Scientific Forecaster Discussion
Area forecast discussion
National Weather Service northern Indiana
602 am EST Friday Feb 27 2015
issued at 441 am EST Friday Feb 27 2015
High pressure building southeast from the northern plains will
result in generally dry and cold conditions today. The only
exception to the dry weather will be near Lake Michigan where
light lake effect snow showers are expected to end later today.
Highs today should be around 15. Lows tonight are expected to
drop below zero as a high pressure area moves across the region.
Short term...(today and tonight)
issued at 442 am EST Friday Feb 27 2015
The latest surface observations and satellite showed lake clouds
spreading south over Southern Lake Michigan into the northwest
corner of Indiana early this morning. Although Delta T values over
ice free areas were over 20c...shear in the cloud layer was
increasing and will weaken remaining snow showers this morning. Snow
showers have temporarily shifted west and have apparently reoriented
themselves more with the ice free areas of the lake aggregate trough.
Bitterly cold temperatures early this morning were around -10 over
several areas...however calm to light winds were not adding much of
an additional chill to the air...so have held off with a Wind Chill
Advisory. The center of high pressure will move across the area
tonight allowing one more night for temperatures to fall below zero.
These lows are in range of breaking the record lows for February 28
of -4 and -1 at South Bend and feet Wayne respectively. Given ideal
conditions for radiational cooling with a bitterly cold airmass...
light winds...clear/mostly clear skies and deep snowpack especially
over northern areas...favor record or near record lows for this
Long term...(saturday through thursday)
issued at 442 am EST Friday Feb 27 2015
..two significant weather systems to impact region this period...
Two separate weather systems will bring a plethora of weather types
to the region during this long term period with significant impacts
possible. Heavy snow...freezing rain...sleet...heavy rainfall and
possible flooding all are possible between Sunday and Wednesday.
Large temperature swings also expected with potential for warmest
temperatures in weeks followed by more Arctic air.
First system still on track for late Saturday night through Sunday
and weather type should be all snow for this event. Water vapor this
morning showing atmospheric transition over North America with
digging energy helping to carve out trough over western Continental U.S..
downstream flow beginning to respond with transition to
southwesterly flow aloft. This will set the stage for increasing
moisture transport into our region via subtropical connections.
Jetstream will strengthen over the Midwest Sat night and Sunday
while low level jet develops and Gomex becomes tapped. Nam12 showing
2 to 3 g/kg mixing ratios over the area by Sunday while GFS in the
3-4 g/kg range. System relative isentropic lift is deep and winds
remain nearly orthogonal to pressure surfaces from about 09z-21z
Sunday. Garcia method still showing 4 to 8 inch snow potential with
these values. The question still remains as to exactly how much
moisture will be advected into our area and where the strongest band
will develop. Latest guidance still favors southern half of our area
for heaviest snow and wpc has gone along with this as well. Models
have been very consistent from run to run and with each other for
several days now. Wind does not look to be significant Sunday with
light southerly flow in the 5 to 10 knot range and snow will be wet
with liquid to snow ratios around 13 to 1 on average. Dgz remains
high but deep and this supports near climatology ratios. No plans to issue
headlines this early but certainly something will be needed
Focus quickly shifts to second storm system likely to impact the
region anywhere from late Monday night through Tuesday night. Western
trough and embedded energy begins to lift out and head our way. This
system still looks to tap deep subtropical moisture plume from south
central Pacific in addition to Gomex connection as low level jet
ramps up quickly Monday night. Over 50 knots low level flow being
indicated by models which helps spread 6 to 8 g/kg mixing ratios and
precipitable waters over an inch into the area Tuesday. When these ejecting lows
tap the subtropical plume...they never disappoint with precipitation amounts.
00z European model (ecmwf) remains slower and holds off most precipitation until Tuesday
afternoon. This would help reduce chances for significant icing at
onset as surface temperatures would warm to near or above freezing by the time
precipitation arrives Tuesday. GFS and Gem remain faster with precipitation onset and thus
more significant icing potential early Tuesday morning. Most medium
range solutions suggest temperatures to warm above freezing by middle day
Tuesday as warm surge overwhelms low level cold wedge. Will keep the
mention of mixed precipitation in for Monday night and Tuesday morning due to
model differences and uncertainty. This warm surge will also help
temperatures climb into middle to upper 30s north and possibly
approaching 50 in the south Tuesday afternoon.
Given the anomalous deep tropical connection heavy rain will be a
threat with this system Tuesday. Quantitative precipitation forecast amounts on both European model (ecmwf) and GFS
impressive with 2 to 3+ inches being generated. The question looks
to be where it will occur and not if it will happen. European model (ecmwf) puts 2-3
inches of rain over southern forecast area where fresh snowpack may
exist from Sunday system. GFS keeps this heavy rain axis further
south but still generates over an inch in our area. These amounts
coupled with melting snowpack and frozen ground may generate
significant runoff. Rivers will likely still be frozen from ongoing
sub zero temperatures so threat for flooding and ice jams could be
significant. Will mention all these potential hazards/impacts in severe weather potential statement
but as mentioned timing and storm track will be critical as to where
all this plays out. Continue to monitor future forecasts.
Another cold surge in wake of this system expected with 850mb temperatures
dropping back toward -20c Thursday with chance for snow showers.
Highs likely to only make the lower 20s followed by single digit
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 122 am EST Friday Feb 27 2015
The latest observation and satellite showed lake clouds spreading south
over Southern Lake Michigan into Northwest Indiana overnight. Shear
was increasing...and will limit snow shower development. Expect
the lake clouds and showers to gradually translate east. It is
possible sbn may become MVFR for a short period this morning as
lake clouds translate east... but for now...kept both sites VFR.
Any MVFR ceiling should be very brief.
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