Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Miami Florida
1039 am EDT Thursday Sep 18 2014
Update...no significant changes were made to the forecast this
morning, except to increase temperatures a tad for the East Coast metropolitan
areas. Convective activity will continue to focus mainly south of
Alligator alley where the highest moisture is present. So kept the
pop trend for the afternoon, but lowered probability of precipitation for the morning
hours with no mention of showers/thunderstorms from the lake region to
the Palm beaches...but a few showers and storms could develop
across this area during the afternoon. The morning Miami sounding
shows weak middle level lapse rates, but with dry air aloft moving in
from the Lake Okeechobee region, a few thunderstorms could produce
gusty winds. /Gregoria
Previous discussion... /issued 745 am EDT Thursday Sep 18 2014/
the winds will be light and variable this morning before becoming
easterly at 5 t0 10 knots after 18/19z today over the East Coast
taf sites due to the East Coast sea breeze pushing inland.
This will allow for some thunderstorms to develop over the East
Coast taf sites this afternoon. Therefore...a thunderstorms in the vicinity will continue
after 18/19z today over the East Coast taf sites. Kmia and ktmb
could even see some showers this morning. So a vcsh has been added
for this taf sites for this morning.
Kapf taf site will also be light and variable in the winds this
morning before becoming westerly at 5 to 10 knots after 17z today.
Vcsh will also continue this morning over kapf taf site before
changing to thunderstorms in the vicinity after 17z today.
The ceiling and visible will also remain in the VFR conditions for
most of today at all of the taf sites. But the ceiling and visible
could fall down into MVFR conditions during any showers or
thunderstorms that do move through. But at this time it looks like
the reduced visible and ceiling will be less than 30 minutes. So will
not put a tempo group in at this time for the taf sites.
Previous discussion... /issued 436 am EDT Thursday Sep 18 2014/
an upper level trough is pushing through the area today. Looking
at the water vapor, there is plenty of drier air and subsidence
behind it. Models are showing precipitation today. However,
looking back at what happened behind the front yesterday, which
doesnt look like much, have kept the higher probability of precipitation in the southern
portion of the County Warning Area. Precipitable waters will still be near 2 inches. There is
some weak 500mb vorticity advection across South Florida as well.
Boundary layer cape looks to be between about 800 j/kg and 1500
j/kg. The normalized cape is low from Opa-Locka north, at only
1. South of there, it does go up to around .17. So, better
parcel accelerations to the south, near the front, which would
make sense. Lapse rates are poor all across South Florida on the
NAM soundings, with most of the values in the 850mb-500mb less
than 6 c/km. The 500mb temperatures are fairly warm, with the NAM S model
sounding showing around -6c. Also, there may be some low level
directional shear along the Atlantic coast, most likely from the
sea breeze, which, if clouds persist, may struggle to develop. If
heating is sufficient, they could develop, and may act to cause
some training over some of the Miami metropolitan area this afternoon.
So, mainly expect showers with a few thunderstorms, with the
better chances across the southern portion of South Florida. The
lake region has the lower probability of precipitation today, being behind the upper level
front. South of i75 will see probability of precipitation generally around 50 and higher.
The front looks to stall somewhere over the southern tip of
Florida. This will keep some convective activity into the
overnight hours tonight. Tomorrow, precipitable waters increase ever so
slightly, and a 500mb short wave rounds the base of the trough.
This will serve to provide better dynamics for convection
tomorrow, although, cloud cover could curtail daytime heating,
causing the potential for convection not to be maximized. However,
given model guidance and the better dynamics due to the shortwave,
have kept a blended model guidance of likely probability of precipitation for Friday,
mainly in the afternoon.
By Saturday, an upper level and surface low try to develop off the
Florida Atlantic coast. This will keep moisture and instability
over the County Warning Area into the weekend. Precipitable waters on Saturday go up to near 2.2
inches. Interestingly, the GFS and the NAM are showing the
boundarylayer cape dropping to less than 500 j/kg Saturday
afternoon. So, Saturday may see mainly showers with a chance of
thunderstorms across South Florida, with the Atlantic coast
somewhat more favorable than the Gulf Coast due to the westerly
component in the flow above 700mb.
Saturday night through Wednesday...
convective activity will become more diurnal this weekend as the
developing low exits to the northeast Saturday night. By Sunday,
the trough over the eastern US weakens and pulls back to the
north, allowing high pressure to slowly build back in over the
Gulf. With a lighter flow over South Florida, the convection will
once again become diurnal, with the main trigger, the sea breezes.
The interior areas look to be the most favorable for development
each day, although either coast could also see showers and
thunderstorms each day through the middle of next week.
scattered showers remain across the area early this morning
resulting from a weak surface trough. Most of the activity remains
across the area waters and should not impact any of the East Coast
sites this morning. Kapf on the other hand should continue to see
intermittent rain showers for the next hour or so with resultant ceilings
briefly down in the MVFR to IFR range. Otherwise...weak northerly
flow will turn more easterly into the afternoon with a Gulf Breeze
a low pressure trough will bring another round of shower and
thunderstorms to the South Florida waters today. The trough will
stall over the area, keeping the showers and storms in the
forecast through at least Saturday. The long period swell will
begin to diminish today. While the period will continue to be
around 11 seconds, the swell height will be around 1 foot in the
Atlantic waters, mainly off the Palm Beach coast.
Preliminary point temps/pops...
West Palm Beach 89 75 85 74 / 40 20 70 30
Fort Lauderdale 88 77 86 75 / 50 30 70 40
Miami 88 76 86 75 / 60 20 70 40
Naples 87 76 86 74 / 50 40 70 30