Special Weather Statement, NWS Miami, FL

Area Forecast Discussion
National Weather Service Miami FL
734 PM EDT Mon May 18 2020

.Aviation...
A line of showers and thunderstorms will continue to push eastward
over the next several hours. Periods of MVFR and IFR will occur
especially across the east coast terminals as this line moves
through. There will remain a chance of showers through the
overnight across the east coast terminals. Light and variable
winds will increase once again out of the southwest by the middle
of Tuesday morning. Scattered thunderstorms will develop again on
Tuesday afternoon especially interior and east.

&&

.Update...
Updated the forecast based on current radar trends. A line of
thunderstorms will continue to push into the east coast metro
areas over the next hour or two. There could be strong gusty
winds, funnel clouds, and heavy rainfall which could lead to some
flooding of the urban area especially in locations that have
received heavy amounts of rain earlier. Behind this line, showers
will linger for the rest of the evening hours and into the
overnight hours across the eastern and southern areas. Patchy fog
cannot be ruled out across portions of the interior sections late
tonight into early Tuesday morning. Winds on Tuesday will once
again increase out of the southwest as another round of showers
and thunderstorms will develop later Tuesday morning and Tuesday
afternoon. The highest chances will be across the interior and
east coast sections. Some of the thunderstorms could become strong
once again along the east coast and they could contain gusty
winds, heavy rainfall, small hail, and frequent lightning.

&&

.Prev Discussion... /issued 344 PM EDT Mon May 18 2020/

..Strong to severe storms this afternoon/early evening...
..Another round possible overnight in Southwest Florida...

Short Term (Rest of Today and Tonight)...

Tropical Storm Arthur is currently moving over the outer banks of
North Carolina and it will continue to move northeastward off of
the Mid-Atlantic coastline through the rest of the afternoon and
evening hours. At the same time, the tail of Arthur, basically a
trough of low pressure will extend into the southeastern portion
of the country. This also combined with a mid to upper level area
of low pressure moving into the Tennessee Valley and a trough of
low pressure moving into the eastern Gulf of Mexico will allow for
a south southwesterly flow to continue across South Florida. The
Gulf coast sea breeze will be allowed to push inland as the east
coast sea breeze will remain pushed up against the coast. All of
this will result in the development of scattered to numerous
showers and thunderstorms especially over the east coast and the
Lake Okeechobee region. Some of these thunderstorms could be
strong to severe this afternoon and this evening as 500mb temps
will be between -9 and -11C across the region along with steep
lapse rates and high MLCAPE over the area. The primary impacts
with these storms will be heavy rainfall, frequent lightning
strikes, small hail, and gusty winds. The potential for some minor
street flooding is possible as well across the east coast with
the heavy rainfall from thunderstorms.

The trough of low pressure over the eastern Gulf of Mexico will
slowly move across the region and into the Atlantic waters later
this evening and into the overnight hours. This will help to keep
the chances of showers and thunderstorms around during this time
frame. The greatest chances of showers and thunderstorms will
continue to be across the east coast as well as the Lake Okeechobee
region.

Long Term (Tuesday through Sunday)...

The mid-level trough will push into the Gulf waters late tonight
into early Tuesday morning likely spurring additional convection.
Portions of Southwest Florida could see vigorous showers and
thunderstorms overnight spurred by the mid and upper level support
provided by the trough moving through with activity pushing
eastward through the morning. The afternoon focus will remain
along the east coast where the sea breeze, the surface boundary,
and remnant moisture could interact to produce some convection.

The mid-level low will emerge over the Tennessee River Valley late
Tuesday into Wednesday. The surface boundary and associated moisture
should slip to the southeast of the area which may allow a brief dry
slot for portions of the area to emerge. Any dryness should be short-
lived as the mid-level low pulls back northward with the moisture
returning in time for the sea breeze convection to potentially
produce thunderstorms with the potential for heavy rain.

The generally unsettled pattern will persist as the rich moisture
remains across the region through the rest of the week. Diurnal
heating and the presence of sea breezes due to a light synoptic flow
should allow mentionable thunderstorm chances most days except for
those brief winds where a dry slot of air could move through
portions of the region. A ridge of high pressure may try to build
over the aloft over the Gulf but it may not be able to cap
convection as much as a stronger high centered closer to the area.
Temperatures will remain warm with upper 80s and 90s on most
afternoons and mid 60s to 70s for minimum temperatures.

Marine...
Southwesterly flow will continue through the middle of the
week across the Atlantic and Gulf coastal waters. Seas will
generally remain at 3 feet or less across the local waters.
Scattered to numerous showers and thunderstorms will be possible
each day through the end of the week. Winds and waves will be higher
in and around any showers or thunderstorms.

