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Tropical Storm Norma Discussion Number  10
NWS National Hurricane Center Miami FL       EP172017
300 PM MDT Sat Sep 16 2017

The convective stucture has continued to degrade during the day
with symmetric, but ill-defined bands in place within 180 nm of the
center.  An Air Force reconnaissance mission this afternoon has
measured peak flight-level winds of only 58 kt, which supports an
initial intensity of at most 55 kt. Thus Norma is downgraded to a
tropical storm.

While the vertical shear is currently low and should remain that way
for the next couple of days, the lack of motion over the last days
would likely induce ocean upwelling and cooler waters underneath the
core of Norma.  Additionally, a prominent tongue of dry air seen in
the total precipitable water imagery has wrapped around the southern
semicircle about 200 nm from the center. Model guidance suggests
that the mid-level humidities should dry substantially while Norma
encounters increasingly cooler SSTs during the next few days.
Additionally the vertical shear is now anticipated to substantially
increase around days 4 or 5 (if the GFS model is correct) or a day
or two earlier (if the ECMWF/UKMET models are correct).  The
official intensity forecast thus no longer indicates any
intensification, especially given the reduced initial maximum winds.
Correspondingly, a faster decay is shown compared with the previous
advisory and Norma is expected to become a remnant low in five days,
if not sooner.  This forecast is close to the mean of the tightly
packed statistical and dynamical intensity guidance.

Norma is moving toward the north at about 3 kt, as it is being
steered by a developing deep-layer ridge over central Mexico/Gulf of
Mexico. A motion toward the north-northwest or northwest should then
continue for about three days.  By days 4 and 5, a substantially
weaker Norma is anticipated to meander while located just west of
southern Baja California.  The official track forecast is based upon
the HFIP Corrected Consensus Approach and is about the same as the
earlier advisory through 48 hours and shifted westward beyond that
time.

Observations from the Air Force Hurricane Hunters show that Norma
has a smaller tropical-storm-force wind radii than earlier
estimated.  The official size foreast is based upon the RVCN
multi-model consensus.


FORECAST POSITIONS AND MAX WINDS

INIT  16/2100Z 19.7N 110.2W   55 KT  65 MPH
 12H  17/0600Z 20.4N 110.3W   55 KT  65 MPH
 24H  17/1800Z 21.4N 110.7W   50 KT  60 MPH
 36H  18/0600Z 22.2N 111.3W   45 KT  50 MPH
 48H  18/1800Z 22.9N 112.0W   40 KT  45 MPH
 72H  19/1800Z 23.6N 112.9W   35 KT  40 MPH
 96H  20/1800Z 23.8N 113.1W   30 KT  35 MPH
120H  21/1800Z 24.0N 113.0W   20 KT  25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Landsea

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