Scientific Forecaster Discussion
fxus62 ktae 221726
Area forecast discussion
National Weather Service Tallahassee Florida
1226 PM EST Mon Jan 22 2018
Aviation [through 18z tuesday]...
Onshore S-southeast winds were keeping a stable marine layer in place
across much of the region so far today, with IFR to low-end MVFR
cigs being reported at all stations. Cigs will improve to MVFR
levels later this afternoon as a ragged, north-south oriented band
of rain showers/thunderstorms and rain moves east, affecting kecp and kdhn mid to late
afternoon, then ktlh and kaby this evening, then finally kvld
later tonight. There will be a 3-6 hour window for fog/low cigs to
develop between the end of this rain and the cold front passage
late tonight, but the cold front will bring a wind shift to the northwest
and gradually clearing skies Tuesday morning.
Previous discussion [1106 am est]...
Near term [through today]...
The 9 am EST surface analysis showed a cold front extending
southward through la and the western Gulf of Mexico from a 995 mb
low centered along the Iowa-MO border. Vapor imagery and upper air
data showed a deep upper low very near the surface low. A ragged
band of showers and thunderstorms, not especially intense in terms
of lightning, preceded this front and will move into our Florida
Panhandle and southeast Alabama counties mid to late afternoon then
into south Georgia and the Florida Big Bend this evening. This timing is
based mainly on the hrrr and a few other local cam runs which
appeared to have a good handle on this system so far.
The prospects for severe storms in our forecast area appear low
due to several factors, including (1) poor thermodynamics (owing
to weak mid-layer lapse rates and a moist, stable planetary boundary layer which has
spread inland from the cool coastal waters of the Gulf, and (2)
lack of a strong, well-defined 850 mb jet stream. Although
forecast deep- layer shear values would support some storm
organization, it seems unlikely that these storms would be purely
surface-based. Indeed, our local ecam shows no probabilities of
severe storms here.
Short term [tonight through wednesday]...
The occluded upper low will move into the Midwest overnight, with
the surface front making it just about halfway through the tri-
state region by dawn. As mentioned above this is a typical high
cape/low shear event which tend to carry a marginal risk for
severe weather. It'd be hard to imagine anything but elevated
storms east of the Apalachicola and Chattahoochee rivers with
water temperatures in Apalachee Bay hovering right around 50
degrees and a full day of modest onshore flow. West of the rivers,
instability will likely be diurnally driven and should be waning
in the evening hours. Thus expect the threat for any strong to
severe storms to be lessening through the evening hours. Though a
low-end threat, all types of severe weather would be possible,
where surface based convection is favored, given 50-60kts of deep
layer shear, 25kts of low-level shear and favorable hodographs,
rather steep mid-level lapse rates and cool temps aloft. Storms
will likely be more of the discrete variation as they'll be pre-
frontal, at least prior to midnight.
Showers associated with the front should clear the southeast Big
Bend by early afternoon, with highs ranging from the low 60s in
southeast Alabama, up to the low 70s in the southeast Big Bend. In
the warmer Big Bend and south-central Georgia locations, highs
will be met prior to the passage of the front and will gradually
cool through late afternoon. Expect lows to dip into the upper 30s
to near 40 degrees Tuesday night, with sunny skies and highs in
the upper 50s to low 60s on Wednesday.
Long term [wednesday night through monday]...
Expect dry conditions to continue through the end of the week as
deep layer ridging prevails. Highs will gradually climb through
the 60s, with lows in the upper 30s. Over the weekend, a northern
stream trough moving through the eastern Continental U.S. Will merge with the
southern stream, and eventually push a cold front through the
region by Monday morning. Ahead of the front, veering low layer
flow and a southern stream wave will support a large area of
isentropic rain spreading into the northeast Gulf, at least as
far as North Florida. At this time models differ considerably
regarding the placement of this rain shield and thus our average
rain amounts with the weekend system could range from an inch or
less to 1-3".
Cautionary level winds and seas are forecast to begin on Tuesday
in the wake of a cold front and last until the end of the week
when conditions should continue to deteriorate to advisory levels
ahead of our next frontal system. Expect showers and thunderstorms
late today through tonight and again this upcoming weekend.
Dry air will move into the region by Tuesday morning as a cold front
moves through. Minimum relative humidity values will likely remain
above 30 percent this week. Patchy fog is possible this morning.
Rain is likely this evening and overnight.
Average rain amounts associated with today and tonight's frontal
system will range from 0.25-1", with the highest amounts expected
in the southeast Big Bend of Florida. Isolated higher amounts
would likely be less than 2". This weekend brings another chance
for rain, though just how much is quite uncertain. Additional
amounts could range from an inch or less to 1-3".
Spotter information statement...
Spotter activation is not requested. However, spotters are always
encouraged to safely report significant weather conditions when they
occur by calling the office or tweeting US @nwstallahassee.
Preliminary point temps/pops...
Tallahassee 58 68 40 61 36 / 90 10 0 0 0
Panama City 57 63 43 57 39 / 70 0 0 0 0
Dothan 49 62 36 58 33 / 70 0 0 0 0
Albany 55 64 37 58 34 / 80 0 0 0 0
Valdosta 62 69 40 60 35 / 80 20 0 0 0
Cross City 62 72 41 62 37 / 80 40 0 0 0
Apalachicola 60 66 45 58 40 / 70 0 0 0 0