Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 280844
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
344 AM EST Sat Feb 28 2015

.SHORT TERM (Now through Sunday)...
Issued at 330 AM EST Sat Feb 28 2015

...Light Snow or a Wintry Mix Possible Tonight...

The short term period will be characterized by southwesterly flow
aloft as a deep trough develops over California. At the surface this
morning sprawling high pressure sits just to the north of the
forecast area. This high pressure will shift eastward with a weak
trough developing across the region today into tonight.

Quiet weather is expected through this afternoon. Mid level clouds
are present across the region this morning and bases should lower
through the day. Highs today will range from the lower 40s near the
Tennessee border to near freezing across southern Indiana and
portions of north central KY.

This evening and tonight pose more of a challenge as a weak upper
level wave and isentropic lift result in increasing moisture and
precipitation. Precipitation looks to move into southern Indiana
from the west during the evening hours and spread across the IN
counties through the evening. The models remain in a bit of
disagreement with how far south the precipitation shield will
spread. The 0Z NAM is the farthest north and would keep the precip
across our northernmost counties for most of the night. Most of the
other models spread it further south and bring at least a slight
chance to the entire forecast area by morning.

The other question will be precipitation type. The NAM is the most
aggressive with bringing in a warm layer aloft, but as mentioned
before, the precipitation shield is much farther north. Leaned
slightly towards the cooler side of things. Thus, there could be a
brief chance for some freezing rain towards morning Sunday, but
looking at thermal profiles, it does not look too likely at this
point. If we do get any, accumulations would only be a few
hundredths of an inch or less. The best chance for accumulating snow
looks to be across our northernmost counties in southern IN. Think
1-2 inches of snow will be possible tonight across this region, so
will issue a Winter Weather Advisory. Some light accumulations look
to reach as far south as the river, but they look to stay under an
inch.

Regardless of what happens tonight, temperatures on Sunday should
warm quickly under southerly flow. All areas should transition to
rain by mid morning with rain continuing through the day on Sunday.
Highs will range from the upper 40s in the south to around 40 in the
north.

.LONG TERM (Sunday night through Friday)...
Issued at 330 AM EST Sat Feb 28 2015

...Very Active Period of Weather Setting Up for the Ohio Valley...

The long term period continues to look very active as two system
will bring precipitation to the region.

System 1: Sunday night through Monday Night

System number one will be in progress at the start of the forecast
period.  As an area of low pressure continues to move northeastward
and away from the area, a surface cold front will sag southward
through the region.  Rain showers will be the predominant
precipitation type early Sunday night across the region.  However,
colder air will work into the region from the northwest as the
precipitation shield moves eastward.  This will result in the rain
changing over to a wintry mix and then over to plain snow before
ending.  Initially thought that we`d have more of a freezing rain
threat as the atmospheric column dried out and we lost moisture in
the ICZ.  However, the latest runs have trended a little more moist
in the column which would support more of a snow/sleet mix going
over to snow before ending.  Precipitation still looks to move out
quicker than the cold air can get in, so the length of time of
wintry precipitation late Sunday night looks to be a short window.
Nonetheless, a dusting or minor accumulation may be possible.  Lows
Sunday night will cool into the mid-upper 20s in the north with
lower 30s in the south.

We still expect to see a modified Arctic airmass spill into the
region for Monday and into Monday night which will provide the area
with a brief reprieve in the weather.  This break in the weather
will be short lived as the next system will quickly move into the
region late Monday night.  Highs Monday will warm back into the
mid-upper 30s in the north with upper 30s to the lower 40s across
the south.  By late Monday night, the cold front to the south will
retreat northward as a warm front and result in precipitation moving
back into the region from the southwest.  Temperatures look to
bottom out in the upper 20s in the north with near freezing
temperatures down across the south just after midnight.
Temperatures look to rise in the later half of the night, which
should result in mostly rain across the south.  Further north,
perhaps from the Parkways northward, a wintry mix of rain and
freezing rain will be possible for a short period.  This may result
in some minor icing accumulations before temperatures rise above
freezing after sunrise Tuesday.

