Area Forecast Discussion
Issued by NWS Louisville, KY
FXUS63 KLMK 200746
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
346 AM EDT Wed Aug 20 2014
.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 (06Z TAF Update)...
Issued at 110 AM EDT Wed Aug 20 2014
The main TAF concerns will be any light br this morning and
convection timing. First of all, the convection in the area early
this morning looks to impact perhaps the LEX TAF with a rain shower
over the next few hours before moving out of the region. Will need
to keep an eye on the convective complex over central Indiana as it
slowly makes progress southward toward SDF/LEX this morning. It is
weakening and most short range models do not have it making it into
these TAF sites early this morning so will not include in the
SDF/LEX TAFs at this point although will watch closely.
The next concern is light br formation around sunrise. Went ahead
and backed off of vsbys some with this TAF issuance as the mixy
atmosphere with a 20 kt LLJ expected during the pre-dawn hours
should limit fog density. Will hold BWG/LEX at MVFR conditions
around sunrise and SDF at VFR.
This evening another convective complex is expected to develop to
our northwest and dive south/southeast into the area. Ahead of this
complex of storms, additional cells may form over central KY. Felt
confident enough to include VCTS at the TAF sites for this issuance
as models have been persistent with this round of convection.
However, timing may need to be adjusted throughout the day today.
Winds will remain out of the SSW through the TAF period generally 7
kts or less except in t-storms.