Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service northern Indiana
1221 PM EST Sat Feb 28 2015

Synopsis...
issued at 100 am EST Sat Feb 28 2015

High pressure will drift across the area today with dry
conditions...but below normal temperatures. The first of 2 winter
systems will then take aim on the area starting tonight...bringing
accumulating snow to the area. Highs this afternoon will be in the
lower 20s... with lows tonight in the teens.

&&

Short term...(today and tonight)
issued at 411 am EST Sat Feb 28 2015

A large surface high pressure area with a center reading close to
1040 millibars will move east out of the area as return flow
increases on the back side of the high pressure area. A large upper
trough will shear out east as the remainder of the longwave trough
remains over the west Continental U.S.. favor the GFS with deeper saturation.
Expect snow to develop tonight given favorable upper level jet
support and isentropic lift. The best lift and moisture will be over
southern areas where around 2 inches of snow is likely by late
tonight. Lesser amounts are expected farther north including far
Southern Lower Michigan. Otherwise...this will be the last of the
frigid mornings for at least a little while given moderating
temperatures and increasing clouds ahead of this next system.
Temperatures early this morning were somewhat similar to Friday
morning with 4 am EST readings close to -10 in some areas. This
shallow inversion will mix out rapidly allowing temperatures to
quickly recover.

&&

Long term...(sunday through friday)
issued at 411 am EST Sat Feb 28 2015

As we have seen numerous times this winter latest model guidance has
trended slightly southward with both strong storm systems coming up
for Sunday and Tuesday. Still expecting impacts from both but latest
guidance and trends suggest we will be on northern fringe of both
systems. Still very cautious with wavering solutions...especially
with the Tuesday event as main energy for that still in largely void
region of upper air network.

Snow will be ongoing Sunday as low level jet and strengthening upper
level jet combine to bring low level moisture and broad lift across
the area. System relative isentropic remains impressive with strong
wind convergence and orthogonal component to pressure surfaces along
with 2-3 g/kg mixing ratios and 4 g/kg just to our south. Best lift
from this remains focused along our southern border counties during
the day. This also same region where near surface temperatures will rise to
near freezing by early afternoon. This should aid in compaction as
well as helping melt some of the snow on roads...limiting
accumulations as well as impacts. Highest axis of quantitative precipitation forecast has shifted
south of our area but still coming up with just over a tenth of an
inch north...quarter inch central and near a half inch south. Nam12
snow ratios start around 15 to 1 but down to near 10 to 1 in south
with afternoon warming. This should help lower accums south despite
uptick in quantitative precipitation forecast during the day. Dgz still depicted to be rather high
and near the top of deeper moisture. All these signs point toward
total accums of 1 to 3 north...2 to 4 central and 3 to 6 far south.
Any further southward shift will push accums toward the lower end of
these values. Must remind everyone to focus on the range of possible
accums and not the high numbers. After collaboration with
neighboring offices decided confidence better further south for
headlines and northern tier offices opted to hold off until later
today for possible additional advisories on northern fringe.

Second system for Monday night and Tuesday still looks on track. GFS
starting to back off on precipitation arrival until Tuesday. Kept low to middle
chance probability of precipitation going late Monday night. This later arrival will
actually allow low temperatures to drop Monday night into teens and lower
20s. Still expect actual surface temperatures to lag warming aloft Tuesday given
the current Arctic air in place with deep and fresh snowpack and a
frozen ground down to a depth of nearly a foot. Therefore period of
freezing precipitation Tuesday morning still looks possible at onset and will
keep the mixed type in forecast. Also a little concern that despite
measured 2m air temperatures rising above freezing...actual surface temperatures may
remain at or below freezing a little longer due to the extreme cold
and snow pack. This is similar to an event last winter in northern
Illinois where freezing rain continued on surfaces despite air temperatures in
the middle to upper 30s for several hours. This is rare but has
occurred following prolonged extreme cold events. Something to keep
in mind Tuesday especially across our far north and northeast. For
now kept trend of ending freezing precipitation all areas by 18z as warm air
aloft increases and deepens. Rain then expected through afternoon
and into early evening. Again...highest quantitative precipitation forecast axis has shifted south
but models still generating respectable amounts for southern half of
County Warning Area with an inch or so possible. This would still be enough for
flooding concerns with frozen ground...storm drains blocked by
snow...and rivers being frozen leading to possible ice jam flooding.

Cold air will slowly spill in from northwest Tuesday night changing any
precipitation over to light snow from west to east. A return to deep freeze
then expected with temperatures back well below normal for early March with
possibility of single digits and even some more below zero temperatures.

&&

Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 1214 PM EST Sat Feb 28 2015

Little change to previous tafs. Moisture over-running cold-dome
associated with departing Arctic high should cause snow to
overspread northern Indiana by late evening. Stronger moisture advection
and forcing at FWA suggests highest snowfall rates at that
terminal where IFR forecast late tonight and Sunday morning with just
MVFR conditions expected at sbn. Accums through 18z expected to be
around an inch at sbn and 2-3 inches at FWA.

&&

Iwx watches/warnings/advisories...
in...none.
Michigan...none.
Ohio...none.
Lm...none.
&&

$$

Synopsis...skipper
short term...skipper
long term...Lashley
aviation...jt



Visit US at www.Weather.Gov/iwx (all lower case)

Follow US on facebook...twitter...and youtube at:
www.Facebook.Com/nwsnorthernindiana
www.Twitter.Com/nwsiwx
www.Youtube.Com/nwsnorthernindiana

National Weather Service Glossary of Abbreviations