Prev Discussion... /issued 135 PM EDT Mon May 18 2020/

Aviation...
Mainly VFR conditions will prevail throughout the forecast period.
The exception to this will be in and around any showers or
thunderstorms that develop this afternoon and this evening where
periods of MVFR or even IFR conditions are possible. South to
southwesterly winds will continue through this evening between 10
and 15 knots. Patchy fog development is possible across the
interior sections during the overnight hours.

Prev Discussion... /issued 1121 AM EDT Mon May 18 2020/

Update / Mesoscale Discussion...

Midlevel water vapor loops indicate a well-defined southern-stream
cyclonic-flow perturbation centered over the Middle Gulf waters well
off the Tampa Bay Area coast.  This feature will continue drifting
eastward or even southeastward, as the centroid of its accompanying
cyclonically-curved jet streak still lies upstream of the trough
axis.  As a result, CVA-associated height falls and related DCVA-
enhanced midlevel cooling preceding this shortwave trough will
continue to overspread the NWS Miami County Warning Area through the
day -- with 500-mb temperatures falling to -8C to -10C from
southeast to northwest.

Meanwhile the low-level mass response preceding the wave is favoring
enhanced isentropic ascent and regenerative convection within a
corridor from the east-central Gulf toward areas from Tampa to Fort
Myers.  This activity will progress northeastward to eastward, with
diurnally destabilizing inflow favoring an increasing potential for
robust convection to accompany this activity across Glades and
Hendry Counties toward/across the Lake Okeechobee region from late
morning into early afternoon.  Meanwhile, areas of cloud-thinning
and partial clearing southeastward of this regime toward southeast
Florida will favor strong destabilization by this afternoon.
Concurrently, differential-heating-induced baroclinicity and
solenoidal circulations along the edges of thicker/denser mid/high-
level cloud decks and old outflow accompanying the more pronounced
isentropic-ascent regime, coupled with decaying outflow from
dissipating convection presently ongoing over Biscayne Bay vicinity,
along with some potential for a modest/broad east-coast sea-breeze
circulation to ensue, will favor a marked uptick in robust
convective potential this afternoon across southeast Florida.
Convection will preferentially regenerate into areas of highest
theta-e through the afternoon into the early evening -- i.e.,
southeastward to southward.

Given the extremely moist planetary boundary layer -- associated
with lowest-100-mb mean mixing ratios of 15-16 g/kg -- convective
inhibition will be low amid sufficient low-level ascent -- aided by
the larger-scale synoptic ascent -- to support high confidence in
the aforementioned regenerative convective regime taking place
today.  Moreover, it appears that an elevated mixed layer has
overspread South Florida from the Sierra Madre Mountains of Mexico --
 accompanied by maximum midlevel lapse rates of 7.0-7.5 C/km.
Related enhanced midlevel CAPE density will favor intense parcel
accelerations, offsetting the effects of precipitation loading / dry
entrainment, in support of robust updrafts.  While modest, the
presence of 20-30 kt of effective shear -- accompanying the enhanced
midlevel height gradient peripheral to the shortwave trough -- will
combine with 2000-3000 J/kg of midlevel-instability-enhanced
boundary-layer buoyancy for intense multicell clusters. In addition
to sporadic instances of small hail, these parameters suggest large
hail -- to around quarter size -- may occur especially where
updrafts remain in more separated/discrete phases.  Moreover, the
enhanced downward buoyancy accompanying the EML could favor damaging
wind gusts where more substantial cell interactions take place among
the multicell clusters.  Confidence continues to grow in isolated
severe winds and hail affecting southeast Florida this afternoon --
especially from Downtown Miami / Miami Beach area northward through
coastal/metro Palm Beach County.  The most-likely timing is 1-7PM.
The latest output from the Statistical Severe Risk Assessment
Model (SSCRAM) depicts both conditional and unconditional
probabilities for damaging wind gusts and severe hail across these
coastal and metro areas of South Florida this afternoon, further
increasing confidence in these convective hazards materializing in
the strongest storms that develop. However, earlier scattered
strong thunderstorms may also affect Hendry/Glades Counties
through the Lake Okeechobee region.

In addition to severe hail/wind potential today, there is also a
reasonable-worst case scenario for a slight inland mass flux in the
lowest 1 km AGL to favor effective SRH near 100 m2/s2 in immediate
proximity to the east coast.  Related streamwise-vorticity
enhancements could conditionally favor the potential for convection
to briefly attain supercellular characteristics within 10-15 miles
of the immediate Atlantic coast -- locally boosting the hail/wind
potential, and possibly supporting a brief/weak tornado.  The
enhanced midlevel lapse rates and related parcel accelerations would
also contribute to enhanced vorticity stretching in the low-levels,
and thus there is a reasonable worst-case scenario for brief tornado
with the east-coast-area convective regime.

