Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 212313

613 PM EST Sun Dec 21 2014

...Updated Aviation Discussion...

.SHORT TERM (Now through Monday Night)...
Issued at 235 PM EST Sun Dec 21 2014

As of mid afternoon, mostly sunny skies prevailed across central
Kentucky and southern Indiana, thanks to the influence of high
pressure that was centered to the northeast. High level clouds were
spilling east across the area associated with a weak shortwave
trough moving through the Mid-Mississippi Valley. Temperatures were
seasonable, ranging from the lower to upper 40s.

Plan on mostly clear skies this evening, then increasing clouds
overnight as warm advection and southeasterly/southerly flow begins
to push clouds currently over southern TN and northern AL/GA
northward. The best moisture transport and isentropic lift with this
system looks to stay east of the forecast area, but soundings show
up to 200 mb worth of saturation and weak lift from the Cumberland
to Bluegrass regions, so will continue slight rain chances. There
may end up being some drizzle as well. Areas along/west of I-65 are
expected to be dry overnight. Warmer lows are on tap thanks to the
clouds and southerly winds as readings range from the lower 30s
across southern Indiana and northern Kentucky to mid/upper 30s

On Monday, we`ll begin to feel the effects of the developing and
strengthening storm system across the central Plains. Stronger warm
air advection is expected to push 850 mb temps from around 0C
tonight to +9C Monday evening, giving highs in the low/mid 50s
despite a mostly cloudy day. There will be some low/mid level dry
air to overcome initially and the greater moisture transport doesn`t
begin to nose into Kentucky until closer to 23.00z, so much of the
day should be dry. The exception will be the far western forecast
area, where slight rain chances in the afternoon were continued. Rain
chances become likely across the entire area after sunset and
increase through the night associated with the passage of an upper
level shortwave. The clouds, precipitation and milder air mass
should keep overnight lows in the low 40s.

.LONG TERM (Tuesday through Sunday)...
Issued at 325 PM EST Sun Dec 21 2014

Strong upper low will slowly open up as it moves from the Upper
Midwest across the upper Great Lakes Tuesday and Wednesday. Expect a
mild but very wet pattern both days as a secondary surface low taps
copious Gulf moisture, and cuts almost due north up the I-65/I-75

Will carry categorical POPs Tuesday and Tuesday night, then begin to
drop precip chances Wednesday as the low-level jet and the strongest
moisture feed move into eastern Kentucky. With low-level jetting, a
bit of elevated instability, and the falling pressures ahead of the
deepening surface low, will carry isolated thunder Tuesday into
Tuesday evening.

Most of the bust potential is late Tuesday night and Wednesday, as
that hinges strongly on the timing of the low and the associated
cold front. Model trends have generally been slower, and therefore
warmer. The 12Z ECMWF takes the surface low a bit farther east, but
has had some issues with run-to-run consistency. Therefore will
continue with the slower trend. The result will be a very sharp temp
gradient from west to east, and a non-diurnal curve where the 12Z
Wed temp will go in the books as the Tuesday night low and the
daytime high for Wednesday. Based on the latest temp curve, will
further delay the changeover from rain to snow, pushing the
transition closer to midnight Wednesday night. Accumulations look to
be minor at best, but given the low-level lapse rates and wind
fields, we could be looking at wind-driven convective snow showers.

Progressive ridging will make for a mild and dry Christmas Day and
Boxing Day, with temps running on the high end of normal. Next
system will already be taking shape by late Friday as a shortwave
trof in the northern stream moves into the Great Lakes. Trailing
cold front will swing through the Ohio Valley by Friday night, but
we could get gapped as it struggles to tap into any moisture source.
Have trended a bit drier with only a 20-30% POP on Friday night, and
any precip of consequence will be far enough south to be all rain. A
Canadian high will build in for the weekend, with temps back on the
low side of seasonal normals.


.AVIATION (00Z TAF Update)...
Issued at 613 PM EST Sun Dec 21 2014

Surface ridge extending from New England to the southern Mississippi
Valley at 23Z will drift east during the period.  As it does so,
surface winds at all three TAF sites will veer to the southeast,
then gradually more to the south by the end of the period. This
return flow will juice up the lower atmosphere, with low end VFR
cigs moving in by 06Z and possibly MVFR cigs making their way into
BWG by 12Z.

Southerly flow will deepen and intensify during the day in advance
of a closed surface low building into the central MS Valley. As the
moisture layer deepens in the southerly flow, -RA or -DZ could
reduce vsbys into MVFR range at times, leading up to IFR conditions
beyond 22/0000Z.




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