Area Forecast Discussion

FXUS63 KLMK 241316
Area Forecast Discussion
National Weather Service Louisville KY
916 AM EDT Sat Mar 24 2018
.Forecast Update...
Issued at 914 AM EDT Sat Mar 24 2018
The warm nose continues to progress slowly eastward. Area webcams 
show that some areas that saw snow previously such as Frankfort are 
seeing melting now. A mix of rain, snow, and sleet continues mainly 
across areas in the Winter Weather Advisory with snow still likely 
falling in much of the KY portions of the Winter Storm Warning area. 
Jefferson, IN looks to be more of a mix. No changes to headlines are 
planned at this time. 
South central KY currently has a break in precipitation, but this 
will fill back in over the next couple of hours as showers and 
storms to the west move in. 
Issued at 639 AM EDT Sat Mar 24 2018
850-700 mb warm nose had been progressing nicely to the east across 
our CWA, however has recently stalled roughly along a line from 
Scottsburg, IN down down through Richmond, KY due to dynamic 
cooling. The stage is now set for a battle between the warm nose 
aloft and undercutting cold advection at and near the surface. Areas 
under the SPS and Winter Weather Advisory will likely see a mixture 
of sleet, snow, rain, and even a bit of freezing rain this morning. 
Some light accumulations mainly on grassy/elevated surfaces are 
expected, although rates can be briefly strong enough to cover 
roadways. Getting a few reports of heavy sleet mixing in over areas 
where high reflectivities are occurring. So far, these areas are 
covered under the Special Weather Statement and/or Winter Headlines. 
The Winter Storm Warning area still looks good at this time as the 
slower progression of the warm nose appears to be taking shape, and 
longer residence time in mainly snow p-type appears to be setting up.
Ran some new numbers here in the last hour, and getting some 
slightly higher totals in the Advisory and SPS counties. Not ready 
to upgrade the SPS counties yet, but will consider one more tier of 
Advisory counties if we start getting some accum reports.
.Short Term...(Today through Sunday)
Issued at 320 AM EDT Fri Mar 23 2018
...Another Round of Wintry Weather Expected For Some Today...
A surface low currently over east central Kansas will work its way 
SE to the Ohio/Mississippi River confluence by this evening, 
weakening slightly as it does so. Meanwhile, a warm front at the 
surface will sneak into our far SW CWA near Bowling Green with a 
large scale overrunning event occurring to the north and east of this 
feature. The overrunning will be driven by a 850-700 mb SW-NE 
oriented low level jet responding beneath the right entrance region 
of an upper level jet. 
This entire event will be driven by the location of the 850-700 mb 
warm nose, and its role on P-type as we move into the morning and 
afternoon hours. It should be noted that the warm nose is going to 
be quite easy to track through the day as there should be a very 
intense band of precipitation that sets up along it given a very 
strong convergence zone along the elevated front. This band should 
serve as the divider between rain on the SW side and heavy snow 
along and on the NE side, with perhaps a period of sleet in between. 
Currently the warm nose can be seen stretching from central IL down 
through SW Indiana.
The big question with this forecast is exactly where the band of 
snow is going to set up and persist the longest. The latest data 
still supports our far NE CWA picking up the heaviest snow with 
Warning criteria expected along and NE of a line from Madison, IN 
down through Georgetown and Paris, KY. 3 to 5 inches is expected 
across Jefferson County Indiana, and Scott/Bourbon counties KY, with 
4 to 6 inches expected across Harrison/Nicholas counties. Will also 
buffer these counties 1 tier to the SW where a tight 1 to 4" 
gradient is possible across the counties. In coordination with IND, 
will add Scott county to the Advisory where 1 to 2 inches could fall 
in the NE parts of the county, but taper to nearly nothing in the SW 
parts of the county. We're dealing with a very tight gradient here.
Did want to bring up the potential for some sleet and/or freezing 
rain in the Advisory/Warning counties. The main wintry component 
should be snow, but forecast soundings show a very deep isothermal 
layer around 0-1C, and if enough melting occurs hydrometeors could 
fall back into a re-freeze layer in the near-surface region. 
Therefore, a brief but intense period of sleet is possible. In 
addition, surface temps look to be just above freezing (~33F) in our 
far NE, but if they are a degree or two cooler we could be dealing 
with a period of freezing rain once the warm nose gets established 
later this afternoon and evening. Don't think this is the likely 
scenario, especially with precipitation rates being pretty intense 
at times, but didn't want to mention as chances aren't zero for a 
glaze of ice on some trees.
Expect the warm nose to become established well enough over the CWA 
for all rain by 5 PM EDT. The exception would be the northern halves 
of Harrison and Nicholas county KY where snow could hang on through 
sunset. This is the area where we have the highest snow totals 
forecast. 4 to 6" with locally higher amounts not out of the 
The event is mostly over by sunset, although the warm nose does look 
to collapse later in the evening with mid level cold advection. So, 
if there is any deeper moisture left by this point, we could see a 
last round of light snow. Like the idea of keeping the far NE 
headlines going until 06z.
