Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 201737
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
137 PM EDT Wed Aug 20 2014
...Updated Forecast and Aviation Discussions...
Issued at 135 PM EDT Wed Aug 20 2014
Complex across central Illinois is holding together fairly well
early this afternoon. Have a pretty thick canopy of cirrus out ahead
of the storms, but it may hold together enough as it moves into our
forecast area. High-res models are not too gung ho about rain
chances in this area this afternoon, but with a very unstable
surface-based atmosphere in our region, SPC mesoanalysis has 4-6 k
CAPEs, just cannot rule out storms developing/maintaining. Have
shifted the highest pop axis a little south, to a Huntingburg, IN to
Danville, KY line. Will continue to watch incoming pattern for
updates through the afternoon.
Issued at 1135 AM EDT Wed Aug 20 2014
Partly cloudy skies were noted across southern Indiana and central
Kentucky this morning. Temperatures ranged from the upper 70s to
near 80- in the east/northeast with lower 80s generally in the I-65
corridor and points west. From the KY Mesonet data and visible
satellite imagery, there appears to be boundary draped across the
southern part of the forecast area. This evident in the dewpoint
readings across southern KY which are in the middle 70s with only
upper 60s to lower 70s across our NE sections.
A decaying MCS over central IL will continue to spread mid-high
level cloudiness down into mainly western KY this afternoon. These
clouds along with the boundary in the region may serve as a focus
for convective development later this afternoon. The high
resolution convection allowing models have vastly different
solutions. The latest runs of the HRRR keep much of our region dry
thought the day with convection mainly developing out to our west.
While the NSSL WRF ARW runs develop convection up near CVG and drop
it SE into WFO JKL`s region. The mixed signals in the guidance make
this forecast particularly challenging this afternoon.
For now, have kept the current forecast on track. Current thinking
is that convection will probably start to develop over the next few
hours along the left over boundary...given multiple convectively
induced lifting mechanisms cross the region. Damaging winds and
perhaps some large hail will be possible with the storms this
afternoon as moderate instability combined with around 30 kts of
WNW flow aloft will be in place across the region.
.SHORT TERM (Now through Thursday)...
Issued at 340 AM EDT Wed Aug 20 2014
A complex short term forecast is in store with multiple rounds of
strong to potentially severe convection possible as potent
shortwaves topple a central CONUS ridge and dive southeast into the
Ohio Valley. In addition, the eastern edge of a warm front will
slowly lift north over the region over the next few days providing
focus for convection. Also left over boundaries from storms
will provide focus for renewed convection over the next few days
with convective activity peaking during the afternoon and evening
hours during best instability.
As of 315am this morning, convection over the Bluegrass region was
causing mainly very heavy rainfall and lots of lightning. A flood
advisory is effect for northern Harrison County KY through 6am this
morning due to training storms over that area. The Harrison County
mesonet has received 1.34 inches of rain so far with impressive
rainfall rates which may cause minor flooding issues.
Think that this complex of storms will move east of the area within
the next 1-2 hrs and we may see a dry period through the morning
hours. Expect a left over boundary from convection over central
Indiana yesterday evening to settle somewhere near the Ohio River
over portions of southern Indiana and north central Kentucky.
Believe this boundary will be key for redevelopment of convection
Most short range models indicate a shortwave will dive south into
the area this evening. Ahead of this wave, expect the atmosphere to
become very unstable with only partly cloudy skies in the wake of
morning convection. Bulk shear will increase ahead of the wave as
early as 1-3pm this afternoon. This is when we may see convection
begin to fire in the region of the left over sfc boundary.
Convection should be enhanced in strength and coverage during the
evening hours as the shortwave passes through. It is a bit unclear
how long into the evening and overnight hours convection will
continue. It may survive for a prolonged period of time if a good
cold pool and outflow boundaries get going. At some point late
tonight into tomorrow morning expect a break in showers/storms.
Another notable shortwave looks to impact mainly northern KY and
southern Indiana with storms for Thurs afternoon/evening as well.
Models generally do not have a good handle on ongoing convection and
evolution so confidence is rather low on exact timing. However,
model soundings indicate increased bulk shear values with each wave
and excellent instability so storms will likely be strong to severe
in nature today through Thurs with damaging winds, large hail, and
torrential rainfall all possible. PWATs will hover in the 1.6-2
inch range resulting in some heavy rainers and potential flood
As for temperatures, think that highs will range from the upper 80s
to lower 90s today and tomorrow with locations west of I-65 likely
seeing max heat indices near 100 each afternoon. Low temperatures
tonight will be mild in the lower 70s.
.LONG TERM (Thursday night through Tuesday)...
Issued at 311 AM EDT Wed Aug 20 2014
Thursday night through Tuesday...
Expect a hot spell throughout the entire extended period, at least
through the middle portion of next week.
In an upper air pattern that we`ve not seen much of this summer,
strong 500mb ridging will develop initially across the lower
Mississippi Valley by Thursday evening, with longwave troughs
located over the Pacific Northwest and New England. The wavelength
of this trough-ridge-trough pattern will lead to a stagnant pattern.
The GFS for example holds this ridge in place through at least late
Monday, building it northwards towards the Commonwealth. We should
expect probably a week`s worth of hot sticky humid weather with
highs well into the 90s and warm overnight lows in the lower to mid
70s over the weekend and into the middle of next week. Rising Heat
Indices may be a concern for early next week.
In this type of pattern, shortwaves moving along the periphery of
500mb ridging can bring episodic convection. Initially, Thursday
night through Sunday, scattered thunderstorms may develop within a
northwest flow pattern along the northeastern edge of this building
ridge. This would favor southeastern Indiana, southern Ohio and the
northern Bluegrass for possible scattered thunderstorms,
concentrated during the afternoon and evening hours. Southwestern
Kentucky, closer to the warm temperatures aloft associated with this
ridge, will likely stay dry.
By Monday and Tuesday, think the entire region will be dry, as
ridging builds northwards into the Commonwealth.
.AVIATION (18Z TAF Update)...
Issued at 125 PM EDT Wed Aug 20 2014
Confidence in convective forecast this period is low, as cirrus
shield from storms to our northwest quickly moves into the region.
Have some cumulus developing over the central forecast area this
hour, and SPC mesoanalysis indicates plenty of instability across
the region. However, the cirrus canopy should provide some
subsidence to inhibit convection. There is still a possibility that
the storms to our northwest can remain organized enough to bring
those storms to the terminals later this afternoon. Thus have gone
with vicinity storms over a larger period of time and will update as
the afternoon progresses. May even get into another round of precip
later tonight, as additional disturbances move in from the
northwest, but a lot will depend on what happens this afternoon. Fog
forecast for Thursday morning will depend on that as well. Given
very high dewpoints across BWG this hour, have enough confidence to
go with a period of MVFR conditions, as in current guidance.