Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 152113
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
413 PM EST Mon Jan 15 2018

.SYNOPSIS...
Issued at 409 PM EST Mon Jan 15 2018

Light to moderate snow will dissipate for most inland areas
gradually overnight. Minor accumulations are expected. Otherwise,
heavy lake effect snow is possible for snowbelts impacted by north
to northwest winds tonight into Wednesday morning. Accumulations
greater than 6 inches are possible, especially in Berrien and La
Porte counties. See the latest winter weather message for details.
Otherwise, temperatures will fall to the single digits tonight.
Areas south of US 30 could see wind chills as low as 10 to 15 below
zero.

&&

.SHORT TERM...(This Evening through Tuesday)
Issued at 409 PM EST Mon Jan 15 2018


...Significant Lake Effect Snow Possible Late Tonight into Wednesday
Morning...

Expect a diminishing trend to the snow ahead of the clipper system
as the best isentropic ascent and upper level forcing weaken
extensively, and shift eastward out of the area overnight. The dry
slot has already advanced across much of the CWA and expect this
will continue as the low lifts NE. As far as the lingering trough
axis behind our clipper, things look on track as far as the larger
scale pattern. Aloft, we`ll see the upper level low/potent PV
anomaly move over southern Lake Michigan, which gives us ample
support for snow development. Accordingly we see models develop
precipitation along the WSW-ENE oriented trough axis that develops
at the surface, with winds becoming more WSW and N-NW by early
Tuesday morning. With this convergence over the lake, expect a
mesolow to develop to our north. This will drift southward on the
long-axis of Lake MI with the surface trough through the evening. As
winds become north-northwesterly behind the low, expect our single
band to develop and move into the CWA. Though I`m confident in the
development of lake effect snow tonight as describe above, the
details on location/amounts with lake effect can be very difficult
to narrow down with any real confidence. At this point, the question
seems to be if, when, and where the mesolow will come onshore, and
what condition it will be in once it gets there.

Agree with previous shift in that it will likely be more over
Berrien county late tonight into early Tuesday morning (9-12Z) as
most of the high res model guidance seems to agree on this, and that
is when we see the trough axis move overhead. Michigan counties to
the east of Berrien may see light snow at this time, but don`t think
it will be anything of significance at this point.

From there, however, the guidance falls into disagreement. Some of
the models keep the mesolow and the trailing single band further
west across the lake, bringing it onshore in LOT`s CWA (Porter, Lake
& eastern Cook Counties)Tuesday morning and taking it eastward into
our area Tuesday afternoon into Wednesday morning. Others have the
band developing over LaPorte County, and have it drift eastward
before transitioning to more of a multiband, NNW Wind set up. Models
furthest west (including GEM), have the band not even reaching our
CWA until Tuesday evening, which could really mess up the forecasted
higher snow totals. The GEM has had a westward bias with lake effect
set ups in the past, but with the trough and potent PV anomaly in
the upper levels not sure this is a more traditional set up, and it
could be onto something.

Given these concerns, and collaboration with neighboring offices,
opted to stay with current winter weather headlines (Advisory
through 6z for Berrien, then winter storm watch for Starke,
Marshall, St. Joseph, IN, and La Porte from 12Z Tuesday into Wed
AM). Storm totals, assuming both the mesolow and the trailing
snowband come onshore in Berrien/LaPorte and drift eastward, could
be up to 6-8" with highest totals in southwestern Berrien County and
in La Porte County. Convective depths are 6-9K ft at times using the
NAM, and lift is centered within or just slightly below the DGZ-
which would yield us higher ratios. Additionally, there is a lake
superior connection, and abundant lingering moisture associated with
the clipper.

HOWEVER...If the band sets up further westward and the mesolow never
makes it far onshore in Berrien, we`ll be seeing much less.
Additionally, with the current advisory in effect, did not want to
issue a double advisory headline. Will let the evening and midnight
shifts monitor the short term trends and update accordingly.

Outside of lake effect concerns tonight into Tuesday, expect quiet
weather for inland areas as surface ridging builds in. Low
temperatures will drop into the single digits, with some areas
dropping slightly below zero. Wind chills will range from -5 to -
15F, with areas south of US 30 seeing the lower wind chills of -10
to -15. This is sub-wind chill advisory criteria, but will be
something we need to monitor closely overnight for those southern
counties.


&&

.LONG TERM...(Tuesday Night through Monday)
Issued at 409 PM EST Mon Jan 15 2018

Diminishing Lake Effect: Wednesday into Friday Night

High pressure builds overhead, and winds gradually shift to the WSW,
forcing the lake effect band eastward into Lower MI/northern IN
before it finally dissipates late Wednesday morning. Conditions will
be dry, but still relatively cold. High temperatures Wednesday and
Thursday will be in the 20s. Overnight lows will be in the teens. On
Friday, we`ll warm up some with highs nearing 30 degrees.

