Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
1146 PM EST Friday Dec 26 2014
The aviation section has been updated below.
issued at 226 PM EST Friday Dec 26 2014
A frontal system is expected to affect the area this weekend. Over
the weekend. In the wake of this frontal passage...high pressure is
expected to build into the area for most of next week. A low
pressure system may affect the area towards the end of next week.
Near term /updated discussion/...
issued at 919 PM EST Friday Dec 26 2014
Forecast still on track. Skies have clouded over. Rains still long
ways away and still not expected until near daybreak.
Previous discussion issued at 226 PM EST Friday Dec 26 2014
By the early morning hours of Saturday...model data suggest the
precipitation bearing layer gets close to...but doesn/T fully
saturate over the western zones as lift increases. Will go with
small chance probability of precipitation for light rain over the western zones after
Based on low level thickness forecasts...the GFS MOS lows for tonight
look reasonable...so won't stray too far from them.
Short term /Saturday through Monday/...
issued at 226 PM EST Friday Dec 26 2014
Model data indicate upper trough currently over The Rockies will
shear out across the area during this period as it encounters
confluent flow between an upper low over southeast Canada and an
upper ridge over the southeast parts of the United States. Appears
the best threat for precipitation will be Saturday and into Saturday
evening as a frontal system passes across the area. Will go with
rather high probability of precipitation during those periods. Precipitation threat expected
to diminish later in the night on Saturday as deeper moisture shifts
off to the southeast.
Some of the model data hint that there may be another piece of The
Rockies trough that shears out over the Tennessee Valley on
Sunday...which may serve to keep some precipitation threat over the
southern zones into Sunday. Will add some chance probability of precipitation in these areas
on Sunday to cover this possibility.
Given the sheared out nature of the trough...the disturbances that
may affect the area during this period should be weak.
Thicknesses suggest this will mostly be a rain event...although
enough cold air may sneak in late Saturday night/Sunday morning for
the potential for mixed precipitation over the northern zones.
Little or no accumulations expected at this time as quantitative precipitation forecast looks light
and temperatures will be marginal.
Based on low level thickness forecasts...the GFS MOS highs for Saturday
and Sunday look OK...but the lows may be a bit on the cool side.
Will nudge up the guidance lows Saturday and Sunday nights a few
Long term /Monday night through Friday/...
issued at 200 PM EST Friday Dec 26 2014
Main focus to the extended will be on temperatures and also on a
late week southern system that could bring precipitation to the area
by next Friday.
Models in good agreement that strong Canadian high pressure will
spread across the Ohio Valley bringing a colder but dry pattern to
central Indiana to end the old the old year and start the new one.
They also agree that an upper low will move into the Southern Plains
next Friday with return flow...on the back side of the departing
surface high...bringing increased moisture to the Ohio Valley for
late next week and next weekend.
Although...the deterministic models and ensembles have some timing
and strength issues regarding the upper low and its surface
reflection...the 12z GFS and 00z European model (ecmwf) both bring deep moisture
northeast and overspread central Indiana by Friday afternoon. This
reflects in the model quantitative precipitation forecast fields along with the regional blend.
Will go with a chance of a rain and snow mix for now as varying
model thermal profiles do not come close to adding any reasonable
confidence in precipitation type 7 days out between the warmer Euro
and colder GFS.
Regional blend appears to be doing a good job with temperature
trends with the coldest air in place new years evening and New Years Day
with highs then only expected to be in the 20s and 30s and lows in
the teens. With the southerly flow increasing but clouds
increasing...middle and upper 30s look OK for Friday.
Aviation /discussion for the 27/06z tafs/...
issued at 1146 PM EST Friday Dec 26 2014
VFR conditions likely rest of the night at all sites but ind
briefly...and laf where MVFR will likely predominate much of the
Approaching frontal system will cause MVFR or worse conditions and
rain to overspread the area near daybreak...with conditions likely
getting toward the low end of MVFR and perhaps even IFR for a period
on Saturday. Will only take things down to ovc010 at this time
however as confidence in timing is low.
Winds will be generally out of the south or south southwest through
tomorrow afternoon...with winds becoming westerly as the front
passes late tomorrow afternoon into the evening. Winds will likely
be 10kt or less the majority of the period.
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