Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 200909

Area Forecast Discussion
National Weather Service Northern Indiana
409 AM EST Sat Jan 20 2018

Issued at 400 AM EST Sat Jan 20 2018

Warm, moist air will continue to spread over the area through the
weekend. Expect highs today from the upper 30s to around 40, the
lower to mid 40s on Sunday, and into the 45 to 50 degree range for
Monday. Today will be dry, but expect fog and drizzle overnight.
Rain is possible Sunday, but the best chances will be Sunday night
into Monday. Areas of fog will continue into Sunday evening.
Minor flooding is possible from rain and snowmelt by early next


.SHORT TERM...(Today and Tonight)
Issued at 354 AM EST Sat Jan 20 2018

Overall not much change in the short term forecast, with eventual
development of low clouds later this afternoon, and transition to
drizzle and fog later tonight or early Sunday morning as the main
forecast stories.

Progressive, broad, low amplitude wave skirting across the eastern
Great Lakes this morning has resulted in stalling of low level front
from the upper MS Valley to the western Great Lakes. With low
amplitude flow continuing today, this boundary is not expected to
move much over the next 24 hours, keeping local area on the "warm
side" of the front. Westerly flow regime will continue today in
advance of this front, although thermal advection fields will
continue to weaken through the morning. Most model low level thermal
progs actually indicate slight cooling today, possibly aided in part
by some uptick in low level upward motion fields on warm side of
weak thermally direct circulation. Increasing low level moisture
advection anticipated today which should yield eventual
development of low clouds across southern Illinois/southern
Indiana that will lift north across the local area late this
afternoon, or tonight. Given above factors, have opted to go on
cool side of guidance spread today, with a blend of raw model 2m
temps and consensus guidance. This blend yields highs generally in
the 37-41 degree range.

Low level warm/moist advection will start to become more
pronounced tonight as long advertised southern stream amplified
upper trough begins to emerge from the Four Corners region.
Eventually this should increase drizzle fog/potential, although
have pushed timing back slightly, and this more delayed idea may
need to be continued in future forecast amendments. Not much
movement in temperatures tonight given warm/moist advection, with
lows remaining in the 30s.


.LONG TERM...(Sunday through Friday)
Issued at 354 AM EST Sat Jan 20 2018

Low level theta-e advection to ramp up more sharply later Sunday
afternoon into Sunday night with increasing light rain coverage
expected by later Sunday afternoon. Will continue with mention of
fog through Sunday as higher dew point air (upper 30s-lower 40s)
advects over snowpack. Some subtle changes in forecast thinking
over the past 24 hours as deterministic/ensemble solutions have
steered more toward more southern idea hinted at by last night`s
EC operational run. This more southerly track of mid/upper level
height minimum should put northwestern portions of the area in
more optimal zone of upper forcing (more pronounced
diffluence/jet coupling) for Sunday night/early Monday). Low
level mass convergence with speed/directional convergence
associated with low level jet may also be more optimal across NW
Indiana/SW Lower Michigan. Storm total rain amounts from late
Sunday night into Monday morning of 0.75-1.25 inches appear
possible across the northwest. Water equivalents of snowpack with
recent synoptic/lake effect event vary quite a bit across the
northwest from 0.3 to perhaps up close to an inch for areas
impacted by heavier lake effect snow recently. The combination
runoff from rain and snowmelt could lead to some minor flooding
issues, but possible mitigating factor could be melting of this
snowpack over a relatively long period from today through early
Monday morning. If rainfall amounts over an inch verify, some
minor flooding issues certainly a possibility and will continue to
mention this via HWO this morning, along with potential isolated
ice break up/ice jam flooding.

Dry slot to push across the area Monday afternoon allowing
widespread rain to transition to light rain/drizzle from west to
east before wrap around moisture works into the area for later
Monday night into Tuesday. The aforementioned trends to a more
southerly solution have resulted in more substantive changes to
the forecast for this period. Broad low level thermal ridge
associated with occluding cyclone will keep low level profiles
warmer for a longer duration than previous indications, and this
should cut into snow accumulations in wrap around late Monday
night through early Tuesday afternoon. Near sfc wet bulbs will be
hovering at or above freezing for much of this period which should
also cut into snow ratios quite a bit. TROWAL signature also not
overly impressive, with better signature well to the
north/northwest of the area. At this time would suspect
accumulations of an inch or less, but will continue to monitor
this period for possible impacts. A period of gusty winds in the
25-35 mph range appears possible late Monday night into Tuesday as
colder low level air wraps back across the southern Great Lakes.
In terms of temps, Monday still shaping up to be the warmest day
of the period with highs into the 45 to 50 degree range.

Remainder of the work week should be on the quiet side as a few
moisture starved clippers could drop across the Great Lakes
region. Trend back to seasonable temps Wed-Fri before heading back
above normal for the weekend. Another period of more active
weather could be in store for next weekend as another large scale
upper trough takes shape across western CONUS.


.AVIATION...(For the 06Z TAFS through 06Z Saturday Night)
Issued at 1248 AM EST Sat Jan 20 2018

For the rest of tonight and into early Saturday morning, expect VFR
conditions with mainly LLWS expected as a low level jet ramps up
over the region. Otherwise, expect deterioration in conditions
through the remainder of the TAF period at both sites, with low
level moisture and warm air advecting into the region. Additionally,
melting snow will add greater potential for MVFR/IFR conditions.
That being said, still considerable disagreement among guidance as
to the timing of lowering ceilings/visibility, but based on
observational trends opted to push back the MVFR/IFR conditions to
later this afternoon. I`m feeling confident in the IFR ceilings/vis
occurring, but have lowered confidence in the exact timing. For now,
have both sites dropping to MVFR around 22Z, then to IFR after 00z,
with KFWA dropping first.




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

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