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Area forecast discussion 
National Weather Service northern Indiana
646 am EST Wednesday Mar 4 2015

issued at 434 am EST Wednesday Mar 4 2015

Mostly cloudy skies are expected today as an upper level disturbance
moves across the region. This disturbance may result in some light
snow from central Indiana into portions of Northwest Ohio. Arctic
air will filter into the region today and tonight...with many
locations expected to reach high temperatures this morning. Highs on
Thursday will only reach into the teens for most locations...with
subzero low temperatures for Thursday night. A moderating
temperature trend is then expected for the upcoming weekend as
departing high pressure allows south winds to develop.


Short term...(today and tonight)
issued at 434 am EST Wednesday Mar 4 2015

Temperature trends given strengthening low level cold advection
today and with precipitation chances across the far south/southeast will
continue to be the primary short term forecast challenges today and

Axis of deeper middle level moisture appears to be translating to the
southeast of the local area early this morning which should provide
at least a temporary lull in light precipitation chances across southeast
portions of the forecast area this morning. Water vapor/rap analyses
depict a low amplitude...highly sheared vorticity maximum across southern
Iowa this morning...which should track across the southern Great
Lakes this afternoon. Approach of this wave will be accompanied by a
narrow middle/upper level fgen axis along with a brief northward surge
of more pronounced middle level moisture across southeast portions of
the forecast. 03z sref probability of precipitation for this afternoon also seem to key in
on this feature with temporary upswing in the 18z-00z period across
the southeast. While low level thermal profiles would not support
-sn with lack of saturation above frontal inversion...middle level fgen
forcing and middle level cloud deck could provide seeder feeder
mechanism to introduce a low end chance of some flurries or light
snow. Not much change made to previous forecast...and will focus
these low probability of precipitation across the southeast this afternoon.

Elsewhere...hourly temperature trends through this afternoon will be
a challenge given sky cover and advection considerations. Low clouds
have held temperatures in the upper 20s to middle 30s through much of
the night...with only limited temperature drops anticipated over the next
several hours. Perhaps some window of clearing may exist toward
daybreak across western locations that could allow temperatures to drop
into the lower to middle 20s. Strong low level cold advective surge to
work across the southern Great Lakes this afternoon as broader
positively tilted upper trough begins to work across the northern
Great Lakes. This should allow for steady or slowly falling
temperatures early this afternoon...with a sharper temperature drop
late afternoon/early evening. Most locations will reach maximum temperatures
this morning due to low cloud deck and timing of stronger low level
cold advection later this afternoon.

Low level thermal trough to become entrenched tonight with upper
level trough tracking through the region. Some increase of at least
scattered lake effect snow showers is expected...although large
scale synoptic subsidence and relatively large dew point depressions
above the inversion should limit overall threat. Some light
accumulations of less than an inch are possible...but will maintain
high chance probability of precipitation for northwest fetch lake effect prone areas
tonight. Lack of decoupling and possibility of low clouds persisting
should keep mins in the 5 above to 12 above range tonight...with
wind chills in the 5 above to 5 below range.


Long term...(thursday through tuesday)
issued at 434 am EST Wednesday Mar 4 2015

One last Arctic surge of bitterly cold will invade the area before
the the northern stream finally becomes more high zonal and allows
temperatures to recover to near and above normal early next week.
Inherent challenges this period include weak upper level troughs that
may be able to produce some light precipitation and briefly tap some
cooler air. Very cold air has been over the area for nearly 3 weeks
with the average temperature at feet Wayne only 11.5 since February
12th. During this period...NCEP temperature guidance for highs have
had a marked cool bias in the cold air with clear to partly cloudy
skies. The coldest record cold highs for Thursday...March 5th in
over a hundred years are 20 and 17 for South Bend and Fort Wayne
respectively. Therefore...with all of this considered...have gone 4
to 5 degrees above NCEP guidance for highs Thursday. For Thursday
night...record lows are -7 and -4 at South Bend and feet Wayne
respectively. Expect ideal radiational cooling Thursday
night...which should allow temperatures to drop below zero. Have
lowered lows 3 to 4 degrees to come in line closer to these record
lows given recent huge diurnal swings in this cold pattern.
Otherwise...a weak lake response is possible with favorable Delta T
values...but low inversion heights Thursday into Thursday night.
Much warmer weather is expected starting this weekend. Deep snow
cover may initially inhibit the warmup over northern went
a little colder over far northern Indiana and southwest lower
Michigan. Kept conditions dry except for a chance for snow Sunday
night with the passage of a weak upper trough.


Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 630 am EST Wednesday Mar 4 2015

Post frontal dry air advection is expected to work across northern
Indiana later this morning into this afternoon. Steep low level
lapse rates and persistent frontal inversion should support
continued MVFR ceilings however...generally in the 2-3k feet range.
Cannot completely rule out a brief period of ceilings below 2k feet for a
time this morning...but confidence in occurrence is too low for
inclusion in the 12z tafs. Reinforcing low level dry air advection
this evening with secondary Arctic boundary should allow for
primarily VFR conditions tonight...although later forecasts may
need to assess potential of scattered lake effect snow showers at
ksbn...which could provide periods of MVFR visibilities tonight. Steep
low level lapse rates/cold advection should allow for some
west/northwest gusts into the 15 to 20 knot range this
afternoon...before diminishing again this evening.


Iwx watches/warnings/advisories...


short term...marsili
long term...skipper

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