SPECIAL UPDATE…2:00 P.M. EDT…SEP. 12, 2013:
INVEST 93L continues to become better organized. The NHC has increased the probability of this becoming a Tropical Cyclone during the next 48 hours to HIGH (80%).
Indications are that a Tropical Depression may be forming as I stated in this morning’s forecast:
TROPICAL WEATHER OUTLOOK
NWS NATIONAL HURRICANE CENTER MIAMI FL
200 PM EDT THU SEP 12 2013
FOR THE NORTH ATLANTIC…CARIBBEAN SEA AND THE GULF OF MEXICO… THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON HURRICANE HUMBERTO…LOCATED SEVERAL HUNDRED MILES NORTHWEST OF THE CAPE VERDE ISLANDS…AND ON TROPICAL STORM GABRIELLE…LOCATED A COUPLE OF HUNDRED MILES WEST-NORTHWEST OF BERMUDA.
1. SATELLITE AND SURFACE OBSERVATIONS INDICATE THAT A TROPICAL DEPRESSION COULD BE FORMING IN THE SOUTH-CENTRAL BAY OF CAMPECHE…AND IF THIS TREND CONTINUES…ADVISORIES WILL BE INITIATED LATER TODAY OR TONIGHT. AN AIR FORCE PLANE IS SCHEDULED TO INVESTIGATE THIS DISTURBANCE LATER TODAY. EVEN IF A TROPICAL CYCLONE DOES NOT FORM TODAY…ENVIRONMENTAL CONDITIONS ARE FORECAST TO REMAIN VERY FAVORABLE FOR DEVELOPMENT…AND THIS LOW HAS A HIGH CHANCE…80 PERCENT…OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS…AND A HIGH CHANCE…90 PERCENT…OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 5 DAYS. THIS DISTURBANCE IS FORECAST TO MOVE VERY SLOWLY ACROSS THE SOUTHERN GULF OF MEXICO PRODUCING LOCALLY HEAVY RAINS OVER A LARGE PART OF EASTERN MEXICO DURING THE NEXT SEVERAL DAYS. THESE RAINS COULD CAUSE LIFE-THREATENING FLASH FLOODS AND MUD SLIDES.
Good day everyone!
First, a quick update on Gabrielle and Humberto (with tracking maps).
Tropical Storm Gabrielle was downgraded earlier to Tropical Depression status. Convection had flared up nicely a few hours ago, just on the eastern edge of the LLC, but now appears to be diminishing with warming cloud tops. Should this convective activity remain sustained, the NHC could again upgrade her to Tropical Storm. There are basically two scenarios put out by the NHC, in that some modest strengthening could occur, as a frontal trof approaches, creating baroclinic forcing, allowing her to become a tropical storm once again before being absorbed. The second is, she just could decay and become a remnant. The NHC for now indicates that the more favorable factors outweigh the negative, and they are calling for some strengthening prior to becoming post tropical.
NOTE: Just prior to releasing this forecast, Gabrielle has regained tropical storm status.
Hurricane Humberto has maintained his strength during the past 24 hours.
The NHC indicates Humberto is over <26C sst’s. However ample moisture and instability have allowed him to maintain strength and structure. This is what is termed as CISK (Conditional Instability of the Second Kind).
Based on forecast steering, and sst’s in the projected path, I agree with both the NHC forecast track and intensity on Humberto. At the moment, It does not appear Humberto will ever threaten the U.S.
Tropical Disturbance INVEST 93L is now becoming better organized,and is located just in the southern extreme portion of the BOC.
Currently, upper level winds are improving, but are still somewhat marginal, based on the wind shear map from CIMSS. However the current GFS wind shear forecast indicates conditions will become more conducive for development over the next 12 hours, as the upper level anticyclone just to the south, moves over the southern BOC. My first thought as far as strength on this, based on the shear forecast, was to call for slow intensification. However, based on the current satellite loop imagery with improving structure, and numerous overshooting cloud tops noted in the RGB imagery, I believe the NHC may upgrade this to a tropical depression later this afternoon. Once 93L clears land more, given the shear forecast, I am not ruling out a more steady intensification rate (eg., FERNAND, and the recent TD that made landfall in Mexico).
Based on the current steering layers map, the flow is basically westward at the moment over the BOC. However, it is noted the ridge over the U.S. centered near OK., has weakened. Averaging the two, it produces a motion of WNW to north of West. This can be verified by the last reported position of the center, which has been located over the past 2 days at latitude 18.0N, and is now at 19.5N. Analysis of forecast steering layers maps from the PSU e-WALL site, valid for 12Z, indicate this motion to continue for the next 72-84 hours. Based on this, landfall of this system could be Tampico, MX., which is in good agreement with the 12Z Dynamic Model guidance, which has made somewhat of a significant northward shift in track. The current run of the GFS and FIM models support this scenario. The CMC is further south, and weaker with the system, and I am ruling this out at the moment based on numerous developing systems we have witnessed in that area over various hurricane seasons.
A word of caution however… IF this system continues to move very slowly, or stall briefly, this would allow for it to feel a weakness in the western periphery of the U.S. ridge as it slides eastward. This could change the track guidance to a south TX. threat. Should it pick up speed, then the CMC landfall scenario would most likely occur. Extreme south TX. could pick up some rainfall in about 6 days, as what may be left of this system could backtrack toward the NE around the western periphery of the U.S. ridge. Albeit forecast steering does not go out to that time, generally, if you follow the upper level anticyclone associated with a system, you can approximately pinpoint location…which in this case winds up over Brownsville in 5-6 days.
I will be monitoring this system closely over the next 4-5 days for any significant changes.
Elsewhere, I am beginning to monitor an area east of the Lesser Antilles. Shower activity is limited, however upper level winds are conducive with an upper level anticyclone overhead. Upper level winds are forecast to remain favorable during the next 48-56 hours, before westerly shear increases in the path of this weather. The limiting factor however is dry air surrounding this area.
I will continue to monitor this for any changes.
Ok…this is only one run of the models…there is a possibility we could encounter a Gulf Coast Threat in about 8-10 days from now. The GFS is bullish on developing a tropical storm in the BOC, and bring it ashore near LA. The CMC is supporting development as well, and the ECMWF is showing a general lowering of MSLP in the same time frame. The GFS wind shear forecast supports this as well with an upper level anticyclone over the GOMEX. Again, being longer range, I will be looking for consistency in the scenario over the next 72-96 hours. Albeit not much weight should be placed on this at the moment…do not let your guard down.
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST
MEMBER WEST CENTRAL FL AMS
CERTIFIED SKYWARN OFFICIAL STORM SPOTTER (advanced)