Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 282116
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
516 PM EDT Fri Apr 28 2017

...Mesoscale Forecast Update...
Issued at 511 PM EDT Fri Apr 28 2017

Convection is struggling at the moment due to 750mb inversion
evident on latest AMDAR soundings, and marginal mid level lapse
rates. This first round of showers and a few storms is being driven
by a subtle wave moving up out of the lower Ohio River Valley. Given
that the cap and marginal lapse rates are likely inhibiting
convection a bit, severe weather doesn`t appear to be imminent just
yet.

As we move toward the 22-23z time frame, we are watching as
secondary subtle wave coming out of southern MO that could be the
focus for more intense convection across southern IL/western KY
where strong moisture convergence is occurring. In addition, a
noticeable instability axis also feeds into this area, and would be
the best bet to break the cap over the next couple hours. If this
occurs, expect that convection would intensify as it moves into our
CWA as we move into the evening hours. Will be monitoring trends
over the next couple of hours.

.Short Term (Now through Saturday Night)...
Issued at 215 PM EDT Fri Apr 28 2017

...Severe Storms Forecast for Late this Afternoon through Tonight...

Have a nice clear slot over southern KY early this afternoon,
allowing for a nice pocket of instability for the warm front that`s
lifting quickly northward now to take advantage of. Expect storms to
start developing in the next hour or two with an increase in
coverage especially along the front this evening. Forecast soundings
continue to increase confidence in supercells developing in this
environment, with large hail and damaging winds and a tornado
threat, especially along the warm front. This evening, NCAR ensemble
0-3 km storm relative helicity was over 300 m2/s2 with 0-1 km shear
vectors around 40 knots, indicative of good low-level rotation
potential. These values would satisfy a local study on potential for
tornadoes developing within supercells.

Tonight, we could see a brief break in precip before they redevelop
again as a low-level jet focuses convergence somewhere over or just
north of the Ohio River. This new development and its potential
persistence was the main reason a Flash Flood Watch was issued
earlier today. Precipitable waters in the 1.7-2 inch range will make
for very heavy rains coming out of these storms, so rainfall totals
around 3 inches are possible, if not a little more, over the watch
area by this time tomorrow.

By late morning Saturday, we should see those rains taper off,
especially as the warm frontal boundary looks to lift a little
farther north. By afternoon, we should get another pool of surface-
based CAPE. Near record temperatures are forecast for highs, and
even warm lows (see Climate section below). A cap should be in place
tomorrow though, that will keep most from taking advantage of it.
The GFS and WRF-ARW are the only models showing any QPF in our area.
Some of the better performing models for rain chances the last few
months would argue for a narrow stripe of isolated to scattered
storms across our northern counties in Indiana (i.e., Dubois to
Jefferson) and a secondary isolated area over Lake Cumberland.

.Long Term (Sunday through Friday)...
Issued at 215 PM EDT Fri Apr 28 2017

...Another Round of Heavy Rain and Some Severe Potential Late
Sunday...

After another warmer morning Sunday, again potential breaking warm
min records, we should be dry for most of the day. Models are in
pretty good agreement on this scenario, bringing in storms ahead of
a north/south-oriented cold front Sunday night. Given another near
record warm day, these storms should have plenty of available
instability to allow for strong to severe potential at least in the
evening hours, perhaps late into the overnight as well. Precipitable
water levels once again will surge into the 1.7-2 inch range, so
another round of flash flooding is possible.

Monday looks like a chillier day, as a large upper low moves into
the Great Lakes. We`ll have southwest winds, but much lower low-
level thicknesses, to bring temperatures to below normal for highs.
The cooler weather likely will persist most of the week, as that
upper low slowly heads northeast and then gets replaced by another
meridional trough for late week. We may see some cool precip with
this transition from mid to late week. Did not make any changes to
the blended forecast for now.

&&

.Hydrology...

Issued at 1100 AM EDT Fri Apr 28 2017

Heavy rain is expected over the weekend over much of the middle
Ohio Valley. Between two to five inches of rain are expected along
the Ohio River over the next 72 hours, with the heavier amounts in
the west. This will lead to quick rises on all stream and rivers in
the area with minor flooding possibily in the Green, Rough, and
Muscatatuck basins. In addition, heavy rain tonight could lead
to flash flooding in parts of southern Indiana and northern
Kentucky. If you live in a flood prone area, watch water levels
closely this weekend.n&&

.Aviation (18Z TAF Update)...
Issued at 115 PM EDT Fri Apr 28 2017

Widespread mid and high-level clouds persist at this time over
southern IN and at SDF, while increasing low-level moisture over
south-central KY is leading to low-level cumulus cloud development
including at BWG. As low-level moisture surges northward this
afternoon, cumulus clouds will develop at all 3 TAF sites. Winds
from the SE to S will slowly increase this afternoon and evening to
10-20 kts.

Latest high-resolution models suggest scattered to numerous
thunderstorms developing by mid or late afternoon over parts of
central and western KY, then moving quickly to the northeast through
early evening. Given that no storms have currently developed, it is
hard to determine areal coverage and exact timing. Thus, have kept
only VCTS in all TAFs until better delineation can be made later
this afternoon. Any storm could produce strong wind gusts, different
from the prevailing wind in the TAF, hail, and brief heavy rain.

For tonight, storms should move off to the northeast, and there
should be a pause in activity. Later tonight after 05 or 06z,
nocturnal convection is expected close to and north of a warm front
which should lie out near the Ohio River or in southern IN. Storms
could affect SDF again, but are expected to stay mainly north of BWG
and LEX, except for a possible stray storm. Southerly winds may gust
over 20 kts for a time this evening/tonight.

On Saturday, convection will lift north of SDF, with all 3 TAF sites
remaining dry along with a southerly wind of 10-15 kts. Cant rule
out an isolated storm or two in the afternoon, but not included in
the TAFs at this time do their isolated nature, if at all.

.Climate...
Issued at 830 AM EST Fri Apr 28 2017

Temperature and rainfall records for the weekend:

            4/29 Warm L   4/29 Record H   4/30 Warm L  4/30 Record H
Louisville   67 (1951)      89 (1899)      70 (1899)    91 (1894)
Lexington    67 (1899)      86 (1899)      70 (1899)    91 (1942)
Bowling G.   67 (1899)      91 (1894)      68 (1899)    92 (1942*)
Frankfort    63 (1899)      89 (1914)      66 (1910)    91 (1942)

            4/29 Rainfall 4/30 Rainfall
Louisville   2.02" (1927)   2.37" (1983)
Lexington    1.31" (2014)   3.21" (1909)
Bowling G.   2.40" (1912)   3.00" (1911)
Frankfort    2.20" (2002)   2.62" (1909)

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...Flash Flood Watch through Saturday morning for KYZ023>025-
     029>038.

IN...Flash Flood Watch through Saturday morning for INZ076>079-083-
     084-089>092.

&&

$$

Mesoscale......BJS
Short Term.....RJS
Long Term......RJS
Hydrology......CMC
Aviation.......TWF
Climate........RJS



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