Area Forecast Discussion
Issued by NWS Northern Indiana

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FXUS63 KIWX 191117

Area Forecast Discussion
National Weather Service Northern Indiana
617 AM EST Fri Jan 19 2018

Issued at 616 AM EST Fri Jan 19 2018

Partly cloudy to sunny skies are expected today as the moderating
temperature trend continues. High temperatures this afternoon
will range from the mid to upper 30s. The warming trend will
continue into the weekend with highs on Saturday in the upper 30s,
and into the lower to mid 40s by Sunday. Chances of light rain
and drizzle will begin to increase late Saturday into Sunday, with
rain becoming widespread later Sunday night into Monday as a
storm system emerges from the southwest United States. High
temperatures on Monday will range from 45 to 50 degrees, with
conditions turning colder for Tuesday behind this system.
Precipitation will likely end as a period of snow showers Monday
night into Tuesday.


.SHORT TERM...(Today and Tonight)
Issued at 341 AM EST Fri Jan 19 2018

Quiet weather to continue in the short term, with primary forecast
challenge continuing to be temperature forecasts.

A broad, low amplitude southern Canadian short wave will continue to
skirt across northern Ontario this morning, with a strong zonal flow
pattern continuing from the High Plains to the western Great Lakes.
This history of stronger zonal flow over the past few days has
allowed for enhancement of low level thermal ridging in the lee of
the Rockies. Eastward progression of this Canadian wave will allow
for primary low level thermal ridge to sink southeast across the
region later this afternoon in this evening.

Complicating factors for max temp forecasts once again today are
presence of very strong low level inversion with warm advection atop
lingering snowpack, and shallow mixed layer depths. Modest gradient
this morning and ongoing low level warm advection has allowed for
temps to hold in the lower to mid 20s most locations. Even with
shallow mixing today, magnitude of this incoming low level air
mass and good deal of insolation (some passing higher level clouds
today) should allow temperatures to reach above freezing for all
locations. Warmest low level temps are anticipated across the
northwest where more snow needs to be melted. Changes in snow
cover from northwest to southeast across the forecast area could
result in a smearing of the temp gradient to a broader region of
mid to upper 30s highs this afternoon. Once again, confidence in
how this will play out somewhat on the low side for first period
forecast given the competing factors at play. Despite shallow
mixing, similar wind profiles in place today with afternoon gusts
into the 20 to 25 mph range expected.

Passage of Canadian short wave trough will allow associated sfc
boundary to stall just to the north-northwest of the local area
this evening, with gradually slacking low level height gradient
tonight. Did contemplate inclusion of some patchy fog late tonight
into early Saturday with slackening of this gradient and melting
snow, but confidence is too low to include at this time with
better surge of low level moisture holding off until later in the
weekend. Relatively mild mins tonight in the mid-upper 20s are


.LONG TERM...(Saturday through Thursday)
Issued at 341 AM EST Fri Jan 19 2018

For Saturday, splittish eastern Pacific flow will allow amplified
southern stream upper trough to emerge across the Four Corners
region. Isentropic ascent to gradually ramp up through the Saturday
night-Sunday period. Relatively shallow moisture depths Saturday
afternoon/evening could yield more of an increasing drizzle/fog
potential. Did add some mention of fog to the forecast for
Saturday night-Sunday with good deal of near sfc moisture via
snowmelt as higher dew point erodes remainder of snowpack across the
region. May still need to watch for a narrow window of freezing
drizzle potential Saturday evening across far north, but still
feel that near sfc/sfc wet bulbs will transition to more
supportive of liquid p-types due to strength of advective fields,
and not expecting much of any impacts at this time with very low
mixed, light precip potential Saturday night.

Period of strongest moisture transport still appears to be
in the later Sunday night-early Monday period as channeled 50 knot
LLJ allows impressive, but narrow axis of deeper moisture to advect
across the area. Will maintain categorical rain PoPs through this
period with good upper support/dynamics and some hints of coupled
jet structure early Monday morning. Some indications of weak
elevated instability with some non-zero chance of an isolated
thunderstorm. However, best quality moisture should get pinched
off south of the area late Sunday night, which should keep more
favorable elevated moisture profiles for isolated thunder south of
the region and will continue to omit for this forecast. Precip
amounts not expected to be overly impressive with this system due
to progressive nature of moisture axis/forcing, and with gradual
snowmelt over the next few days, not anticipating any major hydro
issues at this time. Will have to monitor for some isolated ice
jam issues/break-ups however. Rain should taper from west to east
Monday afternoon as dry slot noses across Ohio Valley, but wrap
around moisture and eventual approach of broad deformation zone
will allow for rain showers to transition to snow showers Monday
evening. Some light snow accumulations are possible, particularly
northern half of the area. While true lake effect is not expected
to be a major factor in this event, lake enhancement with this
deformation band could be something to watch for at least a couple
of inches of accumulation Monday night-early Tuesday for
southwest Lower Michigan.

Some trend over the past few cycles for weaker low level reflection
with this system, and thus higher end wind gusts in cold advection
occluded portion of the storm may not be as much of a concern as
previously thought. Wnd gusts into the 25 to 35 mph range still
appear to be a likelihood by later Monday into Monday evening

Temps will moderate into the 40s by Sunday with warmest day of
the period immediately preceding frontal passage on Monday with
highs from 45-50. Turning colder for Monday night-Tuesday but
nothing of the arctic intrusion variety that we have been dealing
with recently. Near seasonable temperatures should hold through
the middle of next week before moderating again late week as next
larger scale western CONUS trough takes shape. A few weak clipper-
type systems may be progressing across northern tier for the Wed-
Fri timeframe, but moisture availability will probably be an
issue and nothing more than lower chance mention warranted.


.AVIATION...(For the 12Z TAFS through 12Z Saturday Morning)
Issued at 616 AM EST Fri Jan 19 2018

VFR Conditions are expected at the TAF sites. High pressure ridging
over KSBN/KFWA will keep weather dry through the period. Somewhat
gustier southwest flow will continue through Saturday morning as low
pressure moves across Ontario and into Quebec, tightening the
pressure gradient. Gusts up into the low 20s are expected at KSBN
this afternoon.




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

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