Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 121726

Area Forecast Discussion
National Weather Service Northern Indiana
1725 PM EDT Mon Mar 12 2018

Issued at 355 AM EDT Mon Mar 12 2018

Snow showers are expected today with highs in the upper 30s. Little
to no snow accumulation is expected during the day today. Lake
effect snow will then be possible from tonight through Wednesday
morning. 2 to 5 inches is possible across extreme southwest Michigan
and northwest Indiana during this time. The peak of the event will
be Tuesday morning with lighter...more widespread snow showers
through Tuesday evening. Accumulations away from the lake will be an
inch or less. Drier conditions anticipated for the second half of
the week with moderating temperatures through the weekend.


Issued at 827 AM EDT Mon Mar 12 2018

Based on radar and observation trends have had to speed up arrival
of burst of light snow to impact most locations for at least a few
hours this morning. Leading edge of visibility impacts (albeit
minimal) already through South Bend and working rapidly SE. KBEH
down to just above a mile as we speak suggesting some light
accumulation will be possible. Have kept to a few tenths of an
inch of snow at best given it won`t last much more than an hour or
2. No other changes made as attention will then shift to lake
effect snowshowers tonight into Tuesday.


.SHORT TERM...(Today through This Evening)
Issued at 355 AM EDT Mon Mar 12 2018

Morning water vapor shows robust shortwave diving almost due south
into the Great Lakes on cyclonically sheared side of 100kt jet
streak at 400mb. Corresponding PV perspective shows 1.5 PVU surface
extending to almost 600mb. A healthy wave to be sure but 00Z KDTX
sounding also shows very dry/stable antecedent airmass that will
limit the response somewhat. Local forecast soundings for today
remain dry above 700mb with just enough saturation to support ice
nucleation. Low level convergence and ascent will support saturation
along with rapidly decreasing stability by afternoon as CAA begins.
Expect most places (especially north of US-30) will see some precip
today but likely only a few hundredths at best. Surface temps also
expected to be in the mid/upper 30s today. Upper 20s dewpoints will
likely support primarily snow as precip type (especially in heavier
showers) but any lasting accumulation will be difficult given March
sun angle and relatively warm surface. Little to no impact
anticipated on the roads for some brief vis reduction/
slick spots in the (isolated) heavier showers. These showers will
exit the area by late evening and should be a brief gap before lake
effect ramps up around 06Z.


.LONG TERM...(Late Tonight through Sunday)
Issued at 355 AM EDT Mon Mar 12 2018

Second shortwave pinwheels into the area early Tue morning as closed
midlevel circulation develops over the eastern Lakes. Meanwhile...
steady CAA continues with 850mb temps dropping to around -13C by
midday Tue. Deep cyclonic flow with fetch down long axes of both
Michigan and well as ample background moisture/
instability to aid tepid thermal differentials...will set the stage
for a decent lake effect event. NAM12 0-2km theta-e lapse rates well
below zero across the lake by late tonight and extend well inland
during the day Tue. With strong low level flow...this will support
substantial inland penetration of lake bands Tue and widespread
(lighter) snow showers across the entire CWA as third shortwave
approaches. Forecast soundings are a bit ominous by Tue morning with
inversion heights near 10 kft. Only negative factor/cause of
uncertainty is that strong flow and cold water limit lake-induced
pressure trough/convergence and formation of a single, dominant
band. Hi-res guidance supports a Type III event with 2-3 bands
that oscillate in both area and strength during the day. Some
disruption is anticipated during the middle part of the day as
flow backs slightly with some subtle ridging in between
shortwaves. Second round of good lake effect expected Tue evening
but inversion heights start to fall with increasing dry
air/stability noted in theta-e profiles. Winds quickly back by Wed
morning with rapid increase in subsidence and dry air that could
shut off lake effect entirely by 12Z.

Putting it all together...expect the peak of the event to be 09-15Z
Tue with more disorganized/lighter/widespread snow showers Tue
afternoon and evening. 2-5" expected in our NW/W zones from tonight
through Tuesday night. Advisories may be needed but confidence in
band evolution is still not high and with main event still 24+ hours
away and amounts spread over a 36 hour period...have decided to hold
off for now and let day shift make final call.

Rest of long term period is very quiet. Latest guidance dry from
Wednesday afternoon through Sat with initial weekend wave now
staying south of our area and strong ridge/high pressure folding
over the Great Lakes. First low chance of rain not until Sun but
models continue to struggle immensely with evolution of southwest
CONUS trough and further changes certainly possible. Temps steadily
moderate through the period as well with 50s anticipated by Sun.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday Afternoon)
Issued at 125 PM EDT Mon Mar 12 2018

Post frontal convective snow showers continue to expand with limited
diurnal heating and will briefly impact terminals for a time early
this aftn before drier air upstream overspreads late aftn.

Otherwise secondary sw trough will cross the terminals around
daybreak with another band of synoptically driven snow and lake
effect invof KSBN. VFR outside snow showers with brief/transient
primarily MVFR within most snow showers.


LM...Small Craft Advisory from 2 PM this afternoon to 11 AM EDT
     Wednesday for LMZ043-046.




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