Area Forecast Discussion
Issued by NWS Indianapolis, IN

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FXUS63 KIND 181424
AFDIND

Area Forecast Discussion
National Weather Service Indianapolis IN
922 AM EST Fri Jan 18 2019

.UPDATE...

The Aviation section has been updated below.

&&

.SYNOPSIS...
Issued at 252 AM EST Fri Jan 18 2019

High pressure is expected to quickly move across Central Indiana
today and depart this evening...bringing dry and cool weather to
Central Indiana to end the work week.

A powerful storm system is then expected to push out of the
southern plains states and sweep into the Ohio valley on Saturday.
The low will spread snow across the northern parts of Indiana and
a mix of precipitation across Central Indiana before changing over
to snow on on Saturday Afternoon. Significant snowfall amounts are
expected as the storm passes.

Much colder conditions are expected on Saturday night and Sunday
as arctic high pressure builds across the Ohio Valley from
Canada. This will result in some of the coldest temperatures of
the winter season so far.

The changeable weather is expected to continue as the work week
begins as warmer air arrives for Monday and Tuesday ahead of
another approaching cold front that may bring more precipitation
on Tuesday Night...before yet another cold high pressure system
arrives for the middle of the next work week.

&&

.NEAR TERM /Today/...

Issued at 252 AM EST Fri Jan 18 2019

Surface analysis early this morning shows large high pressure in
place over Minnesota...stretching south through the Mississippi
Valley to Arkansas. A deep low pressure system was over the high
plains of eastern Colorado. Water Vapor showed a plume of moisture
stretching across Texas aiming for the Tennessee Valley. Meanwhile
aloft a deep trough was found over the Rockies...exiting into a
more zonal flow across the plains and Ohio Valley.

GOES16 shows extensive lower level cloud remaining across
Indiana...with stratus in place. Dew points were in the middle 30s
with light westerly flow across the forecast area. Snow pack and
the weak warm air advection...higher dew points and snow pack were
resulting in some fog across the area.

Forecast soundings today show a steep inversion in place aloft
with trapped lower level moisture lingering throughout the day.
Time heights also show this lingering lower level moisture. Models
both suggest that the surface high will quickly pass across the
region today...providing little in the way of any kind of forcing
while the flow aloft remains nearly zonal. Thus will keep the
trend of cloudy skies today as the lower level flow gradually
becomes easterly ahead of the approaching snow. Given the expected
cloud skies and easterly flow will trend highs today at or below
the forecast builder blends.

&&

.SHORT TERM /Tonight through Sunday Night/...

Issued at 252 AM EST Fri Jan 18 2019

Dry weather will be expected this evening as the high pressure
system departs. However the powerful storm system is expected to
arrive overnight and early Saturday morning. Thus will keep a
cloudy sky this evening but ramp up pops overnight as the the low
arrives.

GFS 290K Isentropic surface shows excellent Isentropic lift
across Central Indiana on Saturday morning. Specific Humidities
along this surface are near 4 g/kg...plenty of moisture. This
strong lift arrives by 09Z but quickly shifts east by late
morning as the the surface low is expected to quickly pass east of
Indiana and wrap around flow begins on Saturday afternoon. Strong
forcing remains in place aloft as the GFS shows a coupled jet
aloft ahead of a sharp upper trough passing across the area. Other
problem is the amount of warming across the area at
IND...forecast soundings show hardly any layer above
freezing...but as one goes farther south...more warm air is
present aloft. Even so...surface temps at places like BMG look to
remain at or above freezing overnight. Even at IND and HUF...the
warm layer aloft is minimal even though surface temps look to be
barely below freezing. Thus the freezing rain threat seems to be
minimal...however...confidence on the rain/snow line is much
smaller...and this will have large impacts on final snow totals.
At this time this looks to be near I-70 and points south.

Chance over to snow looks widespread on Saturday afternoon as a
deformation band looks to set up across Central Indiana. Favorable
Heavy snow trough axis at 700mb looks to pass across the area
during the afternoon. Before departing east after 00Z Sunday.
Through the afternoon...forecast soundings show a saturated column
as wrap around precip continues to fall. If warning criteria is
reached...it will likely be reached on Saturday Afternoon as this
deformation band passes. Confidence is high for warning criteria
to be reached across the North and northeast parts of the
forecast area where there is no question that precip type will be
snow. Farther south across the watch area...things are much more
fuzzy. Will trend toward categorical pops for now and highs
cooler than the blends on Saturday. Continue working with
neighboring offices on watch/warning decisions.

Upper support is lost on Saturday night as the upper trough axis
exits east of Indiana. Forecast soundings dry out dramatically
overnight as subsidence resumes in the wake of the system and the
arrival of arctic air. May keep some low pops east for the first
few hours after 00Z...but will overall trend toward a dry forecast
and lows at or below the blends due to the new snow and cold air
advection.

High pressure will then build across Indiana for Sunday and Sunday
Night. Models suggest ridging in place aloft by Sunday night amid
NW flow. This will allow the intrusion of very cold air. Will
stick with a dry forecast here along with temps at or below the
blends given the cold air advection. 850mb temps are suggested to
fall to -14c by 00Z Mon.

&&

.LONG TERM /Monday through Friday/...
Issued at 333 AM EST Fri Jan 18 2019

Long term guidance depicts a continued progressive pattern.
Anticyclonic flow aloft and surface high pressure will keep things
briefly quiet at the start of the week before precipitation
chances ramp back up again in earnest Tuesday onward. Model
differences in handling of the various impulses in the overall
strongly cyclonic flow later in the week will have a significant
impact on expected precipitation chances, but at these time
horizons see little to be gained in departing from the blend.
There may be chances for additional light snowfall accumulations
at times during the week, particularly mid week onward.

Temperatures may briefly recover Tuesday before dropping back to
near or just below seasonal normals again for the rest of the
week.

Blended initialization was reasonable and required only minor
adjustments throughout.

&&

.AVIATION /Discussion for the 18/15Z IND TAF Update/...
Issued at 922 AM EST Fri Jan 18 2019

Flying conditions have improved to VFR, so changed the TAF to
reflect that.

Previous discussion follows...

Issued at 617 AM EST Fri Jan 18 2019

Borderline MVFR/IFR conditions likely to continue throughout the
day before conditions deteriorate again to widespread IFR as
winter storm approaches late in the period.

Conditions today will likely straddle the borderline between MVFR
and IFR. An approaching winter storm will spread precipitation
across the sites late in the period, and conditions will
deteriorate with this precipitation.

Precipitation will likely begin as rain at HUF/BMG, and snow at
IND/LAF, with IND transitioning to a -FZRASN mix near daybreak
Saturday. HUF and perhaps BMG will also experience mixed
precipitation with this storm, but likely beyond the extent of
this TAF period.

Winds will be light and variable early in the period, becoming
more easterly and northeasterly late in the period, with gusts
into the low to mid 20KT range.

&&

.IND WATCHES/WARNINGS/ADVISORIES...

Winter Storm Watch from late tonight through late Saturday night
for INZ021-028>031-035>049-051>057.

&&

$$

SYNOPSIS...Puma
NEAR TERM...Puma
SHORT TERM...Puma
LONG TERM...Nield
AVIATION...Nield/MK



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