Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 200523

Area Forecast Discussion
National Weather Service Louisville KY
123 AM EDT Tue Mar 20 2018

...Updated Aviation Discussion...

.Mesoscale Update...
Issued at 553 PM EDT Mon Mar 19 2018

Surface low is currently just west of Nashville, with a warm front
analyzed eastward from it along the KY/TN border. Mesoanalysis data
over the past few hours had shown a nice trend northward with the
warm sector, however it has slowed with it`s northward progression
this hour. Given that the surface low is now pushing east across
north central TN, we may not see much more advancement of the warm
sector/surface based environment. Will continue to monitor these
trends over the next couple of hours.

With that said, watching a storm along the KY/TN border capable of
gusty winds to 40 mph, and nickel sized hail. Given low freezing
levels/-20 C heights, some large hail is possible with stronger
cells in our area. The elevated or only near surface based
environment along the warm front could be capable of some stronger
wind gusts, but also might be mitigated a bit by current convection
and a slight low level stable layer just north of the more
established warm sector to the south. Looks like our best window for
any strong to severe storms would be between now and 10 PM EDT.


.Forecast Update...
Issued at 849 PM EDT Mon Mar 19 2018

Surface low is now centered over south central TN and should
continue to track east this evening. Warm sector was never able to
get established over our southern CWA and storms in that area have
stayed sub-severe.

Across our southern CWA, the bulk of showers and a few storms should
push east of the area over the next 1 to 2 hours, with an impressive
dry slot working in. A few showers could linger in the area as the
exit region of an upper jet noses in, but things should stay mostly
dry overnight. Across southern Indiana and north central Kentucky,
showers will last into the overnight as a closed upper low tracks up
the Ohio River.

The forecast is on track, just made some adjustments to the timing
of precipitation exit. Temps should drop into the 30s in most areas
by dawn, with our far SE hanging onto the 40s.


.Short Term...(This evening through Tuesday night)
Issued at 315 PM EDT Mon Mar 19 2018

...Strong Storms Possible Through This Evening Across Southern KY...
...Accumulating Snow Likely Tuesday Afternoon through Wednesday...

A little bit of everything weather-wise going on across the lower
Ohio Valley the next 48 hours, just in time for the end of winter
and the first day of spring.

First, this afternoon/evening`s storm potential. A 998 mb surface
low was analyzed near Springfield, Missouri this afternoon with a
warm front draped across northern Alabama, Mississippi, and
southwestern Tennessee. Easterly winds continue across Kentucky and
southern Indiana and visible satellite shows some breaks in the

For the remainder of the afternoon and evening hours, the near-term
and hi-res guidance still suggests the warm front will surge to
around the Kentucky/Tennessee border by 6-8 pm EDT. By this point, a
line of showers/storms is expected to cross through middle TN and
south central KY. Surface based instability is forecast to reach
around 500 J/kg and with 0-6 km shear still 50-60 kts, isolated
strong to marginally severe thunderstorms are possible. Obviously
the greatest risk is further south into TN and AL/GA, but will need
to watch closely storms across far southern Kentucky with the warm
front in vicinity. The timing for any strong/severe storm would be
from around 4 pm EDT to 9 pm EDT. Main hazards are strong wind gusts
and hail.

Elsewhere, showers and some elevated thunderstorms will be possible
up to the Ohio River this evening for a 3-6 hour period as an arc of
precipitation moves through. As the surface low races across the TN
Valley, a dry slot is likely across portions of central Kentucky
while a west/east oriented band of rain continues across southern
Indiana and northern Kentucky.

This band of precipitation will begin to sag southward across more
of the forecast area overnight into Tuesday morning once the surface
low moves into

Winter Weather:

For the first official day of spring, accumulating snow is likely
across southern Indiana and central Kentucky.

