Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 231741 CCA

FXUS63 KIWX 231738

Area Forecast Discussion...Correction
National Weather Service Northern Indiana
650 AM EST Mon Jan 23 2017

Issued at 400 AM EST Mon Jan 23 2017

Areas of fog will dissipate later this morning. There is a chance
for light rain later today...mainly east of Interstate 69. Highs
will hold in the mid 40s. Dry conditions expected on Tuesday but
chance of rain returns on Wednesday with cooler conditions and
chances for light snow showers by the end of the week.


.SHORT TERM...(Today and Tonight)
Issued at 400 AM EST Mon Jan 23 2017

Another morning with areas of fog as main sensible weather concern
in the very near term as abundant moisture remains trapped in pesky
inverted surface trough draped over the Great Lakes. Setup for this
morning is a bit different than Sunday though as increasing
northerly gradient from impressive southeast CONUS cyclone leading
to more of an advection fog/very low stratus event aided by total
lack of mix out across Michigan yesterday. Surface winds holding in
the 5-10 kt range and this is leading to much more variability in
surface vis, which in turn makes for difficult decision with respect
to pre-existing dense fog advisory. Many sites hovering around 1
mile with very few quarter-mile reports seen. Trend in the last
couple hours has also been up with more of a transition to very low
stratus as most surface webcams and observations indicate improving
conditions. Areas of fog will certainly continue through the morning
but dense fog threat appears to be dwindling and after coordination
with surrounding offices, decided to go ahead and cancel the
advisory. Will issue an SPS for areas of 1SM fog/BR. Also expect
more meaningful improvement (compared to yesterday) by late morning,
given better mixing seen in nearly all sources of forecast soundings.

Other main story for today will be chances for some light rain.
Leading midlevel vorticity spoke and some gentle 290-295K isentropic
upglide already making a run at our far SE counties. Deep-layer
forcing is awfully weak though with incomparably better upper jet
dynamics and positive PV advection confined well to our E/SE. Precip
chances locally will depend on muddled midlevel deformation amid
reduced static stability above 800mb. This will be enough to spit
out a few showers east of I-69 along remnant boundary (worthy of
likely PoP`s in our far east) but amounts will be limited (a tenth
or two at best) and may struggle to measure west of I-69. Central
CWA may have slightly better chance for a few showers this evening
as low level convergence increases a bit ahead of shear vorticity
max. Precip chances taper off overnight as mid/upper level ridge
builds ahead of next system. Some patchy fog may be possible in our
eastern counties late tonight given lingering inverted trough and
better surface moisture. However, steady northwest gradient and
associated dry air advection should limit chance for any widespread/
dense fog, especially west of I-69. Northerly flow will also spread
cooler temps across the entire CWA. Afternoon highs unlikely to be
much warmer than current values under overcast skies with lows
tonight dipping back into the mid/upper 30s.


.LONG TERM...(Tuesday through Sunday)
Issued at 400 AM EST Mon Jan 23 2017

Benign weather anticipated on Tue with stout ridge overhead and
highs holding in the mid 40s. By Tue night/Wed, strong Pacific jet
(150+ kts) will eject into the central CONUS and send elongated/
positively tilted trough through our region. Left exit jet dynamics
will support a healthy swath of midlevel deformation and upper
divergence. However, plethora of latest 00Z guidance (deterministic
and ensemble) has taken a decidedly northward shift with jet core
and associated surface low track. This relegates our precip chances
to just some broad isentropic ascent/theta-e advection and
convergence along trailing vort max and associated diffuse front.
This will also prevent any chance for snow until Wed night when
postfrontal CAA ramps up. By that point, all deep/substantial
vertical motion has exited stage right and we are left with just
broad, lake-enhanced cyclonic flow with associated SCT/light snow
showers. Still does appear we will have an extended period of such
conditions though with deep upper low holding over Quebec through
the weekend and periodic, weak shortwaves rippling through the Great
Lakes in persistent NW flow. Difficult to nail down exact timing of
individual waves and associated periods of greater-than-flurry
activity but any snow accums expected to be light and grinding out
over a period of several days.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday)
Issued at 1240 PM EST Mon Jan 23 2017

Winds generally in from the north-northeast in the cloud-bearing
layer continue to stream in plenty of moisture in the low levels
along with light rain. IFR VISBY and CIGS have remained consistent
at FWA but rain moving in has VISBY into MVFR and VFR so will
include that with arrival of rain. Rain moves out as we head into
the overnight and CIGS and VISBY will trend down to IFR with the
overnight cooling. At SBN, MVFR and VFR VISBY occurred a few times
in the morning with the slight westerly wind component, but feel the
IFR VISBY should prevail from the afternoon into the overnight.
Expect the rain to stay east of SBN. For Tuesday, expecting some
drier air to move in from the south with high pressure affecting the
region and so expect some lifting of CIGS and VISBY, but then a
chance of rain for the overnight.





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