Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 180537

Area Forecast Discussion
National Weather Service Northern Indiana
1237 AM EST Sun Feb 18 2018

Issued at 1232 AM EST Sun Feb 18 2018

Drier conditions are expected overnight through this evening with
lows in the 20s and highs in the 40s. Much warmer and wetter
conditions will arrive Monday through Wednesday. Several inches
of rain are possible during this period which will likely lead to
flooding on area streams, rivers and lowland areas in general that
are prone to flooding.


Issued at 450 PM EST Sat Feb 17 2018

Robust upstream sw disturbance over wrn IL will eject quickly ewd
this evening. Ahead of this system strong pv advection in
combination with vigorous low level theta-e burst yielding a
substantial yet relatively brief period of sustained strong vertical
ascent. No doubt plume of modest lapse rates aloft aiding and
abetting upstream snow rates as observed through ne IL/wrn IN attm
and inferred in widespread 30-35dbz echoes and convective nature of
GOES-R vis imagery. Thus in concert with sharply uptrending near
term guidance trends seen in HRRR/RAP will augment going evening
grids sharply higher as several storm reports out of ne IL
indicating snow rates in excess of 1" per hour. Otherwise diabatic
cooling at onset and heavy precip rates will forego prior rain
mention with quick 1-2" accum expected west of the I-69 corridor
this evening and an inch or less east.


.SHORT TERM...(This Evening through Sunday)
Issued at 320 PM EST Sat Feb 17 2018

Robust midlevel shortwave currently approaching the area with our
next chance of light precip this evening. Strong PV advection to
pass directly overhead with 1.5 PVU surface extending to 500mb.
Dry/stable conditions below 500mb as noted on both KILX and KDTX 12Z
raobs but strong vertical ascent is compensating. Upstream obs show
visibilities 1/2 mile or less at times with a quick 1-2" of snow
accumulation. Surface temp/dewpoint guidance also trending a little
cooler/drier supporting less rain and more snow. With strong precip
rates...will likely be almost entirely snow but some melting
possible at surface. Best moisture advection remains well to our
south with lead shortwave and do still expect a general weakening
trend as precip shield moves east and encounters increasingly
hostile environment. Still...hires/rapid refresh guidance is
trending stronger/wetter in our CWA with each run and could see a
quick inch or so in our NW zones with a dusting for the rest of the
CWA. Time window still anticipated to remain short as wave exits
shortly after midnight.

Little/no substantive CAA behind this wave and expect lows only
around mid 20s for most locations tonight. Low level ridge axis
passes quickly Sun morning and WAA increases substantially by late
Sun. Mid 40s should be easily attainable with mostly sunny skies.


.LONG TERM...(Sunday Night through Saturday)
Issued at 320 PM EST Sat Feb 17 2018

Wet weather still on tap for at least the first half of the work
week. Longwave trough develops over the western CONUS with behemoth
subtropical ridge off the southeast coast. Quasistationary
baroclinic zone to set up in between with copious moisture advection
flooding into the Midwest. Front aligned nearly parallel to the mean
flow and expect several rounds of rain with slow ejection of
southwest CONUS trough. Initial theta-e surge arrives Mon morning
with our first good shot of widespread moderate rain. Could even see
a few elevated thunderstorms with this initial surge...though
instability profiles are a little too marginal to introduce thunder
at this stage. Not expecting much impact with this initial rain.
Potential problems will arise Tue into Wed as several waves ripple
along baroclinic zone. Question will be where exactly this front
sets up. General trend has been subtle NW shift...seen in 12Z
guidance (especially in new NAM12). Another round of moderate
(possibly heavy with embedded convective elements) rain is expected
late Tue into Wed as front sweeps southeast but could be a
substantial break in between if front remains far enough northwest.
While the pattern certainly favors heavy rain somewhere...with PW
values near 1.5 inches along active quasistationary front...eventual
swath of heavy rain is often narrower than what coarse global models
suggest. Certainly worth keeping an eye on but confidence in exact
rain amounts and locations remains low.

Unseasonably warm conditions also expected during this period.
Several guidance sources now suggesting record highs of 70F in FWA
and even upper 60s in SBN on Tue. Record warm lows could also
be easily broken. Bumped temps even warmer for this period.

Dry conditions for Thursday but additional rain possible late Fri
into Sat as another trough ejects out of the southwest CONUS and
intersects very moist environment locally.


.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
Issued at 1232 AM EST Sun Feb 18 2018

Difficult forecast this period as hires models have significantly
slowed the progress of eastward clearing overnight and HRRR
trending toward LIFR cigs and vis toward 12z with some indications
of dense fog closer to KSBN. High pressure will slide in
overnight and winds will become less than 5 knots west of KFWA.
Not ready to jump on the dense fog with stratus appearing to be
left behind and limiting radiational cooling. Will have to monitor
trends overnight but latest satellite loop does show an overall
slowing trend to back edge of clouds. Trended terminals slower
with clearing until at least mid day when winds become southerly
and begin to increase.





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