Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 252011
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
311 PM EST Tue Nov 25 2014
.SHORT TERM (Now through Wednesday Night)...
Issued at 302 PM EST TUE Nov 25 2014
...Wintry mix possible late Wednesday into Wednesday night...
The synoptic pattern this afternoon features broad troughing across
much of the CONUS, making for temperatures continuing to run below
normal. A clipper system will dive southeast through the trough
aloft tomorrow into tomorrow night, bringing the potential for a
wintry mix to portions of the region.
Conditions will remain tranquil tonight as high pressure settles
into the region. High clouds passing across portions of eastern KY
will likely help keep temperatures up a bit there, otherwise most
other locations should dip into the mid and upper 20s.
Wednesday will start off dry as high pressure slowly drifts east of
the region. However, attention will quickly turn to the northwest
as a clipper system dives southeast through portions of the Midwest
and into the Ohio Valley. The left-exit region will help induce a
tightening mid-level thermal gradient, which should lead to a
frontogenetically induced band of precipitation.
The main forecast challenge will be both with the exact track of
this narrow precip band, along with the thermal profiles beneath
it. The latest guidance continues its trend of dipping the bulk of
the precipitation further south into central and southern KY. This
is not a huge surprise given that this northward placement is a
known bias in most numerical guidance. With better convergence of
the model solutions, have gone ahead and upped pops across portions
of southern KY, while dropping them slightly for portions of
southern IN/northern KY. The hardest challenge with this event will
be low-level thermal profiles. The good news is that it appears
surface wet bulb temperatures will remain above zero through the
event. However, if we get heavy enough rates, it could wet bulb the
column down just enough to set up a very narrow band of moderate
snow. Looking at the forcing, it appears to be weakening as it
enters the Ohio Valley, given a weakening upper-level jet structure
and the fgen response. Therefore, while a band of moderate
precipitation will likely still move across portions of
central/southern KY, not sold it will be long-lived enough to wet
bulb the column and give widespread snow. Given the surface low
will also be weakening as it moves into the region, surface cold air
advection will likely not play too much of a role either.
Bottom line is that this event will start as rain and remain mostly
as rain. However, rates may become just enough to wet bulb the
column and allow for some wet snow to mix in. If this does occur, a
moderate burst of snow could drop a few tenths of an inch of
accumulation briefly, before melting again once rates decrease.
All-in-all, not much, if any, impacts are expected from this event.
Ahead of the system, highs will climb into the low to mid 40s
tomorrow (warming ground/road temps ahead of this event). Behind
this system, overnight lows tomorrow night will dip into the upper
20s and lower 30s.
.LONG TERM (Thursday through Tuesday)...
Issued at 300 PM EST Tue Nov 25 2014
Thanksgiving Day may just be the coldest day of the long term.
Models had been divergent on a cold front coming into the region for
the weekend, but now they stall the front to our north Saturday and
Sunday before slowly pushing it through at some point early next
work week. That will mean an above normal forecast for temperatures
this weekend, but also a better chance for rains starting Saturday
night. The front will be to our north, but warm/moist air riding
near that front would promote showers and storms over an extended
period of time, at least into Monday. A lot will depend on the
location of that front Monday and Tuesday, so will lean towards
climatology and start decreasing pops by Day 7 (Tuesday).
.AVIATION (18Z TAF Update)...
Updated at 1212 PM EST Tue Nov 25 2014
VFR conditions will persist through the TAF period. Strato-cu
around FL025 continues to slowly sag to the south towards the Ohio
River. It appears these clouds will remain just north of KSDF, but
can`t completely rule out some MVFR cigs for a time this
afternoon/evening. Otherwise, generally light winds and partly
cloudy skies will persist at all sites tonight into early Wednesday
morning. Clouds will begin to increase ahead of a disturbance
tomorrow, but all precip and cig/vsby restrictions will hold off
until after the valid TAF period.