Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 261903

Area Forecast Discussion
National Weather Service Louisville KY
303 PM EDT Fri May 26 2017

.Short Term (Now through Saturday Night)...
Issued at 245 PM EDT Fri May 26 2017

...Significant Damaging Wind Possible Saturday Night...

Shortwave ridging aloft is building a strong mid-level cap over the
Ohio Valley, keeping us high and dry even as low-level southerly
flow develops behind a departing surface high. An MCV to our north
continues to ride eastward across northern Illinois, and remains the
feature to watch for tonight, with the main question being how far
southward the impending MCS is able to dive. Hi-res models suggest
storms will stay along and north of Interstate 74 as the cluster
pushes across Indiana and into Ohio overnight, but with the warm
front hung up nearby and a 30-40 kt low-level jet, it`s difficult to
rule out precip completely.

Saturday forecast is quite tricky, as sfc dewpoints crank up near
70F in a boundary layer of limited depth beneath the mid-level cap.
This will make the air mass quite unstable with CAPE approaching if
not exceeding 4000 J/kg, and convection to our north tonight could
send boundaries southward into our area. We`ll continue to carry
scattered POPs during the day but this still seems on the generous

Forecast confidence regarding potential severe storms has not
really changed in this cycle, as models are not coming into any
better agreement. Confidence is moderate to high that a complex of
severe storms will develop over Missouri on Saturday afternoon, with
a fast-moving cold-pool-dominated MCS rolling ESE across the Ohio
Valley during the late evening and overnight hours. Timing has
trended a bit slower as convective initiation may be farther west or
perhaps southwest. Downdraft CAPE well over 1000 J/kg deep into the
evening will support a significant wind threat, and mid-level lapse
rates are steep enough for large hail. Brief intense rainfall rates
could also result in localized flash flooding.

Much lower confidence in the timing and placement, as it is
dependent on Sat afternoon convective initiation to our west. Slower
trends in the models are noteworthy, but may be too slow given the
likely acceleration once the cold pool is established. Also, if
initiation is too far south and west in Missouri, the MCS could dive
mostly into the Tennessee Valley and only catch western and south-
central Kentucky, but for now the entire CWA remains under the gun
for severe weather Sat night. These trends won`t likely be refined
until storms fire on Sat afternoon.

.Long Term (Sunday through Friday)...
Issued at 245 PM EDT Fri May 26 2017

...Another Round of Severe Storms Possible Sunday Afternoon...

Warm and humid air mass will remain in place on Sunday, with
continued strong instability supporting severe potential. Limiting
factor in forecast confidence is what actually happens with Saturday
night convection, but suffice to say it will be a boundary-rich
environment, also with a synoptic front moving in during the
afternoon. Expect convection to be locally initiated and start early
in the afternoon given all the boundaries and a weaker mid-level
cap. Mid-level westerlies near 40 kt and strong downdraft CAPE will
support mainly a damaging wind threat, but at this point we still
expect less organization to the convection overall. A secondary
threat will be heavy rainfall, which could cause localized flash
flooding if any locations see multiple downpours.

A bubble of high pressure will build in Sunday night, with picture
postcard weather expected on Monday. Temps will run near climo, with
dewpoints only in the 50s.  The dry and pleasant weather may not
last long, as a broad upper trof sets up over the Great Lakes
by late Tuesday. We`ll continue to roll with a dry forecast for
Tuesday, but 20-30% POPs will creep back into the forecast from
Wednesday onward as a series of disturbances will rotate through the
broader trof.


.Aviation (18Z TAF Update)...
Issued at 123 PM EDT Fri May 26 2017

VFR conditions are expected through this afternoon and evening under
the influence of upper level ridging. For tonight, a system will
pass by to the north of the region. The main batch of precipitation
from this system should not affect the TAF sites, but some trailing
showers could move through overnight, so will put in VCSH to cover
this. In addition, low level moisture will be on the increase. Most
of the short term guidance has ceilings building down to fuel
alternate MVFR by early tomorrow morning. It is possible that a
brief period of IFR cigs may develop, but confidence is lower on
this, so will not mention it in the forecast at this time.

Ceilings should slowly improve through the morning into mid day
tomorrow with VFR cigs expected by the afternoon. Winds will be out
of the southwest through tonight, shifting to a bit more west-
southwesterly by tomorrow afternoon.




Short Term........RAS
Long Term.........RAS
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