Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 251052
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
652 AM EDT Sat Apr 25 2015

...Updated Aviation Discussion...

...Forecast Update...
Updated 621 AM EDT Sat Apr 25 2015

No major changes needed to the forecast this morning, as it is
largely on track.  Still expecting rain showers to continue to
blossom over the region, especially along and north of I-64 this
morning.  This is all in response to a low-level jet and isentropic
ascent veering into the region.  May see a few rumbles of thunder
especially as we get into the late morning hours, but no severe
weather is expected through noon.

The 06Z NAM has come in further north with the warm frontal
placement late this afternoon, which would bring the I-64 corridor
more into play for severe potential.  However, the latest runs of
the HRRR continue to show the warm front stalling south of I-64 and
very close to the current forecast position.  Given the HRRR has a
decent handle on ongoing convection, and the support of the other
hi-res guidance in regards to expected frontal position this
afternoon, see no need to adjust things further north based on the
06Z NAM quite yet.  Will continue to monitor trends through the
morning hours and especially the 12Z guidance suite.

.SHORT TERM (Now through Saturday Night)...
Issued at 345 AM EDT Sat Apr 25 2015

...Severe Weather Likely This Afternoon and Evening...

Overview
========
The current synoptic setup features a split-flow regime across the
CONUS.  The Ohio Valley will be affected by a PV anomaly currently
across eastern Kansas and western MO, which will push through the
region late this afternoon into tonight.

In response to the ejecting PV anomaly out of the southwest flow
aloft, a surface low has formed across portions of eastern KS.  This
low will slide east today, but will begin to fill as the overall
system encounters confluent flow over the Ohio Valley.  A very sharp
warm front associated with this low pressure system will develop
across the Ohio Valley today.

This Morning
============
With the ejecting wave, isentropic ascent has increased across the
region this morning.  This has led to showers breaking out across
the Ohio Valley, and the radar should continue to blossom through
the morning hours as a low-level jet of 40-50 knots veers into the
region towards sunrise.  Forecast soundings show limited elevated
instability, so severe weather is not expected.  Just some showers
with a few embedded rumbles of thunder appears likely.

This Afternoon/Evening
======================
The placement of the aforementioned sharp warm front will be
absolutely critical to how far north the severe weather threat gets
this afternoon/evening.  The 25/00Z guidance shifted this front
south just a bit from previous runs, which would take much of the
I-64 corridor out of the main severe risk.  Will cautiously trend
the forecast towards this more southern solution given the support
on almost guidance members, but still am a bit weary this front
could end up surging further north, especially if there is less
showers/storm coverage this morning than expected (less convective
enhancement to the front).  This warm front will be the main player
that we will continue to monitor through the day.

At this point, expect the front to align itself roughly along a line
from Owensboro, to north of Elizabethtown, over towards Lebanon
by late this afternoon.  To the south of it, a warm and unstable
airmass is likely to materialize given temperatures rising to near
80 and dewpoints in the middle 60s.  This low-level environment
coupled with very steep lapse rates aloft will support MLCAPEs
anywhere from 1500-3000 J/kg, with much of that CAPE residing within
the hail growth zone (-10 to -30C).

The evolution of the convection could be rather complex.  There
seems to be a decent signal in the hi-res guidance that the strong
warm front may lead to convective initiation, which will likely be
initially discrete in nature.  Given 0-6km shear well over 50 knots,
these will be supercellular, if they are to form. The aforementioned
instability coupled with deep-layer shear will lead to a large hail
threat, with golf ball to even baseball-sized hail not out of the
question in some of these storms.  Bunker`s RM storm motion vectors
place the motion of these potential supercells almost parallel to
the warm front, which is worrisome because any storm that forms
along this front will have access to enhanced helicity, given the
backed surface winds.  Therefore, think there is an enhanced tornado
threat right along this boundary wherever it happens to set up,
perhaps more of a threat than SPC is currently highlighting with
their 5% Tornado risk.

