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Area forecast discussion 
National Weather Service northern Indiana
221 PM EDT Sat Apr 25 2015

issued at 232 am EDT Sat Apr 25 2015

Widespread rain will continue this afternoon as a storm system
tracks across the southern Great Lakes and Ohio Valley. A low
pressure area associated with this system will move east tonight
as rain ends across the region. Temperatures through this
afternoon will generally be in the middle to upper 40s...and lows
tonight will drop back into the lower to middle 30s.


issued at 1158 am EDT Sat Apr 25 2015

Only minor tweaks were made to going forecast this morning. Low
level south-southeast flow has allowed for axis of better low
level moisture to overspread the local area this morning.
Isentropic upglide and low/middle level fgen forcing downstream of
approaching upper trough has allowed for extensive rain. Some
thunder has also developed over past few hours across west central
Indiana/east central Illinois at the leading edge of dry slot.
Confidence remains low on northward extent of thunder given more
limited elevated instability across northern Indiana. Upper
forcing should be on the increase over the next few hours in a
brief window before blocking northern stream begins to suppress
forcing south of the local area. Thus...still expecting some
northward expansion of rain showers across Southern Lower Michigan
over next few hours before backing low level winds begin to
transport drier air back southward late this afternoon. Overall
temperature forecast appears to be on track...maybe just a bit
warm in spots which receive steadier rainfall through the
afternoon. Precipitation should rapidly diminish this evening as low
level dry air advection advances southward.


Short term...(today and tonight)
issued at 505 am EDT Sat Apr 25 2015

An upper level disturbance over northeast Kansas early this morning
will eject northeast today and will bring another round of
precipitation to the forecast area. An area of rain has been
translating south and was being supported and enhanced by the
combination of the left exit region of an upper jet along with
surface convergence along a weak boundary. For later today...the
GFS...European model (ecmwf) and NAM have initialized this system very well and are
similar in handling the mass fields of this ejecting system.
However...the GFS was farther north and an outlier among these
models with the development of precipitation. Favor a more
intermediate European model (ecmwf)/new 06z NAM solution which brings 0.75 to 1.25
inches of rain over the southern third of the forecast area....but
much lower amounts farther north. A trowal should translate over the
area with about a 9 hour duration and with a tight northern gradient
in the best depicted on the new 06z NAM. The latest
composite radar was already showing this next round of precipitation
developing over Illinois. However...dry air entertainment and the loss
of upper level jet support was vaporizing the northern returns. As a
result...have cut back rainfall amounts significantly over far north
areas...with less than a tenth over far Southern Lower Michigan to
0.75 over southern areas. The system will move east tonight with rain
ending and some clearing possible late. Frost is possible late...but
for now has left frost out due to uncertainty of clearing times.


Long term...(sunday through friday)
issued at 505 am EDT Sat Apr 25 2015

Couple of shortwaves expected to rotate around closed low off the New
England coast and move S-southeast across the Great Lakes and Ohio Valley
Sunday-Monday. Moisture lacking for precipitation with main impact just to
sustain cool airmass over the region. Upper level ridge expected to
build east into the area Tuesday providing continued fair weather and warmer
temperatures... however weak north-NE low level flow should limit the warmup. Rather
deep trough expected to develop over the eastern Continental U.S. Middle/late week as a
closed low moving east from the Southern Plains phases with an upper level
low/trough digging S-southeast from central Canada. 00z medium range
models continue to have significant differences on timing/evolution of this
development with European model (ecmwf) moving upper low across our area Wednesday while Gem
moves one low south through the MS valley Wednesday with a second low
dropping through the Great Lakes/Ohio Valley Thursday... and GFS indicates an
open wave moving southeast through the lower Great Lakes Thursday. Given the large
differences among the models... followed a highly smoothed model
blend which yielded a slight chance of showers Wednesday... with dry
conditions and a warming trend to above normal temperatures late week as
the deep trough moves off the East Coast and a strong ridge builds
east from the plains.


Aviation...(for the 18z tafs through 18z Sunday afternoon)
issued at 203 PM EDT Sat Apr 25 2015

Rain will continue to affect the terminals through late afternoon
as main upper level trough centered across west central Illinois
tracks across the region. Low level front has likely reached its
approximate maximum northward position...and should start to see
front sag back southward late afternoon/early evening as low level
winds back more northeast. Have maintained trend to MVFR
conditions at kfwa for a brief time late this afternoon/early this
evening. Any lingering MVFR ceilings at kfwa this evening should
improve to VFR as drier low level air advects southward. VFR
conditions to persist into Sunday.


Iwx watches/warnings/advisories...


short term...skipper
long term...jt

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