Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 200505
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
105 AM EDT Sat May 20 2017

...Updated Aviation Discussion...

.Forecast Update...
Issued at 631 PM EDT Fri May 19 2017

Strong to severe convection continues across mainly southern IN and
far northern KY at this time.  Areas along and north of the I-64
corridor will be at most risk for severe weather.  Current trends
suggest that strongest storms and highest severe threat will be
across the Bluegrass region of central KY (north of the I-64
corridor).  Storms will affect Scott/Bourbon/Harrison and Nicholas
counties over the next hour.  Some rotation has been noted in these
storms.  However, intense lightning and torrential rainfall may pose
a flooding risk in those areas.

South of the BG/WK Parkways, convection has subsided greatly.
Overall convective trends in this area suggest that just some light
showers will be possible this evening in that area.

A third area of convection will be watched across our far NW
counties of Dubois/Perry/Crawford/Orange counties where a NW moving
outflow boundary will intersect convection developing along the
actual surface front.  This activity may linger for a few more hours
until diurnal stabilization starts to gain momentum as we approach
sunset.

In addition to the severe convection, heavy rainfall will be a
concern across southern IN this evening.  Repeated training of
thunderstorms over the same area will result in some localized
flooding issues.  Areas at most risk for flooding look to be
Washington/Scott/Clark and Jefferson counties of southern Indiana.

&&

.Short Term (Now through Saturday Night)...
Issued at 345 PM EDT Fri May 19 2017

...Strong to Severe Storms Possible This Afternoon and again on
Saturday...

Convection continues to develop across the region this afternoon,
primarily along remnant outflow boundaries and along a sagging
surface cold front across southern Indiana.  Corridor of instability
was noted along either side of the Ohio River.  Current mesoanalysis
suggests 1500-2000 J/KG of CAPE along with sufficient DCAPE.  The
instability decreases along a gradient as one heads into southern
KY.  This is due to a large amount of convective overturning from
the morning MCS activity.  Temperatures were largely in the upper
70s to the lower 80s across southern IN and central Kentucky.  Some
higher readings were noted in far NW KY where skies had remained
mostly sunny and temps were in the mid-upper 80s.

For the remainder of the afternoon, expect convection to continue to
expand in coverage.  Greatest risk of severe weather looks to
remain mainly confined to southern Indiana and far northern KY where
stronger forcing and higher DCAPE values reside.  Scattered strong
to severe convection will be also be possible down in western KY,
west of the I-65 corridor.  Convection along and east of I-65,
especially in southern KY is questionable given the amount of
convective overturning that occurred.  However, with skies clearing
out, some uptick in instability may develop.  Convection is likely
to diminish with the setting sun as nocturnal stabilization of the
PBL takes place.

Most likely we`ll see a lull in convection late tonight and into
early morning hours.  However, most of the CAM guidance suggests
that convection will develop late tonight across MO and work into
western KY and southwest IN toward dawn.  Best precipitation chances
look to be well west of I-65 tonight, so plan on keeping low PoPs in
the east with higher PoPs out west.   Another mild night is on tap
with lows in the upper 60s to around 70 in most places.

For Saturday and Saturday night, forecast is rather dependent on
convective evolution late tonight.  Current thinking is convection
that is expected to develop over MO will move northeastward into
southern IL/W KY/SW IN Saturday morning and continue to move off to
the northeast.  Frontal boundary to the north is forecast to lift
back northward as a warm front, so the entire forecast area will
remain in the warm sector.  CAMs suggest some scattered development
throughout the day just given the expected diurnal instability.
However, lack of a forcing mechanism looks to keep any activity
rather scattered.  Any storm will be possible of producing damaging
winds, hail, and torrential rainfall.  Highs will warm into the mid-
upper 80s in most spots.

By Saturday night, surface cold front out to the west will surge
eastward and push a line of strong/severe storms through the region.
The line will be coming into the western parts of the region in the
evening where some residual diurnal instability will remain thus,
highest risk of severe weather will likely remain west of the I-65
corridor.  The storms will sweep eastward overnight with a likely
decrease in intensity as the storms head eastward.  Lows Saturday
night will cool into the mid-upper 60s.

.Long Term (Sunday through Friday)...
Issued at 400 PM EDT Fri May 19 2017

As we head into Sunday, some additional trailing showers and/or
storms will be possible as the surface cold front continues to move
eastward across the forecast area.  Current thinking is that
precipitation will largely end by Sunday afternoon/eve with drier
conditions expected for Monday and into early Tuesday.  Highs Sunday
will be cooler with lower-mid 70s expected and overnight lows in the
lower-mid 50s.  Highs on Monday will warm back in to the lower-mid
70s again.  Lows Monday night should cool back into the lower-mid
50s.

The upper level pattern will deamplify on Tuesday as yet another
deep upper trough axis swings in from the west.  This will bring
another round of showers and storms to the region.  Depending on the
instability and quality of moisture, some of these storms could be
strong to severe as well.  Something to watch over the coming days.
This upper trough axis will shift eastward and attempt to close off.
The trough axis will be overhead on Wednesday, likely resulting in
cloudy/cool/showery conditions across the region.  After highs in
the 70s on Tuesday, temps on Wednesday could struggle to get into
the lower 60s.  The blend has been coming down in recent days, so
even cooler readings will be possible.

The upper trough axis is then forecast to push off to the east with
a more zonal flow pattern developing by late week.   Highs Thu/Fri
will warm back in the upper 60s to the lower 70s. Though the
pattern will likely shift back to a southwesterly flow by late next
weekend with disturbances moving through within the flow. So as of
this writing, the Memorial Holiday weekend may start off dry, but
looks to turn stormy by the later half of the weekend.

&&

.Aviation...(06Z TAF Issuance)
Issued at 102 AM EDT Sat May 20 2017

The most challenging part of this TAF forecast will be when storms
will develop and if they will affect the terminals. Early this
morning a cluster of storms is moving across the Bluegrass region of
KY. This looks like it should stay south of LEX.

For the remainder of this morning, the latest short term models show
storms firing along a boundary across north central KY. These storms
could impact SDF and LEX between 15-21Z or so. There then looks to
be a break in the afternoon, with another round of storms developing
in the evening hours and spreading east overnight. Will carry VCTS
for this potential.

Winds will generally be out of the south to southwest through the
day outside of any thunderstorms. Some light fog will be possible
this morning at BWG.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...None.
KY...None.
&&

$$

Update...MJ
Short Term.....MJ
Long Term......MJ
Aviation...EER



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