River Flood Warning 15, NWS Louisville, KY

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
719 PM EDT MON MAR 18 2013

...Updated Aviation Discussion...

.Short Term (Now through Tuesday Night)...
Issued at 315 PM EDT Mon Mar 18 2013

Another busy day weather-wise, but for a different reason. Most of
the morning's rains have exited the area now, just hanging on over
the far east. The cold front associated with a surface low over east
central Indiana now is just east of a SDF to BWG line. This surface
low will continue shifting north as it merges with a deeper low
moving toward Lake Superior this hour. The cold front may drag a
little as the surface low pulls northward, but still expect no
further development along the front because of lack of insolation.

Clouds will linger a little into the evening hours, but time height
sections indicate drying through the column after 06Z, so have skies
clear by daybreak Tuesday. Next up will be precip chances as a
secondary trough crosses the Ohio Valley late Tuesday night. The
only model giving us rain chances is the SREF ensembles, with slight
chance for light precip. Temperatures should be cold enough by then
for light snow over the north half. Given how dry the front is
though, will keep precip chances slight for now.

Temperatures will drop off quickly after Midnight, as the skies
clear. Have lows in the upper 20s over my far northern tier of
counties and low to mid 30s elsewhere. Highs tomorrow under mostly
sunny skies should rise from the mid 40s north to the low 50s south.
Then Tuesday night, with skies starting off mostly clear we should
fall off quickly, then slow as clouds come in association with that
trough. Have lows falling to around 30 degrees around daybreak.

.Long Term (Wednesday through Monday)...
Issued at 332 PM EDT Mon Mar 18 2013

Wednesday...

The upper air pattern will feature a deep eastern CONUS trough on
Wednesday, with fairly good model agreement that a shortwave
embedded in the flow will slide through the region. The system will
generally be fairly slim on available moisture, however given steep
low and mid level lapse rates isolated to scattered snow and rain
showers appear reasonable through the day. Will likely see a mix of
rain and snow as temperatures are forecast to recover to the low and
mid 40s by midday. The fast moving system will exit by Wednesday
evening.

Wednesday Night - Thursday...

A very cold airmass will settle in behind the front associated with
the wave mentioned above as H85 temps in the -12 to -16 C range
settle over the area. Skies become mostly clear with slackening
winds allowing for decent radiational cooling conditions and
temperatures dropping at least into the upper teens and lower 20s.
May end up having to go colder as dew points are projected to be in
around 10 above zero. Could some records be in Jeopardy?

Records:

BWG - 21 - 1986 Forecast - 21
LEX - 15 - 1986 Forecast - 19
FFT - 18 - 1986 Forecast - 18
SDF - 17 - 1876 Forecast - 22

Temperatures on Thursday will struggle only to the the upper 30s and
low 40s despite mostly sunny skies.

Thursday Night - Friday...

Confidence in the forecast begins to wane for late in the week as
there is model uncertainty with how to handle a developing southern
stream system, and how far north it will make it against our polar
air in place. The ECMWF makes the most meteorological sense in
keeping the bulk of the precipitation chances to our south, however
cannot ignore at least small chances of snow falling across our
extreme southwest on Thursday night as temperatures fall into the
low 30s. Will keep Friday mostly dry, although if the northern
solution is correct some light precipitation would scoot across our
southern counties through the day. With temperatures expected to
recover into the low and mid 40s, would expect mostly rain.

Friday Night - Monday...

Confidence really decreases into the weekend as models diverge
greatly in solutions. This time frame will need to be monitored as
the forecast is looking like a wet and much below normal pattern for
the period. In fact, the AO is forecast to go obscenely negative (-5
to -6 standard deviations below normal) adding some confidence that
some of the solutions showing up may have some credibility. A local
study that was performed for the November-March time frame of the
last 11 years has only had 44 days of AO values less than -4
standard deviations below normal. Throughout those 44 days, 48.2" of
snow fell, which is over an inch per day. So in general, we can
assume a pattern like this has an increased chance to result in
accumulating snowfall. All of that being said, its a little early to
get too excited about the possibilities, especially since we have
had an active southern stream all winter and were never able to
realize big snows. So for now, we'll go with the following.

As we move into Friday night, generally zonal flow will be in place
across the eastern CONUS with troughing across the western CONUS as
an impulse in the polar jet dives across the intermountain west. How
models handle the interaction between this impulse and the
subtropical jet will ultimately decide our weather for the weekend
into early next week. The GFS, which has general but not as extreme
support from its ensemble mean, the NAEFS, and GEFS phases the two
jets over the southern Gulf Coast states with a resulting cold and
strong system passing to our south. This would result in a wintery
period of weather from Saturday night through much of Monday. The
more likely solution will probably end up being a lesser degree of
this solution which will still be cold, however not as amplified. A
mix of rain and snow from Friday night through Monday seems like the
best way to go at this point. With highs in the low and mid 40s and
lows in the low and mid 30s.

One thing that can be said with higher confidence is that some sort
of southern stream storm will get going during this time period,
potentially anomalous. If that ends up being the case, our chances
for a wintry period of weather will go up. Stay up to date of the
latest forecasts.

&&

.Aviation (00Z TAF Issuance)...
Issued at 718 PM EDT Mon Mar 18 2013

Of immediate concern is the question of when the low clouds will
clear out.  Based on recent satellite trends, that should happen
from west to east during the 01Z-04Z time frame.  Mostly clear skies
and west winds of 6 to 11 knots will then prevail for the remainder
of the night.

High pressure building from the Plains to the Southeast will keep
skies mostly clear and visibility free of restrictions Tuesday into
Tuesday evening. West winds will continue and may get rather gusty
at the northern sites with gusts reaching 20 to 25 knots around
midday.

&&

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

Short Term.......RJS
Long Term........BJS
Aviation.........13

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