Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tallahassee Florida
946 am EDT Sat Apr 18 2015
Near term [through today]...
Previous forecast is on track with only minor changes made. Currently
in a brief period of middle- level drying for our County Warning Area before deep
layer moisture returns as a large mesoscale complex currently over
southern la approaches this afternoon. Scattered showers and
thunderstorms will spread into the western zones this afternoon
where probability of precipitation are highest (40-60%). Probability of precipitation for the eastern zones have
been trimmed back into the lower chance category (30%). Enhanced
upper ridging over the region ahead of a deep trough along with
some breaks in the cloud cover will push temperatures into the middle to
upper 80s for most of the County Warning Area.
Rainfall this week has resulted in a rise of area rivers with many
reaching action or bank full stage. The Choctawhatchee River is
forecast to reach minor flood stage at Caryville on Sunday night
and at Bruce on Monday. With an additional 1 to 2 inches of rain expected
through Tuesday, other rivers may approach minor flood stage by
the end of the week.
Previous discussion [506 am edt]...
Short term [tonight through monday]...
By late tonight or early Sunday a lobe of middle-upper level
vorticity, associated with a subtle shortwave, is expected to
reach Alabama and the western Florida Panhandle. Coinciding with
this will be the rapid development and strengthening of a 100-120
knot upper level (250mb) jet streak from Alabama into eastern
Kentucky. This would place our forecast area in a region of
positive DCVA and the right front quadrant of an upper level jet
streak. In other words, the stage will be set for vertical motion
by Sunday morning, and we should see showers and storms either
persist in a northeastward trajectory from the Gulf of Mexico, or
develop across portions of Alabama, Georgia, and North Florida.
Models are actually in excellent agreement with the majority of
the convective development taking place between around 09 UTC and
15 UTC Sunday morning. Hi-res models indicate that while initial
convective modes may be more cellular, growth into a linear system
is expected with time with storms eventually pushing east through
much of the area. Given the agreement, probability of precipitation were bumped up to
around 80 percent for much of the area. Although it shouldn't rain
for more than a few hours at a given location it seems quite
likely that much of the area will see rain. Model forecast
soundings indicate a favorable overlap of around 40-50 knots of
deep layer shear and 1000-1500 j/kg of SBCAPE, even in the early
morning hours, across the area. Given the favorable environment,
we included severe thunderstorm wording in the forecast. However,
some uncertainties remain, largely centered around whether or not
this initial round of storms will be able to root quickly in the
boundary layer or will actually be realizing less instability than
what models are projecting. Precipitable water values should be
around the 99th percentile for this time of year, so heavy rain
will also be possible.
Additional showers and thunderstorms will be possible in the short
term period, likely either later Sunday night or Monday.
Subsidence in the wake of the morning mesoscale convective system could lead to a lull in
the late afternoon and evening before a surface cold front and
another upper level shortwave begin to approach. Forcing will not
be as strong or focused from Sunday night into Monday, so rain
chances are not as high. However, instability is likely to be
higher with continued moderate-strong deep layer shear and steeper
middle level lapse rates as an eml plume arrives relatively in tact
from the High Plains. Therefore, additional strong to severe
storms will be possible. Given lower overall likelihood of storms,
severe wording was not included in the forecast at this time.
Long term [monday night through saturday]...
The surface cold front should push through the area on Monday
night with high pressure settling into the Gulf Coast region
through Wednesday morning. Although the front should not make high
temperatures significantly cooler, it should bring in a slightly
drier air mass thereby also leading to slightly cooler overnight
lows. Some small rain chances enter the forecast again on Thursday
as the surface high erodes.
[through 12z sunday] ceilings will fall to IFR/LIFR levels at all
terminals by daybreak. The potential exists for LIFR
visibilities as well, but confidence remains lower with respect to fog. For
now, IFR visibilities are advertised. Ceilings will be slow to
scatter tomorrow morning. Scattered convection will develop and
spread from west to east later today becoming widespread late
tonight through Sunday morning.
Southerly winds will slowly increase this weekend and could reach
scec levels of 15-20 knots over parts of the coastal waters late
tonight or early Sunday. Lighter winds will return behind the cold
front on Tuesday and Wednesday. Showers and storms will be
possible ahead of the front from today through Monday. Some storms
on Sunday could be severe with strong winds.
No red flag conditions are expected at least through the middle of
Preliminary point temps/pops...
Tallahassee 88 70 81 68 84 / 40 30 80 20 30
Panama City 83 73 78 72 79 / 50 50 80 20 30
Dothan 82 68 80 67 82 / 50 70 80 20 30
Albany 83 69 81 67 82 / 40 70 80 20 30
Valdosta 87 69 84 67 83 / 30 30 80 20 30
Cross City 86 69 84 68 83 / 30 20 70 20 40
Apalachicola 84 73 78 73 80 / 40 30 80 20 30