Post-Tropical Cyclone Gamma Forecast Discussion

WTNT45 KNHC 060231

Post-Tropical Cyclone Gamma Discussion Number  15
NWS National Hurricane Center Miami FL       AL252020
1000 PM CDT Mon Oct 05 2020

Gamma was entirely devoid of convection for most of the day. Shortly
before sunset, a few disorganized thunderstorms developed to the
southeast of the cyclone's center, however these appear to be forced
at least in part by a sea breeze boundary and are not exclusively
associated with Gamma. A few other small cells of convection have
developed to the west of Gamma's center during the past hour or so,
but not nearly enough to be considered sufficiently organized to
meet the requirement for a tropical cyclone. Gamma is therefore now
considered to be post-tropical and this will be the last NHC

The cyclone could still produce some additional disorganized 
convection and periods of heavy rain overnight as it moves inland 
over the Yucatan peninsula, but this is not expected to result in 
significant regeneration. Some of this rain could impact areas that 
are preparing for the much more significant approach of Hurricane 
Delta in a day or so. The cyclone is moving southwestward near 
5 kt, and this should continue for another day or so until it 
dissipates. The winds associated with Gamma's remnants should 
gradually weaken through that time, though the system could still 
produce a few areas of heavy rain over southeastern Mexico.

It is worth noting that several model trackers, and consequently
the consensus aids, depict that Gamma will move northward over the
Gulf of Mexico and strengthen significantly in a couple of days.
This is because the trackers lose track of Gamma when it dissipates
and start following nearby Hurricane Delta instead. In reality, no 
models forecast that Gamma will remain a well-defined cyclone for 
more than another day or two.


INIT  06/0300Z 21.6N  88.4W   30 KT  35 MPH...POST-TROPICAL
 12H  06/1200Z 21.1N  88.9W   30 KT  35 MPH...POST-TROP/INLAND
 24H  07/0000Z 20.2N  89.6W   25 KT  30 MPH...POST-TROP/INLAND
 36H  07/1200Z...DISSIPATED

Forecaster Zelinsky

Tropical Storm Gamma Forecast Discussion

WTNT45 KNHC 051455

Tropical Storm Gamma Discussion Number  13
NWS National Hurricane Center Miami FL       AL252020
1000 AM CDT Mon Oct 05 2020

Air Force Reserve reconnaissance aircraft investigating Gamma found 
maximum 850 mb flight-level winds of 37 kt and surface SFMR peak 
winds of 40 kt, and the central pressure has increased to 1003 mb.  
Therefore, the initial intensity is lowered to 40 kt for this 

Stiff, persistent southerly shear, and an inhibiting thermodynamic
surrounding environment should prevent Gamma from restrengthening, 
although, the Decay SHIPS intensity model does indicate a very 
brief period of decreasing shear magnitude just prior to landfall 
around the 24 hour period.  Afterward, the shear, once again, 
increases.  Gamma is expected to make landfall over the 
northwestern Yucatan peninsula on Tuesday and further weakening 
to a remnant low is forecast Wednesday as the cyclone remains 
inland over the northern Mexican State of Yucatan.  Dissipation is 
likely to occur Friday over the region, if not sooner.

The initial motion is estimated to be a south-southwestward drift, 
or 205/2 kt. A turn southwestward or west-southwestward is 
expected by tonight and this general motion should continue through 
Wednesday morning while moving inland over the northern Mexican 
State of Yucatan.  The large-scale models still insist on some 
binary interaction occurring with Tropical Storm Delta commencing 
Tuesday night/Wednesday morning.  This should cause Gamma to 
gradually turn cyclonically through Thursday while remaining over 
the Northern Yucatan peninsula.  If there is a delay in Gamma's 
landfall, or the cyclone does remain just offshore, similar to the 
outlier UKMET, then Tropical Storm Delta could end up absorbing the 
smaller Gamma tropical cyclone toward the end of the week.  For 
now, the NHC forecast will side with a landfall scenario which 
agrees with the HCCA consensus and the GFS/ECMWF solutions.


1. Gamma is expected to produce heavy rainfall through midweek over 
portions of southeast Mexico, including the Mexican states of 
Yucatan, Campeche, and Tabasco. This rainfall could result in 
significant flash flooding. 

2. Even though Gamma is just offshore, tropical storm conditions
are likely along portions of the northern coast of the Yucatan
Peninsula later today.


INIT  05/1500Z 22.3N  87.6W   40 KT  45 MPH
 12H  06/0000Z 22.1N  88.4W   40 KT  45 MPH
 24H  06/1200Z 21.6N  89.2W   35 KT  40 MPH
 36H  07/0000Z 20.9N  89.9W   30 KT  35 MPH...INLAND
 48H  07/1200Z 20.2N  90.2W   25 KT  30 MPH...POST-TROP/INLAND
 60H  08/0000Z 20.3N  89.8W   25 KT  30 MPH...POST-TROP/INLAND
 72H  08/1200Z 21.0N  89.5W   20 KT  25 MPH...POST-TROP/INLAND
 96H  09/1200Z...DISSIPATED

Forecaster Roberts

Tropical Storm Gamma Forecast Discussion

WTNT45 KNHC 040232

Tropical Storm Gamma Discussion Number   7
NWS National Hurricane Center Miami FL       AL252020
1000 PM CDT Sat Oct 03 2020

Tropical Storm Gamma remains over the northern portion of the
Yucatan Peninsula, and the cyclone has been inland for about 10
hours now.  Although the storm has weakened some, satellite images
show that Gamma still has an intact central dense overcast feature
and spiral bands on the north side of the circulation.  The initial
intensity is estimated to be 50 kt, but there is quite of bit of
uncertainty in this value.  The tropical storm is drenching the
northern portion of the Yucatan Peninsula, and heavy rainfall is
likely to continue there into Sunday.

