Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 172154

Area Forecast Discussion
National Weather Service Louisville KY
554 PM EDT Sat Mar 17 2018

.Mesoscale Update...
Issued at 550 PM EDT Sat Mar 17 2018

Convection is already on a downward trend after getting a few sub-
severe reports from Marion County eastward to Lincoln. The main
boundary of concern runs from near Greensburg to Stanford, and that
will be the main focus of any remaining SVR threat. Will start
peeling off the western/northwestern layer of counties from WW 10
this hour, with the rest of central KY likely to come down next

Issued at 433 PM EST Sat Mar 17 2018

Storms have developed along a well defined surface boundary
stretching across central KY in an environment characterized by 1000-
1500 J/Kg surface based instability coincident with 50-55 knots of
effective bulk shear. Given strong speed shear with height along
with some veering with height, supercell structures (some splitting)
are possible. The main concern is large hail given freezing levels
around 8K feet and -20C heights around 18 K feet. A damaging wind
threat will also exist given steep low level lapse rates with a 20
degree T/Td spread at the surface. There is a possibility that
storms could congeal, and if a cold pool is established the wind
threat would increase.

An isolated tornado can`t be ruled out, however low level surface
winds aren`t overly backed to promote horizontal streamwise
vorticity ingestion. In addition, LCL`s are in the 1000-1200 M
range. Latest mesoanalysis shows effective SRH values around 50-100
M2/S2 along the boundary so there is some localized enhancement of
the environment along the front.

Severe Thunderstorm Watch #10 was just issued and expires at
Midnight EDT. However, expect storms to move east of our area by
around 6-7 PM EDT.


.Short Term...(This evening through Sunday night)
Issued at 215 PM EDT Sat Mar 17 2018

As of 2 PM, a surface analysis showed an area of low pressure
centered near Owensboro, Kentucky. GOES-East mesoscale visible
imagery depicted an arch of enhanced cumulus denoting the warm front
location that was along an Owensboro-Elizabethtown-Lexington line.
South of that boundary, spring-like weather prevailed with mostly
sunny skies, breezy southwest winds, and temperatures in the 70s.
Northward, across southern Indiana, lower clouds and easterly flow
prevailed and readings were in the 40s. Across the forecast area,
there is about a 30-35 degree temperature gradient.

A pool of near-60 degree dewpoints lie across south-central
Kentucky, contributing to about 500 to 1000 J/kg of surface-based
CAPE, per recent RAP mesoanalysis and GOES-East CAPE derived
product. Aloft, the presence of a passing shortwave is providing
modest deep-layer shear, on the order of 40 to 50 kts.

In the next 3-5 hours, a brief window of opportunity exists for
strong to marginally severe thunderstorms where the axis of
instability remains as the surface low tracks east through central
Kentucky providing the necessary lift to initiate convection. The
available hi-res models show the area where the risk exists is from
around Lexington southward toward Lake Cumberland, and as far west
as Glasgow, Campbellsville, and Springfield Kentucky. The latest SPC
Day 1 outlook nudged the marginal/slight risks westward from the
early morning issuance.

The main hazards for any storm that does develop is large hail
(quarter sized) thanks to the lower freezing levels (8-10 kft),
steep low-level lapse rates, and drier air aloft. If storms
organized into small clusters, a damaging wind threat also exists.

The main time frame is from around 3-4 pm through 7 pm as the storms
develop and quickly move eastward into eastern KY/TN.

The surface low moves east of the area this evening and drags the
cold front back through the area. There is a drizzle / light shower
chance across the northern areas and northern Bluegrass later this
evening, then temperatures tonight will fall into the upper 30s to
lower 40s with mostly cloudy to overcast skies.

On Sunday, seasonable readings are expected as high pressure builds
into the region. Drier air working in from north will erode the
cloud layer initially over southern Indiana and north central
Kentucky, where a mostly to partly sunny day is expected. Further to
the southwest, across south-central and west-central Kentucky, the
effects of that dry air are limited and time-height cross sections
show more moisture lingering, so a mostly cloudy to overcast day is
forecast. The clouds will impact high temperatures, and southern
Indiana may end up slightly warmer than south-central Kentucky by a
few degrees. Most areas will range from 55 to 60.

The next weather system is quick to move into the region as the
upper level pattern remains quite progressive. Showers are likely
Sunday night, especially after midnight across south central and
central Kentucky. Lows will fall into the 40s.

.Long Term...(Monday through Saturday)
Issued at 220 PM EDT Sat Mar 17 2018

An unsettled weather pattern is in store for the upcoming work week
across the lower Ohio and Tennessee Valleys. The progressive but
active pattern could bring both spring and winter weather to the

The 17.12z models are in reasonable agreement showing a compact
shortwave tracking across the Tennessee Valley Monday. At the
surface, a low pressure looks to organize and track across middle TN
during the afternoon and evening hours. This setup will put a warm
front somewhere along the KY/TN border or into south-central KY. The
best dewpoint surge, instability, and potential severe weather
threat continues to be across TN/AL/GA. However, a time-trend
analysis over the last several model runs shows a slight northward
jog in the surface low and shortwave, and if this trend continues,
portions of south-central KY could be under a threat for stronger to
severe storms Monday afternoon/evening.

Further north, a frontogenetical band of rain is likely to setup
across southern Indiana and north central Kentucky. As temperatures
cool behind the front Monday night and Tuesday morning, a brief
changeover to wet snow is possible but at this time, the colder air
lags the precipitation enough to preclude much if any snowfall

The Tuesday-Wednesday period has more uncertainty and lower
confidence. The potential phasing or not of the upper level energy
and the southern stream system could spread light precipitation
across portions of eastern and central Kentucky. It does appear to
be much colder with highs in the 40s, and potentially staying stuck
in the 30s. For now, continued with a model consensus but will need
to watch this period for the potential for light snow accumulations
across the Bluegrass.

The remainder of the work week looks more benign with high pressure
and seasonably cool temperatures. Toward next weekend it appears
another system may track through the Midwest and OH Valleys and
bring chances for showers and thunderstorms to the area.


.Aviation...(18Z TAF Issuance)
Issued at 112 PM EDT Sat Mar 17 2018

Surface warm frontal boundary is located just south of an HNB-SDF-
LEX line this afternoon.  This boundary looks to move a little
further northward this afternoon.  However, a surface low pressure
area will move eastward along the front and drag a cold front
through the region this afternoon.  IFR/MVFR conditions will be seen
at KHNB since they are on the cool side of the front.  The winds
there will shift to the northwest this afternoon and low clouds will
remain in place through the evening and into the overnight hours
before clearing out near sunrise.

Over at KSDF and KLEX, some clearing may work into the terminals
this afternoon as the warm front inches northward.  With the surface
low moving eastward, a threat of showers, and perhaps a thunderstorm
or two will be possible through 18/00Z.  The best chance of storms
though look to be south and east of those terminals.  Winds will
shift to the northwest this evening with MVFR cigs lingering through
early Sunday morning.

Down at KBWG, warm front is already past the terminal and VFR
conditions are expected through the period.  Gusty southwest winds
of 13-18kts with gusts to 25-27kts will be seen this afternoon.
Winds will shift with the cold frontal passage this afternoon and a
period of MVFR/VFR cigs will be seen this evening and into the
overnight with some clearing by Sunday morning.




Short Term...ZT
Long Term...ZT
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