Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 040524
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
1224 AM EST Mon Dec 4 2017

.SYNOPSIS...
Issued at 308 PM EST Sun Dec 3 2017

The warming trend will continue overnight and into Monday, with lows
tonight only dropping into the 40s. Highs Monday will climb into the
upper 50s and low 60s. Rain, gusty winds, and even a few
thunderstorms will move in Monday as a strong low pressure system
crosses the Great Lakes. Wind gusts up to 35 to 40 mph are possible,
especially Monday afternoon into Tuesday. Temperatures will fall
into the 30s by mid Tuesday afternoon. For the rest of the week,
expect continued cold and lake effect snow. Highs will be in the
30s and lows will be in the 20s. Another system may move through
the area Thursday night into Saturday, bringing chances for snow
inland.

&&

.UPDATE...
Issued at 1221 AM EST Mon Dec 4 2017

Substantive update to more greatly delay/lower pops/qpf through
Monday. Low level jet focus held well west of CWA within 60-70 kt
core from nw MO into far sw WI through 12 UTC today. Leading edge
of richer bulk moisture /1000-850mb mixing ratios/ does increase
in excess of 8 g/kg by 00 UTC Tue, though low level condensation
pressure deficits remain generally at or above 100 mb, save for
far northwest CWA clipper by shortwave, presently over IA, in the
12-15 UTC timeframe. Backed sse surface flow into midday also
serves to delay onslaught of moisture advection. Slight increase
for high temps as well Monday given placement of low level
thermal ridge and upstream max temps Sun in addition to lowered
shower potential.

&&

.SHORT TERM...(This Evening through Monday)
Issued at 308 PM EST Sun Dec 3 2017

It`s the final countdown, with one last day of unseasonably warm
temperatures for the short term forecast. Monday`s high temperatures
will climb to the upper 50s and even a few spots hitting the low 60s
as warmer air advects into the region.

The strong sfc low pressure system forecast to bring the return of
winter will begin its rapid deepening by Monday morning as it moves
into IA/MN. The 500mb closed low and negatively tilted trough at
this time will be situated over the Dakotas, providing ample support
for deepening of the surface low per 850-500mb qvector convergence.
Additionally, a strengthening jet at 250mb will round the base of
the upper most trough. At this point, it will be around 135 knots,
and with the surface low in the left exit region, and an amplifying
ridge to our east, this jet will strengthen to 150 knots by the time
we reach Tuesday afternoon: which is when it will be over the Ohio
Valley. Long story short, things are lining up for our low pressure
system to lift into the western UP and drop nearly 12 mb in 12 hours
(at least according to the most extreme guidance, there are slight
differences). In other words, this is a strong system. As for our
forecast area, we`ll be in the warm sector by late this evening
as the surface warm front lifts northward, with the more abundant
moisture and favorable isentropic ascent to our west.

What does that mean for tonight and Monday`s forecast? We`ll see a
little bit of rain shower activity associated with the warm air
advection (mainly west of I 69 and north of I 80-90), and a steady
increase in southerly winds through Monday afternoon. From the
latest guidance, expect gusts to be in the 30-35 mph range by then,
especially west of I 69. A gale warning is in effect from 4 pm
Monday for our nearshore zones, where wind gusts could reach as
high as 40-45 mph.

&&

.LONG TERM...(Monday Night through Sunday)
Issued at 308 PM EST Sun Dec 3 2017

The first concern for the long term will be the rapidly deepening
low pressure system to our northwest, which will bring strong winds,
rain, and even a few thunderstorms to the Great Lakes Monday night
into Tuesday. The second concern will be lake effect snow, which is
mainly within west to west-northwesterly wind favored snowbelts. The
final concern will be a low pressure system that may or may not
impact the area with snow Friday-Sunday.

First and foremost, our strengthening low pressure system. Monday
evening we`ll be in the warm sector, with increasingly gusty
southerly winds. Some of the models suggest precipitation associated
with a pre-frontal trough that develops as the low lifts into
western Lake Superior, deepening from 993mb to roughly 981mb in 12
hours. The cold front at this time will be to our west, storming
eastward through Illinois (literally). The rain with this pre-
frontal feature will be pretty light and inconsequential compared
to the cold front, which will enter our west by around 10 pm
Monday night and continue eastward, exiting the forecast area NLT
10 am Tuesday. There is a slight chance of thunderstorms with the
rain along the front, but the 150-300 J/kg of MUCAPE forecast for
Illinois wanes as it heads eastward into Indiana, and dissipates
by the time it reaches Ohio. Limited to slight chance of thunder
for now. Given the favorable set up for a rapidly deepening
surface cyclone, I am tempted to side with the more progressive
GFS (versus the still slower ECMWF). Given that it is still just
over 24 hours out, I slowed down front timing just a bit to
account for the discrepancies between models. Plus, the 12z GFS is
an outlier when compared to the others-both in westward shifted
track and faster deepening.

