Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 252224
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
624 PM EDT Wed Mar 25 2015

...Forecast Update...
Issued at 622 PM EDT Wed Mar 25 2015

A quasi-stationary boundary has stalled just south of the Ohio River
at this hour. Meanwhile, a warm front extends from just southwest of
Owensboro into southern Illinois. Will watch this feature and our
stationary boundary lift back north through the evening hours as
surface low pressure ejects out of the southern Plains. This wave
moving along the boundary, along with increased low level jetting
will lead to more widespread convection into the overnight mainly
across southern Indiana and north central Kentucky.

In the meantime, will watch slightly developed cu in the unstable
air south of the boundary over central KY this evening. Some hi-res
data suggests we`ll get enough convergence and destabilization for
some storms in the next few hours. However, looking at latest
mesoanalysis destabilization is slow going and there isn`t much of a
trigger yet. If any storms are able to fire and get organized, the
large T/Td spread would allow for gusty winds as soundings do show
an inverted V structure near the surface. At this point, feel the
likelihood is low.

.SHORT TERM (Now through Thursday Night)...
Issued at 210 PM EDT Wed Mar 25 2015

Broad SW flow and warm advection well established over the Ohio
Valley, with most of the area near 70 degrees with the exception of
Dubois County. A surface wave taking shape over the southern Plains
is just starting to fire convection over SE Kansas and SW Missouri,
and the main challenge will be the timing and impact of this system
as it swings through tonight.

There should be enough convergence along a quasi-stationary front
that precip will blossom ahead of the surface wave tonight, with
POPs ramping up to categorical after midnight. SPC has extended a
narrow Slight Risk as far east as I-65, with a marginal into the
Bluegrass. Main threat appears to be damaging wind as there is a
deep layer of 50-60 kt winds, but lack of surface-based instability
will really limit the severe potential especially as you head east.
Will highlight gusty winds in the Hazardous Weather Outlook but not
play it up too much.

Cold front moves NW-SE across the area on Thursday, with widespread
precip continuing through at least the morning in a classic anafront
setup. While it will be mostly stratiform in nature, the overall
duration of precip including tonight`s convective precip will push
QPF over 1 inch in southern Indiana. Could see some nuisance
flooding in typically prone locations.

Thursday max temps will be reached early, especially north of the
Kentucky Parkways where recovery will be almost nil. By late
afternoon most locations will be in the 40s. Precip should finally
end Thursday evening in east central and south central Kentucky,
with the cold air chasing the moisture out before any change to snow
can occur.

.LONG TERM (Friday through Wednesday)...
Issued at 145 PM EDT Wed Mar 25 2015

The focus in the long term forecast is on the unseasonably cold
temperatures expected this weekend.

The synoptic pattern Friday is expected to feature an anomalously
deep upper trough axis across the eastern CONUS while at the surface
Canadian high pressure will be centered west of the Ohio Valley.
Unseasonably cold air will be moving into the area and highs are
expected to remain in the 40s, ranging from the low 40s across
southern Indiana to upper 40s across southern Kentucky. The
mid/upper trough axis swings through during the day and 850 mb
temperatures are forecast to fall from around -5C Friday morning to
-10 to -12C Friday night. Steepening low-level lapse rates on the
order of 7-8 C/km combined with some low level moisture still
warrants the slight chance for instability/diurnal showers
along/east of I-65. Rain/snow/graupel all look possible based on
thermal profiles. Nothing significant or impacts expected, but
conversational winter precipitation may be falling in the afternoon
at times. It will also be blustery with northwest winds 10-15 mph.

Lingering precipitation Friday afternoon should wane in the evening
hours with the loss of daytime heating and as the upper trough axis
pushes east of the forecast area. A time trend analysis shows the
overall progression of this system a 3-6 hours slower, which might
keep the pressure gradient and northerly flow across the area
through Friday night. This may prevent the boundary layer from
really decoupling and hold lows up a few degrees. Nonetheless, still
looks like 20s regionwide, with the sheltered locations possibly
falling into the lower 20s. Those with agricultural interests or
sensitive vegetation should take note of these forecast freezing
temperatures.

The surface high is expected to drop into the Ohio Valley Saturday
which should provide a mostly sunny but another unseasonably cold
day. Highs in the mid/upper 40s to near 50. With deep troughing
aloft, a quick moving shortwave is forecast to slip through the
mid-Mississippi River Valley, staying west of the forecast area.
Plan on another unseasonably cold night Saturday as readings fall
once again into the upper 20s to near 30 degrees.

High pressure moves off to the east during the day Sunday and we`ll
see a slow push of southerly winds. 850 mb temperatures recover from
-10 to -13C Saturday morning to 0 to +3C by Sunday afternoon. Look
for highs in the mid/upper 50s.

25.12z guidance in fairly similar agreement showing a cold front and
shortwave trough coming through the Upper Midwest into the Ohio
Valley late Sunday into Monday. Moisture return ahead of the system
is limited but there looks to be sufficient forcing along the front
to warrant chance POPs across the area. Guidance shows the upper
level flow tries to turn zonal to slightly southwesterly, so
seasonable temperatures with highs early next week in the 50s to low
60s looks reasonable at this time.

&&

.AVIATION (18Z TAF Update)...
Issued at 1250 PM EDT Wed Mar 25 2015

VFR strato-cu ceilings and gusty SW winds will prevail for the rest
of the afternoon. Could scatter out at times, especially at SDF, but
ceilings are high enough that operational impacts are limited.
Winds will lay down and back to due south this evening, and we could
see ceilings lift slightly but will remain close to status quo.

Main challenge is timing and impact of precip that is expected along
a slow-moving cold front that enters Kentucky after midnight
tonight. Elevated instability supports thunder potential, but
difficulties in timing will limit the mention to VCTS. Stable
boundary layer will keep gust potential in check. Will keep ceilings
at least a high-end MVFR until the actual fropa after daybreak
Thursday.

Much of the day on Thursday will be marked by a classic anafront
setup with low clouds and rain. IFR ceilings are a good bet from
mid-morning through mid-afternoon, with some late improvement
expected at SDF by the end of the 24-30 hr planning period.

&&

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

$$

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




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