Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 050053

Area Forecast Discussion
National Weather Service Northern Indiana
753 PM EST Mon Dec 4 2017

Issued at 315 PM EST Mon Dec 4 2017

Strong winds are expected this evening through tomorrow. A few gusts
of 40 to 45 mph may be possible across the area tonight. Also
expecting rain to move over the region tonight with isolated
thunderstorms possible. Much colder air will then filter into the
area tomorrow and persist through the weekend with highs in the 30s.
Some lake effect snow is possible through the week but any
accumulations will be light.


Issued at 735 PM EST Mon Dec 4 2017

Pre-frontal convergence and strong low level moisture transport
has resulted in scattered shower development across northeast
Indiana/northwest Ohio. Trend over the next few hours should be
for more favored pre-frontal convergence to slowly shift eastward
and affect mainly northwest Ohio through 06Z. Otherwise, strong
southerly flow will continue across the area this evening in
advance of approaching cold front. Still expecting some additional
uptick in these southerly winds to the 25-30 mph, gusts to 35-40
mph range over the next few hours. Meanwhile, a strong upper vort
lobe will lift northeast across northern Wisconsin later this
evening helping to drive cold frontal boundary eastward. An axis
of 250-500 100 mb MLCAPE along MS River has been sufficient for
broken line of convection to form early this evening. Through the
late evening hours, these limited instability magnitudes should
wane with eastward extent, and expecting thunder coverage to be
much more limited than is currently being observed across western
Illinois. Still some concern as noted in previous forecast of some
stronger winds aloft mixing down with these showers, but greater
concern for winds will be mainly synoptic in nature given
subsidence and pressure rises accompanying frontal passage.
Guidance does continue to depict some diminishing magnitude to
this isallobaric contribution as front tracks across the area
overnight, and still feel gusts should remain below advisory
criteria for the most part.

No major changes anticipated to the forecast at this point with
only changes of note to add iso thunder mention early evening for
pre-frontal forcing portion of this event, and to slightly slow
onset of higher PoPs with frontal forcing overnight.


.SHORT TERM...(This Evening through Tuesday)
Issued at 315 PM EST Mon Dec 4 2017

Rapid deepening of Upper Midwest cyclone is now underway as Canadian
PV anomaly dives south and phases with main shortwave ejecting out
of the High Plains. This wave will take on a negative tilt by
tonight with strong, coupled upper level jet streaks and good
mid/upper diffluence allowing efficient UVM and deepening of surface
low to around 972mb. Trailing cold front still poised to cross our
CWA around 09Z late tonight and herald arrival of stark pattern
change that will bring much colder conditions for the foreseeable

Main weather concern will be the strong winds developing this
afternoon and continuing through tomorrow. Very tight gradient ahead
of the wave generating a prefrontal LLJ of 60+ kts. Main question
for this evening will be how much of this can we mix into. Always
tough to get advisory criteria (gusts to 45 mph) in stable/WAA
regime. Forecast soundings show some impressive wind speeds just a
few kft off the surface but tapping into the best wind speeds may
prove difficult given stable conditions and waning insolation. GFS
is most bullish and would suggest a few hours of 40+ kt gusts late
this afternoon right around sunset. A few gusts may approach 40-45
mph this evening but based on upstream obs, our SE displacement
from main low, dwindling daylight, and steady WAA/stable profiles
prefer to hold off on advisory for now.

Bigger concern for some brief gusts to 45 mph will be later tonight
with actual frontal passage. Signal for convection is very weak but
enough low level moisture advection to support a few hundred J/kg of
MUCAPE. Regardless of actual thunder, concern is that convective
showers could mix down stronger gusts in isolated spots. Also of
particular concern is strong/abrupt surge of isentropic descent seen
on 285K surface immediately behind surface front 08-12Z.
Conceptually, this is a much better scenario for downward momentum
transfer of strong winds. However, the signal is brief as
postfrontal winds relax (to under 40kts anyway) quickly. Given
confidence in widespread advisory criteria gusts is low and
potential for a short window, prefer to let later shifts make final
call with benefit of upstream obs. Current thinking is it would be
best to handle with SPS`s as needed with/just behind the front.
Strong winds then continue into tomorrow with deep mixing in dry/CAA
regime. Winds speeds at top of 5-6 kft boundary layer are only
around 40 kts though and we should remain below advisory criteria.

Rainfall amounts tonight will not be particularly impressive given
quick-hitting nature. Still looking at only 2-3 tenths for most
locations. Sharp temp drop still expected with early morning temps
in mid 50s...dropping to the 30s by 12Z and remaining there through
the day. Wind chill values will be in the 20s. Slight chance for a
few flurries late tomorrow with passage of strong vort max. Profiles
are very dry though and not expecting any accumulation/impacts.


.LONG TERM...(Tuesday Night through Monday)
Issued at 315 PM EST Mon Dec 4 2017

Story for the long term remains cold with just a few low chances for
light snow, mainly lake effect. 850mb thermal differentials
certainly supportive but flow on Wed is almost due west (even WSW)
with significant dry air entrainment and limited time for theta-e
flux given strong flow. Flow does veer Wed night Thu as weak
shortwave passes and this may be best time for lake response but
forecast soundings show relatively low inversion heights and still
some dry air entrainment concerns. Kept close to previous forecast
with 1-2" possible through Thu.

Also a chance for a few flurries inland on Wed as shortwave passes
through. Moisture is extremely limited so held with just flurries
for now with no accumulation/impact expected. Slightly better chance
for light snow on Sat but moisture remains limited and dynamics
relatively weak. Models still struggling with exact evolution as
well so will hold with chance concensus PoPs. Any accumulation will
be light if it does occur.

Of course other main story will be continued cold. Lowered concensus
blends again based on raw guidance. Highs likely struggle to get
above freezing with wind chill values in the teens through at least
Sat and possibly longer.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday Evening)
Issued at 735 PM EST Mon Dec 4 2017

Pre-frontal forcing will allow a few showers to persist across
northeast Indiana over the next hour or two, but majority of
period through 04Z should be dry. Sfc low pressure continues to
deepen across far western Lake Superior this evening which will
allow for maintenance of 20G30-35KT southerlies in advance of the
cold front. Fairly strong pressure rises with the cold front and
steeper low level lapse rates should support a brief 2-3 hour
window of strong westerly gusts, possibly as high as 35 knots in
the 08Z-12Z timeframe. Strong pre-frontal low level moisture
transport may allow for a brief period of MVFR cigs ahead of the
front, followed by cold advection induced MVFR cigs early Tuesday
morning before improving to VFR toward mid morning. Frontally
forced showers will work from west to east across northern Indiana
in the 04Z-10Z timeframe, with generally a 3-4 hour period of
rain at each terminal. Current observational data suggests good
coverage of thunder across MS River but waning instability with
eastward extent overnight should make thunder more isolated in
nature. Given diminishing coverage to thunder overnight, will
refrain from mention with the 00Z TAFS. Gusty winds will continue
into Tuesday, but gusts should be limited to the 25 to 30 knot


LM...Gale Warning until 7 AM EST Wednesday for LMZ043-046.




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