Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 250734
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
334 AM EDT Thu May 25 2017

.Short Term...(Today through Friday)
Issued at 305 AM EDT Thu May 25 2017

A deep elongated trough aloft with two low centers, one roughly over
southeast IN and the other over northern AL, dominates the weather
early this morning and today. A persistent area of rain associated
with the northern low and deformation zone was nearly stationary
over southern IN into northern KY (including Trimble and Henry
counties) earlier, but has now started to move southeast. This is
the area where the Flash Flood Watch is in effect until 11 am EDT
today. Currently, there are patches of moderate rain, but no
thunderstorms or persistent heavy rain, and none are expected. No
significant flood problems have been reported. Farther east within
the Watch over east-central KY, precip has been more spotty and
light so far.

For today, short range models show that this whole system will
translate eastward. This will sweep the area of rain southeast over
the northern and eastern parts of central KY. Areas of moderate rain
will continue with additional rainfall amounts up to around 0.5 inch
over the Bluegrass area. Overall, the existing Flash Flood Watch
does not appear justified at this time, and will cancel it. Any
flood problems at all with the rain will be local ponding and low
spots, not flash flooding.

By late today with the system moving into the upper OH Valley and
mid Atlantic States, rainfall will diminish over our eastern
forecast area, while some breaks of sun may occur over western
counties in southern IN and central KY. High temperatures will only
be in the lower to mid 60s east with the clouds and rain, and around
70 or lower 70s west with some late day sunshine.

Tonight, the trough pulls well to our east as ridging aloft builds
into the OH Valley. Therefore, a clearing trend will occur from west
to east with lows by Friday morning mainly in the lower and mid 50s.

Friday will be partly sunny and warmer with afternoon highs in the
upper 70s to mid 80s. The air mass will become somewhat unstable in
the afternoon as lapse rates aloft steepen. Isolated to scattered
thunderstorms may develop in the afternoon mainly just north of our
forecast area over IN near a warm front, but our area should remain
dry for the most part during the daytime hours as model soundings
show capping to preclude much development other than possibly an
isolated storm or two. Additional storms could develop Friday night
as discussed below.

&&

.Long Term...(Friday night through Wednesday)
Issued at 315 AM EDT Thu May 25 2017

...Severe Weather Potential Late Saturday and Sunday of the
Holiday Weekend...

An active and potentially volatile weather pattern may be setting up
for late Saturday into Sunday for the lower OH Valley. The pattern
suggests one or more bowing mesoscale convective systems (MCSs) are
possible.

Friday night...

Model soundings show good elevated instability, especially over our
northern counties. Models vary somewhat, but suggest that a cluster
of thunderstorms/MCS could occur near or just north of our forecast
area in IN into OH associated with isentropic lift/warm advection. A
tail of scattered thunderstorms could occur to the south in the
northern half of our area late Friday night. If this occurs, then
hail may occur with the stronger cells given steep mid-level lapse
rates, along with brief heavy rain. However, severe weather is not a
major hazard.

Saturday and Saturday night...

Model guidance varies during this time frame, and the weather is at
least somewhat dependent on potential convection Friday night and
any leftover boundaries. Nevertheless, models have been consistent
the last few runs showing potential development of strong
instability during the day, capped at first as shortwave ridging
builds into the area. However, the cap will erode during the day as
conditions become warm and humid (dewpoints rising to near 70 or
lower 70s in afternoon). Mid-level lapse rates remain quite steep
given some dry air aloft, and low-level lapse rates steepen with
moistening during the day. Model instability values include lifted
indices around -10 or -11 and mixed layer CAPE over 3000 J/kg (most
unstable CAPE at least 4000). These values will depend highly on low-
level moisture values (i.e., less moisture means less instability).

At the same time, westerly a mid/upper-level jet streak is progged
to cut eastward from the mid MS to lower OH Valley during the day
and evening with SW 850 mb flow. The resulting vertical wind profile
is one that is often associated with a forward propagating MCS with
embedded bowing segments that could produce a swath of potential
wind damage where the MCS develops and tracks. Model RH and QPF
progs suggest such an MCS as well, initially developing in the mid
MS Valley just north of the jet steak then driven ESE into our area.
Model low-level shear values are not overall impressive, and with
steep lapse rates and probably robust downward CAPE (DCAPE), this
would suggest an outflow-dominated MCS (assuming one develops) with
strong winds and hail as primary hazards, along with lots of
lightning.

We looked at CIPS analogs for this pattern, and a mean composite for
the top 15 analogs suggest an axis of heavy rain and severe weather
roughly from St. Louis to Louisville (but with definite spread in
individual past events). Many of the reports were wind and hail.

Model timing differs for sure, but agree that sometime late Saturday
into Sunday morning could be potentially quite active across parts
of the OH Valley, including our forecast area.

Sunday...

Convection Saturday night into Sunday morning will complicate
potential redevelopment Sunday afternoon along an actual synoptic
cold front, with any outflow boundaries playing a role as well.
However, models suggest more destabilization and at least scattered
thunderstorms again, with isolated severe storms possible. Hard to
pin down detail of Sunday at this time.

Monday though mid next week...

Progressive shortwaves will move east through the Great Lakes region
with another chance or two of scattered showers and thunderstorms.
However, model spread increases resulting in a lower confidence
forecast. Severe weather potential will be less during this period,
which is good news. Highs will be in the mid 70s to lower 80s.

&&

.Aviation...(06Z TAF Issuance)
Issued at 153 AM EDT Thu May 25 2017

An expansive upper low is currently situated from the Great Lakes
down to the northern Gulf states. More specifically, a deep low
pressure centered is noted north of CVG. Showers will continue to
rotate around this feature early this morning and through early to
mid afternoon before it lifts east later today. Showers will be most
prevalent at SDF/LEX later this morning and early afternoon, with
only VCSH noted at BWG until midday.

The bigger impacts will be with low ceilings hovering on the
IFR/MVFR threshold until early evening at SDF/LEX, before rapidly
improving. BWG should stay VFR until sunrise, before a brief period
of MVFR until midday, then back to VFR. Expect steady winds mostly
out of the west between 10 and 15 mph during this cycle. A few gusts
are possible later this afternoon.

&&

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

$$

Short Term...TWF
Long Term....TWF
Aviation.....BJS



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