Excitement has spread throughout social media about weekend runs of the GFS model depicting a tropical storm or hurricane making landfall along the Texas coast next Sunday. No other deterministic weather model is predicting such a scenario and only a handful of ensemble members of the GFS and the ECMWF are predicting any system in the Gulf of Mexico at all next weekend. Deterministic models provide one scenario whereas an ensemble provides one scenario for each of its members. Despite this, the naive hype spread like wildfire, even prompting an unusual response by the NWS Forecasting Office at Tallahassee, FL, warning – “Obviously, it’s important to pay close attention to the tropics during hurricane season, just do so responsibly ; )” in their forecast discussion. Forecast discussions do not typically contain emojis. As of the latest three model runs, the GFS has lost this hurricane threat, exemplifying the inconsistent nature of long-range weather forecasting. But that does not necessarily mean the Gulf Coast is entirely in the clear from tropical threats this weekend into early next week.
The Atlantic basin has been quiet since Subtropical Storm Alberto made landfall on the Florida panhandle two weeks ago. Dry air over the eastern Atlantic has hampered development over the ocean and strong upper-level high pressure over northern Mexico and the southern Plains has blocked access to the Gulf of Mexico from the Caribbean. After almost two weeks of being mostly stationary, the high pressure system is expected to wobble to the northeast this weekend. A southerly flow will develop behind it over the Gulf of Mexico which will pour moisture into the Southeast US and invite complexes of thunderstorms over Central America and the western Caribbean to migrate northwards.
Convection over Central America and over the western Caribbean coast is expected to move northward over the Yucatan Peninsula with the change in wind direction. Most of these storms are forecast to stay over Mexico. While no deterministic model currently depicts a tropical cyclone in the Gulf of Mexico, there are ensemble members of the GFS and ECMWF that do, indicating that there is a chance that some of these storms may survive or redevelop in the western Gulf of Mexico. Sea surface temperatures in the Gulf have warmed substantially since Alberto, ranging from 80 to 87 degrees throughout the entire basin, warm enough to support tropical development no matter where any potential complex of thunderstorms were to track.
The images below, courtesy of weathermodels.com, depict the ensemble mean sea level pressure (MSLP) and cyclone locations of each members that forecasts one for Sunday morning from the 12 UTC June 11 GFS and ECMWF ensembles, respectively. Each ensemble member is a slightly different version of the parent deterministic model. There is significant disagreement among the models, which is not unusual considering the forecast has nearly a week’s lead time. The GEFS has 10 of its 20 members predicting some sort of tropical disturbance in the western Gulf whereas the EPS has just 3 of its 51. In other words, the GEFS has a 50% chance of there being a tropical disturbance in the Gulf of Mexico Sunday morning whereas the EPS only has a 5.9% chance at the same time.
To be sure, a side-by-side comparison like this can be misleading. Each ensemble member develops weather systems differently. Some members of both the GEFS and the EPS develop cyclones into Tuesday. However, even accounting for these temporal differences, spaghetti plots (not shown here) show that only 7 EPS members have a land-falling tropical system in the next 10 days. The contrast between the ensemble spreads of the GEFS and EPS represents the significant uncertainty in the Gulf of Mexico.
Given the favorable upper-air pattern and ensemble guidance suggesting the possibility for cyclone development Sunday, the western Gulf of Mexico needs to be monitored closely. The NHC has issued a 20% chance that a broad area of storms located just north of Barranquilla, Columbia as of 2pm EDT Monday will develop into a tropical cyclone by the time it crosses the Yucatan Peninsula into the Gulf of Mexico by Saturday. Residents of the Gulf Coast should check back as we know more.