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Area forecast discussion 
National Weather Service northern Indiana
339 PM EST Friday Feb 27 2015

issued at 332 PM EST Friday Feb 27 2015

High pressure will drift across the area through Saturday with dry
conditions...but below normal temperatures will prevail. The first
of 2 winter systems will take aim on the area starting Saturday
night into Sunday...bringing accumulating snow to the area. Lows
tonight will drop below zero once again with highs on Saturday
rebounding into the 20s.


Short term...(this evening through saturday)
issued at 332 PM EST Friday Feb 27 2015

Surface high pressure over the area will move slowly east tonight
providing nearly calm winds. Any lingering cumulus/strato-cu should
dissipate this evening but an increase in high clouds expected late
tonight as a weak shortwave over the Central High plains moves east
into the Midwest. Combination of dry airmass... light winds...
snow cover... and at least some periods of clear skies should
result in another night of strong radiational cooling allowing
temperatures to fall into the -3 to -8 range across the area by Saturday
morning (record low at both sbn/FWA for 2/28 is -4). As high
departs to the east on Saturday... warm air advection will commence. Fair weather/dry
airmass will persist... but backing winds aloft associated with
deepening western trough will advect middle-high level moisture northeastward which
should lead to thickening high clouds overspreading the area in
the afternoon. High temperatures expected to be a little warmer than today...
in the upper teens/l20s.


Long term...(saturday night through friday)
issued at 332 PM EST Friday Feb 27 2015

..focus of forecast on impacts from first of two significant
weather systems to impact region this period...

A weak upper level wave...aided by left front quadrant of a 125 knots jet
streak...will induce modest isentropic lift allowing for near
saturation across much of the forecast area with light snow breaking
out quickly Saturday night. Accumulations should be somewhat limited
ranging from an inch or so north to around 2 southwest. Somewhat
stronger wave will move in Sunday into Sunday evening...bringing
more widespread snow to the area as 150+ knots jet streak moves quickly
across the area enhancing lift across the region. 2 to 3 g/kg mixing
ratios advect into the area with highest in S/southeast locations. Some
negatives for heavy snow fall are 1) surface temperatures in the middle-upper
20s keeping snow ratios limited 2) highest moisture and lift located
below a much higher level dgz 3) isentropic lift plentiful but not
perpendicular to pressure surfaces...possibly limiting mesobanding.
4) lack of closed off low/middle level features. NAM/GFS/ECMWF have all
come into some agreement with quarter inch north to half inch or
more south/southeast of quantitative precipitation forecast with general 3 to 6 inch accumulations still
looking reasonable. 9z sref pushed the rain/snow line well north
into the forecast area...reducing snow amounts in the south and
shifting northward highest accums. Initial frames of 15z run appears
to be trending back south with the rain/snow line again. At this
point...previous forecast still on track in terms of accumulation
potential. Could be argued to shift northward somewhat but with
concerns outlined above best option is to leave alone. Did increase
probability of precipitation to higher categorical and realigned timing Saturday night to
better reflect trends. Collaboration with surrounding offices was to
hold off on any headlines but advisory will likely be needed for
most if not all of the area...especially on Sunday when the heaviest
snow will occur.

Region will get a brief breather before wave currently over Alaska
drops south along the coast and begins to deepen as it
interacts/phases with orphaned closed upper low over SW states.
GFS/European model (ecmwf) continue to vary on handling/phasing of these features and
subsequent timing of ejection towards the region. GFS remains faster
with precipitation moving in as early as Monday night with European model (ecmwf) waiting
till middle day Tuesday. Despite this...both models do show mix
potential at the onset of the event as warmer air aloft is faster to
move...with an initially slow displacement of cold surface dome of air.
This struggle will be key in determining how fast any
freezing/frozen precipitation changes to all rain (assuming it does
everywhere) and potential snow/ice accumulations. Period of frozen
precipitation at the onset should be quiet brief as soundings saturate and
depth of warm air increases. Most favorable time frame still looks
to be in the first 6 to 9 hours of the precipitation arriving...meaning
late Monday night into Tuesday am per the GFS and Tuesday afternoon on the
European model (ecmwf). Given the strength of the overall system...European model (ecmwf) cannot be
dismissed as full phasing of the trough and SW closed low would slow
things down considerably. Surface low track has shifted southeast somewhat
compared to 24 hours ago with track now shifted to a central
Illinois to Southeast Michigan line. This also shifts highest quantitative precipitation forecast further
south and increases potential for significant icing for parts of the
area as colder surface air would be harder to dislodge especially northwest. No
changes made to ptype from what was inherited from overnight shift.
However...did increase to higher end Cat probability of precipitation with growing
confidence of something reaching the ground. System will need to be
closely watched in the coming days with trough of concern yet to
form. In addition to ptype concerns noted above...runoff form any
new precipitation and snow melt could cause issues both inland and
over river basins with frozen ground/rivers across the area. Last
frost depth for the office on Thursday was 10 inches deep.

Colder air will filter back in on Wednesday with highs occurring
early in the day and then falling temperatures with readings on Thursday
only in the teens. Some hints that temperatures could rise back to near
normal levels again towards the end but for now have stayed


Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 122 am EST Friday Feb 27 2015

Mainly VFR through the period... though brief MVFR ceilings possible this
afternoon. High pressure drifting across the area will provide light
westerly winds this afternoon becoming south by Saturday morning.


Iwx watches/warnings/advisories...


short term...jt
long term...Fisher

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