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Area forecast discussion 
National Weather Service northern Indiana
600 PM EST sun Dec 21 2014

issued at 1217 am EST sun Dec 21 2014

High pressure over the eastern U.S. Will continue to cause fair
weather across our area tonight. A low pressure system will move
southeast into the Central Plains Monday. Rain associated with
this system will likely spread into western portions of the area
Monday afternoon... while eastern counties should stay dry through
early evening. Lows tonight will be in the upper 20s and lower
30s... with highs on Monday in the lower and middle 40s.


Short term...(this evening through monday)
issued at 332 PM EST sun Dec 21 2014

Pocket of drier air in low levels has spread north-northwest from the Ohio Valley
into our area as surface ridge continues to move slowly east across the eastern
Continental U.S.. broad upper level trough over the plains will amplify tonight and
Monday as shortwave over Montana and accompanying strong jet over the Pacific
northwest/eastern Pacific digs southeastward with system closing off and becoming
vertically stacked in the vicinity of Oma by Monday evening. Warm front
extending southeast from the low to the lower Ohio Valley Monday morning
will lift north-NE during the day. Best forcing/moisture transport
associated with the warm front will overspread western portions of the
County Warning Area and points westward Monday suggesting rain likely moving into
this area during the afternoon... while just a slight chance of precipitation
reaching the eastern counties.

Although warm air advection will strengthen across the area overnight... just some
high thin clouds during the evening with still fairly light winds should
allow temperatures to fall into the u20s/l30s. Stayed close to previous
forecast highs Monday which were generally on the warm end of 12z MOS
guidance as expect continued warm air advection and increased gradient winds/mixing
will allow temperatures to reach the l-m40s.


Long term...(monday night through sunday)
issued at 332 PM EST sun Dec 21 2014

Frustrating model inconsistencies continue with respect to potential
Christmas evening storm. Trend seen in last night's 00z data for a later
phase/more progressive surface low in our area continued to some
degree into this morning but still a high degree of intermodel and
run-to-run variability in the fine scale details. 170+ knots upper
level jet that will drive this event is just now coming onshore.
Hopefully will have some more definitive answers by tomorrow as this
jet is fully sampled by the noam radiosonde observation network.

Increasing warm air advection ahead of lead wave will bring widespread rainfall to
the area Monday night into Tuesday. Efficient moisture transport and
290-295k isentropic ascent associated with increasing southerly low level jet
support categorical probability of precipitation despite some lingering uncertainty as to
exact timing and total quantitative precipitation forecast amounts. Main jet core and attendant pv
anomaly will then round the base of longwave trough Tuesday night
into Wednesday and support secondary cyclogenesis somewhere over the
Ohio Valley. It is the exact timing and location of this secondary
wave that has caused so much difficulty for the various nwp
solutions. As noted earlier...overall trend in the past few cycles
has been for a more progressive/open trough through our area...not
becoming negatively tilted/rapidly deepening until it reaches
Canada. As has been mentioned frequently...there is no good source
of cold air upstream and a weaker/more progressive surface low will
struggle to pull in sub-freezing air before the best forcing for
ascent exits. And while the latest consensus track is slightly
further east...still looks like a limited window for any significant
snowfall accumulations late Wednesday. That will be especially true
given warm/wet ground conditions and very marginal thermal profiles.
A quick look at the new 12z European model (ecmwf) shows a stronger Ohio Valley low
but surface wet bulbs still hovering right around freezing. Taking a
more holistic approach...decided on no major changes to the going
forecast given the level of uncertainty. Still suspect at least a
brief period of light snow and gusty winds Wednesday evening but the
magnitude of which is yet indeterminable.

While chances for a significant event appear low...there is still
little confidence in any one particular solution. Anyone with
Christmas evening travel plans should still keep an eye on the latest
forecasts. Even an advisory-level event can cause large disruptions
on a busy travel day.

There is another chance for rain or snow on Friday but will have to
get Christmas evening storm sorted out before any meaningful discussion
of Friday. Does appear to get colder heading into the weekend and
early next week though...with a return to northwest flow and Arctic
air at least making a run for our local area.


Aviation...(for the 00z tafs through 00z Monday evening)
issued at 600 PM EST sun Dec 21 2014

Drier boundary layer air had filtered through the County Warning Area this afternoon and has
scoured out stratus plague of late. However fairly intense low pressure
prognosticated to organize through the Central Plains on Monday west/resultant
sig downstream moisture advection taking shape. Thus expect flight conds will
deteriorate considerably Monday afternoon as rain develops.


Iwx watches/warnings/advisories...


short term...jt
long term...agd

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