Further south (across far southern KY, near TN border), strong
mixing coupled with the fact that the surface low will be filling
will lead to more veered surface winds, which should cut down
somewhere on the effective SRH.  However, with westerly low-level
jet expected to strengthen towards 26/00Z, hodographs will be
long/curved enough to support an isolated tornado threat early this
evening in areas south of the warm front.  Coverage of these storms
not tied to the warm front will likely be rather isolated, given a
lack of convergence and the presence of a weak cap in the warm
sector.

By a bit later this evening, the actual surface low will push into
west-central KY.  Near the triple point of this low, enhanced
convergence will likely lead to a more concentrated area of
convection, perhaps a mix of bowing segments and supercells which
will push into the I-65 corridor toward 02-04Z.  By this time,
daytime instability should be beginning to wane, but still think
this second round will carry a severe threat of large hail, damaging
winds, and isolated tornadoes as it pushes southeast across
south-central KY.

Bust Potential
==============
There remains some potential bust factors with this setup.  The
first and most obvious, is the morning convection.  We will need
this to clear out and some sunshine to break out south of the warm
front this afternoon for us to destabilize.  Given the general
guidance agreement, do have pretty high confidence this will occur
and we will become quite unstable across southern KY late this
afternoon.

The other potential bust factor will if discrete development will
occur late this afternoon/early evening.  Warm fronts can be tricky
in initiating surface-based convection, so we could go through the
day not seeing much becoming sustained along the front.  In
addition, the weak cap could hold through the rest of the warm
sector giving very little coverage of discrete convection until the
more concentrated line of storms approaches with the surface low
well after peak heating.

Summary
=======
While there is bust potential, have fairly high confidence in severe
weather this afternoon and evening.  Think the main threats will be
hail and damaging winds, but an enhanced tornado threat will be
found right along the warm front.  This is something we will
continue to monitor closely this afternoon.  Even north of the warm
front in Lexington and Louisville, forecast soundings show quite a
bit of elevated instability above the stable boundary layer, so
severe hail will certainly be a possibility even in areas north of
the warm front.

.LONG TERM (Sunday through Friday)...
Issued at 230 AM EDT Sat Apr 25 2015

Cool and mostly dry conditions are expected from Sunday most of the
way through next week. Upper pattern looks fairly blocky as the main
player is an eastern CONUS trof that keeps getting reinforced.
Around mid-week a bowling-ball closed low in the southern stream
will push across the Deep South, with precip chances in the Ohio
Valley Wednesday and Wednesday night. Timing is still a bit
uncertain, and the ECMWF is more bullish in phasing this system with
a northern stream disturbance, so will just paint low-end chance
POPs for Wednesday and Wednesday night. Temps will run below climo
for most of this period.

Toward the end of the week, upper ridging over the Plains starts to
build into the Ohio Valley. Look for dry weather and exceedingly
pleasant temps with highs in the 70s and lows either side of 50 just
in time for the first weekend in May.

&&

.AVIATION (12Z TAF Update)...
Updated at 651 AM EDT Sat Apr 25 2015

Deteriorating conditions will push in through the TAF period, as an
area of low pressure approaches from the west.  Showers will
continue to expand in coverage this morning, bringing MVFR vsbys at
cigs at times to all terminals.  Will leave in VCTS wording given
some marginal elevated instability, but think any thunderstorm
activity will be rather isolated through the morning hours.

A warm front will sharpen and lift northeast into the area today.
Just north of this front, a period of IFR cigs can be expected at
all sites.  The latest guidance suite continues to suggest this
front will likely stall just to the south of KSDF and KLEX, meaning
low cigs may persist in these locations through much of the day,
especially at KLEX. KBWG will likely scour out this afternoon and
mix down SSW wind gusts around 25 knots.  Late this afternoon into
this evening, severe thunderstorms are expected to break out over
central KY, mainly affecting KBWG.  However, there is enough
elevated instability to warrant VCTS wording at KSDF and KLEX as
well with this activity.  Winds will turn more northerly late in the
TAF period as the area of low pressure pushes to the southeast.

12Z TAF Forecaster Confidence
=============================
Ceilings    : Medium
Visibilities: Medium
Winds       : Low at KSDF and KLEX
=============================

&&

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

$$

Update.........KJD
Short Term.....KJD
Long Term......RAS
Aviation.......KJD





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