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Tropical Storm Bonnie Forecast Discussion


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WTNT42 KNHC 020858
TCDAT2

Tropical Storm Bonnie Discussion Number  19
NWS National Hurricane Center Miami FL       AL022022
400 AM CDT Sat Jul 02 2022

Overall, Bonnie's structure is holding together quite well as the 
storm moves across Central America. While the coldest cloud tops 
near the center have warmed somewhat, radar from the Nicaraguan 
Weather Service in Las Nubes shows a healthy reflectivity structure. 
In fact, after the last center fix the Air Force Reserve Hurricane 
Hunter aircraft made indicated a formative eyewall was developing 
with Bonnie, a similar signature is now also currently observed with 
Bonnie over Lake Nicaragua. Assuming there has been some weakening 
of the wind field since Bonnie began moving over land, the 
intensity is being reduced to an uncertain 35 kt for this advisory.

Bonnie was moving just north of due west from the recon fixes last 
night, and the initial motion has been maintained at 275/14 kt. A 
strong deep-layer ridge poleward of Bonnie should continue to steer 
the cyclone west to west-northwestward for the majority of the 
forecast period after it moves offshore. This track continues to 
take Bonnie roughly parallel to the southern coast of Central 
America and Mexico. The track guidance remains in good agreement 
this cycle, and only slight changes to the forecast track were made, 
mostly a somewhat faster motion at the end of the forecast period. 
While the relatively small size of the tropical cyclone is currently 
forecast to keep the highest winds offshore, interests in coastal El 
Salvador, Guatemala, and southwestern Mexico should continue to 
monitor Bonnie's track for the next few days, as any northward track 
adjustments could require tropical storm watches for portions of 
this coastline.

So far Bonnie's structure does not appear to be that adversely 
affected by its ongoing land interaction crossing Central America, 
perhaps because it is traversing a relatively flat gap across Lake 
Nicaragua, in between higher terrain to its north or south. However, 
one thorn in the system's short-term intensity forecast is that sea 
surface temperatures (SSTs) just offshore Nicaragua are not all that 
warm, only between 26-27 C with a very shallow depth of these 
marginally warm waters for the first 24 h or so. For this reason, 
only slow intensification is forecasted early on, which is under the 
majority of the guidance in this time frame. Afterwards, SSTs 
markedly warm to above 28 C, and vertical wind shear is expected to 
remain only light to moderate. Thus, a faster rate of 
intensification is anticipated after 36 h, and Bonnie is still 
forecast to become a hurricane in about three days. The latest 
intensity forecast has been adjusted downward in the short-term, but 
still peaks the storm as a 75-kt hurricane at the end of the 
forecast period.

It is worth mentioning that Bonnie's low-level circulation is 
expected to survive its passage across Central America, similar to 
that of Hurricane Otto back in 2016. Thus, the system is expected to 
retain its name even after it moves into the far eastern Pacific 
later today.  The intermediate advisory at 1200 UTC will be issued 
under the same Atlantic header as before.  Product headers will 
change to eastern Pacific headers beginning with the next complete 
advisory at 1500 UTC, with the ATCF identifier changing from 
AL022022 to EP042022.


KEY MESSAGES:

1. Heavy rainfall is expected across portions of Nicaragua and
Costa Rica today. Life-threatening flash flooding and mudslides are 
expected.

2. Tropical storm conditions are expected along the Caribbean
coasts of Costa Rica and Nicaragua within the Tropical Storm Warning
areas for the next several hours, and along the Pacific coasts of
Costa Rica and Nicaragua today.


FORECAST POSITIONS AND MAX WINDS

INIT  02/0900Z 11.3N  85.1W   35 KT  40 MPH...OVER LAKE NICARAGUA
 12H  02/1800Z 11.3N  87.1W   35 KT  40 MPH...OVER PACIFIC OCEAN
 24H  03/0600Z 11.5N  89.4W   40 KT  45 MPH
 36H  03/1800Z 12.0N  91.9W   45 KT  50 MPH
 48H  04/0600Z 13.0N  95.0W   55 KT  65 MPH
 60H  04/1800Z 13.9N  98.0W   60 KT  70 MPH
 72H  05/0600Z 14.7N 101.0W   65 KT  75 MPH
 96H  06/0600Z 16.0N 106.4W   75 KT  85 MPH
120H  07/0600Z 16.4N 110.5W   75 KT  85 MPH

$$
Forecaster Papin