Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tallahassee Florida
436 am EDT Sat Apr 25 2015
..severe thunderstorms possible today and Sunday across the
Near term [through today]...
the primary forecast concern today is related to thunderstorm
chances and severe weather potential. The short version is that
rain and storms are expected to be most likely over the northern
half of the forecast area, and that severe weather is a distinct
possibility (particularly in those same areas).
As of 08 UTC, a broken line of strong to severe thunderstorms
extended from near Birmingham, to Jackson MS, to nearly Natchez MS
along the Mississippi River. These storms were advancing east,
and could conceivably reach the northern fringes of our forecast
area around 12 UTC (7 am cdt). These storms were situated along
the northern edge of a larger Reservoir of instability, and likely
just north of a low-middle level capping inversion. This cap was
sampled by the 00 UTC lix and lch soundings in southern Louisiana
with 700mb temperatures between +10c and +12c, and the latest rap
analysis indicates that similar values are currently nosing into
the Florida Panhandle. This should limit the southward extent of
morning convection, although a few isolated storms can't be ruled
out even down to the Gulf Coast. What unfolds in the afternoon
will largely depend on the convective evolution this morning. The
vast majority of convection-allowing models (cam) indicate
afternoon initiation along a trailing west-east oriented outflow
boundary. How far north or south that occurs will depend on the
area swept out by storms this morning. Current indications are
that it will be close to the northern border of our forecast area,
plus or minus a few counties.
Severe weather will be possible where storms occur today, but
should be more likely over the northern half of the area as that
is where storm chances are currently highest and there may be a
lingering boundary. By middle-morning, a broad-based average of
models shows just over 1000 j/kg of cape in the warm sector,
increasing to around 2000- 2500 j/kg by afternoon. Combined with
0-6km shear values of around 50 knots, the environment should be
very favorable for severe storms with the possibility of
supercells or bowed line segments. We have maintained severe
wording in the forecast north of I-10. All severe hazards would be
possible, with large hail and damaging winds the main threat but
tornadoes also possible. It is Worth noting that even the cams
that produce morning showers and storms indicate very strong
storms later in the day. Steep middle-level lapse rates and a moist
boundary layer should aid in quick destabilization in the event of
morning rain and cloud cover. The primary thing that would
decrease our severe risk is if the main outflow boundary remains
off to the north of our area, but that seems the less likely
scenario right now. Finally, we could see some localized heavy
rainfall or flash flooding given storm motion vectors roughly
parallel to the expected outflow boundary orientation.
Short term [tonight through monday]...
on Sunday...a weak frontal boundary may be lingering across the Gulf
Coast states. Middle level lapse rates will likely remain steep with
westerly flow continuing with an unstable airmass south of the
front. Weak convergence along this leftover boundary combined with
the afternoon seabreeze may be enough to generate isolated to
scattered thunderstorms across the Florida Panhandle and Big Bend.
The 00z GFS and NAM both forecast SBCAPE values over 3000 j/kg south
of the front on Sunday afternoon...so we will need to watch for
isolated severe weather on Sunday afternoon...mainly across the
Florida Big Bend and pandhandle. However...the lack of strong
forcing will likely limit the coverage of storms...thus the forecast
pop is only around 30 percent Sunday afternoon. The Storm Prediction Center outlook
calls for a marginal risk of severe storms on Sunday. Isolated to
scattered afternoon convection is also possible on Monday afternoon.
Temperatures will continue above average with highs generally
ranging from the middle 80s to near 90 and overnight lows ranging from
the middle 60s to near 70.
Long term [monday night through saturday]...
active weather will continue through middle-week as another trough
will emerge eastward from the Desert Southwest. The associated
surface low will track eastward along the Gulf Coast...giving our
forecast area a good chance of rain on Tuesday into Wednesday.
There remains some uncertainty with the exact track of the
low...which will affect the severe weather threat across the area.
A track too far south would limit the warm sector and give US more
of a moderate to heavy rain threat vs severe storms. The 00z European model (ecmwf)
cuts the low across the Florida Big Bend...which would likely keep
the threat of stronger storms limited to the southeast Big Bend.
The 00z GFS mslp pattern looks a bit strange with a large mesoscale convective system
tracking southeastward off the coast of Louisiana. This may be
feedback and could be resulting in a surface low a bit too far
south initially. Overall...something like the 00z European model (ecmwf) seems
reasonable across the area with some uncertainty in the exact
timing...provided that the large mesoscale convective system does not develop like the GFS
thinks. This system should be gone by the end of the week with a
slightly cooler and drier airmass in its wake.
[through 12z sunday] some low cigs, primarily in the MVFR range,
may affect ecp and dhn early this morning. However, generally VFR
conditions are expected at the terminals through around or shortly
after 00 UTC. After that time, low ceilings should redevelop and could
affect most or all of the terminals with IFR-MVFR conditions.
Showers and storms will be possible today, most likely across the
northern half of our area which includes dhn and aby. Some of the
storms could be severe, and scattered to numerous storms may
persist around the dhn or aby terminals for several hours today.
south to southwest winds will remain at moderate to locally higher
levels through this weekend. Thereafter...an area of low pressure is
expected to affect the Gulf Coast states with increasing winds and
seas on Tuesday and Wednesday with cautionary to advisory conditions
red flag conditions are not expected with high relative humidity expected this
weekend and into next week. Rain chances are in the forecast
through Wednesday and some wetting rains are likely. High
dispersions are expected today areawide.
river levels will remain elevated for the next several days with
scattered convection across the area. A more widespread moderate
to locally heavy rain event remains possible for Tuesday into
Wednesday. The placement of the heaviest rainfall remains
uncertain with some models placing it in the lower parts of the
basins with lesser impacts and other models placing it farther
north with greater impacts.
Preliminary point temps/pops...
Tallahassee 87 72 89 70 86 / 30 20 30 20 30
Panama City 81 73 81 71 78 / 30 20 30 20 30
Dothan 84 70 88 65 85 / 60 20 10 10 20
Albany 82 70 91 64 83 / 60 30 10 10 10
Valdosta 88 71 91 68 85 / 30 30 30 10 20
Cross City 86 73 86 71 82 / 10 30 30 20 30
Apalachicola 81 74 83 72 80 / 20 20 30 20 30