The tropical storm is moving northwestward at 6 kt on the
southwestern periphery of an Atlantic subtropical ridge.  Gamma is
expected to move very slowly to the northwest or north-northwest
toward a weakness in the ridge during the next 12 to 24 hours, and
that should take the core of the tropical storm over the extreme
southern Gulf of Mexico by early Sunday.  By Sunday night and
Monday, a combination of a building low- to mid-level ridge over the
Gulf of Mexico and a developing low pressure area over the
northwestern Caribbean Sea should cause Gamma to turn fairly sharply
to the west or southwest early next week.  A slow southwest or west
motion is likely to continue into the middle of next week, and some
of the models suggest that Gamma could become nearly stationary by
the end of the forecast period.  The NHC track forecast is largely
an update of the previous one and lies near the TVCA and HCCA
consensus models.

The intensity forecast seems tricky.  The tropical storm still
appears well organized despite its time over land and since it will
be moving back over water soon, it could re-strengthen.  However,
none of the intensity models show much strengthening, in fact, most
of them show Gamma gradually weakening throughout the forecast
period.  On one hand, the storm is going to be over the warm Gulf
of Mexico waters for several days, which favors strengthening. On
the other hand, there is a fair amount of dry air over the western
Gulf of Mexico and this could entrain into the circulation. Another
complicated factor is how Gamma interacts with a developing low over
the northwestern Caribbean Sea and a disturbance to its south.
Based on these complexities, the NHC intensity forecast shows no
change in strength through the forecast period, and lies near the
high end of the guidance near the SHIPS model.


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 inland, tropical storm conditions will
continue across portions of the Yucatan Peninsula tonight
and Sunday.


INIT  04/0300Z 21.3N  88.0W   50 KT  60 MPH
 12H  04/1200Z 21.8N  88.2W   50 KT  60 MPH
 24H  05/0000Z 22.2N  88.5W   50 KT  60 MPH
 36H  05/1200Z 22.1N  88.9W   50 KT  60 MPH
 48H  06/0000Z 21.8N  89.9W   50 KT  60 MPH
 60H  06/1200Z 21.3N  90.9W   50 KT  60 MPH
 72H  07/0000Z 20.7N  91.7W   50 KT  60 MPH
 96H  08/0000Z 20.0N  92.5W   50 KT  60 MPH
120H  09/0000Z 19.9N  93.3W   50 KT  60 MPH

Forecaster Cangialosi

Tropical Depression Twenty-Five Forecast Discussion

WTNT45 KNHC 021456

Tropical Depression Twenty-Five Discussion Number   1
NWS National Hurricane Center Miami FL       AL252020
1100 AM EDT Fri Oct 02 2020

Visible satellite images show that cloudiness and showers associated 
with the low pressure area over the northwestern Caribbean Sea have 
become significantly better organized since yesterday, with 
convective banding features becoming prominent. Moreover, low cloud 
motions suggest that a closed circulation has become better defined. 
 Therefore, advisories are being initiated on Tropical Depression 
Twenty-Five at this time.  The initial intensity estimate is 30 kt 
based on Dvorak T-numbers, but an Air Force Reserve Hurricane Hunter 
aircraft is scheduled to investigate the system later today to 
provide a better intensity estimate.  Sea surface temperatures are 
very warm, near 30 deg C, and vertical shear should remain low for 
at least the next couple of days, so the cyclone is likely to become 
a tropical storm by the time it nears the Yucatan Peninsula 
tomorrow.  The main impediment to strengthening over the next few 
days should be the interaction with land.  Given the uncertainties 
about how far offshore the center will be over the next several 
days, the official intensity forecast is conservative.

Since there is still a lot of scatter in the center fixes, the 
initial motion estimate, 315/8 kt, is rather uncertain.  For the 
next couple of days, the system is expected to move northwestward to 
north-northwestward on the southwestern edge of a mid-level high 
pressure area.  This would take the center near or over the 
northeastern Yucatan Peninsula.  After about 48 hours, the steering 
currents are not well-defined and there is considerable spread in 
the track models.  At this time, it appears the cyclone should move 
slowly westward over the latter part of the forecast period in 
response to weak ridging over the north-central Gulf of Mexico.  The 
official forecast is near or a little north of the corrected and 
simple model consensus predictions.

Tropical Storm Warnings and Watches have been issued for a portion
of the Yucatan Peninsula.


1. The system is expected to produce heavy rainfall that could
result in life-threatening flash flooding over portions of the
Yucatan Peninsula, far western Cuba and well away from the center in
the Mexican states of Campeche, Tabasco, and northern Chiapas.

2. The depression is forecast to become a tropical storm and bring
tropical storm conditions to portions of the Yucatan Peninsula on
Saturday, where a Tropical Storm Warning is in effect.


INIT  02/1500Z 18.1N  84.7W   30 KT  35 MPH
 12H  03/0000Z 18.9N  85.6W   30 KT  35 MPH
 24H  03/1200Z 19.8N  86.6W   35 KT  40 MPH
 36H  04/0000Z 20.6N  87.2W   40 KT  45 MPH...INLAND
 48H  04/1200Z 21.4N  87.5W   40 KT  45 MPH...INLAND
 60H  05/0000Z 22.0N  87.6W   40 KT  45 MPH
 72H  05/1200Z 22.0N  88.2W   45 KT  50 MPH
 96H  06/1200Z 21.5N  90.5W   45 KT  50 MPH
120H  07/1200Z 21.5N  92.5W   45 KT  50 MPH

Forecaster Pasch


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