Finally, given widespread 1.9-2.0-inch precipitable water, enhanced
outflow establishing foci for convective regeneration/training amid
low MLCINH, and anticipated slow multicellular-convection motion,
heavy rain and flooding will be a concern -- especially for the east-
coast metro areas and vicinity.  If training convection were to
become established over areas that experienced several inches of
rain over the past few days, the potential for significant urban
flooding could occur this afternoon.  Such potential would be highly
localized, though antecedent conditions may bolster this risk.

Ultimately, there remains some uncertainty regarding the exact
details of most-likely strong/severe risk, along with a bit of
uncertainty about the maximum magnitude of potential convective
intensity. This is especially the case given the modest large-scale
ascent over the region. However, there is growing confidence in a
severe-thunderstorm and flooding potential for the east coast metro
areas and vicinity, and all interests across South Florida are
encouraged to monitor weather conditions closely today.
&&

.Preliminary Point Temps/PoPs...
West Palm Beach  72  87  72  87 /  50  60  20  60
Fort Lauderdale  75  87  74  86 /  50  60  20  60
Miami            74  88  74  87 /  50  60  20  50
Naples           72  85  73  86 /  60  50  10  20

&&

.MFL WATCHES/WARNINGS/ADVISORIES...
FL...None.
AM...None.
GM...None.
&&

Update...55/CWC
Aviation...55/CWC

Hazardous Weather Outlook for South Florida
National Weather Service Miami FL
421 PM EDT Mon May 18 2020

AMZ610-630-650-651-670-671-FLZ063-066>075-168-172>174-GMZ656-657-676-
190830-
Lake Okeechobee-Biscayne Bay-
Coastal waters from Jupiter Inlet to Deerfield Beach FL out 20 NM-
Coastal waters from Deerfield Beach to Ocean Reef FL out 20 NM-
Waters from Jupiter Inlet to Deerfield Beach FL from 20 to 60 NM-
Waters from Deerfield Beach to Ocean Reef FL from 20 to 60 NM
excluding the territorial waters of Bahamas-Glades-Hendry-
Inland Palm Beach-Metro Palm Beach-Coastal Collier-Inland Collier-
Inland Broward-Metro Broward-Inland Miami-Dade-Metro Miami-Dade-
Mainland Monroe-Coastal Palm Beach-Coastal Broward-Coastal Miami-
Dade-Far South Miami-Dade-
Coastal waters from Chokoloskee to Bonita Beach FL out 20 NM-
Coastal waters from East Cape Sable to Chokoloskee FL out 20 NM-
Waters from Chokoloskee to Bonita Beach FL from 20 to 60 NM-
421 PM EDT Mon May 18 2020

...STRONG TO POSSIBLE SEVERE STORMS THIS EVENING...
...HEAVY RAINFALL AND LOCALIZED FLOODING POSSIBLE THIS EVENING...

This hazardous weather outlook is for South Florida.

.DAY ONE...TONIGHT

Thunderstorms: Scattered to numerous thunderstorms are possible
across all of South Florida and its waters this evening. A few of the
thunderstorms could become strong, to even severe, with gusty winds,
hail, frequent lightning strikes, and heavy rainfall. A weak
isolated tornado is possible across eastern portions of South
Florida.

Tornado: A weak isolated tornado may be possible this evening with
the strong to severe storms, primarily across the east coast of South
Florida.

Wind: The strongest thunderstorms could produce wind gusts up to 60
mph.

Hail: The strongest thunderstorms could produce hail up to quarter
size.

Flooding: Heavy rainfall from thunderstorms, and repeated rounds of
showers/storms, can produce flooding in low lying and poorly
drainage areas, especially for the east coast where it has been
raining the past few days.

Waterspouts: There is a slight chance of waterspouts with any
thunderstorms and showers that do develop over the local waters.

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY

There will be a chance of thunderstorms across South Florida
throughout the week. Gusty winds, locally heavy rainfall, lightning,
and waterspouts will be possible with the strongest storms.

The threat of rip currents for Atlantic beaches will also increase
towards the end of the work week as winds become more easterly over
South Florida.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed. Please relay any information about
observed severe weather to the NWS while following all local, state,
and CDC guidelines.

For more information...visit the National Weather Service in
Miami website at www.weather.gov/miami.