System 2: Tuesday - Friday

By Tuesday, high pressure will be located to our east and we should
be firmly in a return flow/southerly wind pattern.  A fairly potent
upper level trough is forecast to eject out of the western US and
result in lee-side cyclogenesis.  This low looks to track from the
Plains through the upper Midwest and into southern Great Lakes.  We
will be on the warm side of this weather system and will see our
temperatures spike up for a short period of time.  Aloft, a fairly
impressive 300 hPa jet streak will be in place aloft combined with
an increasing low-level jet near 850 hPa will support strong
moisture advection into the region.  Instability is somewhat in
question given the large amount of cloud cover.  CAPE values are
still fairly low (100-200 J/Kg), but in the cool season, the high
shear/low CAPE environments often produce severe weather.  The SLU
CIPS analogs have been suggesting the threat of severe weather over
the SE US for the last several days.  We still think that is very
possible with the highest threat a little south of here, but it
would not take much to advect some of the higher instability values
up this way.

Moderate to heavy rainfall out ahead of the synoptic front looks
likely.  That combined with embedded thunderstorms could pose a risk
of flooding across the region during this time period.  Current QPF
forecasts suggest 2-3 inches of rainfall across the area...though
the OP GFS and Euro have slightly different placements on the QPF.
The timing for the heaviest rainfall would be Tuesday afternoon
through Tuesday morning.  Flooding issues are likely to develop
across the region.  Those concerns are addressed in the Hydrology
section below.  Highs Tuesday will likely warm into the upper 50s to
the lower 60s.  Highs Wednesday will likely peak early in the day
with falling temps through the remainder of the day.  Early day
highs in the upper 40s to the lower 50s look likely at this point.

As the synoptic front presses southward, modified Arctic air will
sink into the region Wednesday afternoon and Wednesday night.  The
models have been signaling the potential for several waves of low
pressure moving northeastward along the front, which looks to bring
yet another wave of light to moderate precipitation into the
region.  With the Arctic air pushing in from the north, this may
result in significant wintry weather across the region.  This is not
an unprecedented setup for wintry weather.  Something similar to
this set up occurred back in March 1995 where we had 65+ degree
temps that quickly transitioned back into the upper 20s/lower 30s
with several inches of snow as the Arctic air slipped back into the
region.  This will be something to watch over the coming days.

Drier and colder conditions look to return to the region by Friday
as high pressure builds into the region from the NW.  Generally
stuck close to the multi-model consensus here for temps.

&&

.HYDROLOGY...

Issued at 330 AM EST Sat Feb 28 2015

The snowpack in the area contains between one half and two inches of
liquid.  The snow is expected to continue melting this weekend as
surface temperatures are expected to warm above freezing.
Additional precipitation is expected across the region with totals
of up to a half inch.  This precipitation combined with the snowmelt
will saturate the ground and result in rises on small streams/creeks.

A secondary storm system is expected for Tuesday and Wednesday.
Moderate to heavy rainfall with embedded thunderstorms is expected.
Around two to as much as two and half inches of rainfall is expected
with this storm system.

The combined rainfall from the two systems mentioned could trigger
widespread flooding across the region, especially in areas with
heavier snowpacks.  Given current forecast QPF projections, flooding
on the major river stems would likely begin Wednesday and continue
for the remainder of the week.  There is still quite a bit of
uncertainty with regards to where the heavy QPF axis will set up.
Though the latest forecasts suggest it will be along or slightly
south of the Ohio River main stem.  We will continue to watch these
storms closely over the weekend and will continue coordination with
the RFC.  Residents in flood prone areas should closely monitor
water levels in the coming days and prepare for possible flooding by
midweek.
&&

.AVIATION (06Z TAF Update)...
Issued at 1153 PM EST Fri Feb 27 2015

Flying conditions are expected to remain VFR tonight and through the
afternoon Saturday. A mid deck of clouds is currently spreading
across the area. Bases should lower to around 5-6 kft overnight and
stay in that range through the afternoon Saturday. Northeasterly
winds will become easterly as high pressure to the north of the
region shifts east today. As the next weather system approaches from
the southwest tomorrow evening, ceilings will begin to lower. LEX and
SDF look to drop to MVFR early Saturday evening and stay down
through the night.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...NONE.
IN...WINTER WEATHER ADVISORY from 7 PM EST /6 PM CST/ this evening to
     10 AM EST /9 AM CST/ Sunday FOR INZ076>079-083.

&&

$$

Short Term........EER
Long Term.........MJ
Hydrology.........MJ
Aviation..........EER





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