680 
fxus63 kiwx 091116 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
615 am EST Friday Jan 9 2009 


Aviation... 
large area of snowfall from NE Illinois back westward into eastern Iowa continues to 
propagate eastward this morning and will overspread taf sites by middle 
morning. Dry ll Theta-E feed out of departing polar ridge slowing 
eastward top/down saturation yet still expect a several hour period of accumulating 
snow and LIFR conds will pass through ksbn vicinity this morning. 
Shorter duration expected at kfwa west/deeper ascent and btr moisture passing 
north of the terminal yet still a period IFR restriction expected. 


Some improvement this afternoon before conds deteriorate further middle late 
evening in conjunction west/strong SW trough over the central rockies 
ejecting eastward and deepening surface low lifting out of northern Texas into the Ohio 
River valley. Will hold off on LIFR mention yet at kfwa for brevity 
sake but certainly expected at ksbn by 06z. 


&& 


Short term...through Sat night 
intense SW trough over the central rockies this morning will push eastward 
today while surface low pressure consolidates across the Southern Plains before ejecting 
northeastward up through the Ohio Valley lt tonight and Sat. 


In light of continued model trends west/00z guidance...several changes to going 
forecast in order this morning. Most sig is a continued slwr...further south 
shift in surface low track. This aides in sig more Gomex moisture rtn 
entrainment...especially Friday night/Sat into developing middle level deformation band 
commensurate west/amplifying middle level trough. In addn...strong downstream 
warm air advection induced ascent leading to a expanding area of lt/moderate snow across 
the middle Mississippi Valley which 00z runs were too slow in developing. 
This area will continued eastward and overspread County Warning Area this morning before 
diminishing generally south of Highway 30 this afternoon as ll baroclinic 
zone and associated ll ascent maximum mix northward to along the in/Michigan border. 
After much deliberation west/surrounding offices...will upgrade previous 
watch issuance to a warning...particularly for tonight and Sat and 
which generally encompasses area along/north of Highway 30. Advisory south 
of here starting by middle evening. West/model trends in mind and blended 
GFS/Gem ensemble quantitative precipitation forecast/S fairly Stout...favorable collocation of dgz 
within deep ascent zone and expected dominant aggregate snow growth lean 
ME to upgrade to a warning vs a high end advisory west/gridded snow 
amts on the conservative side. However problem lies west/afternoon period today as 
warm air advection snow burst works through the area and things diminish west/southward 
extent from the border. Nevertheless...more sig snow develops through 
this evening ahead of ejecting SW trough and more sig moisture flux. In 
addn...ECMWF/GFS both indicating coupled jet streak aloft and 
further deepening of ascent plume within increasingly 
frontogenetical middle level thermal ridge Sat suggestive of a period of 
heavy...banded snow likely before breaking down Sat afternoon and which would 
push borderline accums along southwestern periphery of warning well into 
warning criteria. 


System shld clear eastern areas by Sat evening west/quick backing Sat night ahead of 
another quick moving SW trough dropping southeastward through the northern lakes. Little 
time to mess west/temperature grids this morning but southern areas may clear enough 
to radiate considerably colder than currently advertised. 


&& 


Long term... 
Sunday through Thursday...the main challenges this period include 
snow chances ahead of clipper type systems and Arctic air intrusions 
behind these systems. A long wave trough will become established over 
the eastern Continental U.S. By the end of the period as very cold Arctic air 
spreads into the region. It looks like there will be a significant 
surge of Arctic air around the middle of next week. This surge will 
be behind a stronger clipper...with temperatures plummeting below 
zero behind the associated front. The GFS ensemble plume diagrams 
show relatively good agreement with this stronger system moving 
through about Tuesday night. Both GFS and Canadian Gem ensemble 
members support a very cold period by Thursday. The airmass well 
upstream into the Canadian and alaskan interiors was bitterly cold 
with temperatures -50f to -60f in the alaskan interior. As the upper 
level ridge amplifies off the West Coast...the downstream trough over 
the eastern Continental U.S. Will deepen. The GFS ensembles...Canadian 
ensembles...European model (ecmwf) operational run and GFS analogs all solidly 
support these very cold conditions late in the period. 
Therefore...have adjusted temperatures down accordingly late in the period. 
Given the active clipper type of pattern...ongoing snow chances with 
lake effect are reasonable. 


&& 


Iwx watches/warnings/advisories... 
in...Winter Storm Warning from 2 PM this afternoon to 5 PM EST 
Saturday for inz018. 


Winter Storm Warning from noon today to 3 PM EST Saturday for 
inz006>009-016-017. 


Winter Storm Warning from 10 am EST /9 am CST/ this morning to 1 
PM EST /12 PM CST/ Saturday for inz003>005-012-014. 


Winter Weather Advisory from 10 PM this evening to 3 PM EST 
Saturday for inz013-015-020-022-023. 


Winter Weather Advisory from 10 PM this evening to 5 PM EST 
Saturday for inz024>027-032>034. 


Michigan...Winter Storm Warning from noon today to 3 PM EST Saturday for 
miz080-081. 


Winter Storm Warning from 10 am this morning to 1 PM EST 
Saturday for miz077>079. 


Ohio...Winter Storm Warning from 2 PM this afternoon to 5 PM EST 
Saturday for ohz002-004-005-015-016-024-025. 


Winter Storm Warning from noon today to 3 PM EST Saturday for 
ohz001. 


Lm...none. 


&& 


$$ 


Short term...holsten 
long term...skipper 
aviation...holsten