Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 222010

310 PM EST Mon Dec 22 2014

.SHORT TERM (Now through Tuesday Night)...
Issued at 310 PM EST Mon Dec 22 2014

Deep upper low is just about to close off over the Northern Plains
this afternoon, with deep=layer S-SW flow increasing over the
Mississippi and Ohio Valleys. This pattern will result in mild but
unsettled weather over the next 36 hrs or so, as a series of
disturbances will touch off rounds of showers.

Moisture transport is just getting established, so the main precip
shield associated with the lead disturbance has remained confined
mainly to the Wabash Valley and points west, with very little in the
way of precip over central Kentucky or southern Indiana. A
smattering of showers did develop over northern Alabama, but not
showing much organization as it lifts through Middle Tennessee.

Still expecting precip to blossom tonight as additional upper waves
interact with a 40-45 kt low-level jet, but QPF looks fairly modest.
Therefore will still carry categorical POPs, even though confidence
in getting any substantial rain amounts is decreasing. An isolated
rumble of thunder still can`t be completely ruled out, but continues
to look less likely as this system appears to come at us in pieces.
Therefore will go forward without mention of thunder at this time.

By daybreak the best feed of deep moisture will be shunted to our
north and east, so would expect a break of a few hours in the precip
Tuesday morning. However the next wave will take shape late in the
day, spreading rain back in from south to north late afternoon/early
evening. Deepening surface low still progged to run up between the
I-65/I-75 corridors, with tremendous bust potential depending on
exactly where it tracks. Categorical POPs and steady temps in the
50s along/east of the track, while farther west, precip chances are
barely 50-60% and temps could drop into the lower 40s by Wednesday
morning. There will be a tight gradient in between but the placement
is a little iffy, and for now we will assume that to be somewhere
near I-65.

.LONG TERM (Wednesday through Monday)...
Issued at 300 PM EST Mon Dec 22 2014

An active long term period with the main forecast challenge being
temperatures and precipitation timing/types Christmas Eve through
Christmas morning, followed by more precipitation chances this

The 22.12z guidance is trending toward a similar solution with the
main surface low track Wednesday morning. Outside of the NAM, which
was an eastern outlier, the ECMWF/GFS/GEM show a strengthening low
lifting along a Nashville to Louisville to Indianapolis track during
the course of the day. A time trend analysis shows this is a slight
shift west and about 3-6 hours later than the previous consensus.
The impacts this has to precipitation timing and type were very
minimal, as there has been good consensus for steady rain to move
across the region during the course of the day. However, the change
does allow for warmer air to move through central/eastern Kentucky
and as a result, high temperatures were increased to the mid/upper
50s from the Cumberland to Bluegrass region, including the Lexington
metro area. For the Louisville area, the tighter baroclinic zone
will be right along the I-65 corridor, making Wednesday morning
temperatures difficult to pinpoint at this time. Nonetheless, the
area should warm into the low 50s before cold advection kicks in and
temperatures crash into the low/mid 40s by afternoon. Western
forecast area should see the coolest readings, likely stuck in the
upper 40s, then falling into the upper 30s by mid afternoon.

The main message/impact for the public will be a wet, unsettled
Christmas Eve with falling temperatures during the afternoon. It`ll
also be windy, both ahead and behind the surface low. Not ideal
holiday travel conditions, even if it`s just all liquid.

Soundings show a potential changeover from rain to snow west of I-65
during the evening hours /after 7 pm/. However, the greater
frontogenetical forcing and deformation precipitation band should be
west of the forecast area and we`ll be dealing with scattered, light
precipitation. Will continue to advertise a rain to rain/snow mix to
light snow during the nighttime hours. Little accumulations likely
as wet/warm surfaces and temperatures in the low/mid 30s will limit
accumulations. For Christmas, plan on clearing skies west to east as
a Pacific airmass high pressure quickly builds in from the
southwest. Temperatures will be around normal in the mid 40s.

The weekend forecast has some uncertainty as the whole upper level
flow remains amplified and active across the CONUS. At least
initially, there is some consensus that a surface low lifts through
the Great Lakes Friday night, dragging a cold front through the
local area Saturday. 30-40 percent rain chances look reasonable at
this time. Differences arise between the ECMWF/GFS/GEM as the 22.12z
ECMWF/GEM camp develop a secondary low along the Texas gulf coast
Saturday night, lifting it toward eastern Tennessee Sunday. This
would spread precipitation back across the area. Temperature
profiles this time would support a rain/snow mix for the northwest
half of the forecast area. However, the forecast details, including
any specific storm system/track, beyond Saturday have low confidence
as there are a wide range of solutions in this type of upper level
pattern. Next week looks to featuer similar uncertainity as both
ECMWF and GFS show varying storm systems to possibly impact the
area. An active stretch through the end of the year.


.AVIATION (18Z TAF Update)...
Issued at 100 PM EST Mon Dec 22 2014

Increasing low-level southerly flow over the Ohio Valley is having
minimal impact thus far, with only BWG having dipped into a high-end
MVFR ceiling. Lower clouds poised upstream over western Kentucky and
Tennessee, and the main near-term challenge is timing just how
quickly conditions will deteriorate this afternoon and evening.

Based on 925-950mb condensation pressure deficits, look for
fuel-alternate MVFR conditions to develop in BWG and SDF by late
afternoon, at which point we`ll also include VCSH as the precip
shield edges closer. Rain arrives in earnest by late evening, with
ceilings dropping into IFR. Low-level jetting will pick up as an
inversion sharpens around 2000 feet, which will put us into marginal
LLWS for much of the night. Can`t rule out embedded thunder
overnight either, but it`s a low enough probability that we`ll leave
it out of the TAFs.

Forecast confidence really decreases after roughly 09Z, as models
diverge on just how much of a break we`ll have in the precip. NAM
has a substantial break in the action, while the GFS appears to hang
up a SSW-NNE moisture feed across central Kentucky through the day.
Will try to depict modest improvements in ceiling/visibility,
perhaps above fuel-alternate thresholds. However, will hang on to
VCSH mention, out of respect for the possibility that precip would
not shut off.




Short Term........RAS
Long Term.........ZBT
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