Tropical weather analysis - August 15, 2012
Remnants of TD7
The remnants of Tropical Depression Seven appear to have moved inland over Central America near the Honduras/Nicaragua border based on satellite and surface observations.
Figure 1. Latest infrared satellite image of former Tropical Depression Seven. Image credit: NOAA
The system is expected to move generally west-northwestward at about 15 mph over the next few days. Global model guidance suggests that it could enter the far southern Bay of Campeche on Thursday. Although the upper wind pattern looks favorable at that time, it is expected to move inland on Friday or Saturday, and the system will more than likely fail to significantly redevelop due to limited time over water.
Regardless of development, the system will continue to produce locally heavy rains and possible flooding over regions of Central America over the next day or two. These rains will begin to overspread portions of southern Mexico by the weekend.
Probability of development within 48 hours: Near 0%
A tropical wave over the central Atlantic centered about 800 miles southeast of Bermuda ("93L") is producing disorganized showers and thunderstorms to the west of the wave axis. Based on satellite imagery, the circulation associated with this wave appears to be becoming better-defined.
Figure 2. Latest infrared satellite image of Invest 93L. Image credit: NOAA
93L is well-embedded in a weakness in the subtropical ridge along 50W. An unseasonably deep trough is in the process of amplifying across the east coast of the United States. The various models suggest that this trough will ultimately pick up 93L and recurve it well east of Bermuda on Thursday or Friday. Given current trends, I think this could occur on Thursday.
While upper-level winds are currently favorable for development, water vapor imagery suggests that dry air still surrounds the system. Although the SHIPS gradually moistens the ambient environment over the next few days, the relative humidity parameter at 0z never goes above 57%. I think SHIPS is overdoing it a notch, and I think there is potential for 93L to become a tropical depression within the next few days. This is supported by the global model forecasts, which in general bring the system to tropical cyclone status, albeit small, on Friday while the system moves toward the Azores. If 93L obtains a better structure throughout the day today (Wednesday), it is possible that genesis could occur about a day sooner.
Interests in the Azores should begin to monitor the progress of this system, as most of the models bring it through that archipelago on Monday.
Probability of development in 48 hours: 60%
Hector continues to meander as a weak tropical storm. As of the latest NHC advisory, the following was posted on the storm:
Wind: 40 mph, with higher gusts
Location: 17.2°N 115.1°W
Movement: W at 2 mph
Pressure: 999 mb
Hector remains disorganized on satellite images, although some convection has recently developed to the southwest of the low-level center. SHIPS diagnoses 20 kt of easterly shear over the cyclone at 0z. CIMSS had a similar estimate.
Figure 3. Latest infrared satellite image of Tropical Storm Hector. Image credit: NOAA
The GFS/SHIPS suggests that this relentless shear pattern is not going to abate through at least the next few days. I personally think the shear is going to abate some. But even if it does abate, Hector will also be passing over cooler waters and their associated stratocumulus cloud field as early as Thursday morning, which should continue the current weakening trend. Hector is expected to cease being a tropical cyclone in about 48 hours, and dissipate completely by 96 hours.
Water vapor imagery and objective steering analyses indicates that Hector is embedded in a col region between an upper low off the California coast and a narrow low- to mid-level ridge over central Mexico. The models respond to this pattern by forecasting Hector to turn northwestward very soon. Given that steering currents have been rather weak with this particular cyclone, I have opted to go a little slower with the forward motion, but agree that Hector should turn soon.
5-day intensity forecast
INITIAL 08/15 0600Z 35 KT 40 MPH
12 hour 08/15 1800Z 30 KT 35 MPH
24 hour 08/16 0600Z 30 KT 35 MPH
36 hour 08/16 1800Z 30 KT 35 MPH
48 hour 08/17 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
72 hour 08/18 0600Z 25 KT 30 MPH...POST-TROPICAL/REMNANT LOW
96 hour 08/19 0600Z...DISSIPATED
5-day track forecast
Figure 4. My 5-day forecast track for Hector.
Gulf of Mexico development on the horizon?
Over the last several days, the GFS has been consistent in developing a weak tropical cyclone somewhere in the western Gulf of Mexico over the next 7 - 10 days. I am unsure where this development is supposed to come from; following the 850 mb vorticity tracker on that model, I am hard-pressed to believe it is the remnants of Tropical Depression Seven. It could perhaps be from the tropical wave approaching the Windward Islands, given the timeframe on that particular model. It also appears that a southward-drifting cold front stalls out and at least partially contributes to an area of disturbed weather in that area.
Since the GFS is really the only one that is enthusiastic about this development, I am not getting excited right now, but it bears watching.
The GFS/ECMWF suggest that a tropical depression could form in the eastern Atlantic from a tropical wave over the next 5 - 7 days. The GFS suggests recurvature, while the Euro suggests a more westward motion. It is impossible to favorably pick one over the other at this time.