergence). The wave axis could move westward and boost vorticity when it interacts with the Bahamas system, too. I think we may see some favorable parameters for cyclogenesis off SE Florida (this area in the Bahamas) and the Antilles (current wave). The sfc trough remains in the vicinity of southern FL, Bahamas, and Gulf of Mexico.

I'm not making a definite statement, but I believe this area may become a potential "cradle" for possible development over the next several days, especially if the convective parameters remain favorable. Gabrielle's effects are negligible, and shear could gradually subside over the region. The NWS Miami radar has been indicating persistent thunderstorm activity over the region. Look at IR imagery, 850 mbar vorticity, and shear trends. Instability has been aiding diurnal convective activity over the Bahamas, and the next few days could provide some insight; I think this area could become a player. I think it could become the dominant area, since 90L doesn't contain sufficient convection over the Gulf of Mexico. It's under higher shear, too. I would not be surprised if the Bahamas convection becomes better organized over the coming days.

What do you think?

[boca]

We now have 90L in the Gulf associated with this trough. I wonder how everything will interact with each other.