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Area forecast discussion 
National Weather Service northern Indiana
745 PM EST Sat Dec 20 2014

Synopsis...
issued at 1210 am EST Sat Dec 20 2014

High pressure will continue to provide the region with dry
conditions...light winds...and mostly cloudy skies this weekend.
Highs are forecast to reach the low to middle 30s today and the middle
to upper 30s on Sunday. Chances for rain return Monday into
Tuesday as an upper level trough digs into the plains and Gulf
moisture streams north into the region.

&&

Short term...(this evening through sunday)
issued at 347 PM EST Sat Dec 20 2014

Surface ridge extending from Quebec to Texas this afternoon continuing to cause light
low level wind flow across our area resulting persistence of
stratus trapped beneath inversion based around 3kft. Middle level
moisture/lift associated with a weak shortwave moving east across the area
was also resulting in some flurries but a layer of dry air between
stratus and middle deck should prevent any measurable precipitation from
occurring.

Upper level trough crossing The Rockies this afternoon expected to gradually
amplify over the plains over the next 24hrs as strong west-east oriented
jet over the eastern Pacific begins to overspread the western Continental U.S.. this
will result in gradual backing/increase of low level flow across our
area which may scour out some of the low clouds by Sunday evening but
still expect another mostly cloudy day as high clouds should also be
moving into the area from the west. The clouds should result in a
small diurnal temperature range over the next 24hrs with lows
tonight in the M-u20s and highs Sunday in the M-u30s.

&&

Long term...(sunday night through saturday)
issued at 347 PM EST Sat Dec 20 2014

Solution space with respect to Christmas evening storm is beginning to
narrow a bit. Important subtleties have yet to be resolved but
generalized expectations are becoming more illuminated with at least
better consistency in the latest deterministic models. Intense
surface low looks to track almost directly overhead Wednesday
morning...forcing a changeover to snow sometime during the latter
half of the day. Snow amounts will depend on exact track of surface
low and time of transition. This is difficult to pin down more than
a day or two in advance but does appear that our area will see at
least some light snow late Wednesday into Christmas morning. Perhaps
the most noteworthy aspect of this event will end up being the
strong winds...possibly gusting over 40 miles per hour...expected late
Wednesday.

Here are the details:

Latest satellite imagery shows the impressive North Pacific jet and
moisture plume already impinging on the West Coast. Models should
continue to hone in on the details of the event as this jet is
better sampled by the noam upper air network. Lead wave still
expected to enter the upper Midwest on Monday. Increasing warm air advection/290k
isentropic ascent ahead of this wave will bring increasing rain
chances to our local area by late Monday. Main jet core and pv
anomaly will then dig into the Southern Plains on Tuesday.
Associated tightening of the thermal gradient from Texas to Michigan
will lead to continued rain chances through Tuesday. Could even see
some decent rainfall totals...especially late Tuesday as upper jet
rounds the base of the trough and rapid cyclogenesis commences over
the Tennessee/Ohio valleys. Surface low still expected to "bomb" out over the
region on Wednesday...supported by good diffluence aloft and coupled
150+ knots upper level jet streaks. Trough becomes negatively tilted
and closed off by Wednesday as well. Final snowfall amounts will
depend greatly on exact timing/path of surface low and associated
thermal profiles. To this point there is still enough disagreement
to lead to lower confidence in any snowfall prediction. Operational
GFS and Gem are notably slower than the parallel GFS and European model (ecmwf) which
show a more progressive surface low in the u.P. By 18z Wednesday. As has
been noted in prior discussions...these highly dynamic/negatively
tilted systems are often slower and further west. Though that is not
always the case...have nudged the forecast in that direction with a
later changeover to snow. Actually neither Camp is particularly
conducive to significant snowfall for our area with a more westward
track delaying the cold air and a faster solution lifting the best
forcing into the northern lakes. There would be a significant
difference in wind speeds though. Operational GFS shows a much
tighter pressure/height gradient in our area. Even more ominous is
the dry slot/isentropic descent seen in the GFS Wednesday afternoon
that could promote efficient downward momentum transport. Will
continue to monitor the latest numerical solutions and tweak the
forecast as necessary in the coming days.

To re-emphasize the bottomline...still expect a significant storm in
the Great Lakes on Christmas evening. Snowfall amounts are uncertain but
there does appear to be at least a brief period of light snow
possible late Wednesday into Thursday. There is also the potential
for strong winds...making travel difficult even with light snow
accumulations. Anyone with travel plans should stay tuned to the
latest forecasts as we pin down the details of the Christmas evening
forecast.

&&

Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 740 PM EST Sat Dec 20 2014

Ll moisture trapped beneath weakening ll thermal inversion will give way
this evening as better subsident induced mixing develops in wake of
weak SW that produced afternoon --sn. However while VFR conds expected for the
period some MVFR visibility restriction possible in vicinity of the terminals overnight
into sun am within continued weak but moist boundary layer follow.

&&

Iwx watches/warnings/advisories...
in...none.
Michigan...none.
Ohio...none.
Lm...none.
&&

$$

Synopsis...steinwedel
short term...jt
long term...agd
aviation...T



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