Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 200731

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

.Short Term...(Today through Wednesday)
Issued at 321 AM EDT Mon Mar 19 2018

...Accumulating Snow Likely This Evening through Wednesday...

Scattered to numerous light showers will mainly be along and north
of I-64 this morning as a closed upper low rotates through the Ohio
River Valley. As this feature moves off to the east, we`ll see a
brief period of upper ridging take hold which will likely give us a
brief lull in precipitation activity from mid morning to mid
afternoon. During this time, expect a trace to a couple tenths of
rainfall with temps mostly staying or dropping into the upper 30s.

...Setup/Timing/Confidence In Accumulation Ability...

As we move into the mid to late afternoon hours, a secondary upper
low will close off over central Kentucky. Models have been
consistent in showing a deformation band developing and pivoting
over the region as we move into the evening and overnight hours.
Surface temperatures are expected to initially be in the mid 30s
with precipitation onset in the afternoon, however as we work into
the evening hours temps should cool down to around freezing, and
then below freezing past Midnight. As a result, accumulating
conditions will progressively get better as the event wears on.

There is a lot of question how well we will be able to accumulate in
the afternoon across our western counties given temps several
degrees above freezing and already warm surfaces. That being said,
have seen a recent trend in hi-res models to start the deformation
band off further west where decent rates in moderate banding could
overcome the warm surface temps. As a result, have expanded the
Winter Weather Advisory further west by 2 tiers of counties, and may
have to expand it more if the trend continues. Still think the best
totals will be where the original Advisory was placed as this
snowfall will mostly occur after sunset.

Snow slowly diminishes from W to E late morning Wednesday into the
early afternoon as the upper low pushes out of the area. Temps will
likely struggle given a stiff NW surface wind, some cloud cover, and
expected snow on the ground.


Thinking that the original Advisory area stretching east of a line
from Scottsburg, IN down through Louisville and over to Somerset, KY
could see in the 2 to 4 inch range. Would like to make note of a
small cluster of counties around Madison/Scottsburg, IN where some
locally higher amounts over Warning Criteria might be possible given
some hi-res models suggesting a moderate to heavy snow band pivoting
for quite a while. Don`t have enough confidence to quite go with a
Warning in that area at this time, but it will be something to watch.

The expanded Advisory area to the west should generally plan on 1 to
2". Again, there is a lot of question with how much accumulation
ability will occur in the late afternoon hours given surface temps
above freezing and some sunlight influence.

Will include the rest of our western counties in a Special Weather
Statement in coordination with PAH for amounts ranging from a
dusting up to 1 inch.


Since we aren`t planning on much in the way of impacts in the
afternoon, will keep the advisory start time at 8 PM EDT. If we do
start to see some accumulations in the afternoon given better rates,
there is a chance the advisory could start a bit earlier. Otherwise,
anticipate gradually deteriorating road conditions into the late
evening and overnight hours as temps fall and moderate snowfall
rates occur at times. The Wednesday morning commute will likely be a
slow endeavor, so prepare accordingly.


.Long Term...(Wednesday night through Monday)
Issued at 321 AM EDT Tue Mar 20 2018

Wednesday Night - Thursday...

Dry NW flow aloft and high pressure at the surface take hold for a
short time through mid week. This will be a cold but dry period as
lows on Wednesday night drop into the low to mid 20s. Highs on
Thursday are only expected to recover into the mid and upper 40s,
possibly touching the low 50s in our SW.

Thursday Night - Friday...

As we head toward the end of the work week, some light precipitation
could overspread mainly central and wester portions of our CWA as
the left exit region of an upper jet dives into the region.
Associated frontogenetical forcing combined with deep moisture in
the 1000-500 mb range would be enough to squeeze out some light
rain. Thermal profiles look to support mostly rain, but can`t rule
out some light frozen precip during this time as well. At this
point, wouldn`t expect it to be impactful.

Temps on either side of the freezing mark Friday morning should
recover to the low and mid 50s by the afternoon.

Friday Night - Monday...

The weekend into early next week looks to be an active time period
as a stronger storm system ejects out of the Plains and into region.
Widespread rain (and possibly some thunderstorms) are expected
during this time. The upper pattern will then switch to more of a SW
flow, where an active subtropical jet will bring several waves of
deep moisture riding along a quasi-stationary boundary somewhere in
the region. Continued periods of widespread rain under this pattern
could lead to some eventual flooding concerns. Something to keep an
eye on as we move forward.

Highs are generally expected to be in the 50s and 60s during this
time, with lows mostly in the 40s.


.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 EDT /7 PM CDT/ this evening to
     2 PM EDT /1 PM CDT/ Wednesday for INZ076>079-084-089>092.

KY...Winter Weather Advisory from 8 PM EDT /7 PM CDT/ this evening to
     2 PM EDT /1 PM CDT/ Wednesday for KYZ024-025-027>043-



Short Term...BJS
Long Term...BJS
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