Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 011422
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1022 AM EDT Wed Apr 1 2015

...Forecast Update...
Issued at 1015 AM EDT Wed Apr 1 2015

Current forecast looks to be in good shape for today.  Only tweaked
temps/dewpts some for this afternoon.  Better mixing over south
central and east central KY will result in slightly lower RH values
than previously forecast. Tweaked high temps slightly for this
afternoon.  A slightly cooler airmass is in place today but mostly
sunny skies and stronger spring sun angle should help us reach the
upper 60s to lower 70s for highs.

.SHORT TERM (Now through Thursday)...
Issued at 315 AM EDT Wed Apr 1 2015

Surface high pressure just this side of the Appalachians will
provide a mild and dry spring day, only with a lot less wind than
Tuesday. Northerly flow overnight has cooled low-level temps by
several degrees, so max temps this afternoon should fall a bit short
of yesterday. Still expect to be above MOS guidance, with most
locations getting into the lower 70s, except for the Bluegrass
region which will be in the mid/upper 60s.

As the surface high retreats to the east today, the door opens for
return flow to draw Gulf moisture back into the area tonight. An
increasing SSW low-level jet will keep the boundary-layer mixy
enough to prevent temps from falling too far, and in this pattern
models are usually too cool. Therefore continued the above-guidance
theme with tonight`s min temps.

As low-level warm advection strengthens and precipitable water
values jump up over an inch, look for slight precip chances to creep
into areas west of I-65 just before daybreak Thursday. Will continue
to ramp up POPs through the day as moist SW flow dominates. Look for
likely POPs by late afternoon northwest of a Bowling Green to
Frankfort line. Can`t rule out strong/severe wind gusts as
thunderstorms increase in coverage late in the day, but the deeper
dynamical support for stronger storms arrives too late to coincide
with any marginal low-level instability. Heavy rain threat doesn`t
become a factor until the surface front gets into the area Thursday
night.

.LONG TERM (Thursday night through Tuesday)...
Issued at 325 AM EDT Wed Apr 01 2015

...Heavy rain and the potential for a few strong storms Thursday
night into Friday...

The synoptic pattern at the start of the long term period will
feature broad southwest flow across much of the Central Plains and
into the Ohio Valley.  Embedded disturbances within this southwest
flow aloft will interact with a slow-moving cold front Thursday
night into Friday, bringing several chances of showers and
thunderstorms.

A surface low passing well to the north across southern Canada will
bring a trailing cold front across portions of IN/OH by Thursday
evening.  Widespread convection will likely accompany this front
just to the north of the LMK CWA early Thursday evening, before
sagging south into southern Indiana and central Kentucky Thursday
night.  As this occurs, the front will be aligning itself with the
mid/upper-level flow, which means it will be very slow to push south
through the overnight hours.   A southwesterly low-level jet of
45-50 knots will feed right into this stalling boundary through the
overnight hours, setting the stage for training convection along and
just to the north of the front.  Just how much rain falls will
depend on the mesoscale details and whether or not the front
completely stalls or is able to slowly sag south. However, the
synoptic pattern of a slow-moving front, seasonably high PWATs, and
continuous ascent in association with the low-level jet impinging on
the front all point to a narrow axis of heavy rainfall setting up
somewhere across southern IN or central KY, where a few inches of
rain could fall.  This could certainly lead to some flooding issues,
so we will continue to monitor this potential closely as the
mesoscale details become more apparent.

A more minor threat with the Thursday night convection will be the
potential for a few stronger storms.  However, MUCAPE values will be
waning given the lost of diurnal heating and a cooling boundary
layer.  Additionally, with forecast soundings being so saturated,
severe hail seems quite unlikely.  Given the 50-knot low-level jet,
cannot rule out a stronger wind gust or two mixing to the surface
given only an expected weak low-level inversion, but don`t
anticipate a widespread severe threat with the overnight Thursday
convection.

Quite a bit of uncertainty remains apparent for Friday.  A rather
strong PV anomaly will rotate through the southwest flow aloft,
helping to induce a surface low across the Ozarks Friday morning.
This low will slowly deepen as it pushes into the Ohio Valley.  The
main question mark will be just how far south the cold front makes
it Thursday night given the expected convection, and what effect
this ongoing convection Friday morning will have on the potential
for destabilization ahead of the next system Friday afternoon.

As of now, think the overnight/Friday morning convection will likely
win out across much of southern IN and northern KY, limiting the
potential to destabilize Friday afternoon.  However, central and
southern KY will have a better chance to sufficiently
destabilize by Friday afternoon ahead of the next surface front
associated with the Ozarks low, with MLCAPES approaching 500-1000
J/kg per the NAM/ECMWF.  The right exit region of a west-to-east jet
streak of 145 knots will nose into the Ohio Valley by Friday
afternoon.  0-6km shear vectors of 55-65 knots will be oriented
largely orthogonal to the cold front, which would suggest cellular
convection will be possible if sufficient destabilization can be
obtained.  This is a setup to be monitored as the synoptic forcing
mechanisms are there to support severe weather, but instability is a
big question mark at this point in time.

Otherwise, the cold front will push through the region Friday
evening, bringing cooler air Saturday into Sunday.  Highs on
Saturday will be in the 50s, rebounding back into the mid 60s by
Sunday.  Even warmer air is expected for the beginning of next week,
but rain chances will return as well Monday into the middle of next
week.

&&

.AVIATION (12Z TAF Update)...
Issued at 655 AM EDT Wed Apr 1 2015

Expect clear skies through the day and fairly deep into the evening,
with light E-SE winds. Approaching southern stream impulse will
start to pull deeper moisture northward well after midnight, with a
mid-level ceiling and a wind shift to southerly shortly before
daybreak on Thursday. Low-level jetting will make it a close call
for LLWS, but will not include it in the TAFs this far ahead of
time.

SSW winds will increase to 12-15 kt with higher gusts in the 24-30
hr planning period at SDF. Can`t rule out a stray shower on Thursday
morning either, but probabilities remain too low to mention in the
TAF.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...NONE.
IN...NONE.
&&

$$

Update.........AMS
Short Term.....RAS
Long Term......KJD
Aviation.......RAS





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