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Tropical Storm Gamma Discussion Number 11
NWS National Hurricane Center Miami FL AL252020
1000 PM CDT Sun Oct 04 2020
Gamma has moved much farther northeast than previously anticipated.
With little in the way of low or mid-level large-scale steering flow
currently in place, it seems likely that upper-level
southwesterlies, also responsible for the shear affecting the
tropical storm, are causing this recent northeastward motion. This
may be either through reformations of the center closer to the
convection or a direct contribution to the net steering. An Air
Force Reserve Hurricane Hunter aircraft is currently investigating
Gamma and so far has found max SFMR winds of 47 kt but flight level
winds of only 45 kt. Given the data so far, there is no indication
that Gamma is any stronger than the 50 kt intensity, but it seems
prudent to let the plane finish its pattern before lowering the
winds at this time.
Gamma's future track is highly uncertain. The track guidance spread
is much higher than usual, and confidence in the forecast is
accordingly low. Most of the global models indicate that Gamma will
soon begin moving generally west-southwestward as a mid-level ridge
briefly builds over the eastern and central Gulf of Mexico.
Unfortunately, there is little agreement on exactly when this will
happen or how fast Gamma will move once it turns. After about 48 h,
Gamma could interact with Tropical Depression Twenty-Six, which is
forecast to move over the northwestern Caribbean Sea at that time.
Current dynamical models are notoriously bad at forecasting such
interactions, but if the two systems do interact it will likely
cause Gamma to move inland over the Yucatan.
Whether such an interaction occurs or not, continued shear and
proximity to land should cause the tropical storm to gradually
weaken, and Gamma could dissipate if it moves over land (and stays
there) in a few days, as shown by the GFS. The ECMWF shows the
vortex remaining over water but dissipating nonetheless, while a
couple other global models maintain the vortex longer and show it
moving north over the central Gulf of Mexico late in the forecast
period. The NHC forecast is a compromise between those solutions,
showing a remnant low moving north over the southern Gulf of Mexico
by day 5. In general, the NHC intensity forecast is similar to the
previous one, but slightly lower to account for the system
potentially moving inland.
Due to the uncertainty in the forecast at this time, future
advisories may feature larger than normal changes to the track or
intensity forecast. It is also worth noting that several dynamical
model trackers appear to lose Gamma and jump to tracking Tropical
Depression Twenty-Six when it reaches the Gulf of Mexico, resulting
in unrealistic depictions of the intensity and track forecasts.
1. Gamma is expected to produce heavy rainfall for several days over
portions of southeastern Mexico, the Yucatan Peninsula, Central
America, and far western Cuba. This rainfall could result in
life-threatening flash flooding and mudslides, particularly in the
mountainous regions of southeastern Mexico and Central America.
2. Even though Gamma is just offshore, tropical storm conditions
are likely along portions of the northern coast of the Yucatan
Peninsula on Monday.
FORECAST POSITIONS AND MAX WINDS
INIT 05/0300Z 22.8N 87.2W 50 KT 60 MPH
12H 05/1200Z 22.7N 87.7W 45 KT 50 MPH
24H 06/0000Z 22.3N 88.5W 45 KT 50 MPH
36H 06/1200Z 21.8N 89.5W 40 KT 45 MPH
48H 07/0000Z 21.3N 90.3W 40 KT 45 MPH
60H 07/1200Z 20.3N 90.7W 35 KT 40 MPH
72H 08/0000Z 20.2N 90.1W 30 KT 35 MPH...INLAND
96H 09/0000Z 22.0N 90.7W 30 KT 35 MPH...OVER WATER
120H 10/0000Z 24.0N 91.0W 30 KT 35 MPH...POST-TROP/REMNT LOW