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Area forecast discussion 
National Weather Service northern Indiana
555 am EST Friday Dec 26 2014

issued at 350 am EST Friday Dec 26 2014

High pressure over the southeast United States will maintain dry and
warm weather today. Highs will be in the upper 40s with lows tonight
staying in the low 40s. There will be a chance of rain late tonight
into Saturday as low pressure moves over the Great Lakes. A brief
mix of snow is possible Saturday night as colder air filters into the
region but no accumulation is expected.


Short term...(today and tonight)
issued at 350 am EST Friday Dec 26 2014

Main forecast challenges for this short term period revolve
around cloud forecast for today and precipitation chances/timing late
tonight. Ohio Valley will remain firmly under the influence of
broad subtropical ridge over the southeast Continental U.S. For one more day.
Approach of longwave trough currently over The Four Corners will
cause subtle low level warm/moist air advection to continue
through the day. Was a little concerned that boundary layer
moisture would be trapped beneath low level inversion and lead to
stratus development later this morning. That scenario is hard to
buy into based on latest forecast soundings and upstream
observations though. Most of the 00z numerical guidance shows low
potential for stratus given overall weak warm air advection/inversion strength
and insufficient low level moisture. No reports of low clouds
upstream and regional 00z soundings indicate very dry environment
below 18 kft. Will therefore hold with mostly sunny/partly cloudy
forecast with just some thin/high based clouds early and low
clouds not increasing until this evening. Given expectation for at
least some sun and 925mb temperatures approaching 5c...have leaned on the
warmer side of guidance with generally upper 40s. Some lower 50s
definitely not out of the question.

Main pv anomaly will then eject northeast by late tonight with weak
surface low in northern Illinois by 12z Saturday. Elevated warm
front will pass through our local area shortly after 06z. This
feature is clearly seen on 290k Theta surface with a well defined
kink in the isobars and a sharp increase in the net adiabatic Omega.
Suspect we will remain dry until this front moves into the area.
Even then...precipitation chances are not great due to persistent dry air
in the midlevels. Lack of good middle/upper level support through 12z
warrants a continued cautious approach to probability of precipitation. Wouldn't be
surprised if a large part of the forecast area sees some sprinkles
and/or drizzle early Saturday morning but chances of measuring are
still fairly low. Increased probability of precipitation a bit but remained in chance
category. All liquid ptypes expected with overnight lows comfortably
above freezing. Doesn't look like a great setup for true drizzle so
left light rain as the weather type. Suspect there will be some br
with the precipitation but persistent surface winds should prevent any
dense fog and will therefore make no mention of it in the gridded


Long term...(saturday through thursday)
issued at 350 am EST Friday Dec 26 2014

Overall very few changes needed for the long term period...with main
concerns on chances of rain Saturday/Saturday night followed by a
transition to colder conditions for next week.

Better chances of measurable light rain should begin to shift into
particularly northwest portions of the area by 12z Saturday as
initial short wave trough across the Central Plains becomes
influenced by deformation zone across the western great/middle MS
valley. Brunt of low level moisture will be relatively shallow in
nature and still expecting light rainfall amounts through Saturday
morning. Rain chances somewhat complicated on Saturday by this lead
short wave becoming increasingly sheared in nature and weakening as
it lifts across the western/southern Great Lakes during the day.
Guidance does seem to be hitting at a secondary area of forcing more
tied into lower level moisture convergence associated with broad
weak confluent low level flow across the Ohio Valley. Tried to
indicate these trends in pop/weather grids for Saturday but net result
should still be for high chance/likely probability of precipitation most areas.

Brief duration of deeper moisture confined to passage of dampening
middle/upper trough should keep rainfall amounts quite light through
Saturday night. Item of lower confidence for this forecast still
deals with ending time of higher precipitation chances. Highly sheared
nature of upper flow suggests that low/middle level frontogenesis
forcing could persist across at least southeastern two thirds of the
area well into Saturday night...although low level moisture profiles
likely to become less conducive for precipitation. Have maintained likely
probability of precipitation across the southeast into Saturday evening...tapering to lower
chance by Sunday morning. Still expecting bulk of precipitation to fall as
-ra...with a small chance of a brief period of light snow late
Saturday night if stronger middle level fgen forcing can materialize
..but no accumulation is expected.

Next eastern Pacific jet streak to carve out more substantial negative
upper height anomaly across western Continental U.S. Early next week...which
should cut off across southwest Continental U.S.. remainder of eastern two
thirds of country to be characterized by low amplitude long wave
troughing and transition to colder conditions. Upper short wave
diving over eastern Pacific Ridge should provide reinforcing shot of
low level cold air advection by middle week with below normal temperatures setting up.
Outside of the potential of scattered lake effect snow
showers...synoptic precipitation chances will be hard to come by through
this forecast period as broad low level anticyclone maintain its
grip. Weak northwest flow wave could yield downstream weak fgen response
for Monday...but moisture looks to be a main limiting factor for
anything other than Chance Lake effect snow showers. Some temperature
recovery expected toward the end of the period...with cut-off
southwest Continental U.S. Pv anomaly eventually getting ingested back into
main westerlies. This also should provide better chance of precipitation
just outside of this forecast period...although good deal of
uncertainty remains regarding how this cut-off feature
interacts/phases with the northern stream.


Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 555 am EST Friday Dec 26 2014

VFR conditions will persist for much of today with a strong ridge
of high pressure in place. Stratus potential for this morning still
appears very low based on upstream surface observation and satellite.
Deteriorating conditions expected by later tonight though as
midlevel shortwave and associated surface low approach the Great
Lakes. Stratus and br are likely but confidence in exact flight
category is still low. Will hold above fuel alternate for now with
the best chances for rain and lower flight category after 12z


Iwx watches/warnings/advisories...


short term...agd
long term...marsili

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