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Area forecast discussion 
National Weather Service Miami Florida
951 am EDT Wednesday Sep 17 2014

minor changes were made to the previous forecast. The latest
short range model guidance looks on track with showers and
thunderstorms developing over the interior by the late morning and
early afternoon hours as a cold front continues to move through
northern and central Florida. With the southwesterly flow in
place, these showers and thunderstorms will then move over the
East Coast metropolitan areas throughout the afternoon and then dissipate
as they move over the Atlantic coastal waters during the evening.
The strongest storms could produce wind gusts up to 40 miles per hour as well
as frequent lightning. Therefore probability of precipitation have been raised a little
bit over the East Coast and lowered a little bit over the West
Coast for today.


Previous discussion... /issued 745 am EDT Wednesday Sep 17 2014/

the wind flow today will be from the westerly direction at all of
the taf sites with wind speeds of 5 to 10 knots. Showers and
thunderstorms should start to develop over the West Coast late
this morning and push east across the area through the early
afternoon hours. The showers and thunderstorms should then be
moving into the East Coast taf sites between 19z and 01z today.
So vcsh will continue over the East Coast taf sites from 15z until
19z before thunderstorms in the vicinity between 19z and 01z. For kapf taf site...thunderstorms in the vicinity from
16z until 21z then vcsh for rest of the evening hours.

The ceiling and visible will remain in the VFR conditions at this time
for the taf sites across South Florida. However...any showers and
thunderstorms that do move through the area could reduce the
ceiling and visible down to MVFR conditions which should only last
about 30 minutes. If it looks like the reduced visible or ceiling will
last longer with the showers and thunderstorms then a short tempo
group will be needed.


Previous discussion... /issued 411 am EDT Wednesday Sep 17 2014/

Short term...
high pressure over the Gulf is weakening as a trough over the
eastern United States digs south. This will keep mainly afternoon
showers and thunderstorms in the forecast today. A broad low over
the Atlantic, just off the Georgia coast, will slowly deepen and
drift eastward through the day today. The developing low and
upper level trough have changed the flow from northerly yesterday
to more west to southwesterly today. This will cause most of the
convection to be in the interior and East Coast, as opposed to
the Gulf side yesterday. Models are showing it may be a little
drier today, with precipitable waters around 1.8 inches, rather than just over 2
inches yesterday. 500 mb temps, on the NAM forecast sounding,
looks to be a little warmer than yesterday, with it sitting around
-6.0c. Normalized cape looks to be around .1, compared to the .18
yesterday. That would indicate weak vertical accelerations. So,
expect showers to develop this afternoon. Thunderstorms will also
be possible, given the upper level trough in the area. Most, if
not all the storms should be garden variety thunderstorms today.

By tomorrow, precipitable waters do go back to just over 2 inches and there is
an inverted trough over South Florida, according to the GFS. The
European model (ecmwf) doesnt show the inverted trough, but it does have weak low
pressure over the area. Winds tomorrow will begin southwesterly,
but should turn more southerly in the late afternoon. This would
make the Eastern Lake region the most likely to see active
weather tomorrow. Models also show a weak cold front trying to
push south into the area. However, it is still pretty early in
the season for a front and would not be surprised to see it stall
over central Florida instead. But this will bring some additional
instability to the area, especially in the lake region.

As we move forward to Friday, models are still indicating the cold
front will push south of the lake in the early Friday morning
hours. But again, it may not make it this far south. But, ahead
of it there should be general instability and weak surface flow.
This should allow for the sea breeze and lake breeze to develop.
This will cause some areas of convergence Friday, that will
trigger convection by the afternoon hours. Precipitable waters remain around 2
inches and the upper level trough does look to dig further south.
So, have somewhat higher probability of precipitation in the forecast for Friday.With
light flow, and plenty of moisture, the showers and storms could
bring heavy rain and possibly some minor urban flooding Friday
afternoon to the South Florida Atlantic coast metropolitan areas.

Long term...
for Saturday through Tuesday night...
the models bring a stronger upper level low to the area by
Saturday morning. This may bring increasing chances for showers
and storms. However, both the GFS and the European model (ecmwf) are indicating
better chances just to the north in central Florida. Given this,
have kept probability of precipitation at 50 percent for now. By Sunday morning, the low
weaken considerable and an upper level front is prognosticated to move
across South Florida. The models are in some disagreement how fast
it will move through , with the GFS being more aggressive. The
European model (ecmwf) actually looks to stall it over the southern portion of the
County Warning Area on Sunday. Either way, it will serve to bring a somewhat
better chance for precipitation on Sunday. So have kept likely probability of precipitation in
the forecast that day.

By the beginning of the week, the boundary will either be out of
the area, or washing out. However, there will still be a trough
over the eastern US, with the base in the area. This will keep
chances of showers and storms elevated into next week, with likely
probability of precipitation each afternoon through Tuesday. The trough will begin to weak
late Tuesday, allowing probability of precipitation to drop off through the night and into
Wednesday morning.

weak high pressure across the southeastern Gulf of Mexico will
turn flow more southwesterly across South Florida through the day.
This should keep the majority of the afternoon convection across
the interior and east coastal areas. Kept southwest to south-
southwest flow across all East Coast sites through the
afternoon. However...any East Coast sea breeze that develops
should not push too far inland but could briefly affect any of the
East Coast sites.

high pressure over the Gulf is weakening today as a weak cold
front slowly progresses towards the region. This will allow for
continued chances of showers and thunderstorms through at least
the weekend, as the front should stall between central Florida and
Lake Okeechobee. Also, a long period swell will continue in the
Atlantic waters for at least the next couple of days. The highest
swell, of around 3 feet, will be off the Palm Beach County coast.


Preliminary point temps/pops...
West Palm Beach 89 73 87 74 / 70 50 60 30
Fort Lauderdale 89 76 88 77 / 70 50 60 40
Miami 90 76 88 76 / 70 50 60 30
Naples 88 76 87 75 / 40 40 50 30


Mfl watches/warnings/advisories...


Short term...55/cwc
long term....55/cwc

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