Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 242258
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
658 PM EDT Wed May 24 2017

...Updated Forecast and Aviation Discussions...

.Forecast Update...
Issued at 540 PM EDT Wed May 24 2017

Surface low now centered over central Kentucky with warm front
boundary draped across the Bluegrass region. Main concern has been
tornado threat as cluster of storms originating out of middle TN
have lifted northeast through south-central Kentucky into portions
of the Bluegrass and eastern Kentucky. A stripe of better
instability thanks to earlier breaks in the clouds and enhanced
effective shear led to a few rotating storms which did produce some
large hail and damaging winds.

Believe the tornado threat is on the downward trend as we lose the
remaining instability this evening. However, the severe threat will
transition to a flash flooding as training convection slowly pivots
around the low pressure system over central Kentucky. Earlier storms
across Hart and Larue counties produced 2-2.5" of rain in a short
period. The thunderstorms are efficient rain producers with PWATs
running at 1.25".

Latest model guidance shows highest QPF axis through tomorrow
morning likely to setup across portions of southern Indiana and
north central Kentucky. HRRR and WRF ARW show convection pivoting
slowly across the area this evening and tonight. Additional amounts
of 1 to 2 inches not out of the question. Locally higher amounts
possible as well.

As a result, in coordination with ILN and IND, will issue a Flash
Flood Watch in effect now through mid morning Thursday for areas
mainly east/northeast of Louisville metro. Some of these areas
picked up quite a bit of rain last weekend and overall, Flash Flood
guidance values are low in these counties.

Updated products and graphics will out shortly.

&&

.Short Term (Now through Thursday Night)...
Issued at 315 PM EDT Wed May 24 2017

...Severe Storms Possible Until Early Evening East of I-65...

Closed upper low over the mid-Mississippi Valley is starting to fill
in, but a vort lobe lifting through the Tennessee Valley ahead of it
has really sharpened and may be taking over as the main player in
our weather. Surface reflection is a deepening 995mb low now lifting
through south-central Kentucky. Axis of marginal instability running
just west of the I-75 corridor, with backed low-level winds,
continues to support a severe threat that includes the potential for
brief tornadoes. Thus we remain under a Tornado Watch roughly east
of a line from Frankfort to Greensburg to Tompkinsville, though we
will spend the late afternoon hours trimming counties from the south
end of the watch as surface winds shift to NW.

Precip shield associated with the upper system will take a bit
longer to exit, with additional rainfall approaching an inch along
the I-65 corridor, though locally higher totals are quite likely in
the more convective activity to the east. Expect the pattern to slow
down due to some Fujiwhara effect between the two upper lows, and we
will end up with a stacked system somewhere near Cincinnati by
daybreak on Thursday. POPs tonight will taper from likely
along/north of I-64 to a low chance across south-central Kentucky.

Elongated north-south trofiness early on Thursday will support
scattered to numerous showers with the highest POPs across the
Bluegrass region, closer to the low pressure center. Precip chances
will taper down fairly sharply in the afternoon, and no mention of
thunder given a lack of instability. Clouds and precip will hold
temps in check, especially in the Bluegrass where we may not get out
of the 60s. Expect dry conditions, gradual clearing, and seasonable
temps Thursday night as shortwave upper ridging takes over.

.Long Term (Friday through Wednesday)...
Issued at 305 PM EDT Wed May 24 2017

Expect unsettled weather for much of the holiday weekend, bookended
by dry and pleasant days on Friday and Monday. Shortwave ridging on
Friday will keep us dry for most of the day, but a weak disturbance
passing just to our north will warrant a slight chance POP for
southern Indiana late in the day. A weak front will drop into the
Ohio Valley Friday night and hang up parallel to the flow aloft.

For most of Memorial Day weekend, the front will remain nearly
stationary over the Ohio Valley, and a series of disturbances will
interact with this front to spawn multiple rounds of showers and
storms. Best chances will be Sat night into Sun as an upper trof
digs into the Plains, and the better dynamics aloft could allow for
a few stronger storms as well. Cold front looks to pass through
Sunday night, with a mid level dry slot bringing drier conditions
into Memorial Day.

Another chance for showers arrives Tuesday, with a few afternoon
thunderstorms possible as a shortwave embedded in the parent trough
rotates through our region.

&&

.Aviation...()
Issued at 657 PM EDT Wed May 24 2017

Low pressure over north central Kentucky will move into southwest
Ohio overnight. Waves of showers and some thunderstorms will pivot
around this low, mainly impacting SDF and LEX through this TAF
period. Expect the steadiest rain at SDF and LEX through late this
evening, then a lull, before rain bands pivot back over the area for
Thursday morning.

Ceilings overall will begin to lower as saturation and cooling take
effect. Good consensus in the model forecasts that IFR ceilings will
encompass much of central Kentucky after midnight then linger
through Thursday morning. Improvement to MVFR then VFR expected late
in the period Thursday afternoon.

Winds will become westerly and generally stay in that direction
through the period. As the pressure gradient tightens Thursday late
morning and afternoon, wind gusts 20 to 25 kts are likely.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...Flash Flood Watch through Thursday morning for INZ077>079-092.

KY...Flash Flood Watch through Thursday morning for KYZ031>037-
     039>043-048-049.

&&

$$

Update...ZT
Short Term........RAS
Long Term.........RAS
Aviation...ZT


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