From Mike Powell@618:250/10 to All on Sun Mar 15 15:11:12 2026
AWUS01 KWNH 151842
FFGMPD
FLZ000-160000-
Mesoscale Precipitation Discussion 0063
NWS Weather Prediction Center College Park MD
240 PM EDT Sun Mar 15 2026
Areas affected...South FL
Concerning...Heavy rainfall...Flash flooding possible
Valid 151840Z - 160000Z
SUMMARY...Slow-moving clusters of showers and thunderstorms with
extreme rainfall rates may cause flash flooding going through
early this evening. This could include some urban impacts.
DISCUSSION...The latest GOES-E IR satellite imagery along with
radar shows slow-moving clusters of very heavy showers and
thunderstorms impacting portions of Miami-Dade and Collier Counties.
This convection is focusing within a moist and unstable
environment characterized by MLCAPE values of 1500 to 2500 J/kg
and PWs of 1.5 to 1.7 inches. Some modest shear is in place with
as much as 20 to 30 kts of effective bulk shear. Much of the
activity recently though is tending to get its focus along the
presence of multiple mesoscale boundaries including outflow and
sea-breeze boundaries where there is small-scale convergence
working with the favorable thermodynamic environment.
Some modestly divergent flow aloft is noted also given proximity
of a weak shortwave west of the FL Straits and Cuba. This modest
deep layer ascent coupled with colliding surface boundaries should
tend to keep the convective threat maintained going into the
evening hours before sufficient levels of instability exhaustion
occurs to promote a weakening trend of the convection.
Already parts of the FL Everglades in Miami-Dade County have seen
over 5 inches of rain, with much of the activity staying away from
the urban areas. However, over the next few hours there will still
be a threat locally for some of these stronger thunderstorms and
heavier rainfall rates to impact the more populated areas of
southern FL which in this case will extend to the the southwest
coast including areas from Ft. Myers down through Naples. The
southeast coast of FL meanwhile from Miami down to Homestead will
still need to closely watch some of these slow-moving cells.
Rainfall rates of 2 to 4 inches/hour and storm totals locally
exceeding 5 inches are expected where any cells become anchored.
Earlier runs of the HRRR and RRFS were suggesting this near parts
of southwest FL in particular. As such, some areas of flash
flooding will be possible given the very high rainfall rates and
totals which may include some urban impacts.