Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Louisville Kentucky
1156 PM EST Sat Dec 20 2014
..updated aviation discussion...
issued at 840 PM EST Sat Dec 20 2014
I got burned by rapid erosion of stratocumulus, so have updated taf
for ksdf. It's not altogether unexpected...just put too much Faith
in the very narrow saturated layer around 025 agl, but with no
dynamic forcing, diurnal subsidence prevailed. Klex is still on the
edge of this cloud shield. Will hold off there for now, but update
likely there as well, along with tweaks to sky grids.
Short term (now through Sunday night)...
issued at 200 PM EST Sat Dec 20 2014
The synoptic pattern this afternoon features a quasi-zonal upper
level flow east of The Rockies while an active northern and southern
jet moves across the West Coast. A few weak shortwaves have been
moving through the flow across the Great Lakes and Ohio Valley,
which were responsible for this morning's light snow. At the
surface, weak high pressure is loosely centered northeast of the
area. Temperatures are running around 5 degrees below mid-December
normals, mainly in the mid 30s to lower 40s.
A dry evening/night is in store for the area with the main forecast
challenge cloud cover. Subsidence in the wake of the departing upper
level shortwave and drier mid levels should keep clouds variable.
Areas along/south of the Kentucky parkways are likely to see more clearing
while southern Indiana locations should be partly to mostly cloudy.
Lows will vary greatly depending on where clear pockets develop, but
are expected to range from the mid 20s to the lower 30s. Patchy
light fog may develop across southern/central Kentucky due to the light
wind field, residual low level moisture and expected clearing skies.
For Sunday, much of the day we'll be under the influence of high
pressure as it slowly moves to the northeast. Plan on light
easterly/northeasterly winds and high temperatures in the mid 40s.
Southeasterly to southerly flow develops Sunday night with weak warm
air advection beginning. Clouds will be on the increase during the
night from south to north and with the boundary layer remaining a
bit more mixed, lows will only fall into the mid/upper 30s.
Long term (monday through saturday)...
issued at 315 PM EST Sat Dec 20 2014
..active weather likely Christmas eve...
Upper pattern will begin to amplify on Monday, and will open the
door for a mild but unsettled first half of the week. Confidence in
the overall pattern is fairly high, but limiting factors are in the
sensible wx impacts on Monday and more in the timing thereafter.
SW flow will deepen on Monday, but the best moisture feed will
remain off to our west. Still could see weak isentropic lift with a
warm advection Wing, so will hold onto a slight chance for some
Increasing warm advection with pops continuing to ramp up Monday
night and Tuesday. Leaned on the warmer end of guidance for temps,
and that might not be warm enough. Will need to rely on the onset of
rain and resulting wet-bulb effects to get temps below 50 on Monday
Likely/categorical pops will run Tuesday into Wednesday, with the
best chances centered on Tuesday night under a strong moisture feed.
New uncertainty with the timing creeps in as the GFS tries to close
off a 500mb low over the Southern Plains. This solution also tracks
the deepening surface cyclone right up the I-65 corridor, which
would enhance our chances for thunder. Will not jump on this
solution at this time, but continue to trend slower and therefore
warmer. Net result is temps in the 40s at 12z Wed, which will be our
Tue night low and Wed high. This moves the window for a rain/snow
changeover more into Wednesday evening, with minor snow accumulation
possible Wednesday night if any snow showers are heavy enough as
Santa makes his rounds.
Precip should cut off before dawn on Christmas day, with mild and
dry conditions late in the week as progressive upper ridging
dominates. Next system works into the plains on Friday without
really phasing the northern and southern streams. Will run chance
pops Friday night into Saturday as the cold front pushes through the
Ohio Valley. Temps close enough to freezing to keep an uncertain
precip type near and north of the Ohio River, but confident in all
liquid farther south.
Aviation (06z taf update)...
issued at 1153 PM EST Sat Dec 20 2014
Skies have cleared at the taf sites tonight with just some high
clouds streaming across the region. With clear skies and light and
variable to calm winds some light fog will is expected overnight.
Visibilities should stay mainly in the MVFR range. Winds will be
light through the day out of the east to east-southeast with mostly clear skies.