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Area forecast discussion 
National Weather Service northern Indiana
1232 am EST Sat Dec 27 2014

issued at 350 am EST Friday Dec 26 2014

Low pressure will cross the Great Lakes this weekend...bringing rain
to the area on mainly Saturday. Until then...expect increasing
clouds. Lows tonight will be in the upper 30s and low 40s. Next
week...expect lake effect snow showers near Lake Michigan and normal
to below normal temperatures.


Short term...(this evening through saturday)
issued at 320 PM EST Friday Dec 26 2014

The Atlantic subtropical ridge had edged inland and was helping to
allow return flow with limited moisture to spread northeast. with a similar bias in a recent event...NCEP models
were trying to spread too much moisture too fast to the northeast.
Upstream aircraft observations at St Louis showed low level moisture
was not as robust as the models were showing. Also...upstream low
clouds were not as progressive to spread north. Therefore...adjusted
back the timing of rain. However...isentropic lift should be
sufficient in time with some moisture to allow a fairly widespread
but light rainfall across the area Saturday. Made some changes in
this regard. Otherwise...seasonally very mild weather tonight will
keep lows from fall much below 40...and prevent slippery roads that
occurred this past night.


Long term...(saturday night through friday)
issued at 320 PM EST Friday Dec 26 2014

Quiet weather is in store for the long term period. We start off Sat
night/sun with a 500 mb low over northern Ontario and a subtropical
ridge anchored over the southeastern United States. As the low
departs to the northeast and the ridge drifts eastward into the
Atlantic...flow over the County Warning Area becomes relatively zonal and remains
that way through Tuesday night. At the surface...a weak low pressure
trough drifts across the Ohio Valley to the East Coast...and
building high pressure over the Canadian prairie expands into the
Midwest. As far as sensible weather is concerned...this means
chances of rain/snow associated with the exiting surface low over
Northern Lake Huron Saturday night will diminish by Sunday
morning...leaving a mostly dry forecast in its wake.

However...the expanding ridge will bring a return to more
winter-like conditions to wrap up 2014. After highs in the upper 30s
and low 40s Sunday...normal to below normal temperatures will
arrive. 850mb temperatures drop to near -16c by Tuesday morning...and
remain there through roughly Wednesday night. This puts high
temperatures through Friday in the 20s and low 30s. With the colder
temperatures...zonal to cyclonic flow aloft...and west-northwest
winds expect some clouds and certainly some les near Lake Michigan.
Lake temperatures are around that puts 700mb Delta ts around
20-23c. As far as how much les...amounts look inconsequential. With
flow at the surface becoming increasingly anticyclonic and moisture
limited...dont think things will get crazy. 0-1 km Theta-E lapse
rates only reach -1 to -2 k/km...and with the exception of a few
brief periods...1000-700mb tdds are 10-12c. Inversion heights are
variable...generally ranging from 900mb to 800mb...increasing mainly
with shortwaves passing through the zonal flow aloft. Overall little
change to the going forecast. Kept snow showers Monday-Thursday confined to
west-northwest wind snowbelts near Lake Michigan...with best chances associated with
passing shortwaves aloft and deeper moisture.

Friday into Saturday gets interesting. On Thursday GFS/European model (ecmwf) have an
upper level cut off low swinging onshore in California...and they
both move that low into the Central Plains by 00z Sat. Surface
cyclogenesis takes place in response with both models...but as usual
the exact low track/timing differs. The slower 00z European model (ecmwf) takes the
surface low from Oklahoma at 6z Sat to southwestern lower Michigan by 18z Sat.
The 12z GFS is more progressive as usual...and takes the low from OK
00z Sat to the Ohio Valley by 12z Sat...then to the East Coast by
18z Sat. Will have to monitor trends over the upcoming week as this
system looks to be the next significant weather event for the
otherwise quiet long term period.


Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1232 am EST Sat Dec 27 2014

Highly sheared midlevel wave and associated surface low pressure
will move across the upper Midwest and northern Great Lakes today.
Warm air advection/isentropic ascent ahead of this low will bring light rain to
the area beginning later this morning. Periodic light rain showers
will then continue through much of the day as surface cold front
slowly pushes through the region. Low confidence in fuel alternate
ceiling heights persisting through the whole day but suspect bulk
of the period will feature these conditions and left in as
prevailing. Bumped up the start time though with latest hi-res
models showing precipitation beginning at both terminals shortly before
12z. Also could be some low level wind shear around 12z this morning but did not
put in the taf due to low confidence and short time window for
possible occurrence.


Iwx watches/warnings/advisories...


Synopsis...mesoscale discussion
short term...skipper
long term...mesoscale discussion

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