Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tallahassee Florida
422 am EST Mon Dec 29 2014
Near term [through today]...
A very weak and elongated surface low pressure system centered
over much of MS and Alabama early this morning will continue move
steadily eastward to a position off the coast of the Carolinas by
Tuesday morning. Despite having such a weak and elongated center, it
is fairly atypical in most of these types of low pressure systems in
that it has managed to hold together a fairly impressive light to
moderate rain shield along its path which will be impacting the bulk
of the tri-state area today and into tonight. While storm total
rainfall amounts will not be particularly heavy, many areas may
receive an additional 0.30" to 0.80" over a fairly large portion of
the County Warning Area. While this will not cause any flash flooding in and of
itself, it will help to keep rivers in flood stage that have already
reached such levels from last weeks rains. High temperatures are
expected to be slightly above normal as well, with maximums ranging
from the middle to upper 60s across much of Alabama and Georgia to the lower
to middle 70s over the Florida Big Bend and Panhandle.
Short term [tonight through wednesday]...
The short term period will feature a transition from a rather
amplified upper level pattern to a more zonal one locally. The
northern stream shortwave that has been responsible for a more
progressive frontal system over the past 24 hours or so will have
advected off the northeast coast by this evening. As a result, the
rather large northern/southern stream jet will pull northeast out
of the region as well. This will remove all deep layer synoptic
forcing for ascent locally.
At the surface, the cold front associated with the aforementioned
system will be finishing up its trek across the tri-state region
overnight. Dry air will already be overspreading the area aloft,
so all moisture and forcing should be confined to below 700mb.
Thus, although a well defined line of rain is expected along the
front, quantitative precipitation forecast amounts should be less than a half of an inch. Rain
should be confined to areas south and east of a line from
Apalachicola to Tifton overnight.
Meanwhile, the typical northerly low-level flow regime associated
with high pressure building in behind a front will be slightly
backed across the southern appalachian mountain chain. This will
be a result of the Post frontal stratiform rain associated with
the aforementioned departing jet which will have generated some
weak troughing on the upwind side of the mountains. The resultant
westerly component will favor weak cyclogenesis on the Lee side of
the mountains. This will do a few things locally: (1) it will slow
the progression of dry low-level air into the region on Tuesday
and (2) Tuesday night into Wednesday, it will advect in a slightly
warmer and moist airmass from the northeast as a wedge of high
pressure builds down the East Coast to the north of the trough.
Although the driest surface air will not have advected in on
Tuesday, rain is not expected as all deep layer and surface
forcing will have been removed from the region. On Wednesday
however, while it is unlikely that we'll get any measurable rain,
it is not impossible to imagine a situation where some very light
rain or drizzle is generated along the trough axis as it drops
southwest into the tri-state region. The primary symptom of this
trough will be to bring a rather solid deck of low clouds from
northeast to southwest through the region beginning on Tuesday
night and lasting through much of the day Wednesday.
As far as temperatures are concerned, with the delayed arrival of
dry air tonight, lows will likely only fall to around 50 degrees
in southeast Alabama. They'll be much warmer nearer to the front,
and will most likely remain in the low to middle 60s near Cross
City. On Tuesday, highs will warm to slightly above seasonable
levels under a zonal upper pattern, and with no low-level cold air advection.
Tuesday night will feature another sharp temperature gradient with
upper 30s possible across our extreme northwestern Alabama
counties where northwesterly flow on the leading edge of the
approaching trough will advect in some cooler and drier air.
Elsewhere however, temperatures will likely be between the mid 40s
to the northwest, and mid 50s to the southeast in the warmer region
of the surface trough. Highs on Wednesday will be tricky due to
the expected low clouds, expect afternoon temperatures to be
capped in the upper 50s across portions of our northern Alabama
and Georgia counties, and in the middle 60s across the southeast
Long term [wednesday night through monday]...
Surface high pressure will build further into the southeast on
Wednesday night, bringing in a much drier airmass and lows ranging
from the middle 30s across Alabama, to the middle 40s across the
southeast Big Bend of Florida. On Thursday, a large area of low
pressure will begin to move through The Four Corners region and
into the Southern Plains. This will back middle and upper level
flow to the southwest once again and introduce another period of
unsettled weather beginning on Friday and lasting through the
weekend, when another cold front will pass through the southeast.
How quickly this front clears the region is still quite uncertain
at this time. Rain could linger across the southeast into early
next week. Due to the uncertain timing, pinpointing where the most
rain will fall is too difficult, though several rounds of rain
could result in 1 to 3 inches of accumulation wherever the
heaviest rain falls.
[through 06z tuesday]
With a very low overcast and gloomy conditions with this weak low
pressure system today and tonight, flying conditions are expected
to remain generally in the IFR/LIFR categories throughout much of
the fcst period. Where a few breaks in the broken-overcast conditions do
appear, we could see a glimpse of MVFR or possibly even a small
window of IFR conditions if The Breaks become large enough.
Onshore flow gusting to around 20 knots at times will be possible
today ahead of a cold front. Behind the front, winds will subside
and remain relatively low through Tuesday when the breezy leading
edge of high pressure will build into the northeast Gulf
increasing ocean conditions to at least cautionary levels. A brief
dip back to below headline conditions is expected Wednesday night
through Thursday night before another approaching low pressure
system increases winds and seas once again on Friday.
No fire weather concerns are expected over the next several days as
afternoon relative humidities remain well above any red flag
concerns with most of the area ground across the tri-state area
still well saturated.
At this time, only the Choctawhatchee, Ochlockonee, and
Withlacoochee rivers remain in flood. The most significant impacts
are being felt along the Ochlockonee and the Withlacoochee where
some locations remain in moderate flood stage. A period of dry
conditions through much of this week will allow rivers to either
begin, or continue falling to below flood stage. Later this week
into this weekend another several rounds of rain may bring quantitative precipitation forecast
amounts of 1 to 3 inches to area rivers, especially to the
northwest throughout basins in Alabama and the western Panhandle.
Detailed information pertaining to Current River stages and
forecasts can be found at: http://water.Weather.Gov/ahps.
Preliminary point temps/pops...
Tallahassee 73 59 68 50 62 / 80 40 0 10 10
Panama City 68 57 68 50 64 / 80 10 0 0 10
Dothan 67 51 64 42 56 / 70 10 0 0 10
Albany 67 53 66 45 59 / 80 30 0 10 10
Valdosta 74 57 68 50 62 / 80 70 10 10 10
Cross City 75 64 75 54 64 / 40 60 20 10 10
Apalachicola 72 59 69 52 64 / 70 40 0 0 10