Winds will be the main threat surrounding the time of the frontal
passage, especially Monday night into Tuesday morning. Winds will
shift from the south to the west behind the front. With strong cold
air advection (a 15 degree drop in 12 hours), a steep pressure rise
in the wake of the cold front (more than 8 mb in 3 hours per
NAM/GFS), and mid level lapse rates of around 6-7 C/Km think we
could see some stronger wind gusts. 925mb winds are as high as 40-50
mph by this timeframe, and so it is possible we mix some of this
down to the surface when we lose our stability with the colder air
aloft. In summary, for Monday night I have westerly wind gusts of 35-
44 mph in the forecast, with the highest gusts near the lakeshore
and in the western half. If this pans out, we`ll be on the
doorstep of wind advisory criteria (gusts to 45 mph). For now, I
think any of the higher-end wind gusts would be with the heavier
rain showers or thunderstorms, which could transfer some of the
stronger wind gusts aloft down to the surface. The only other
exception would be those areas along the Lake Michigan shoreline,
where wind will flow relatively uninhibited across the water.

Gusty winds will continue through Tuesday under continued cold air
advection. By early Tuesday afternoon, The ECMWF/NAM/GFS both
suggest higher mixing heights, and potential for continued gusts up
to 30 mph inland, and maybe up to 35-40 mph near the lakeshore.
Temperatures will fall through the day, with early morning highs
in the 50s dropping to the low-mid 30s by late afternoon. Combined
with the wind, it will feel downright chilly, with wind chills in
the 20s in the northwest by the afternoon. I`m also betting that
it will be dry, and that pops during this time from consensus
blend are overdone. During this period we`ll be in the dry slot of
the strong surface low, which by this point is lifting into James
Bay.

Focus then turns to Wednesday and Thursday, where the story is cold
and lake effect potential. Unfortunately, models diverge a little
bit with respect to the strength/timing of several vorticity lobes
rippling through the broad upper level trough that encompasses the
Great Lakes. Favorable factors include the lift associated with the
spurious shortwaves and the broad upper level troughing pattern. Of
course, with a westerly wind fetch is limited in comparison to more
of a northwest flow. 850mb temps fall to around -13 to -14C, and
delta theta-e values are pretty decent, which gives us some
instability to work with. The only limiting factor I think at this
point is moisture availability-as the better moisture is further
north. Some of the latest model guidance keeps the lake effect bands
in this better moisture, which keeps development of snow showers
further north. For now, slightly lowered pops for all but the far
northern portions of the CWA.

Thursday night into Saturday we deal with continued cold
temperatures and another system. Lowered consensus blend
temperatures again, as they are still clinging to climatology. High
temperatures look to be in the upper 20s and low 30s, and lows in
the teens and 20s. As for precipitation chances, models have yet to
get their act together as to the major synoptic features. The GFS
still brings in a closed upper level low by Friday evening and
deeper trough, while the ECMWF keeps our area at the base of a
trough, and delays developing the closed upper low until Saturday,
which it keeps further north in the Eastern UP of Michigan. It
looks somewhat safe to say that we`ll see some precipitation given
the upper level trough, but exact locations are not going to
pinned down until the models become more organized and consistent.
I will say that we`ll see some lake enhancement if there is
moisture around, especially as winds remain largely out of the
west-northwest during this period. So stay tuned, because we`ll
be keeping an eye on things.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday Night)
Issued at 1016 PM EST Sun Dec 3 2017

Low level winds will continue to increase overnight with the
departure of the low level anticyclone across the Ohio Valley.
Broad sfc pressure falls across the Upper MS Valley will also
continue to overspread western Great Lakes overnight aiding in
some uptick in southerly flow. Not much change anticipated with
the 06Z TAFS with shallow mixing on Monday allowing gusts into the
25 to 30 knot range (strongest winds at KSBN). Strengthening
advective forcing overnight should allow for an expanding VFR
cloud deck across southern Illinois to reach northern Indiana
after 09Z. Strongest isentropic lift/moisture transport should
remain situated north and west of terminals through remainder of
overnight, and while some potential of isolated showers will exist
at KSBN toward daybreak, will keep TAFs dry.

A better chance of rain showers will develop by late Monday
afternoon or Monday evening as stronger pre-frontal low-mid level
theta-e advection begins to affect northern Indiana. Relatively
steep lapse rates aloft could allow for a few isolated
thunderstorms, but confidence/expected coverage much too low for
inclusion at this time. This period (Mon evening) will also be
accompanied by greater probabilities of MVFR cigs. Gusty south
winds to 25 to 30 knots should continue into Monday evening in
advance of a strong cold front that will shift winds to the west
just beyond this forecast valid period.


&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...Small Craft Advisory until 4 PM EST this afternoon for LMZ043-
     046.

     Gale Warning from 4 PM this afternoon to 7 AM EST Wednesday for
     LMZ043-046.

&&

$$

UPDATE...Murphy
SYNOPSIS...MCD
SHORT TERM...MCD
LONG TERM...MCD
AVIATION...Marsili


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