Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 201708

Area Forecast Discussion
National Weather Service Indianapolis IN
108 PM EDT Tue Mar 20 2018

The AVIATION Section has been updated below.


Issued at 411 AM EDT Tue Mar 20 2018

A wintry mix across the southern half of central Indiana early this
morning will move off to the east before rain chances return this
afternoon. Tonight this will change over to a mix and then snow.
Eastern counties could see 4 inches of snow by Wednesday morning,
while western counties will probably remain mainly dry. Small
chances for precip will occur over parts of the area Thursday night
into Friday, with a stronger system bringing better chances for
precip for the weekend.


.NEAR TERM /This afternoon/...

Issued at 939 AM EDT Tue Mar 20 2018

Surface analysis late this morning shows a broad area of low
pressure in place across the Carolinas...with cyclonic flow in
place across Central Indiana. Aloft...water vapor showed a trough
over the upper midwest and western Great Lakes. Cold Northeast
Surface flow was in place across Central Indiana...with
temperatures in the upper 20s and low 30s.

Time heights and forecast soundings show a saturated column this
afternoon. A short wave embedded within the trough to the west is
expected to push east into Kentucky and southern Indiana this
afternoon. This should provide enough forcing to allow precip to
begin this afternoon...mainly across the southern parts of the
forecast area. HRRR shows spreading across southern Indiana this
afternoon as this forcing arrives. Thus have trended toward
mostly cloudy skies and ramping up pops this
afternoon...particularly across the south. Given the expected
clouds and precip...have trended highs lower than the forecast
builder blends and LAV guidance.


.SHORT TERM /Tonight through Thursday night/...

Issued at 411 AM EDT Tue Mar 20 2018

Main focus is on changeover to snow tonight and snowfall amounts.
Models in pretty good agreement and generally used a consensus.

Another upper low moves in tonight and keeps the surface low stalled
just southeast of the area. This could set up a strong deformation
zone over the eastern counties of the forecast area. Temperature
profiles indicate a changeover to snow from north to south from
around 8 pm to midnight. The best forcing will set up before
midnight but likely remain across the eastern counties through
much of the overnight. Given the cooler highs today and mostly
cloudy skies, road temperatures will start out cooler then last
night and cool to below freezing without much trouble. Snowfall
rates with the cooler but more saturated profiles and some
possible dendritic growth could approach an inch an hour at their
height, and thus think 4 inch amounts are possible across some of
the eastern counties. With the colder road temps and this bumping
up to the morning commute time, impacts and amounts necessitate a
winter weather advisory beginning at 8 pm and running through
midday Wednesday. Will likely be able to end the advisory a bit
earlier than that but including a buffer in case the system slows
even more as this has been a trend.

This system will finally kick out by Wednesday afternoon and dry
conditions will move in. Temperatures will rebound a bit Thursday
into the mid 40s to around 50, but more chances for precipitation
will arrive in the southwest counties Thursday night as a front
approaches. A wintry mix looks possible at onset.


.LONG TERM (Friday through Monday)...
Issued at 311 AM EDT Tue Mar 20 2018

Mostly wet weather and below normal temperatures are expected this
weekend as Winter refuses to give up without a fight.

Models agree that the long term will start off with an upper low
over the northeastern states, a high amplitude ridge over the
central states and troughing over the west coast. Models also agree,
the central ridge will flatten as it moves overhead on Saturday.
Meanwhile, energy from the western trough and accompanying frontal
system will approach. Models have timing differences with these
features along with timing of QPF. So, confidence is not good on
timing and thus do not have enough to overturn the blend output
which has PoPs starting off Friday afternoon and maximizing Friday
overnight through Saturday evening. After that, upper waves in
southwest flow aloft could bring more showers to the area.

With below normal temperatures, nighttime and morning snow and or
wintry mix looks reasonable. Should see just rain during the
afternoon hours.

Confidence is high on below normal temperatures to start off with.
Meanwhile, temperatures closer to normal look good by Monday.
However, with model differences, confidence is low.


.AVIATION /Discussion for the 201800Z Tafs/...

Issued at 108 PM EDT Tue Mar 20 2018

VFR Conditions are expected for much of this afternoon. A
deterioration to MVFR or worse will be possible after 00Z at IND
and BMG.

An upper level low pressure system is expected to settle across
Ohio Tonight. This Low will wrap moisture across Central
Indiana...with precipitation across mainly the eastern 1/2 of the
state...impacting our IND and BMG Taf sites. Time heights and
forecast soundings show best moisture arriving during the evening
and overnight hours as the GFS shows an inverted trough axis
pivoting across eastern Indiana from NE to SW. Thus precip chances
have been included in the IND and BMG Tafs...but due to a sharp
cut-off with moisture...just VFR Clouds will be expected at LAF
and HUF. MVFR Clouds will persist in the wake of the upper low on
Wednesday morning as time heights shows lingering lower level


Winter Weather Advisory from 8 PM this evening to 2 PM EDT
Wednesday for INZ041-042-048-049-056-057.



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