Area forecast discussion 
National Weather Service Indianapolis in 
325 PM EST Sat Dec 20 2014 


Update... 
the aviation section has been updated below. 


&& 


Synopsis... 
issued at 226 PM EST Sat Dec 20 2014 


High pressure in place over the eastern Great Lakes and Indiana 
through Sunday will keep dry weather in place across central 
Indiana through the weekend. By Sunday night...the high will reach 
New England and warmer...southerly flow will return to Indiana and 
the Ohio Valley. 


High clouds are expected to arrive on Sunday night ahead of a warm 
front...as low pressure builds across the plains states. A warm 
front will then surge into Indiana by Monday night and Tuesday 
providing good chances for rain. 


Low pressure will then move through the Tennessee and Ohio River 
valleys on Christmas evening...bringing chances for light snowfall 
across central Indiana for middle week. Dry but colder weather is 
expected for Christmas. 


&& 


Near term /tonight/... 


Issued at 226 PM EST Sat Dec 20 2014 


Surface analysis early this afternoon shows weak high pressure in 
place across Indiana. Infrared images continue to show extensive 
cloudiness across central Indiana and the entire Midwest. Radar 
was quiet. 


Main forecast challenge tonight will be temperatures. Cloudy skies will 
be expected through the night. Time height sections continue to 
show plenty of lower level moisture lingering through the night. 
Given this and our satellite images...expect cloudy skies to 
remain through the night. Temperature advection through the night 
appears rather neutral. Thus will trend lows at or above 
mavmos...closer to persistence. 


&& 


Short term /Sunday through Tuesday/... 


Issued at 226 PM EST Sat Dec 20 2014 


Main forecast challenge will be temperatures initially...followed by 
precipitation chances on Monday into Tuesday. 


Dry weather is expected through at least Monday as models suggest 
weak ridging in place aloft as troughing builds across the 
Central Plains. Little in the way of upper support appears to 
pass through the Ohio Valley on Sunday and Sunday night. Time 
height sections continue to show lower level saturation lingering 
on Sunday and Sunday night. Even if the stratus does begin to 
break up on Sunday afternoon...the start of warm air advection on 
Sunday night and Monday on the backside of the high should result 
in a return of high cloud. Thus the outlook for sunshine is low. 
Will trend toward mostly cloudy Sunday and Sunday night. As for 
temperatures...given the ongoing warm air advection on the backside of 
the high as 850mb temperatures rise toward 0c by 12z Monday will trend 
highs Sunday cooler than mavmos with clouds and lows Sunday night 
warmer than mavmos. 


On Monday and Monday night GFS and NAM suggests a deep trough 
digging over the Central Plains as te surface high reaches New 
England. Strong warm air advection begins to take shape on Monday 
as southeast winds develop across the state. By Monday 
afternoon...forecast soundings are hinting at deep saturation as 
in initial short wave is pushed across central Indiana as it is 
ejected out of the trough to the west. Meanwhile the 295k GFS 
isentropic surface continues to show very string upglide starting 
Monday afternoon with specific humidities in excess of 3 g/kg. 
Thus feel good ingredients are available for precipitation on Monday 
afternoon. Will trend probability of precipitation higher than mavmos and also trend temperatures 
higher also given. 


On Monday night and Tuesday...strong isentropic lift is expected 
to continues as the strong trough aloft digs through the Central 
Plains meanwhile forecast soundings continue to show good 
saturation. Best saturation appears on Tuesday afternoon as strong 
low pressure pushes into the Mississippi River valley and a surge of 
warm air arrives in central Indiana. Furthermore the GFS suggests a 
50 kts low level jet providing additional lift. Thus with all these 
ingredients will again trend probability of precipitation upwards...and again trend temperatures 
warmer on lows and cooler on highs. Furthermore...being in the warm 
sector...and forecast soundings solidly above 
freezing...precipitation type will be rain. 


&& 


Long term /Tuesday night through Friday night/... 


Issued at 236 PM EST Sat Dec 20 2014 


A strong system is still prognosticated to impact central Indiana during 
the Christmas Holiday. Latest GFS and Euro are differing slightly 
on track of first low pressure system...with the GFS now taking a 
bit more northerly path than the Euro. Tuesday night...the GFS has 
the upper low centered over the northern Great Lakes 
region...while the Euro has it tracking across Wisconsin. 
Meanwhile...a surface frontal system will track across central 
Indiana...and associated southerly flow will advect plenty of 
moisture into the region ahead of the upper low. So...definite and 
likely probability of precipitation are still reasonable for Tuesday night and Wednesday 
across all of the forecast area. 


Precipitation will still be in the form of rain on Tuesday evening...but 
a cold front will quickly move in on Tuesday night...starting the 
precipitation transition from rain to rain/snow on Tuesday night. The 
final transition to all snow should occur on Wednesday afternoon. 
At that point...a secondary low pressure system will approach from 
the Missouri Valley...phasing in with the aforementioned primary 
system. It/S this secondary low that will cause the most impact as 
the best dynamics track across central Indiana on Christmas evening. It 
is too early to list snowfall amounts...but strong winds will 
create slick conditions regardless. 


Further out...high pressure will build into the area on 
Friday...but yet another system will move across Manitoba and 
Ontario late in the extended period...bringing additional chances 
for rain and snow to central Indiana on Friday and Friday night. 




&& 


Aviation /discussion for the 20/2100z taf issuance/... 


Issued at 325 PM EST Sat Dec 20 2014 


Tweaked current conditions only. Previous discussion follows... 


Persistent low level cloud deck continues to affect taf sites. 
MVFR/VFR conditions will prevail for duration of taf period 
despite high pressure over the area. Time cross sections continue 
to show substantial low level moisture. Variable winds will become 
mainly southeasterly tonight with sustained speeds of 2 to 4 kts. 


&& 


Ind watches/warnings/advisories...none. 


&& 


$$ 


Synopsis...puma 
near term...puma 
short term...puma 
long term....tdud 
aviation...tdud/cp 


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