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Area forecast discussion
National Weather Service northern Indiana
646 am EST sun Jan 21 2018

Synopsis...
issued at 646 am EST sun Jan 21 2018

Mild and moist air will continue to lift northward across the Ohio
Valley and southern Great Lakes region today through Monday. Areas
of fog and drizzle with continue today with a chance of some light
rain this afternoon into this evening. Rain will become widespread
late tonight into Monday as a storm system approaches from the
plains. The combination of rain with this system and continued snow
melt may result in some flooding, particularly from Northwest
Indiana to Southern Lower Michigan. High temperatures today will
range from the lower to mid 40s, with highs on Monday from the upper
40s to mid 50s. Showery weather will continue Monday night as cooler
air pushes back into the region.

&&

Short term...(today through monday)
issued at 350 am EST sun Jan 21 2018

Primary forecast concern for the short term will continue to center
on widespread rain event for later tonight into Monday, and
associated impacts when combined with ongoing runoff of snowmelt
across portions of the area.

In the near term, surface boundary this morning remains sprawled
from central Missouri across southern Indiana. Warm moist/advection
continues north of primary sfc boundary this morning, with upper 30s
to near 40 dew points approaching far southern portions of the
forecast area. Have noted widely varying vsbys upstream as this
higher dew point air moves over diminishing snowpack, with a few
patchy instances of vsbys one quarter of a mile across west
central Indiana into east central Illinois. Fog will increase in
coverage this morning as this moisture advection continues with a
greater likelihood of some patchy dense fog, especially across
Indiana portion of forecast area. Will need to monitor for the
potential of a dense fog advisory over the next few hours with
downward trend in vsbys noted. Best chance of light rain/drizzle
early this morning across southern portions of the forecast area
into central Indiana at the interface of this low level Theta-E
gradient. This gradient will shift northward today across
central/northern portions of the area and will continue idea of
increasing chance pops from south to north through the day into
this evening, although rainfall amounts will be quite light due to
relatively shallow depth of better moisture.

For tonight, primary upper level trough across Four Corners region
this morning will begin to emerge across the southern/Central
Plains with corresponding strong 50-60 knot low level jet axis eventually
shifting eastward across eastern Illinois/western Indiana late in
the night. This low level jet axis will mark approach of narrow, but
quite impressive deep moisture axis with pwats around 1 inch
reaching Northwest Indiana in the 09z-12z timeframe. Decent upper
diffluence downstream of the upper trough and some modest upper jet
coupling will support expanding area of rain overspreading
especially northwest half of the forecast area late tonight/early Monday
morning. Low level mass convergence at nose of this stronger low
level jet should also enhance rainfall, although this favored area
of low level convergence will be migratory in nature and lift north
of the area shortly after daybreak.

Some elevated instability should overspread west-southwest
portions late tonight/early Monday with some isolated thunder
potential. Given better elevated instability profiles west/south
of forecast area will not include thunder mention yet, but this
may need to be considered. Strongly forced nature of this event
coupled with unseasonably strong moisture profile will pose at
least a narrow window of moderate/heavy rain potential late
tonight into Monday morning. Greatest rainfall amounts in the 0.75
to 1.25 inch range are expected for west/northwest portions of
the forecast area from Northwest Indiana across south central
lower Michigan where mid/upper forcing is maximized. Please
reference the Hydro discussion below in terms of associated flood
potential for late tonight into early Monday.

Mid level dry slot to quickly overspread the area later Monday
morning/afternoon with coverage/intensity of rainfall diminishing
from west to east, possibly transitioning to a period of
drizzle/light rain as moisture depth decreases. Additional
scattered showers-numerous showers are expected to develop later
Monday afternoon across the west as primary upper low lifts
northeast across northern Illinois.

Very mild temperatures this period as stalled boundary to south
makes retreats northward as a warm front later today, and especially
tonight into early Monday morning. Areas of fog will continue to be
an issue until this warm front lifts across the area Monday. Highs
today will reach into the lower to mid 40s, with steady to slowly
rising temps tonight, and highs in the low to mid 50s for Monday
before cold advection ensues as system occludes.

