Area Forecast Discussion
Issued by NWS Northern Indiana

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244
FXUS63 KIWX 151741
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
1241 PM EST Mon Jan 15 2018

.SYNOPSIS...
Issued at 449 AM EST Mon Jan 15 2018

Light snow over the area this morning will begin to diminish from
west to east this afternoon. Total snow accumulations of 2 to 4
inches are expected, with locally higher amounts closer to Lake
Michigan. Additional significant lake effect snow accumulations
are possible from late tonight through early Wednesday morning
close to Lake Michigan. High pressure will bring quiet weather
through Thursday. A warming trend is expected late this week into
the weekend along with a chance for rain.

&&

.SHORT TERM...(Today and Tonight)
Issued at 449 AM EST Mon Jan 15 2018

Forecast on track for the most part early this morning. Light snow
has spread across the entire area with visibilities generally in the
1 to 3 mile range as weak isentropic lift continues to spread
east. Flake size here at KIWX has been very small through the night
so far, much as we anticipated. Snow has been falling lightly since
about 10 pm EST through 4 am EST and we have maybe an inch on the
ground, and that is likely a generous inch. Upper level jet
streak was beginning to round base of upper level trough and
moving toward central Indiana early this morning. Left front exit
region has helped increase upstream reflectivity on regional radar
mosaic and sfc obs indicate some visibilities below one mile
where stronger lift is occurring with this feature. Models in good
agreement on left exit region moving over western areas toward
daybreak then moving east through the morning. Expect this period
to see uptick in reflectivity and snowfall rates as bufkit
soundings still indicate this deeper lift becoming better aligned
with DGZ to increase dendrites and flake size leading to better
snow ratios and accumulations. Soundings still not supersaturated
during this period so not expecting rates to get out of hand. Plan
on staying course with total accums in the 2 to 4 inch range with
highest accums in the west.

Snow should diminish from west to east this afternoon then focus
shifts quickly to potential lake enhancement and strong lake band
for tonight into Tuesday and Tuesday night. This is a difficult
scenario to forecast given the east-west trough that becomes
stationary tonight near our northern border as upper level
trough drops into the area along with strong PV anomaly. A mesolow
is expected to form along sfc trough over the lake this evening and
low level flow around this wave may keep light snow going across
mainly northern Berrien county. The mesolow is then expected to sag
south overnight and this is where much uncertainty lies with whether
it will be close enough to the shore to keep snow showers going
inland or develop far enough west that circulation would keep most
of the snow offshore. Experience suggest this vortex would remain
close to shoreline taking advantage of thermal contrast and
developing offshore flow from nocturnal processes along the
lake/land border. 00z and 06z NAM12 hinting at this with 925mb
trajectories and omega fields. Thus have started the winter storm
watch for the lake effect at 06z in Berrien as the advisory ends
with transition to more lake effect thereafter. Opted for the
watch headline given the uncertainty and possible break of a few
hours if sfc trough moves farther north and lake effect is delayed
or kept offshore. No easy way to transition from the certain
advisory event to one with more uncertainty.

This still looks like a Lashley-Hitchcock Type VI event though a bit
of a hybrid given the more southern extent of trough and low
formation. Strong single band expected to develop overnight and
likely to be cyclonically curved down spine of lake behind the
mesolow. Sfc-850mb delta Ts still in the low to mid 20s and
0-2km thetaE lapse rates between -1 and -2 K/km will be
ample for low level instability. Strong 925mb convergent flow
over mainly open water will take advantage of this instability plus
plenty of resonance time for moisture flux to produce an intense
single band. The question remains where and when this band will
curve inland on Tuesday. Have trended forecast toward the NAM12 and
3km NAM along with SREF probabilities which bring this band into
Laporte county Tuesday morning. Strong enough winds and
trajectories suggest decent accumulations once again reaching
Starke and Marshall counties before band moves east into St.
Joseph later Tuesday and Tuesday night. For simplicity, started
the Watch for these counties at 12z Tuesday.

Snowfall amounts from lake effect are always difficult but expect
rates of 1 to even 3 inches per hour given the above parameters
so significant accumulations could occur in a very short period of
time, very similar to the event a few weeks ago. Berrien also a
bit tricky as most of county likely to see a lull on Tuesday
behind mesolow before single band moves back east later in
afternoon and especially Tuesday evening. This can be refined a
bit more later today with more hires guidance to look at and
eventual warning or advisory headlines.


&&

.LONG TERM...(Tuesday through Sunday)
Issued at 449 AM EST Mon Jan 15 2018

Obvious main focus for this forecast package has been on short term
concerns. Lake effect snow will be ending Wednesday morning as flow
quickly backs and inversion crashes with warming aloft. Models in
good agreement on warming trend through the end of the week into the
weekend. GFS showing fast moving compact short wave Thu night but
this currently an outlier compared to most other models so will
discount for now and keep forecast dry. Next chances for pcpn arrive
over the weekend as trough begins to develop over the southwest
CONUS and moisture spreads north ahead of it. Potential does exist
for some moderate to possibly heavy rainfall late in the weekend as
strong area of low pressure develops and lifts northeast. Models
generally have us in the warm sector and showing a long and deep
tropical connection which could really increase precipitable water
values. Still a lot of time for this to be sorted out but flooding
issues a possibility by early next week if this scenario pans out.
This also more of a La Nina favored wet pattern which does lend some
credibility to model solutions.

&&

.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 late tonight through Wednesday morning
     for MIZ077.

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

$$

SYNOPSIS...Lashley
SHORT TERM...Lashley
LONG TERM...Lashley
AVIATION...AGD


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