Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 260458
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
1258 AM EDT Thu Mar 26 2015

...Updated Aviation Discussion...

...Forecast Update...
Issued at 908 PM EDT Wed Mar 25 2015

Nearly stationary boundary that is currently stretching horizontally
across the center part of the state will work back north as a warm
front tonight. Expect to see showers/storms beginning to develop
along this boundary over the next 1 to 2 hours as low level jetting
increases and convergence continues ahead an ejecting surface
surface low currently over NW Arkansas. Instability across our area
is quite meager and is not surface based at the moment. Do expect to
see some low and mid 50s dew points pool once the warm sector gets
established later tonight, however likely not enough to allow
convection to become surface based. Still can`t rule out a few
pockets of gusty winds, but think that the overall severe threat is
quite low. Will continue to monitor trends over the coming hours and
update as necessary.

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 (06Z TAF Update)...
Issued at 1254 AM EDT Thu Mar 26 2015

A boundary stretched across central KY this morning is just to the
south of SDF and LEX. This boundary will lift slightly northward
overnight shifting winds to southerly at LEX for a time, but looks
to stay just south of SDF. A low pressure system will then drag a
cold front through this morning, shifting winds to northwesterly at
all sites.

Ahead of the cold front a few storms may still be possible at SDF.
Have moved back the timing of the VCTS to 07Z. Any chance for
thunderstorms will quickly diminish this morning as the cold front
moves through. However, rain will continue through much of the day
at all sites.

VFR ceiling will deteriorate as precipitation begins today. Expect
ceilings to eventually lower to low end MVFR or IFR. They should
improve late this afternoon into the evening as rain moves off to
the east.

&&

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

$$

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




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