Warm and Wet Weekend Ahead: Saturday into Monday

WAA really ramps up late Fri into Sat as next major Pacific trough
moves into the southwest CONUS and downstream ridge-building/SW flow
envelope the Ohio Valley. Expect highs to climb into the 40s and
snowmelt to accelerate. Increasing isentropic ascent could also
bring some light precip on Sat. A very brief mix may be possible at
the onset but precip type looks to be primarily rain through the
weekend. Best chance for precip will be late Sun as primary trough
axis/cold front passes. Models in reasonable agreement for this time
range but still too early to nail down exact details. Some concern
for additional river rises with rain and melted snow but current low
track would suggest a relatively short time window of moderate rain.
Brief return to cooler temps and snow showers on the backside but
will be short-lived given prevailing W/SW flow aloft.

Entering a fairly active weather period with several high amplitude
waves ejecting out of the southwest CONUS and impacting our area.
First such wave already beginning to lift NE and will bring a chance
of rain later this evening and overnight. Chances for
widespread/moderate rain are on the low side, outside of our far
western CWA. Midlevel wave dampens and pivots more NNW overnight
while best upper level support remains just to our NW. Trailing cold
front does not arrive until diurnal minimum of instability and will
be intersecting increasingly hostile environment as it tracks east
with residual dry air and loss of parent forcing for ascent. Still,
most places will probably see some light showers with morning cold
frontal passage and latest hi-res guidance does show convection
presently entering southern Illinois will clip our western zones
this evening. A few strong storms with gusty winds may be possible
with the evening activity but instability is very limited by the
time this wave arrives and not expecting any severe threat.
Convection should gradually trend down as it lifts NNE late this
evening.

Anticipate dry conditions by Thursday afternoon. Non-diurnal temp
trend expected given morning cold front passage. Overnight temps
will likely hover in the 60s and then hold steady or slightly
decline during the day as CAA wing rotates through.

Of slightly more concern is potential for heavy rain over the
weekend. Strong Pacific jet (around 160 kts at 300mb) will carve out
a healthy PV anomaly over the southwest CONUS by early Saturday with
downstream ridge amplification in our our region. Rain chances begin
again Friday afternoon with initial surge of low level theta-e and
WAA but this initial rain will be light/SCT with lackluster moisture
advection and weak isentropic upglide. LLJ strengthens Friday night
as trough inches closer but most guidance sources keep focus of
moisture convergence/precip to our south, which makes sense given
the pattern aloft.

Main timeframe to watch will be later Saturday into Sunday and
Sunday night. Several waves of moderate/heavy precip expected during
this time as upper low approaches, sends warm front north, and then
eventually cold front arrives late Sunday. Deterministic QPF taken
at face value would suggest prodigious rain but smaller scale
convective elements often wreak havoc with final solution and latest
12Z GFS actually shows main axis of heaviest rain will set up to our
west and southwest. Periods of rain certainly expected, and may be
heavy at times, but given recent (relatively) dry conditions, spring
green-up, and low levels on area rivers, suspect it will take a lot
to cause any significant flooding. Moisture advection/PW values
don`t really ramp up in our CWA until late Sunday, ahead of quick-
hitting cold front, and orientation of front with respect to
mid/upper level pattern does not seem favorable for a long duration
of continuous heavy rain. Given very low confidence on flooding
potential, will hold off on any ESF issuance at this point.

Some, very low, threat for severe weather on Sunday but late arrival
of cold front and very marginal instability to begin with will
severely limit potential. Some light wraparound showers possible
through Monday but main story will be cooler temps. Mainly dry
conditions expected midweek, though will have to watch next trough
swinging through the southern CONUS. Latest runs keep bulk of this
precip to our south but something to watch given wet weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday Afternoon)
Issued at 1240 PM EST Mon Jan 15 2018

Conditions will continue to hover around fuel alternate MVFR
through the period. Strongest snow has lifted north and expect
just light snow through the afternoon...slowly tapering off by
this evening...with visibilities of 2-5 SM. Expect slight
improvement overnight as drier air advects into the area but
ceilings will likely remain in high-end MVFR category. Lower
ceilings and lake effect snow anticipated by tomorrow morning as
primary vort max swings through. Significant single band of lake
effect snow possible at KSBN but best chances appear just outside
of this TAF period.


&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Weather Advisory until 7 PM EST /6 PM CST/ this evening
     for INZ003>009-012>018-020-022>027-032>034.

     Winter Storm Watch from Tuesday morning through Wednesday
     morning for INZ003-004-012-014.

MI...Winter Weather Advisory until 7 PM EST this evening for
     MIZ078>080.

     Winter Weather Advisory until 1 AM EST Tuesday for MIZ077.

     Winter Storm Watch from 1 AM EST Tuesday through Wednesday
     morning for MIZ077.

OH...NONE.
LM...NONE.
&&

$$

SYNOPSIS...MCD
SHORT TERM...MCD
LONG TERM...AGD/MCD
AVIATION...AGD


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