Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 201723
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
123 PM EDT Sat May 20 2017

...Updated Aviation Discussion...

.Forecast Update...
Issued at 1240 PM EDT Sat May 20 2017

Visible satellite shows a cu field developing quickly in unstable
airmass along and east of remnant MCS cold pool boundary in western
KY and western TN. A regional radar mosaic shows a thin band of
showers/storms going up as well in these areas. Further east across
the LMK CWA, radar/satellite both remain quite clear thanks to drier
air aloft and a lack of any real forcing mechanism south of a warm
front (other than pure convective heating). In addition, a 1607z
AMDAR sounding over SDF suggests cirrus blowoff and subsidence from
the morning MCS in IL/IN has left a modest cap at around 600 mb.

The warm front is still draped across southern IN, but has been
making its northward push. Central Kentucky Mesonet stations show SW
surface winds across almost the whole area, with the exception of
the Northern Bluegrass.

The convective boundary is currently in the vicinity of Dubois
County, but will only push slowly to the east. Most of the CWA will
likely remain dry over the next few hours, with the exception of the
northwest counties. We could still see an isolated storm in the warm
sector. But rain and thunderstorm chances will be higher from mid-
afternoon through the evening hours as western KY/TN activity moves
to the northeast. Damaging winds will be the main threat, since the
warm front will likely push north of the area by early evening.
Moderate instability has developed, with the 16z SPC meso page
showing 2000-2500 J/kg SBCAPE near and south of the Ohio River.
Effective bulk shear remains quite low, but will gradually increase
from the west this evening as stronger mid-level flow slides east.
The primary convective mode should be linear, along with some
multicells.

Issued at 945 AM EDT Sat May 20 2017

Tweaked pops down slightly into the early afternoon to keep
shower/storm chances isolated further south and east. One notable
storm capable of small hail was moving north near the Dubois/Orange
county line. Overall, this storm is on southeastern flank of
convection arm feeding into larger, linear remnant MCS from near a
surface warm front. This convection is likely to remain elevated as
it exits the CWA.

Convective trends through the afternoon and evening are expected to
be driven by 1) northward advancing warm front and 2) eastward
advancing MCS cold pool/differential heating boundary. Some
scattered showers have developed across west-central KY, and despite
a fair amount of thin cirrus, we should see a decent temperature
gradient roughly along and east of I-65 corridor. A fair amount of
mid clouds are currently limiting warming across south-central KY,
however. So into the early afternoon hours, most likely area for new
strong development is in southern IN/northern KY closer to warm
frontal boundary. But as afternoon progresses, scattered convection
does look possible near differential heating boundary. As larger
scale MCS cold pool boundary slowly pushes east, this looks to be
the trigger for second round of convection this evening into tonight
across KY/TN.

Marginal shear profiles in the warm sector support primarily a wind
threat, along with isolated severe hail, this afternoon into
tonight. As has been mentioned already, stronger low-level shear
near the warm front does pose a small tornado threat this afternoon
and evening across southern IN/northern KY. Otherwise, convective
mode will likely be linear segments or multicellular.

&&

.Short Term...(Today through Sunday)
Issued at 315 AM EDT Sat May 20 2017

...Severe Storms & Flash Flooding Possible Today...

Early this morning a sfc boundary was lifting northward through
north central KY and southern IN.  Near the boundary isld to sct
convection continues.  AMDAR soundings out of SDF show a good low
level inversion so expect any convection early this morning to
remain mostly elevated.  Still small hail and brief heavy rainfall
will be threats with these early morning storms.  Of particular
concern will be Washington County, IN which experienced record
flooding last night and remains under a Flood Warning this morning.
Should convection get into this county, a Flash Flood Warning may be
considered.

For the rest of today, expect multiple rounds of storms as a cold
front approaches the area.  The latest CAM models indicate perhaps a
break in convection around sunrise until mid morning.  Then,
convection could initiate over north central KY and blossom as it
moves NNE into southern IN/southwest OH from mid to late afternoon.
Then a second round of storms looks to get going over western TN
during the late afternoon hours and push northeast into central KY
and possibly even southern IN this evening and into the overnight
hours.  While it`s hard to pin down exact timing on rounds of storms
today, potential for strong to severe weather does exist as areas
that see a good period of warming should become moderately unstable.
While shear profiles aren`t the greatest, they are sufficient for
multicell clusters and will be maximized this evening just ahead of
the cold front.  The main storm threats today will be damaging
winds, hail, and torrential rainfall.  Areas that received heavy
rainfall yesterday like Washington County, IN and Allen County, KY
will be most susceptible to flooding today if they receive more
heavy rainfall.  Also, multiple boundaries in the area could create
a small tornado threat.

Rounds of showers and storms will continue late tonight through Sun
before moving east of the area late Sun.  While severe weather is
not expected Sun, wouldn`t be surprised to see a strong storm or two
east of I-65 on Sun with strong winds, small hail, and torrential
rainfall.  Will need to continue to watch flood prone areas that
receive multiple rounds of storms through Sun.

Temperatures today should range through the 80s for highs with an
isolated 90 degree reading not out of the question.  Tonight expect
lows in the 60s. The post-frontal cooler airmass will begin to enter
the region early Sun limiting highs to the 70s.

.Long Term...(Sunday night through Friday)
Issued at 335 AM EDT Fri May 19 2017

Expect a dry day Monday with sfc high pressure moving through the
region.  Mostly sunny skies and highs in the low 70s will make for a
pleasant start to the work week.

However, this won`t last long as our active pattern gets going again
for Tues.  An upper trough and cold front will move through the
region mid week bringing more chances for showers/storms Tues/Wed
and maybe even Thu depending on it`s exact evolution.  Temperatures
will be below normal with the strong trough over the Midwest.  Highs
Wed/Thu will be limited to the 60S!

The upper level pattern is expected to deamplify by the end of the
week with temps rebounding into the 70s for highs.  Precipitation
chances for Fri/Sat are low confidence as there hasn`t been good run
to run consistency amongst models. Still will carry 20-40% chances
for rain late next week.

&&

.Aviation...(18Z TAF Issuance)
Issued at 120 PM EDT Sat May 20 2017

Have some boundaries west of a BWG/SDF line that should initiate new
storms this afternoon. As activity should be scattered over us, have
gone with vicinity wording starting around 20Z. The threat will
continue in that scattered range for several hours before becoming
more showery overnight. Winds will shift from south and
southeasterly to southwesterly late in the period, with a period of
fuel-alternate MVFR possible Sunday morning.

&&

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

$$

Update...EBW
Short Term...AMS
Long Term...AMS
Aviation...RJS


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