Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 210706

Area Forecast Discussion
National Weather Service Louisville KY
306 AM EDT Fri Sep 21 2018

.Short Term...(Today and tonight)
Issued at 259 AM EDT Fri Sep 21 2018

This morning is looking to start off relatively quiet before things
get potentially busy this afternoon. A cold front currently in the
Midwest will race toward the lower Ohio Valley, but not make it into
our area until late this afternoon and evening. Several models
indicate storms could fire off by 18z along old outflow and
mesoscale boundaries well ahead of the cold front, though the timing
and placement of these storms remains difficult to forecast this far
out. There is higher confidence that we`ll see a broken line of
storms develop near the cold front late this afternoon, impacting
southern Indiana and north-central Kentucky around or after 21z.

While deep layer shear isn`t particularly strong, it`s nothing to
dismiss either (0-6km bulk shear ~30kts). When you take into
consideration that storms will be firing off during peak heating,
there will be plenty of instability to work with ahead of the front
(MLCAPE between 1500-2500J/Kg). The combination of shear and
instability should be enough to support some strong to severe
storms, mostly in the form of multi-celluar clusters or broken
lines. For what it`s worth, the 00z 3km NAM and HREF ensembles
indicate the potential for one or two embedded supercells within a
line, which isn`t too unreasonable given that 30kts of deep layer
shear is borderline supportive of rotating updrafts. The main threat
with any strong/severe storm would be damaging winds, followed by
hail and heavy rainfall. PWATs are forecast to approach 2" ahead of
the cold front, and localized flooding issues may arise where storm
training occurs.

Models weaken the storms significantly between 00z-03z as
instability wanes after sunset. Winds will switch out of the north
behind the cold front, advecting in some refreshingly cooler, fall-
like air to start off the first day of autumn.


.Long Term...(Saturday through Thursday)
Issued at 255 AM EDT Fri Sep 21 2018

Active weather pattern will set up for the weekend and at least the
first half of next week. High confidence that several waves of
showers and storms will move through the area as a front becomes
hung up over the Ohio Valley, but confidence in the timing and
placement of the heaviest rainfall is substantially less. Highest
rainfall amounts over the coming week could push 4 inches, with the
greatest chance over south-central Kentucky. Even if it happens
steadily over the course of a week, that will likely result in rises
on some area rivers, and especially on some of the smaller creeks
and streams.

By Saturday morning the cold front will hang up somewhere near the
Kentucky/Tennessee border, with most of Kentucky in the shallow cool
and moist air trapped under a frontal inversion. Weak disturbances
will eject out of an upper trof over the southern Plains, producing
waves of mainly stratiform rain with no more than isolated embedded
thunder. Best rain chances will be across south-central Kentucky,
but another band could set up near the Ohio River, with some
potential for heavy rainfall due to a training setup. WPC Excessive
Rainfall outlook has a slight risk in Kentucky west of I-65, and
marginal elsewhere. Similar regime remains in place on Sunday,
perhaps with a bit more southward shift in the precip chances.

By Sunday night the flat upper flow backs SW, allowing the stalled
front to lift northward across the Ohio Valley. Broad southerly flow
on Monday could allow for decent coverage of precip area-wide. For
Monday night and Tuesday the focus for showers and storms shifts
north and west, mainly into southern Indiana as another front

The next approaching front will push through Tue night/Wed, bringing
numerous showers and storms to the area. Could have a few strong
storms, and this pattern will bear watching for whether the tail end
of a squall line could again impact the Ohio Valley. For Wednesday
night and Thursday, we`ll finally see lower humidity and a day
without rain throughout the Ohio Valley as the front pushes far
enough south and east to give way to a bubble of Canadian high


.Aviation...(06Z TAF Issuance)
Issued at 101 AM EDT Fri Sep 21 2018

Forecast soundings and VWP profiles reveal a 25-30kt LLJ ongoing
near 2000ft at the start of the forecast period. Latest model data
indicate this LLJ will remain around these values through the
morning, and with surface winds near 10kts at TAF sites, LLWS should
be marginal and not need mentioning in the TAF.

A cold front will approach from the northwest late this afternoon
and evening. While there is excellent agreement amongst models that
a broken line of storms will develop along the cold front, there
will also be potential for storms to develop ahead of the cold front
early this afternoon. Because confidence in the timing and existence
of these early afternoon storms remains low, VCTS has been used at
all TAF sites for storm potential.

Once the cold front clears the region, winds will switch to the
north and drier air advecting in should allow a break from the




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