Scientific Forecaster Discussion
fxus62 ktae 230120
Area forecast discussion
National Weather Service Tallahassee Florida
920 PM EDT Mon may 22 2017
Near term [through tonight]...
The 7 PM surface analysis showed a quasi-stationary front from VA,
through central al, through coastal la, and into south Texas. The
airmass ahead of this front was warm and very moist with precipitable water
values about 40% above climo for our region. All of the large
scale models forecast weak q-g forcing to persist overnight across
our forecast area. This, combined with favorable thermodynamics,
justifies keeping at least 20% pops. However, early Tuesday
morning the hrrr generates a more widespread area of rain across
the Florida Panhandle and southeast Alabama as a perturbation associated
with an mesoscale convective system (currently over la) reaches our region. This solution
is supported, at least in part, by other cams. At this time we do
not expect this rain to be particularly heavy.
Previous discussion [743 PM edt]...
Short term [tuesday through Wednesday night]...
A closed upper low north of the Great Lakes will open up as it
lifts off to the northeast. Meanwhile, a shortwave over Minnesota will
drop southward with a low closing off over Iowa on Tuesday. This
feature will continue southward into the Missouri Valley on
Wednesday digging a deep trough along and east of The Rockies
before beginning to move toward the East Coast Wednesday night. A
few upper level impulse accompanied by deep layer moisture along
with a surface front stalling near the Gulf Coast will bring a
very wet pattern to our area through Wednesday. The first
shortwave will arrive tonight and linger through Tuesday bringing
with it widespread showers and thunderstorms. Along with the
potential for heavy rainfall, Storm Prediction Center has our entire County Warning Area highlighted
for a marginal risk of severe thunderstorms. Ecam guidance shows
the potential for MLCAPE as high as 1500-1800 j/kg Tuesday and
forecast soundings show bulk shear values and an 850 mb jet of 30-40
kts. Additionally, pw's will increase to around 2.0". The
greatest severe threat will be damaging straight line winds but a
tornado cannot be ruled out.
There should be a short break in convection late Tuesday/early
Wednesday before the next and more significant shortwave arrives
ahead of the main upper trough. Expect another round of widespread
showers/thunderstorms Wednesday with rain chances tapering off
from west to east Wednesday night as the cold front pushes
through the region. Storm Prediction Center has all but a portion of the southeast Big Bend
highlighted for a marginal risk with the southeast Big Bend in the
slight risk. Bulk shear and the low level jet are forecast to be
in the 40-50 kt range but the limiting factor may be the lack of
instability due to the earlier convective complex.
As for the heavy rain threat, we will hold off on issuing a Flash
Flood Watch as latest quantitative precipitation forecast amounts have come down just a bit with
the higher amounts (at or just above 3") shifted to our
easternmost zones. These amounts are spread out through Wednesday
Long term [thursday through monday]...
Much drier air will begin to arrive on Thursday in the wake of the
cold front and remain in place through Saturday. Temperature and
humidity levels will be below normal for late may through Thursday
night with a gradual increase to seasonal levels by the weekend.
Rain chances will return Sunday into the first part of next week
with the approach of the next cold front.
Aviation [through 00z wednesday]...
A few showers and isolated thunderstorms near tlh will gradually
dissipate this evening and VFR conditions are expected across our
area tonight under broken mid-high level clouds. Chances for
showers and thunderstorms will increase across our area from west
to east Tuesday morning, with periods of MVFR cigs and visibility
likely at all terminals through the remainder of the day. Brief
IFR/LIFR conditions cannot be ruled out in stronger thunderstorms,
but VFR conditions are also possible during dry periods between
Modest southwest winds will increase quickly on Tuesday to
cautionary levels. A period of advisory levels winds and seas is
expected late Tuesday through Wednesday night with the approach
and passage of a strong cold front. Winds and seas will gradually
diminish through the remainder of the work week as high pressure
builds over the waters.
Recent rains over the last 48 hours have deposited the heaviest
amounts (3-4 inches) in the upper Choctawhatchee River basin in
southeast Alabama. Elsewhere, rainfall amounts have been lighter
generally 2 inches or less. Most of the guidance suggests that
rainfall over the next 36 to 48 hours should generally be in the
3-4 inch range with isolated heavier totals up to 6 inches. While
this could cause some minor flooding issues, slightly higher
amounts would be needed to cause a greater flood threat. Thus,
will not be issuing a Flash Flood Watch on this cycle, though as
confidence increases in the placement of tuesday's convection a
watch may be needed.
The threat for river flooding from this system remains quite low,
given that many rivers were at significantly low levels from the
ongoing drought. Only the Choctawhatchee River, which has received
modest rains in the upper portion of the basin is vulnerable to
minor flooding should heavier amounts on Tuesday materialize.
Preliminary point temps/pops...
Tallahassee 69 81 68 80 60 / 30 70 70 70 20
Panama City 74 81 71 80 65 / 60 70 70 70 10
Dothan 69 78 67 80 59 / 70 90 70 70 10
Albany 71 78 67 79 60 / 50 80 70 70 10
Valdosta 70 81 67 79 61 / 30 70 70 80 30
Cross City 71 83 71 81 65 / 20 60 70 80 50
Apalachicola 74 80 72 82 65 / 30 60 70 70 10