Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 251053
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
553 AM EST Sun Jan 25 2015
...Updated Aviation Discussion...
.SHORT TERM (Now through Monday)...
Issued at 300 AM EST Sun Jan 25 2015
...A few slick spots possible overnight into Monday morning...
The synoptic pattern this morning features amplified flow across the
CONUS, with a strong western ridge and a downstream trough. A
strong PV anomaly, located across the northern Plains as of this
writing, will help to carve out this trough further through the
short term period. As it does, precipitation will break out across
the region today into tonight.
The day will start off dry, with increasing mid/high cloudiness
through the morning hours. Temperatures across MO are running in
the low 50s this hour, which all models are currently under doing by
5-10 degrees. Therefore, think locations across central/southern KY
should warm into the upper 40s/lower 50s on warm air advection
(despite the increasing cloud cover). Further north over southern
IN, temperatures will struggle in the lower 40s as low
clouds/precipitation will move in there quicker.
The latest 25/00Z guidance has continued to trend the surface low a
bit further south, but the latest southern jog was not too
substantial. Therefore, think we`re getting close enough to the
event that guidance is getting a pretty good handle on things.
Confidence is high that everyone in the LMK CWA will see primarily
rain through the day today, as isentropic ascent envelops the region
out ahead of the approaching system. Have upped pops to categorical
(90%) across the northern CWA, with likelies (70%) across southern
KY where the precipitation will be a bit more showery in nature.
Guidance is even hinting at some elevated instability across
southern KY this afternoon into this evening as this system really
gets wrapped up. Will leave thunder out for now as any rumble will
likely be isolated and confined to far southern KY, but it can`t be
ruled out today.
For tonight, a strong cold front will slide into the northern CWA
after 2-3Z, clearing the eastern edge of the CWA by 8-9Z. Behind
this front, temperatures will crash into the mid and upper 20s.
Model isentropic forecasts suggest there will be a deformation band
that sets up with this system. Interestingly, this system is so
wrapped up that this def band will only lag the surface front by
about 25 miles or so. Therefore, lift associated with this def band
will be ongoing as surface temperatures fall to right around
freezing. It appears that lift associated with this band co-located
with sub-freezing surface temperatures will mainly be across areas
along and north of I-64, especially up near Madison, IN. Near
Madison, IN, up to a half of an inch of snow could fall as
temperatures will drop their first. Other areas could receive a
dusting up to a few tenths of an inch, but this does not appear to
be a big snowfall by any means.
Despite only minor accumulations expected, we will have to monitor
the potential for icy conditions developing. Looking at the "flash
freeze" case of January 26th, 2010, there are several similarities
in that setup and the setup expected tonight. Specifically, the
temperatures around freezing in the early overnight, crashing
quickly into the 20s by rush hour. That event had a bit more snow
potential than it appears this system does (around 0.5 inch in
Louisville), but wet roadways from earlier rains coupled with
rapidly dropping temperatures and a coating of snow could create a
few issues for Monday morning, especially along and north of I-64.
Will highlight this potential a bit more in the HWO and the weather
story, but will hold off on any SPS product for now given the
uncertainty on just how much snow will be able to stick on the
backside of this system.
Otherwise, a few flurries will remain on Monday as mostly cloudy
skies prevail. The clouds coupled with surface cold air advection
will help hold temperatures in the low and mid 30s.
.LONG TERM (Monday Night through Saturday)...
Issued at 248 AM EST Sun Jan 25 2015
A 500 hPa vort max and weakening surface low are still on tap to
dive SE from the upper Mississippi Valley to the Carolinas Monday
night and Tuesday. Scattered light snow is expected to accompany the
system, possibly transitioning to light rain before ending on
Tuesday. Localized dustings of snow could occur in a few areas
after midnight Monday night, but for the most part impact from this
system should be minor. The main thing to watch out for is if there
will be anything on untreated roads for Tuesday morning traffic with
temperatures in the upper 20s and lower 30s.
Low pressure will strengthen as it crosses the Plains mid-week,
reaching the Chicago area Thursday morning. Though most of the
precipitation with this system will stay to our north, we`ll still
see some activity here from late Wednesday night to Thursday night.
We could see a light mix of rain and snow after midnight Wednesday
night, but the main time for precipitation will be on Thursday and
into Thursday evening when it will be all rain. Precipitation will
then likely transition to light snow from north to south as it
tapers off Thursday night.
The next weather maker will likely begin to affect the region this
weekend. Will lean more on the ECMWF for Day 7 (Saturday) as it has
done better at that time range lately. It brings another system in
on northwest flow for Saturday, which makes sense given the pattern
we`ll be in. The 00Z/25 run has actually shifted this system to the
east leaving us dry, but this may be a temporary aberration. The
GFS is dry too, but it was wet on the previous several runs. So,
will keep a mention of precipitation in the forecast for Saturday,
but will restrict PoPs to slight chance.
The warmest day still looks to be Thursday with highs in the upper
40s and lower 50s. Wednesday morning still should be the coldest
time as a small area of high pressure takes temperatures down to the
lower and middle 20s.
.AVIATION (12Z TAF Update)...
Issued at 553 AM EST Sun Jan 25 2015
Conditions will deteriorate through the TAF period as a clipper
system dives southeast into the Ohio Valley today. Conditions will
start VFR for the morning hours, with just some increasing mid-level
cloudiness. However, rain and associated lower cigs/vsbys will
quickly move in by the mid afternoon hours, likely dropping
conditions to IFR thresholds by late this afternoon into this
evening. Highest confidence in IFR is at KSDF and KLEX, with lower
confidence at KBWG. A strong cold front will sweep through all
sites, bringing gusty northwesterly winds (gusts in excess of 20
knots) along with a brief changeover to some light snow at KSDF and
KLEX early Monday morning. Likely won`t see any accumulations from
this snow, but can`t completely rule out a quick tenth of an inch or