Tough to be overly confident with this forecast given the marginal 
temp profiles due to a surging warm nose, and a very narrow swath of 
heavy snow. That being said, the data has been fairly consistent 
over the past 24 hours with where the heaviest snow is expected. 
Please keep in mind that a very subtle change in the warm nose 
position either NE or SW will result in a different amount/p-type 
...Rain and Thunder Chances Elsewhere...
South and west of the Warning/Advisory headlines, expecting all rain 
with even some embedded thunderstorms as forecast soundings show a 
notable amount of elevated instability above 700 mb. Expect a 100% 
chance of rain today with an 80-100% chance of rain tonight. 
Overall, rainfall totals should be between 1 to 1.5" in most spots, 
with some localized areas possibly seeing up to 2". Can't rule out a 
few nuisance flooding issues given recent snow melt contributing to 
more runoff.
It should be noted that a stiff E wind between 15 and 20 mph, 
gusting up around 25 mph combined with cold rain and temps in the 
30s will make for quite a miserable day. Our southern tier or two of 
counties will be a bit milder with highs in the low 50s as the 
surface warm front noses into that area. Fully expect to see a 20 
degree temperature difference between southern KY and northern KY 
Precipitation should largely be gone by Sunday morning with temps 
starting out around 30 north to 40 south. Look for a dry day with 
temps only struggling into the mid and upper 40s in most spots due 
to cloud cover and a steady NE surface wind. The south could touch 
the low 50s.
.Long Term...(Sunday night through Friday)
Issued at 320 AM EDT Sat Mar 24 2018
Sunday Night - Monday Afternoon...
An elongated upper ridge axis will stretch from the Gulf of Mexico 
up through eastern Canada to end the weekend. This will give us a 
brief, but mostly dry stretch into early Monday afternoon before the 
next stretch of wet weather begins later Monday. Sunday night lows 
will be in the mid to upper 30s, likely in the low 40s down near 
BWG. Monday highs will top out in the mid to upper 50s before rain 
sets in.
Monday Evening - Wednesday Night...
A wet period of weather looks to take shape for the remainder of the 
work week as SW flow through a deep layer develops between western 
CONUS trough and east CONUS ridging. Initial rainfall should be 
fairly light although pretty widespread as a warm front lifts over 
the area. Will continue mention of scattered to numerous showers in 
most areas, with categorical pops along and north of the Ohio River 
through Tuesday afternoon.
At this point, the best chances for heavy rainfall look to be later 
Tuesday night into Wednesday night as a plume of deep moisture 
characterized by PWATs approaching 1.5 inches through the column 
works into the region ahead of the upper trough axis. Heavy rainfall 
leading to some hydro problems is a concern considering the rainfall 
we receive this weekend, plus the work week rain. Will have to 
monitor this potential over the coming days both for a Flash/Areal 
Flood as well as a River Flood threat. WPC has a large area of 3 to 
4 inches over our region over the next 7 days, and with locally 
higher amounts possible given the setup bringing attention to the 
potential for flooding seems like a reasonable message to continue.
The early to mid week time frame should be a milder stretch with 
highs mostly in the low to mid 60s. After Monday night lows in the 
mid to upper 40s, look for lows in the 50s on Tuesday night.
Thursday - Friday...
The full latitude trough axis will slowly slide through the area to 
end the work week, keeping lingering showers in the forecast, and 
bringing slightly cooler temperatures. Coverage won't be as 
widespread, but will still be scattered to numerous at times. After 
highs Thursday in the upper 50s and low 60s, Friday highs will be 
back in the mid to upper 50s. Lows find their way back into the low 
40s by Thursday night.
.Aviation...(12Z TAF Issuance)
Issued at 645 AM EDT Sat Mar 24 2018
Warm frontal boundary continues to push eastward into the region 
with a band of widespread precipitation extending from north-central 
IL into eastern KY.  Precipitation is in the form of rain across 
much of the region.  However, areas east of a KMDN to KLEX line are 
likely seeing a wintry mix of SN/PL.  KHNB/KSDF/KBWG will likely see 
on and off rain showers for much of the day.  Winds will be gusty 
out of the east with speeds of 10-15kts and gusts up to 22-23kts at 
times.  A wintry mix is expected at KLEX with a period of snow being 
possible for a few hours this morning.  The warm nose looks to push 
eastward later this morning changing KLEX precip over to mainly rain 
by afternoon.  Winds over at KLEX will remain out of the east at 10-
15kts with gusts to 20kts possible at times.
Later this afternoon, a period of showers with a few thunderstorms 
may affect KHNB/KSDF/KBWG.  Model proximity soundings show the LFC 
hanging around 700 hPa with a bit of elevated instability.  For now 
will continue to carry some VCTS down at KBWG, but may need to add 
it into KHNB and KSDF in later forecasts.
IN...Winter Weather Advisory until 2 PM EDT this afternoon for INZ078.
     Winter Storm Warning until 2 AM EDT Sunday for INZ079.
KY...Winter Weather Advisory until 2 PM EDT this afternoon for KYZ032-
     Winter Storm Warning until 2 AM EDT Sunday for KYZ036-037-042-
Short Term...BJS
Long Term....BJS