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NOAA NOAA United States Department of Commerce

Tropical Storm BERTHA


500 AM AST SUN AUG 03 2014

The overall convective pattern of Bertha has changed little since
the previous advisory.  The center position has been difficult to
determine and is based on a blend of the past motion and available
reconissance data.  An Air Force Reserve reconnaissance was able to
close off an 850 mb center around 0200 UTC, and a dropsonde released
to the southeast of the alleged center indicated a surface wind of
210 degrees at 21 kt, which implies a west wind of at least 10
kt somewhere in the southern quadrant.  These data are the basis
for keeping Bertha as a tropical cyclone for this advisory.  Also
during the outbound leg, the aircraft found rain-contaminated SFMR
surface winds of 53, 57, and 48 kt in the southeastern quadrant.
After adjusting those wind speeds due to rain enhancement yields a
consecutive 3-bin average speed of about 42 kt, which justifies
maintaining the intensity at 40 kt.

The initial motion estimate is 310/18 kt. The NHC model guidance
remains in excellent agreement with Bertha gradually moving
northward and then northeastward around the periphery of a
deep-layer subtropical ridge located along 28N latitude and ahead
of an broad mid-level trough that is located over the eastern
United States through 72 hours. After that, the cyclone is expected
to become embedded in the mid-latitude westerlies and accelerate
northeastward and east-northeastward as it becomes extratropical or
merges with a frontal system in 96-120 hours. The NHC track forecast
is essentially just an update of the previous advisory, only
slightly faster, and follows the trend of the consensus model TVCA.

It is possible that Bertha could briefly open up into a wave this
morning due to strong northwesterly shear of at least 25 kt.
However, the environment ahead of the cyclone for about the next 48
hours is expected to become increasingly more conducive for at
least gradual strengthening to occur, especially at 36-48 hours when
the vertical shear is expected to decrease to near 10 kt while SSTs
and mid-level moisture reach their peak. By 96 hours, extratropical
transition over sub-23C SSTs and/or merger with a frontal system is
expected. The ECMWF indicates frontal merger in about 72 hours, but
this appears be premature based on other available model guidance
and input from the NOAA Ocean Prediction Center (OPC). The new NHC
intensity forecast is consistent with the previous advisory, which
now calls for Bertha to reach hurricane strength by 72 hours.
However, this could occur sooner than indicated given that the most
favorable oceanic and atmospheric conditions become juxtaposed by 48
hours or so. Intensity forecasts during the extratropical phase are
based on input from OPC.


INIT  03/0900Z 20.9N  71.1W   40 KT  45 MPH
 12H  03/1800Z 22.8N  72.7W   45 KT  50 MPH
 24H  04/0600Z 25.6N  74.0W   50 KT  60 MPH
 36H  04/1800Z 28.8N  74.0W   55 KT  65 MPH
 48H  05/0600Z 32.0N  72.7W   60 KT  70 MPH
 72H  06/0600Z 37.8N  66.8W   65 KT  75 MPH
 96H  07/0600Z 42.5N  58.0W   60 KT  70 MPH...POST-TROP/EXTRATROP
120H  08/0600Z 46.0N  47.3W   50 KT  60 MPH...POST-TROP/EXTRATROP

Forecaster Stewart