Alert archive / Florida / Severe Thunderstorm Watches in 2020

Severe Thunderstorm Watch 21

January 31, 20204 counties in Florida

The Storm Prediction Center issued this severe thunderstorm watch at Jan 31, 2020, 8:01 PM EST for 4 counties in Florida. It was in force for 6 hr.

The area

The outline as issued.

The facts

Issued: Jan 31, 2020, 8:01 PM EST (01:01 UTC)
Expired: Feb 1, 2020, 2:17 AM EST
In force: 6 hr
Issued by: the Storm Prediction Center
Largest hail expected: 0.5 in
Strongest gust expected: 60 kt

Counties Broward County, FL, Collier County, FL, Miami-Dade County, FL, Monroe County, FL

The radar

The Key West radar (KBYX), 97 km from the middle of the area, began a volume at 00:59 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

What SPC expected

The probabilities SPC published with the watch, for the watch area over its life.

Two or more tornadoes: 20%
One or more EF2 to EF5 tornadoes: 5%
Ten or more severe wind reports: 30%
One or more gusts of 65 knots or more: 10%
Ten or more severe hail reports: 5%
One or more hailstones of 2 inches or more: 5%
Six or more combined severe hail and wind reports: 50%

Warnings issued inside it

4 tornado, severe thunderstorm and flash flood warnings for its counties while it was in force.

Issued	Product	Office	Counties	Lasted
Jan 31, 9:57 PM EST	Severe Thunderstorm Warning	MFL	Collier, Monroe	78 min
Jan 31, 10:32 PM EST	Severe Thunderstorm Warning	MFL	Miami-Dade	73 min
Jan 31, 10:39 PM EST	Tornado Warning	MFL	Monroe	33 min
Jan 31, 11:09 PM EST	Severe Thunderstorm Warning	MFL	Miami-Dade	96 min

Discussions around it

SPC mesoscale discussions over the same counties from four hours before the watch to its end.

The nearest sounding before it

Key West, FL (KEYW), launched 00:00Z, 1.0 hours before the alert went out and 102 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 1226 J/kg
Mixed-layer CIN: -28 J/kg
Mixed-layer LCL: 915 m above ground
0-6 km bulk shear: 48 kt
0-1 km storm-relative helicity: 116 m²/s²
0-3 km storm-relative helicity: 152 m²/s²
Significant tornado parameter: 0.8
Supercell composite: 4.8

Open this sounding in Storm Lab

The text

As transmitted.

812 
WWUS20 KWNS 010059
SEL1  
SPC WW 010059
FLZ000-CWZ000-010800-

URGENT - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Watch Number 21
NWS Storm Prediction Center Norman OK
800 PM EST Fri Jan 31 2020

The NWS Storm Prediction Center has issued a

* Severe Thunderstorm Watch for portions of 
  South Florida
  Coastal Waters

* Effective this Friday night and Saturday morning from 800 PM
  until 300 AM EST.

* Primary threats include...
  Isolated damaging wind gusts to 70 mph possible
  A tornado or two possible

SUMMARY...A bowing complex of storms, and possibly some additional
strong/severe storms preceding it, will continue to spread
east-northeastward this evening toward the Florida Keys and other
parts of South Florida. Wind damage is the most likely risk, but a
tornado or two cannot be ruled out.

The severe thunderstorm watch area is approximately along and 65
statute miles north and south of a line from 50 miles northwest of
Key West FL to 30 miles southeast of Miami FL. For a complete
depiction of the watch see the associated watch outline update
(WOUS64 KWNS WOU1).

PRECAUTIONARY/PREPAREDNESS ACTIONS...

REMEMBER...A Severe Thunderstorm Watch means conditions are
favorable for severe thunderstorms in and close to the watch area.
Persons in these areas should be on the lookout for threatening
weather conditions and listen for later statements and possible
warnings. Severe thunderstorms can and occasionally do produce
tornadoes.

&&

AVIATION...A few severe thunderstorms with hail surface and aloft to
0.5 inches. Extreme turbulence and surface wind gusts to 60 knots. A
few cumulonimbi with maximum tops to 500. Mean storm motion vector
25025.

...Guyer

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Jan 31, 7:13 PM EST	Mesoscale Discussion	SPC	Miami-Dade	62 min
Jan 31, 10:37 PM EST	Mesoscale Discussion	SPC	Miami-Dade, Monroe	83 min

Nearby

Severe Thunderstorm Watches in Florida, 2020 Every severe thunderstorm watch in 2020

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.