Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 282308
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
608 PM EST Sat Feb 28 2015

...Updated Aviation Discussion...

.SHORT TERM (Now through Sunday Night)...
Issued at 300 PM EST Sat Feb 28 2015

...Snow to Wintry Mix Tonight Into Sunday Morning...

The main focus in the short term is on the snow to wintry mix threat
for southern Indiana and northern Kentucky tonight through Sunday
morning.

As of mid afternoon, latest water vapor imagery overlaid with 500 mb
heights shows broad zonal to southwesterly flow as weak shortwave
troughs move through the Southwest into the central Plains. At the
surface, our air mass is characterized by dry low levels as
observed by the 28.12z BNA/ILN/ILX soundings. Surface high pressure
is slowly moving off to the northeast, resulting in northeast to
easterly flow. Mid afternoon readings are mainly in the 30s.

Current mosaic radar shows a band of precipitation across Missouri,
which will begin to move east into the southern IL and IN over the
coming hours. Increasing isentropic lift on the 290-305k surfaces
and moist, warmer southerly flow between 800-900 mb will begin to
saturate the dry air mass this evening. The focus for precipitation
will lie mainly north of the KY parkways between 00-06z where
thermal profiles support all light snow at the onset.

Model Uncertainty / Preference:

The main challenge is the degree of warming aloft between 06-12z and
impacts on precipitation type. 28.12z NAM and GFS were warm but
still only bring a +1 to +3C layer to northern Kentucky by 09z. What
is concerning is that the low-level jet isn`t overly impressive, so
the degree of warming may be limited. Finally, a time trend analysis
has been cooler and the latest GEM/ECMWF also show the warm nose
barely peaks at +1C through 18z Sunday. As such, leaned toward a
slightly cooler solution aloft and adjusted QPF south to account for
the latest trends. A look at some of the hi-res models such as the
HRRR and ARW/NMM show a tight precip gradient across southern
Indiana.

Timing:

Expecting precipitation to break out in the northwest between
00-03z, although initially it will have to overcome the dry
sub-cloud layer. This should work east/southeast through 06z,
reaching the far eastern areas such as Trimble County after
midnight. The bulk of the snow / wintry mix should fall between
midnight and 9 am before going over to rain.

Impacts / Amounts:

The adjustments above resulted in snow amounts of 1-2 inches from
Dubois to Jefferson County, Indiana with roughly 1 inch or less
possible across the remainder of southern Indiana into far northern
Kentucky. As such, we expanded the winter weather advisory a tier of
counties southward to the Ohio River. The combination of 1-2 inches
of snow and ice accumulations of 0.05 inch or less would make for
slick and potentially hazardous roads overnight into Sunday morning.
Finally, a look at the KY and IN RWIS data shows road temperatures
are in the upper 20s, so even light precipitation will create a
glaze on surfaces. Across northern Kentucky, including the
Louisville metro region, plan on a shorter period of light snow to
light wintry mix overnight, possibly making for some slick roads
Sunday morning. Will highlight this area with a SPS.

Sunday - Sunday Night:

Surface temperatures look to warm above freezing across the entire
area late morning as southerly flow increases ahead of a cold front.
This front will bring showers to the area into the evening hours,
northwest to southeast. Highs look to top out around 40 in the north
to mid/upper 40s south of the KY parkways. After midnight Sunday
into early Monday morning, northerly flow will bring cooler
temperatures aloft and at the surface, potentially changing
precipitation back over to a light wintry mix or snow. This is a
case of the cold air chasing the precip and for the most part,
expecting the precip to win. For now, kept a wintry mix on the outer
fringe of the POPs as it exits the forecast area. Plan on lows to
bottom out in the mid 20s to lower 30s.

.LONG TERM (Monday through Saturday)...
Issued at 300 PM EST Sat Feb 28 2015

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

Progressive upper pattern and an active southern stream will make
for active weather in the Ohio Valley Tuesday through Thursday.
After a quiet but cool Monday, an impulse will eject out of the
deeper trof digging down the California coast. A surface low will
develop in the lee of the Rockies and quickly scoot into the Great
Lakes, dragging a cold front into the Ohio Valley. Strong low-level
jetting will draw copious Gulf moisture northward, with a solid
36-48 hrs of categorical POPs in the forecast Tuesday through
Wednesday night.

