Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service northern Indiana
715 PM EDT Wednesday Apr 23 2014

issued at 1240 PM EDT Wednesday Apr 23 2014

A weak warm front will approach the region tonight and may
generate a few sprinkles across Northwest Indiana and southwest
Michigan. Overnight lows are expected to hold in the upper 30s to
lower 40s. Warmer air will overspread the area tomorrow in advance
of a cold front with highs forecast to reach the middle to upper 60s.


Short term...(this evening through thursday)
issued at 404 PM EDT Wednesday Apr 23 2014

Primarily only midlevel moisture to overtop upstream middle
tropospheric ridge that extends from MO northward into southern
Canada. Middle level moisture along with timing of westerly middle level
jetlet to move from southern Iowa at 00 UTC Thursday to chicagoland/far northwestern in
by 06 UTC to provide focus for possible sprinkles. High low level
condensation pressure deficits...only lower briefly to near 50 mb in
roughly 06-09 UTC timeframe along far southeast extension of
elevated warm advection Leaf. More deeply mixed boundary layer on
Thursday support temperatures about 5-10f warmer than today...least modification
across far northestern County Warning Area where concern for backed/easterly surface
component raises concern for Lake Erie shadow/tightened thermal
gradient. Slight trend toward negative tilt to middle level trough as it
enters eastern ND/middle MS valley by f36 and continued backed southeasterly
prefrontal flow supports narrow Theta-E gradient strongly tied to
frontal boundary and have slowed down late afternoon/even chance rain showers with
lesser thunderstorms and rain mention given poor low level moisture pooling through 00
UTC Friday.


Long term...(thursday night through wednesday)
issued at 404 PM EDT Wednesday Apr 23 2014

A quick quarter to half inch of rain still modeled to pivot east
through the forecast area Thursday night along a frontal boundary.
High probability of precipitation warranted given a relatively deep uvm response with an
associated shortwave...and modest low level Theta-E ridging
(1000-850 mb mixing ratios near 9 g/kg) along and in advance of the
occluding frontal boundary/weak surface reflection. Primary adjustment
to the forecast/grids was to delay timing of higher probability of precipitation through the forecast area
per slower 12z model trends. A breezy/mild/mainly dry Friday
anticipated as well mixed dry slot and subsidence overspreads the
area. Brief fgen response with a secondary cold front/pv filament
on backside of phasing waves may support a few showers Friday

Longwave pattern becomes increasingly blocky/amplified this weekend
into next week. The weekend continues to look mainly dry with active
baroclinic zone likely setting up southwest of the local area.
Also...high pressure attempts to nudge south into the lakes in
between emerging negative height anomalies over New England...and
another over The Four Corners/Central High plains.

Unsettled and trending cooler next week as High Plains negative
height anomaly meanders east...eventually teaming up with vorticity lobe
dropping southwest out of southeast Canada/Great Lakes to force a rather
impressive vertically stacked/cut-off low over the middle MS/Ohio valleys
by the middle of the week. Obviously a complex synoptic evolution
with confidence in daily temperatures/probability of precipitation very low.


Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 714 PM EDT Wednesday Apr 23 2014

VFR conds to continue through this period. Upstream warm air advection Wing and band of
associated high based rain showers from western WI southward into central Illinois expected to dissipate
west/eastward extent owing to eastward weakening of ll Theta-E/moisture flux. In fact
given extremely dry and well mixed boundary layer doubt ceiling heights below 8kft
possible lt tonight. Otrws southeasterly rtn follow to deepen on Thursday ahead of SW
trough ejecting out of the Southern Plains. Deeper/better defined ll
moisture/Theta-E push expected lt Thursday night west/period of rain showers and likely MVFR
conds in association/west frontal boundary and parent upper trough progressing
through the area. Until then sensible weather concerns none.


Iwx watches/warnings/advisories...


short term...Murphy
long term...steinwedel

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