Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 060008

Area Forecast Discussion
National Weather Service Northern Indiana
708 PM EST Mon Dec 5 2016

Issued at 658 PM EST Mon Dec 5 2016

Areas of fog is expected to develop overnight, especially across
southern lower Michigan and northern Indiana. Lows tonight will be
in the upper 20s to lower 30s. A low pressure system will lift
northeastward from the Lower Mississippi Valley into the Middle
Ohio Valley region by midday Tuesday. This system will bring some
rain on Tuesday, especially south of Route 24. Highs on Tuesday
will be in the upper 30s to lower 40s. Then Much colder air will
filter into the region behind the system and set the stage for
lake effect snow showers Thursday through Friday night. High
temperatures by Thursday afternoon are only expected to reach the
mid 20s.


.SHORT TERM...(This Evening through Tuesday)
Issued at 242 PM EST Mon Dec 5 2016

Primary issues for the short term forecast will continue to focus on
fog and precip overspreading especially southeast half of the area
very late tonight into Tuesday.

The more significant fog through early this afternoon has been
confined to locations west of the forecast area across
northwest/north central Illinois. Weak moisture advection over
new snowpack across these locations has proved favorable for dense
fog formation this morning/early afternoon. Some concern tonight
that as low level ridge axis continues to shift eastward that
focus of fog will expand eastward locally across especially
snow covered/recently melted snow areas. Current thinking is that
the northwest portions of the area may be inline for most
significant fog where cannot rule out some dense fog later this
evening/early overnight. Southern edge of low cloud deck also has
been making steady northward progress over past few hours which
could help in some radiational pre-conditioning early this evening
in terms of fog formation. Ramping up of low level flow overnight
and stronger WAA may eventually tend to deter lower vsbys

The next upstream feature to watch for late tonight/Tuesday is
fairly impressive short wave this afternoon tracking across
southwest Texas. Track of this wave should be somewhat suppressed by
an active short wave pattern across the northern Plains/upper
Midwest. Bulk of guidance still indicates that best mid/upper level
forcing along with best low level moisture convergence axis should
propagate through southeast portions of the forecast area early
Tuesday. Thermal profiles at precip onset do appear on the marginal
side in terms of precip type, but confidence in snow of mentionable
significance appears to be dwindling due to initially dry conditions
in ice nuclei production layers. Still feel that rain will be the
dominant precip type, and currently not expecting any snow
accumulation. Confidence in westward extent of measurable precip is
on the low side, although some signals exist for secondary weak fgen
band that could yield some light precip Tuesday morning across the
northwest. Forcing will be progressive but given fairly
impressive moisture and brief period of stronger low level
moisture convergence, could see some rain amounts in the 0.25-0.5
inch range across the southeast.


.LONG TERM...(Tuesday Night through Monday)
Issued at 242 PM EST Mon Dec 5 2016

Mainly dry conditions expected by Tuesday night as short wave pulls
out of the area. Passage of this wave will mark the beginning of a
prolonged cold advection period as a stronger mid/upper level height
minimum tracks across southern Ontario. Persistent inversion and
steep low level lapse rates should keep low clouds around on
Wednesday with highs likely limited to the lower to mid 30s.

It still appears as though secondary surface trough with stronger
low level cold advection to track through the area late Wednesday
night into Thursday morning. This airmass will mark some of the
coldest air this season to date with highs only expected to reach
into the mid 20s. Along with the cold air will come increasing lake
effect snow potential from Thursday afternoon through Friday night.
Moderate to strong lake induced instability is expected and while
fetch may be limited to west-northwest, good deal of flux from
relatively warm Lake Michigan should partially compensate. Early
look at 0-2km theta-e lapse rates suggest values of 1-2 degrees,
with lake/700 mb delta T`s on the order of 27 to 30 degrees. There
does appear to be a window of synoptic enhancement to lake effect
with upper trough passage Thursday night-Friday morning which
could represent window of greatest potential. Magnitude of air
mass moving into the area also should be supportive of favorable
low/mid level DGZ orientation to strongest lift. This certainly
will be a period to watch going forward for lake effect snow
potential, with threat gradually waning Friday night as low level
flow begins to back. Synoptic enhancement could allow for some
fairly good inland penetration to lake effect snow showers
Thursday night-Friday morning. Otherwise, this period will be
characterized by raw/blustery conditions with west winds gusting
to around 25 mph Thursday/Thursday evening.

Waning lake effect snow showers later Friday night will then give
way to fairly active eastern Pacific pattern with next upper jet
streak expected to carve out next upper trough across lee of
Rockies late Saturday/Sunday. There has been some run to run
volatility with this pattern in last several medium range model
runs, but it does appear pattern would be favorable for active
pattern to continue late weekend. At this distance, have just
maintained mid range chance snow PoPs for second half of weekend.
Flow deamplification in advance of this Pacific energy will allow
for some moderation in temps this weekend, but still likely below
seasonal norms for early-mid December.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday Evening)
Issued at 709 PM EST Mon Dec 5 2016

Difficult forecast with slowly north retreat of IFR stratus
presently near Route 6 and making gradual inroads northward. May
take a few more hours to scatter out at KSBN. Then concern with
BR/FG formation with widespread wetting ground/snowmelt and
extremely limited mixing there today. Potential for lower
visibilities than presently forecast with IFR relegated to 4 hour
tempo period 07-11 UTC. A bit more mixing at KFWA and kept above
fueling/alternate criteria through morning hours. Concern with
ceilings lowering as northern edge of rain shield spreads into
KFWA late morning as low pressure moves northeast through the Ohio
Valley. Best potential for dominant IFR ceilings to linger at KFWA
through end of forecast period.




SHORT TERM...Marsili
LONG TERM...Marsili

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