Biggest issue with this system will be QPF. Still some differences
between models in just how far north this will occur, but the front
will hang up somewhere across Kentucky and serve as a focus for 2-3
inches of QPF, with localized 4 inch amounts possible. Given recent
rains and still some remaining snowpack, this could have substantial
flood impacts which are outlined in the Hydrology portion of this
discussion.

SVR potential is limited by a strong warm nose near 850mb that will
keep anything from becoming surface based. However, there is enough
elevated instability to support the mention of thunder Tuesday
afternoon into Tuesday night. Best chance of actually drawing any
sfc-based instability will be across south central Kentucky where
temps punch into the 60s, but any severe probabilities remain quite
low.

Wednesday into Wednesday night could be interesting from a winter
perspective as much colder air spills into the area. but the WSW
moisture feed aloft continues. Went with a non-diurnal curve as
temps will be in free-fall beginning around 06Z Wednesday and crash
through the 40s and 30s during the day. Wednesday afternoon precip
may have a tough time changing over to snow in Kentucky, but expect
a quick change on Wednesday evening with enough QPF remaining to
support at least some accumulating snows. This scenario has played
out before, in March 1995 as noted by the previous forecaster.
Confidence in this solution remains low, so will not get too carried
away with the details beyond a mention in the Hazardous Weather
Outlook.

Drier and colder conditions look to return to the region late
Thursday as high pressure builds into the region from the NW.

&&

.HYDROLOGY...

Issued at 330 PM 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 will warm above freezing.  Additional
precipitation is expected across the region tonight into Sunday with
totals of up to a half inch of new liquid. This precipitation
combined with the ongoing snowmelt will saturate the ground and
result in rises on small streams/creeks.

A second storm system is expected for Tuesday into Thursday.
Moderate to heavy rainfall with embedded thunderstorms is expected
for around 36 hours. Around two to three inches of rainfall is
expected with this new storm.

The combined rainfall from the two systems mentioned could trigger
widespread flooding across the region, especially in areas with
heavier snowpacks.  Given current rainfall projections, flooding on
the major rivers 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 rain axis will set up.
Though the latest forecasts suggest it will be along or slightly
south of the Ohio River.  The National Weather Service will continue
to watch these storms closely over the weekend. Residents in flood
prone areas should closely monitor water levels in the coming days
and prepare for possible flooding by midweek.
&&

.AVIATION (00Z TAF Update)...
Issued at 608 PM EST Sat Feb 28 2015

Expect VFR conditions at the start of this forecast cycle, although
ceilings and visibilities are expected to deteriorate through the
evening and overnight hours.

At SDF, precipitation is expected to start off as very light snow
or sleet between 04-06z then transition to a period of
sleet/freezing rain/snow mix 05-10z before going over to light
freezing rain or rain depending on surface temperatures around
daybreak. Plan on snow accumulation less than 1/2 inch and ice
accumulation of a few hundredths or less. Meanwhile, ceilings will
drop below fuel-alternate levels in the pre-dawn hours and into IFR
around sunrise, persisting through the remainder of the TAF period.
Visibilities are expected to be in the 3-4sm range though periods of
1-2sm are possible.

At LEX, similar forecast thinking as SDF but with less of an overall
threat for wintry mix as surface temperatures are expected to warm
above freezing sooner and overall precipitation shield will be to
the north.  Best timing for a light wintry mix should be between 05
and 11z. Look for IFR ceilings to develop around sunrise Sunday
morning as increasing moisture/saturation takes place. The IFR
conditions are then expected to persist for much of Sunday as the
weather system passes over the region bringing a high chance of rain
during the day.

At BWG, not expecting any wintry precipitation, although do expect
to see below fuel-alternate ceilings around sunrise Sunday morning,
with light rain developing by midday.

Overall, surface winds will remain steady out of the NE this
evening, gradually veering to easterly and then southeasterly through
the overnight. Winds will continue to veer to southerly and then
southwesterly between 5 and 10 mph on Sunday.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...WINTER WEATHER ADVISORY until 10 AM EST Sunday FOR KYZ032.

IN...WINTER WEATHER ADVISORY until 10 AM EST /9 AM CST/ Sunday FOR
     INZ076>079-083-084-089>092.

&&

$$

Short Term........ZBT
Long Term.........RAS
Hydrology.........CMC
Aviation..........BJS




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