Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 171650
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1250 PM EDT Sat Jun 17 2017

...Updated Aviation Discussion...

...Forecast Update...
Issued at 1105 AM EDT Sat Jun 17 2017

This afternoon/evening`s hot and humid but rain-free forecast
remains well on track in the absence of any forcing mechanisms for
convection. Main challenge continues to be what sort of remnant
convection will get into the Ohio Valley toward daybreak on Sunday.

Both synoptic and hi-res models are consistent in firing storms this
evening over central Illinois, but there is less agreement on how
well eastward extent will be maintained as they dive south after
midnight.

Slight risk extending into southern Indiana latest SPC Day 1 outlook
seems conditional on the eastern end of any MCS holding together,
which does not help confidence. Better moisture feed will be in the
western flank given SW flow near 850mb.

Gridded and text forecasts are in good shape other than minor hourly
tweaks. Have updated the Hazardous Weather Outlook to separate out
potential for gusty storms well after midnight tonight. Still looks
like prime time for strong/severe convection in the Ohio Valley
on Sunday will be late afternoon and evening.

.Short Term (Now through Sunday)...
Issued at 325 AM EDT Sat Jun 17 2017

...Another Warm & Dry Day Followed By Strong Storm Threat Sunday...

Currently, things are quiet across the CWA with the only concern
being some patchy fog toward dawn. Latest IR satellite imagery shows
convective debris clouds beginning to overspread the region from a
strong MCS over MO, and this could be a limiting factor to more
widespread and intense coverage. Otherwise, temps will run in the
the upper 60s to low 70s by sunrise.

Today, we`ll remain under a stagnant upper air pattern devoid of
mentionable features as a weak upper low stays stationary over the
Gulf Coast states, and a shortwave trough crosses the northern
Plains. With H85 temps in the 17-20C range, this will open the door
for a hot day with highs in the upper 80s and low 90s. Wouldn`t be
surprised if we go more solidly into the low 90s if the upper debris
clouds from the upstream MCS don`t limit heating too much.

A strong shortwave moves out of the northern Plains and into the
upper Midwest this evening, helping to generate another strong MCS
over the mid Mississippi River Valley. Models have been consistent
with this complex weakening as it approaches our CWA, however we may
have to deal with some remnant cold pool driven storms across our
west by dawn on Sunday. Will continue the chance pops, mainly west
of I-65 for this reason.

Sunday, the threat for some stronger and organized storms will
increase as we destabilize and move through the day. The biggest
challenge with the forecast will be how the overnight and morning
MCS debris clouds will potentially limit destabilization. Given
consistency in models showing enough clearing by afternoon, the
thinking remains that remnant outflow boundary will create a
focusing mechanism as the atmosphere becomes moderately unstable. In
addition, stronger forcing arrives ahead of the parent trough as we
get deeper into the afternoon and evening. Deep layer shear should
become strong enough for a few organized multicells and bowing
segments capable of a damaging wind threat. In addition, a few of
the strongest updrafts could support marginally severe hail. A Flash
Flood threat will also be a possibility as we move into Sunday
night, and will be discussed more in depth in the long term
discussion.

.Long Term (Sunday night through Friday)...
Issued at 320 AM EDT Sat Jun 17 2017

As we move into Sunday night, several rounds of strong to severe
convection will likely affect the forecast area.  Stronger belt of
westerlies will be found just to our north while a bit of a weaker
wind field will be seen across our south.  Nonetheless, ample
buoyancy from the afternoon will be in place result in a threat of
damaging winds and wet microbursts from the strongest storms that
develop.  In addition to the damaging wind threat, PWAT values will
continue to climb throughout the evening as moisture pools out ahead
of the approaching upper trough and surface front.  Plenty of low-
centroid storms will likely produce widespread 1-3 inch rainfall
amounts across the region with some isolated spots seeing more than
4 inches, especially if a couple of rounds of storms train over the
same region.  While the axis of heavy rainfall continues to shift
around within the latest guidance, a Flash Flood Watch may be
required across portions of our central and eastern sections in
upcoming forecasts.

Heading into Monday, most of the convection is expected to be to our
east.  Trough axis will be overhead early Monday and then shift
eastward as upper level ridging builds into the region.  The amount
of ridging remains in question.  Overall, the pattern looks to
remain more of a northwesterly flow pattern with several small
disturbances moving through with the progressive flow aloft.  This
will undoubtably making timing of PoPs rather difficult at this
time.  For now, the blend keeps slight chances of PoP in for the
middle to later half of the week.

Highs during the week will start off below seasonal normals with
readings in the upper 70s to the lower 80s for Monday and Tuesday.
Middle to upper 80s look come back around Thursday and continuing
into Friday.  Overnight lows will remain solidly in the 60s through
much of the week, possibly warming into the upper 60s to near 70 by
weeks end.

&&

.Aviation (18Z TAF Update)...
Issued at 1245 PM EDT Sat Jun 17 2017

VFR conditions and modest SW winds will continue this afternoon and
well into tonight. Decent cu field developing and should go to SCT
on a prevailing basis during the heat of the afternoon, with only a
brief ceiling possible around 4-5K feet. Seeing some gusts already
at SDF but those should stay below 20 kt.

Main challenge overnight is whether remnant convection from a MCS
that is expected to develop over northern Illinois will hold
together into Kentucky. Best chance for any showers/storms around
daybreak Sunday will be at SDF, but confidence remains too low to
include in the TAF. Most likely scenario is plenty of debris
cloudiness to create a mid-level ceiling.

Sunday afternoon convection will be beyond the scope of the LEX and
BWG TAFs, and confidence is still fairly low in the SDF planning
period. Still, will introduce VCTS late in the afternoon mostly as a
heads-up.

&&

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

$$

Update.........RAS
Short Term.....BJS
Long Term......MJ
Aviation.......RAS



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