&&

Long term...(monday night through saturday)
issued at 350 am EST sun Jan 21 2018

Unsettled conditions to continue Monday night as upper level low
lifts across lower Michigan with additional scattered-numerous
showers. Significantly cooler low level air will be delayed until
late Monday night into Tuesday morning as low level thermal ridge
wraps cyclonically across the southern Great Lakes. Wrap around rain
showers will transition to snow showers on Tuesday but previous
limitations to snow accumulations still in place. Rather high
near sfc wet bulbs will make for very wet snow with difficulty
achieving accums on wet ground. Dgz also will be high based with a
lack of clear overlap between lift and dgz. Forecast soundings
even suggest some possible saturation issues in dgz as drier mid
level air begins to push back across the southern Great Lakes.
Remnants of trowal associated with this system will also be
overspreading the area, although in a much less impressive and
very highly elevated state. Will continue with accums of less than
an inch for Tuesday. Cannot completely rule out some patchy
freezing drizzle Tuesday evening as lake enhancement winds down
with loss of deeper saturation, but will hold off on mention at
this time with low confidence in occurrence/impacts.

&&

Aviation...(for the 12z tafs through 12z Monday morning)
issued at 645 am EST sun Jan 21 2018

A dreary forecast for the taf period, as conditions deteriorated
more rapidly than originally planned. Unfortunately, we'll see
little improvement from the vlifr/LIFR conditions currently observed
across the area, with continued warm/moist air advection over the
region melting what remains of the snowpack today. Ksbn as of 1140z
was hovering at 5sm, but just upstream at kppo/kvpz and (pretty much
everywhere around ksbn) we have visibilities of around 1/2 to 1/4
mile, so I suspect that we'll be seeing a drop in visibility within
the next hour or so. With drizzle and rain expected to add to the
scenario, expect conditions to remain poor into Monday morning. The
only hope is for a brief "improvement" this afternoon at kfwa/ksbn,
but model guidance is variable in regards to this. Kept it in the
taf for now, but wouldn't be surprised if it didn't materialize.

&&

Hydrology...
issued at 350 am EST sun Jan 21 2018

Runoff from snowmelt will continue today as temperatures rise
into the upper 30s to mid 40s. Low level dew points will also be
on the increase today allowing for increase rate of melting.
Initial water equivalents of snowpack of 0.4 to 1 inch across the
Kankakee/St. Joseph basins will continue to runoff through tonight
with frozen ground conditions. These higher snow water
equivalents do extend across portions of the Wabash River basin,
with steady rising trend noted at Ora this morning. Snow water
equivalents drop off sharply across most of Maumee basin, where
much of pre-existing snowpack has already melted. Rainfall of 0.75
to 1.25 inches late tonight is expected, especially for areas
which experienced these higher snow water equivalents. Rising
River levels are expected through the middle-latter portions of
the week.

Runoff from the melting snow will have occurred over a fairly long
duration, and potential of minor river flooding will be highly
dependent on extent and distribution of Sunday night/Monday
morning rainfall across these basins. Precipitation may be
convective in nature at times, and this will need to be watched
also in terms of higher flood potential. Ice jam potential will
also need to be monitored all basins due to warmer temps today-
Monday and increased potential of ice break up and localized ice
jam flooding.

&&

Iwx watches/warnings/advisories...
in...dense fog advisory until 1 PM EST /noon CST/ this afternoon for
inz003>009-012>018-020-022>027-032>034.

Michigan...dense fog advisory until 1 PM EST this afternoon for miz077>081.

Ohio...dense fog advisory until 1 PM EST this afternoon for ohz001-002-
004-005-015-016-024-025.

Lm...none.
&&

$$

Synopsis...marsili
short term...marsili
long term...marsili
aviation...mesoscale discussion
hydrology...marsili



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