Tropical Storm CRISTOBAL

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TROPICAL STORM CRISTOBAL DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL042014
1100 AM EDT SUN AUG 24 2014

The overall cloud pattern of Cristobal has changed little since
the previous advisory. Flight-level wind data from the Air Force
Reserve and NOAA reconnaissance aircraft investigating Cristobal
earlier this morning indicated that the low-level and mid-level
circulations were not vertically aligned, suggesting that the
inner-core wind field of the cyclone is still trying to form. The
initial intensity of 40 kt is based on 1000-ft flight-level winds of
49 kt and a few reliable SFMR winds of near 40 kt.

The initial motion is an uncertain 345/6 kt. There are significant
differences noted between the various NHC models. The GFS model and
the GFS ensemble mean shear the mid-level circulation southward from
the current position, and take the remaining shallow and weak
low-level circulation quickly northeastward, which seems unlikely at
this time given the overall large structure of the cyclone noted in
recon, satellite, and upper-air data. In contrast, the ECMWF, UKMET,
and NAVGEM models keep Cristobal as a deeper, stronger, and more
vertically coherent cyclone, moving the system only slowly
north-northwestward or northwestward for the next 72 hours or so as
a strong mid-latitude trough over the Carolinas lifts out to the
northeast. As the trough lifts out, the subtropical ridge is
expected to build back in to the north of Cristobal until a second
trough moves off the U.S. east coast in 4-5 days and weakens the
ridge again, allowing Cristobal to move slowly northward and then
accelerate off to the northeast. More weight has been placed the on
the ECMWF-UKMET-NAVGEM solutions, and the official forecast track
remains near the left side of the guidance envelope.

Light to moderate northwesterly to northerly shear is expected to
affect Cristobal throughout the forecast period. However, the shear
is not forecast to be strong enough to prevent at least slow
strengthening, especially given that the upper-level flow is
expected to be quite diffluent and divergent through at least 48
hours, which will act to enhance convective development. The NHC
intensity forecast remains close to SHIPS guidance.

FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 23.6N  73.1W   40 KT  45 MPH
 12H  25/0000Z 24.2N  73.4W   45 KT  50 MPH
 24H  25/1200Z 24.9N  73.6W   50 KT  60 MPH
 36H  26/0000Z 25.7N  73.8W   55 KT  65 MPH
 48H  26/1200Z 26.6N  74.1W   60 KT  70 MPH
 72H  27/1200Z 29.3N  74.9W   65 KT  75 MPH
 96H  28/1200Z 32.2N  73.3W   70 KT  80 MPH
120H  29/1200Z 35.0N  67.3W   75 KT  85 MPH

$$
Forecaster Stewart

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