Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Indianapolis in
327 PM EST Friday Dec 13 2013
the aviation section has been updated below.
issued at 149 PM EST Friday Dec 13 2013
Accumulating snow will begin over central Indiana tonight and
continue through Saturday as a low pressure system and associated
surface trough track through the Ohio and Tennessee valleys. Winter
storm warnings are in effect for much of central Indiana. Arctic air
will return for the second half of the weekend with much colder
High pressure will bring drier weather for the first part of next
week along with a slow warming trend.
Near term /tonight and Saturday/...
Issued at 149 PM EST Friday Dec 13 2013
Surface analysis early this afternoon shows high pressure over the
Carolinas and low pressure over the Southern Plains. A surge of
moisture...clouds and precipitation was found between these two systems
over Missouri. Water vapor imagery shows a strong upper disturbance
near the Texas Panhandle. Tropical moist flow aloft was found ahead of
the system across Texas and into the deep south. The moist northern
jet stream was found streaming across western Canada into the upper
Midwest and Great Lakes. Visible satellite shows high clouds streaming
into Indiana from the southwest...and surface temperatures have been able
to climb to the upper 20s and lower 30s amid warm air advection.
Many factors remain favorable for snow tonight and Saturday. GFS and
NAM both depict the strong short wave over the southwest to push
quickly east tonight to the Texas Panhandle and to Kentucky and
Tennessee by Saturday morning. Potential vorticity maximum is associated
wit his feature is expected to sweep across Kentucky providing ample
forcing. 285k GFS isentropic surface shows good isentropic lift with
specific humidities over 3 g/kg through Saturday morning. NAM
continues it/S more northerly Route of the surface low...across Kentucky
while the GFS maintains it/S more southerly path. Aloft the 850mb
and 700mb lows in the GFS and NAM both suggest favorable paths for
snow...across central Indiana. Forecast soundings indicate a
saturated column shortly after 06z...and keeps that in place through
at least 18z Sat. NAM time heights also suggest a saturated column
with best lift near 06z. Good lower level jet forcing also is in
place ahead of the low as it tracks from the southwest with a low level jet
around 40 knts feeding into Kentucky and Tennessee. Forecast
sounding precipitable water show values around 0.60 inches. Thus
with all of these factors favorable for snow...will aim for 100 probability of precipitation
tonight and on Saturday morning as these features pass across the
state. Best quantitative precipitation forecast and moisture appears to be north of I-70 and that is
where we have placed the warning. Expect many locations in the
warning area to be right near the 6 inch snow criteria. With less
moisture and not as favorable forcing expected across the southern
parts of the forecast area the ongoing advisory for 3-5 inches of
snow looks reasonable.
As for temperatures will trend lows tonight at or above mavmos lows and
trend highs on Saturday at or below mavmos given th expected clouds
and rain. Metmos still looks too warm in all cases.
Short term /Saturday night through Monday/...
issued at 149 PM EST Friday Dec 13 2013
In wake of the snow...strong cold air advection is expected on
northwest winds. By 06z GFS 850mb values fall to -6c...as weak
surface ridging builds from the southwest. Forecast soundings show
strong drying amid the subsidence. Thus will trend toward a dry
Saturday night under partly cloudy skies. Given our expected new
snow cover amid cold air advection will trend lows colder than
On Sunday through Monday...the GFS continues to suggest the
cold..northwest flow aloft to push out of the upper Midwest into
Indiana and the Ohio Valley. Several weak...poorly organized upper
level disturbances look to push across the region on during this
time period. Surface flow looks to remain slightly cyclonic. Deep
moisture looks to remain unavailable as the lower level flow remains
from the northwest. Given this set-up would not be surprised at all
at a few flurries on Sunday and Monday afternoons as forecast
soundings suggest convective temperatures in the lower and middle
20s respectively. Thus will include chances for flurries on both
afternoons. Again with the cold northwest flow...fresh snow and cold
air advection will trend both highs and lows during this period
colder than mavmos.
Long term /Monday night through Friday/...
Issued at 211 PM EST Friday Dec 13 2013
Tranquil weather most of this time frame with warm up coming late
week along with rain. Models in decent enough agreement for most
periods and have used a blend. In larger picture broad eastern
North America trough gradually shifts off the coast as flow zones
out then turns southwesterly for US as upper trough digs into the
western US and thus the transition from cold to mild.
This period will start out with a decent snowpack that will have
an impact on keeping temperatures a bit cooler than they would otherwise
be for much of the week. Late week strong storm system brings
substantially warmer temperatures beginning Thursday and attendant
precipitation should be as rain relative to recent snowy period.
Teens and lower 20s min temperatures will be replaced by above freezing
mins by Friday morning. Maximum temperatures in 30s vs 20s will feel
relatively mild early week but will warm to upper 30s and 40s
Thursday and then upper 40s and 50s Friday so a late week snowmelt
combined with rains could bring some river issues.
Aviation /discussion for 132100z ind taf update/...
Issued at 327 PM EST Friday Dec 13 2013
No changes needed to the taf.
Previous discussion follows...
Snow storm to quickly take shape around 140000z across central
Illinois and central Indiana. Within first hour of snow...expect
quick development of MVFR visibility with a couple hour lag on
lowering ceilings. Rapid development should also mean snow will
lead to IFR visibility within one to at most two hours after snow
starts and again soon to be followed by IFR ceilings. Expect IFR
to then continue through the forecast period.
Winds will back counterclockwise from south to southeast early in
the forecast period and then to east and northeast as low
pressure moves to eastern Kentucky by 141800z. Winds will remain
at or below 10 kts which should limit drifting of snow on runways.
Snow accumulation rates will approach one inch per hour in two
time frames...the first between 1403000-0600z and the second
Winter Weather Advisory from 7 PM this evening to 6 PM EST
Saturday for inz053>057-060>065-067>072.
Winter Storm Warning from 7 PM this evening to 6 PM EST Saturday
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