A potent upper level shortwave is forecast to pivot through the Ohio
Valley, stall, and become closed Tuesday through Wednesday. As much
colder air pours into the region with breezy north/northeast winds,
forecast soundings show the entire column above the surface dropping
below freezing below. The changeover from rain to wet snow is now
likely to occur earlier than expected, with areas across southern
Indiana and portions of north central Kentucky transitioning to snow
in the mid afternoon. Some models suggest this could be even earlier
(late morning) across southern Indiana.

Deformation band snow will pivot across most the area through the
evening and overnight hours. There has been a noticeable upward
trend in QPF in almost all of the models, so forecast snow amounts
trended upward as well. Snow ratios will likely start off 7-8:1 then
become closer to 10-12:1 as colder air works into the area.

Temperatures: Where the deformation band sets up on Tuesday
afternoon, dynamic cooling will likely keep readings in the mid 30s
or lower. As a result, lowered highs across southern Indiana and
north central Kentucky some, but perhaps not as aggressive as some of
the model guidance suggests. As for Wednesday, trended highs down as
well, especially east of I-65 where snow showers and clouds will be
slow to clear.

Potential amounts: From Tuesday afternoon through Wednesday, 1-3
inches will be possible generally along and east of Interstate 65.
The potential for higher amounts (localized 4 inches) exists across
southeast Indiana (Madison area) through the Bluegrass (Lexington

Impact: Warm ground will limit accumulations at first, but as rates
pick up and with nightfall, accumulations will likely occur on
grassy and elevated surfaces. Over time, secondary and untreated
roads could become slushy and snow covered. The Wednesday morning
commute could be messy and slow.

After coordination with surrounding offices, will issue a Winter
Weather Advisory for areas along/east of the Interstate 65 corridor
across southern Indiana and central Kentucky. While the advisory
starts at 00z Wed / 8 PM EDT, some impacts will be possible for the
evening commute between 4p-8p, especially across southern Indiana.
Refinements to the advisory may be needed to the west too in later
shifts if forecast confidence increases.

.Long Term...(Wednesday through Monday)
Issued at 300 PM EDT Mon Mar 19 2018

By Thursday, the deep upper level trough over the Great Lakes is
forecast to push off the mid-Atlantic coast while ridging begins to
build in from the west. Cold Canadian high pressure will be
elongated north to south over the western Ohio Valley. This could
potentially bring a hard freeze to the area with forecast lows
in the 20s in many locations.

A subtle shortwave aloft heading southeast across the mid MS Valley
could bring light precip to our far western counties in central KY
Thursday night, which could fall as a little light rain and/or light
snow, but this system should not cause much impact. Then heading
into Friday night and this upcoming weekend, the upper level pattern
begins to flatten out as a surface low develops across the central
Plains and heads east. Within a warm advection pattern, numerous
showers and perhaps a few thunderstorms are expected Friday night
and Saturday. Scattered showers could linger on Sunday.

19.12z model guidance offers some variability for late Sunday and
into next week. However, the overall message is for moisture to
return to the Ohio and Tennessee Valleys as the flow aloft turns
southwest. This would mean more active weather with several weather
systems lifting through the region. As a result, leaned on the model
consensus precipitation chances late Sunday and early next week.


.Aviation...(06Z TAF Issuance)
Issued at 121 AM EDT Tue Mar 20 2018

An upper level low over the lower Ohio Valley is bringing a wide
variety of cigs/vis with isolated to scattered rain showers. A
rather large area of IFR/MVFR stratus over western KY and southern
IL will move into central KY and southern IN this morning and affect
all TAF sites. Ceilings may rise some this afternoon, but another
system will move in during the late afternoon and evening hours and
bring rain and snow to the region, as well as decreased cigs/vis.
Any rain should transition to all snow by early evening.


IN...Winter Weather Advisory from 8 PM this evening to 2 PM EDT
     Wednesday for INZ078-079-091-092.

KY...Winter Weather Advisory from 8 PM this evening to 2 PM EDT
     Wednesday for KYZ029>043-045>049-054>057-066-067.



Short Term...ZT
Long Term....ZT/TWF
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