Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service northern Indiana
412 am EST Tuesday Jan 27 2015

issued at 410 am EST Tuesday Jan 27 2015

Some very light snow or light freezing drizzle will be possible
early this morning across far western and northwestern Indiana as
well as Illinois. Any precipitation is expected to diminish
shortly after sunrise. Highs today will be in the upper 20s and
30s with partly to mostly cloudy skies.


Short term...(today and tonight)
issued at 403 am EST Tuesday Jan 27 2015

Another relatively quiet short term but cloud concerns and possible
fzdz this morning make for a few issues. Far western County Warning Area saturated
overnight with flurries or very light snow falling in downshear side
of middle level jet axis. Advisory issued last evening for freezing
drizzle that never really materialized over our area due to
saturated dgz and seeder feeder mechanisms allowing for snow Crystal
growth. However...still a slight concern for light fzdz through
sunrise as satellite shows warming cloud tops upstream across WI
moving quickly south and loss of higher seed clouds expected next
few hours. Hires model soundings indeed showing dgz drying out early
this morning in the west but low levels also drying. Thus chances
for any precipitation will be diminishing but precipitation type of any residual precipitation
could be fzdz. Calls to white and Pulaski County indicate no
problems from fzdz overnight and little to no reports of fzdz
upstream at this hour. With advisory set to expire at 6am anyway
plan to cancel early and handle any spotty freezing liquid
development with special weather statements if needed.

Dry wedge in lower levels has remained in place across remainder of
County Warning Area with light easterly flow aiding in dry air entrainment. This dry
easterly flow along with subsidence today expected to help break up
clouds from northeast to southwest through the day.
However...satellite shows area of stratus over WI moving slowly
southward and models do not have a good handle on this cloud cover.
Low level relative humidity forecasts from various models all seem to support drying
today from the northeast but ruc13 has also shown a tendency for
higher relative humidity across central Ohio to advect west into our eastern area
this afternoon. All these cloud issues make for difficult forecast
today. Most surrounding offices favoring the clearing scenario so
will lean toward the drying and be optimistic with skies becoming
partly cloudy and eventually mostly clear tonight. High pressure to
then slide across the area tonight with light winds and mostly clear
skies. This should allow good radiational cooling and temperatures to drop
back to single digits eastern half and lower teens west where some
middle and high clouds may spill over ridge.


Long term...(wednesday through monday)
issued at 403 am EST Tuesday Jan 27 2015

Next clipper-type system still slated to impact the area late
Wednesday night into Thursday. This wave will actually be aided to
some degree by another vorticity maximum leftover from old Baja California cutoff low
currently being reabsorbed into the mean westerlies. Still some
inconsistencies in the models regarding where exactly these two
waves track and how they interact but our area should see a period
of decent pv advection/cva that will generate at least some forced
ascent and light precipitation. The problem is that upper jet support is
marginal at best and latest consensus track forecast keeps the best
low level isentropic ascent and frontogenesis generally north of our
area. Furthermore...strong low level warm air advection will introduce a melting
layer aloft (and eventually at the surface) that will make
accumulating snow very difficult with a hodgepodge of all different
precipitation types over space and time. Impossible to correctly nail down
exact timing and location of various precipitation types this far out but
did try to put a "best guess" in the grids for now...based on
consensus of forecast low track and thermal profiles. Preference
lies with a GFS/European model (ecmwf) combo. NAM surface temperatures appear too cold and
quantitative precipitation forecast amounts too light. Gem thermal profiles are decent but keeps
precipitation locked up too far north. Suppose this is possible if forecast
track is in large error but solutions have been fairly stable thus
far and suspect most areas will see at least some light precipitation with
a modest pv anomaly passing directly overhead. This is especially
true given superb moisture availability with 700mb mixing ratios
over 4 g/kg and precipitable water values around 0.75 inches. As far as precipitation
types...expect cold surface temperatures to support freezing rain at the
onset...transitioning to all rain from south to north early Thursday
morning. Latent heat release due to freezing and continual warm air advection with
S/SW surface winds should lead to a nondiurnal temperature trend and
a fairly quick changeover to rain with minimal ice accumulation.
Still could be some slick roads during the early morning commute
though...especially north of US-30. Atypical diurnal temperature trend
continues on Thursday with highs probably occurring in the late
morning and then falling through the afternoon as colder air wraps
back around exiting low. This will lead to a change back to snow
from north to south with some transitory sleet mixing in as well.
Expect snow amounts around an inch at most with the highest amounts
in our far northeast. Any accumulation of freezing rain at the onset
will be less than a tenth of an inch. Some light lake effect/
enhanced snow possible Thursday night but by and large event will be
winding down by then.

Next chance of snow looks to be on Sunday as elongated/positively
tilted trough swings through the central Continental U.S.. models have shown a
high degree of variability during this timeframe so forecast
confidence is low. Did focus the pop forecast a bit but will hold
off for a few more runs before latching onto any specifics. Still
looks like at least a glancing blow of much colder air behind this
system on Monday. Nudged the forecast toward colder raw guidance
with highs in the teens and lows flirting with 0f.


Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 1237 am EST Tuesday Jan 27 2015

Another tricky aviation forecast with respect to cloud heights
this period. Terminals remain VFR at issuance but MVFR and IFR
conditions exists just west of ksbn. Kmcy and koxi have dropped
to 25hft while kvpz down to 1kft with some light snow at times.
Low level flow being shown by hires guidance to slowly turn
northeast and increase slightly to around 5-8 knots next few
hours. This should slow or end the eastward expansion of MVFR ceilings
and eventually push them southwest. However...with proximity to
ksbn had to add a few hours of tempo MVFR to account for
possibility of these clouds. Northeast flow should eventually
allow slightly drier low level air to work west with VFR
conditions into Tuesday. Next concern is low level clouds over
central Ohio making a run west as this low level flow changes. RUC
model brings higher relative humidity into kfwa Tuesday morning. Have introduced
a scattered deck at 25hft for now but this will be monitored for
possible addition of MVFR ceilings to later forecasts.


Iwx watches/warnings/advisories...


short term...Lashley
long term...agd

Visit US at www.Weather.Gov/iwx (all lower case)

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations