Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 251049

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

...Updated Aviation Discussion...

.Short Term (Now 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

...Organized 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 with wind damage.

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 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

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 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 to -12
and mixed layer CAPE over 3000 J/kg and most unstable CAPE over 4000
J/kg. 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 S to SW low-level flow. The resulting vertical wind
profile is one that is often associated with a fast forward
propagating MCS with bowing segments that could well produce a swath
of wind damage...potentially significant...where the MCS develops
and tracks. Model RH and QPF suggest an MCS initially developing in
the mid MS Valley just north of the jet steak which is driven ESE or
SE into our area by a strong cold pool. Model low-level shear values
are not overall impressive, and with steep lapse rates, robust
downward CAPE (DCAPE) is expected. This would suggest an outflow-
dominated MCS (where it occurs) with potentially widespread damaging
winds and large hail as primary hazards, along with lots of
lightning and brief but intense rainfall rates.

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
(mainly wind damage and large hail) roughly from St. Louis to
Louisville (but with definite spread among individual past events).

Model timing/location differs for sure, but agree that sometime late
Saturday into Sunday morning could be very active across portions of
the mid MS and lower OH Valley, including all or parts of our area.


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 details 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 (12Z TAF Update)...
Issued at 645 AM EDT Thu May 25 2017

An expansive upper low that is currently stationary just north of
CVG will start to slowly push east through the day. Showers will
continue to rotate around this feature this morning and through
early to mid afternoon before tapering from west to east later
today. Showers will be most prevalent at SDF/LEX through mid to late
afternoon, with BWG only seeing VCSH or a brief light rain shower.
Shower intensity should be light enough to only reduce visibilities
slightly, possible into the MVFR range at times. However, low
ceilings will drive the flight category as the hover on the IFR/low
MVFR threshold during the morning and afternoon. Ceilings gradually
improve from W to E late afternoon into the evening with a return to
VFR. BWG should return to VFR by mid to late morning.

Expect steady winds mostly out of the west between 10 and 15 mph
during this cycle. A few gusts around 20-25 mph are possible later
this afternoon.




Short Term........TWF
Long Term.........TWF
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