Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 152331
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
631 PM EST Mon Dec 15 2014
...Updated Aviation Discussion...
.SHORT TERM (Now through Tuesday Night)...
Issued at 250 PM EST Mon Dec 15 2014
The synoptic pattern this afternoon features a trough sliding
through portions of the central CONUS. This trough and its
associated surface low will be the main focus of the short term
Precipitation continues to slowly ooze into the region this
afternoon ahead of the aforementioned surface low and upper-level
trough, albeit in a broken fashion. This broken band of
precipitation will continue to shift ENE across the region, as
moisture transport and associated isentropic ascent increase over
the area. Do expect this band to stay broken as it shifts through,
so pops are a bit of a challenge. For now, 70-80% coverage looks
good with this initial band pushing in from the southeast. Given
the lack of thunder upstream, have decided to pull thunder mention
from the forecast. Just don`t think we will have the forcing needed
to get updrafts tall enough to support sufficient charge separation,
despite there being some weak mid-level instability.
There may be a slight lull after this initial band pushes through,
but most guidance points to additional showers developing along the
surface cold front tonight as it sweeps through the region. Am a
bit uncertain with coverage along the front as there currently is
very little in the way of precipitation along it in Missouri.
However, the left-exit region of an upper-level jet will slide over
this frontal surface this evening into tonight, which should improve
deep-layer ascent. Therefore, think coverage will fill in a bit in
the 03-09Z timeframe as the front shifts across the Ohio Valley.
Overnight lows will be mild as we sit ahead of the front under a
good amount of cloud cover. Will go with lows in the mid and upper
With the front swinging through, temperatures will slowly fall
through the day on Tuesday. After highs in the morning,
temperatures will fall through the 40s during the afternoon hours.
Guidance suggests there will be some wrap-around precip mainly
across portions of southern IN and the northern Bluegrass through
the day. Forecast soundings show saturation only up to around
800mb, so anything that falls will likely be quite light in nature,
with only an additional hundredth or so of QPF expected.
Things will dry out for Tuesday night, but the clouds will remain in
place. Given winds staying up and the expected cloud cover, went on
the warmer end of guidance with overnight lows. This puts
temperatures in the low and mid 30s.
.LONG TERM (Wednesday through Monday)...
Issued at 300 PM EST Mon Dec 15 2014
High pressure across south central Canada will continue to spill
into the region in the wake of the Tuesday front. Wednesday should
be a dry day with quasi-zonal flow. Late Wednesday night/Thursday
morning we still have a weak southern stream system coming through.
Precip looks like it will hold off until Thursday morning, and again
southern KY has the best chance as in previous forecast. Model
consensus QPF is for very light precip, with low precipitable waters
across the region, so not overly concerned about this system.
However temperatures are marginal for winter precip, so will have to
monitor upcoming forecasts for possible slick spots on area roads.
The next precip chance comes in starting Friday afternoon, as a
surface low develops along the northwest Gulf coast. Model timing
and location continues to vary. The latest GFS has slowed the
development of this low quite a bit, instead allowing for some light
precip ahead of a separate upper level disturbance. The new Euro
coming in also has slowed down the development of a surface low, a
considerable switch from its consistently stronger solution nearer
us. Part of the reason it is still very early in the forecasting
process for the potential for snow accumulations. The strength of
this system is the key to how much precip we get, as the background
moisture field will be pretty dry. A stronger system will be able to
wrap more moisture around to our region, whereas a weaker one would
be little to nothing in the way of precip for us. For now will trend
pops down some, but not much, to keep the forecast from
flip/flopping too much until we can see consistency in the models.
Stay tuned for updates the next few days.
As we move forward in time, the GFS and 00Z Euro keep some precip in
for the day Saturday, with the former because of the slower system
and the latter because of the stronger upper system moving up the
Ohio River. The previous Euro solution would keep some pops in for
Saturday night as well. High pressure building in behind this system
should keep us dry and cold the rest of this forecast period.
.AVIATION (00Z TAF Update)...
Issued at 630 PM EST Mon Dec 15 2014
Spotty light showers ahead of a warm front are moving over the TAF
sites at this hour, causing little to now impact as they move
through. Ceilings have been mostly VFR for the past few hours,
however will watch for lowering ceilings along a warm front as we move
through the evening. Upstream obs do show ceilings below
fuel-alternate and even IFR in many spots.
Late this evening, will likely have a brief period of improved
conditions as the cold front and some light showers move through. A
wind shift to WSW, gusting to 20-25 mph will accompany this frontal
passage. Ceilings are then expected to return to low MVFR (below
fuel alternate) and potentially into the IFR range toward Tuesday
morning and into the day. Will forecast just above this due to the
mixy low level atmosphere behind the front. Have updated times to
account for latest trends.
One other note, will include LLWS mention for the first few hours of
the valid TAF period as 40 knot SSW LLJ overruns the warm front.