Area Forecast Discussion
Issued by NWS Miami, FL

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FXUS62 KMFL 190000

Area Forecast Discussion
National Weather Service Miami FL
800 PM EDT Thu Apr 18 2019

Minimal convection continues this evening, mainly over the
Atlantic waters and portions of Southwest Florida. As the front
approaches on Friday, an opening round of convection is possible
in the morning hours. Shower and thunderstorm chances will build
through the day with an afternoon and evening event moving from
west to east ahead of the front. APF will run a few hours ahead of
the other sites. Storms will be capable of sub-VFR conditions
including IFR/LIFR cigs and vsbys with heavy rain. Strong, gusty
winds are possible with directional variability around convection.
Wind will be southeasterly, becoming southerly and then
southwesterly. Behind the front, wind will turn more westerly to
eventually northwesterly beyond the end of the forecast period.


.PREV DISCUSSION... /issued 400 PM EDT Thu Apr 18 2019/



Afternoon water-vapor loops indicate a well-defined shortwave trough
crossing the Upper Texas Coast vicinity to the lower Mississippi
Valley, now phasing with a northern-stream cyclonic perturbation
extending farther north across the north-central states. Enhanced
moisture accompanying convection within the lobe of differential
cyclonic vorticity advection preceding the trough is assuming a
subtle baroclinic-leaf structure, implying the strengthening
large-scale ascent taking place in association with both
frontogenesis and cyclogenesis well northwest of the local area.
Corresponding surface pressure falls beneath the midlevel ascent
are facilitating continued veering of low-level flow across the
forecast area, with surface observations indicating occasionally
breezy southeast to south winds across South Florida. This is
further being reinforced by the eastward translation of a
southwest North Atlantic surface ridge. The increased southerly-
flow component is responsible for the continued influx of low-
level moisture, with precipitable water reaching 1.3 inches across
the region.

Through the overnight hours tonight, the frontal trough trailing
south of the aforementioned zone of surface cyclogenesis will track
eastward across the West and Middle Gulf vicinity, with a
corresponding tightening of the low-level pressure gradient across
the forecast area. The corresponding low-level mass response will
favor a meridionally-oriented band of broadly confluent low-level
southerly flow along the western periphery of the previously
mentioned surface ridge. Corresponding ascent -- also aided by
modest warm advection -- will be maximized from the Middle and
Upper Keys vicinity northward across South Florida, particularly
from interior sections to the East Coast Metro Areas and over the
nearshore waters. Increasing cloud coverage and chances for
showers are expected to accompany the increase in low-level
lift/moisture -- especially over the eastern half of the forecast
area where the lift is forecast to be maximized. A few
thunderstorms may also occur by late tonight, especially over the
waters and coastal locations.


On Friday, showers, and isolated to widely scattered
thunderstorms, may be ongoing during the morning within the pre-
frontal warm-advection band -- particularly focused over the East
Coast Metro areas and adjacent nearshore waters. While southerly
winds are forecast to increase to 15-20 kt over inland areas, the
forcing for ascent associated with warm-advection-related
convection is expected to be limited. Thus, its predictability is
also presently quite limited. Mesoscale boundaries -- including
those convectively enhanced and perhaps modest sea-breeze
circulations -- will have the potential to greatly influence
convective intensity, for which predictability is presently quite
low. There will be a tendency for this activity to exhibit motion
parallel to, or shift off the coast. As a result, there is
substantial uncertainty regarding its capability to ingest
effective inflow subjected to diurnal heating/steepened low-level
lapse rates.

If pre-frontal convection were to persist through the day over
inland locations, increasing potential for strong wind gusts could
conditionally accompany this activity -- in association with small
multicell clusters, evolving amid generally straight hodographs
through the convective-cloud layer. Weak midlevel lapse rates on the
order of 5.5-6.0 C/km should temper upward-scale convective
accelerations and the overall severe potential. However, with 850-
700-mb wind speeds increasing to 30-40 kt, convective momentum
transport aided by evaporatively-cooled downdrafts within the sub-
cloud layer could support small-scale bowing surges capable of very
isolated damaging wind gusts. If a rotating updraft were to
materialize in association with more discrete convection, favored by
effective bulk shear magnitudes increasing to 30-45 kt, the
potential for a tornado cannot be ruled out -- albeit a very low
potential greatly dependent on mesoscale augmentations to low-
level SRH and vertical vorticity fields.

Present indications are that frontal passage is expected during the
early Friday evening time frame -- perhaps entering the western CWA
as early as late Friday afternoon. While strengthening wind profiles
will favor a conditional risk for organized convection in
association with the front, the strongest midlevel height falls and
related differential cyclonic vorticity advection preceding the
previously mentioned midlevel trough will reside well north of
the area. As a result, the baroclinic circulation accompanying the
segment of the front over the local area is forecast to be
comparatively weaker and more shallow. Moreover, with cloud-
layer mean winds forecast to be oriented primarily front-
parallel, frontal convection will have the potential to be
undercut to some extent within an anafrontal-flow regime.
Additionally, the potential for pre-frontal convection to deposit
theta-e deficits that deplete buoyancy may also limit the
intensity of frontal convection. Nevertheless, the strengthened
lower/middle-tropospheric flow, coupled with a notable cross-
frontal low-level isallobaric wind, may support a few
convectively-enhanced downdrafts capable of strong to perhaps
very isolated damaging wind gusts through the evening. The front,
and associated precipitation chances, are forecast to clear the
region from late evening into the overnight hours Friday night.


Generally dry conditions are expected behind the front from the
weekend into next week, with slightly cooler temperatures
expected. The eventual elongation of the post-frontal surface
ridge to the north of the forecast area is expected to support
occasionally breezy easterly winds becoming established for early
to middle parts of next week, with slight chances for showers
returning for the middle of next week.


Breezy E-SE flow is expected today, likely reaching advisory
levels late tonight into Friday for all waters with gusty S-SSW
winds. Winds will then veer NW behind the front on Saturday. A
Small Craft Advisory for winds and waves will likely be needed for
some portion of this period. Shower and thunderstorm chances
begin on Thursday while becoming likely on Friday into Friday
night, with some stronger storms possible during this time.


A high risk for rip currents is in effect for the Atlantic beaches
through this evening, and the high risk for rip currents may
potentially be extended into Friday and across the Gulf beaches as
southerly flow increases.


West Palm Beach  74  87  65  79 /  30  70  80  10
Fort Lauderdale  77  85  68  81 /  30  60  80  10
Miami            76  86  68  81 /  40  60  80  10
Naples           73  85  67  77 /  30  70  70  10


FL...High Rip Current Risk from Friday afternoon through Friday
     evening for FLZ069.

     High Rip Current Risk through Friday evening for FLZ168-172-173.

AM...Small Craft Advisory from 11 PM this evening to 10 AM EDT
     Saturday for AMZ630-650-651-670-671.

     Small Craft Advisory from 2 PM to 10 PM EDT Friday for AMZ610.

GM...Small Craft Advisory from 11 PM this evening to 4 AM EDT
     Saturday for GMZ656-657-676.


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