Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 041943

Area Forecast Discussion
National Weather Service Northern Indiana
243 PM EST Sun Dec 4 2016

Issued at 240 PM EST Sun Dec 4 2016

An area of snow and rain will move through the region this
evening. Precipitation should fall as mainly snow north of a line
from Monticello to Fort Wayne to Lima with 2 to 4 inches over
northwest Indiana into far southern Lower Michigan. More wet snow
is likely Tuesday before much colder air and lake effect snow
arrive late this week.


.SHORT TERM...(This Evening through Monday)
Issued at 240 PM EST Sun Dec 4 2016

Water vapor shows strong short wave approaching with impressive
moisture feed originating from eastern Pacific. Closed upper low
over Mexico helping tap the Pacific moisture and throw it into
the southern CONUS while northern stream short wave is tapping
this influx. Result is widespread precipitation across the Midwest
with hourly liquid rates of 0.5 to 0.10 in per hour being

Lower levels finally beginning to saturate locally this afternoon
with mixed precipitation occurring. Primarily rain south and snow
north as expected. Will see rain/snow line remain stationary for a
period but should slowly sink south this evening with subtle cooling
as weak sun angle diminishes further. Period of strong system
relative isentropic lift ahead of short wave to generate good UVM
for a 3 to 5 hour window and support moderate precipitation rates
with brief bursts of heavier rates possible. Still looking at accums
mainly on grassy and elevated surfaces of 1 to locally 4 inches
with compaction and melting hampering overall accums. Road temps
this afternoon running in the mid to upper 30s but expect some
cooling this evening. Surface wet bulb temps per RAP/HRRR/NAM
hover around 33-34F this evening which may help keep roads mainly
wet. the higher snow rates develop this evening will
likely see a period where some snow begins to develop on
roads...especially on secondary and those less traveled. No plans
to issue advisory right now with no rush hour to contend with and
low relative impacts. Caution is urged as bridges and overapasses
will likely be slick as well. Evening shift will be monitoring
road temps and conditions in event deterioration is more severe
than currently expected.

A period of drizzle is possible on back side of pcpn shield as mid
layers dry quickly and low stratus is left trapped under strong
inversion. Have kept late tonight and early Monday dry but soundings
show a decent amount of trapped moisture lingering so some freezing
drizzle is not out of question. Otherwise stayed pessimistic with
cloud cover on Monday per 1000-925mb RH fields but some breaks
possible south in afternoon. Lowered temps several degrees with more
clouds and possible snow still on grassy surfaces.


.LONG TERM...(Monday Night through Sunday)
Issued at 240 PM EST Sun Dec 4 2016

...Wet snow possible Tuesday then much colder with lake effect snow
still on track for later this week...

The NAM remains the western most solution on Tuesday with ejection
of upper low out of Mexico and through the Ohio Valley. While models
agree on opening this wave up and accelerating it northeast...NAM is
strongest and therefore further northwest while GFS and ECMWF open
wave faster and allow it to become absorbed in southwest flow
quicker. The question will be how far west pcpn shield extends
Tuesday morning and precipitation type on northwest periphery. GFS
has trended cooler with evap cooling as pcpn moves into drier air.
Have stayed with primarily light snow at onset except for far
southeast...then transition to primarily rain mid day. Some small
accums are possible before turning over to rain.

Arctic intrusion still on tap mid to late week. Few changes to temps
and lake effect snow grids for later periods with broad area of
likely to categorical pops downwind of the lake. If moisture and
trajectories remain favorable...significant accumulations still
possible with extreme instability as sfc-700mb delta Ts approach 30
and upper trough over lakes provides favorable synoptic background


.AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon)
Issued at 1223 PM EST Sun Dec 4 2016

Main focus remains on impacts/ptypes from area of light-moderate
precipitation expected to move through mid afternoon through mid
evening, heaviest in 21-01z window. Pcpn still expected to fall as
mainly snow at KSBN with VFR conditions at TAF issuance quickly
deteriorating into IFR/LIFR range with snowfall.

Pcpn likely starts as a rain/snow mix at KFWA given slightly
warmer near sfc conditions. However, latest model/observation
trends suggest a quicker transition to mainly snow by 22z and then
ending toward 01z. As a result have trended toward lower visby`s
and all snow at KFWA 22-01z. Some slushy accumulations will be
possible on the pavement at both terminals (better chances at
KSBN) in times of heavier snowfall.

Winds veer westerly and drier mid-upper level will bring an end
to more widespread pcpn behind shortwave and associated sfc
trough later this evening. Low stratus in IFR to low MVFR range
then settles in under post-frontal inversion, with some patchy
drizzle possible late evening/early overnight.




SHORT TERM...Lashley
LONG TERM...Lashley

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