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fxus63 kiwx 071725 
afdiwx 


Area forecast discussion 
National Weather Service northern Indiana 
1225 PM EST Wednesday Jan 7 2009 


Aviation... 


Sbn/FWA currently indicating ceilings near the 1kft level with shsn 
impacting both locations. Expect current IFR ceilings to increase 
slightly to low MVFR at sbn with MVFR ceilings at both locations 
persisting through the taf period. Synoptic shsn will diminish 
towards evening as upper wave departs the area. However...cooling 
temperatures aloft will /and already have/ spawn some lake effect shsn and 
the lake moisture should help keep low ceilings entrenched through the 
overnight. Expect little in the way of visibility restrictions at 
FWA...with MVFR visibilities possible at sbn. Expect heavier snow /and IFR 
visibilities/ to remain north of sbn through the overnight. Any snow that 
does fall will allow for the possibility for blowing snow given 
strengthening westerly winds this afternoon/evening. 


Not much in the way of improvement early Thursday morning with MVFR 
ceilings likely to persist /although any shsn will begin to taper off/ 
through 18z. 


Westerly winds will quickly increase this afternoon to 15g25kts with 
18g30kts likely this evening...and only gradually tapering off after 
midnight to 10-15kts with gusts 20-25kts. 


&& 


Previous discussion... /issued 623 am EST Wednesday Jan 7 2009/ 


Short term... 


Today through Thursday night... 


.Icy roads still possible this morning... 


Interesting short term forecast with a mainly light precipitation 
event that will be stretched through most of this short term period. 
Most of forecast area has seen freezing drizzle overnight as 
dendrite growth zone...dgz...has been above cloud layer and in 
relatively low relative humidity. Satellite shows middle and high clouds invading from 
west in association with middle level trough that will move across the 
area today. These higher clouds providing the needed seeding of 
lower clouds for ice nucleation to change precipitation over to light snow. 
Surface observations reflect this with many sites changing over from 
west to east as of this writing. With satellite trends showing the 
higher clouds advancing east...should just be a few hours before 
most sites switch over to all light snow. Will likely carry a 
mention of patchy freezing drizzle for very early this morning but 
change to all snow near daybreak. Given the icy conditions on roads 
and sidewalks...especially those untreated...and the light snow on 
top of this glaze...have decided to continue the Winter Weather 
Advisory through the morning commute. Several calls from observers 
early this morning indicate secondary roads especially are still icy 
and hazardous. Surface temperatures remain in the middle to upper 20s 
per Road sensors and observations...and have seen this type of 
situation before where friction from traffic also melts the light 
snow only to see it refreeze on the glazed untreated surfaces. 


Middle level trough and colder air moving across the region today 
coupled with cyclonic flow should lead to numerous snow showers by 
late morning or early afternoon. Following blend of NAM WRF 12km and 
4km runs along with sref which all show light snow across much of 
the area. Higher chances far north where some lake enhancement 
possible and far east where a little better moisture and afternoon 
trough location will provide additional lift. Looks like a high pop 
low quantitative precipitation forecast event but some light snow accumulations 
possible...especially north and east. 


Attention shifts to northwest flow and marginal lake effect tonight 
into Thursday night. Delta T values marginal just into middle teens 
with some low level wind shear early tonight. Depth of moisture and 
lack of inversion along with possible feeder band from Lake Superior 
should allow for lake effect snow showers...with best window late 
tonight into Thursday morning as low level shear decreases. Light 
accums expected but strung out over these next 4 periods may allow 
total accums to push 2 to 4 inches. No headlines anticipated given 
the long duration and borderline amounts for advisory. 


Used a blend of latest MOS guidance and current grids for temperatures with 
a nod toward the cooler grid temperatures given cold air aloft. 


Long term... 


Friday through Tuesday 


Active northwest flow will allow several clipper systems to affect 
the region with timing/track/and intensity still very much in 
question...especially by late in the period. Pattern transition to 
active northwest flow will allow the region to take a roller coaster 
ride in the temperature department...with temperatures warming to at or 
slightly above climatology in the warm air advection zone ahead of each 
disturbance...then fall well below climatology norms Post frontal. The 
bottom is expected to drop out with respect to temperatures by next week as staunch 
ridging in the eastern Pacific and western US is realized...associated 
with a positive pna regime. This is in direct reponse to the 
reinvigoration of the east Asian upper jet as it/S eastward 
extension supports propagation of the the Gulf of alaskan upper 
trough to the east. End result is highly amplified flow into 
Alaska...with staunch ridging expected there. Northwest flow into 
the region will direct potent middle level disturbances into the local 
area...allowing advection of the modified Arctic air /in place in 
the high latitudes/ into the region. Surface temperatures in portions of Alaska 
and northwest Canada have been dipping to -60f...and this Reservoir 
of cold air has been expanding. European model (ecmwf) does indicate the possibility 
of a siberian express connection which would further support very 
cold air into the northern US. 


GFS and European model (ecmwf) deterministic solutions have been wavering on 
track/intensity/and thermal fields related to the clipper systems 
Friday/Monday/and Tuesday. Mean ensembles of the naefs and gefs not 
much better...with signs of high spread in slope and quantitative precipitation forecast 
fields...although 850 mb T fields have been fairly solid in the last 
several runs with respect to to Sat-sun. FSU GFS ensemble confidence graphics 
highlight the growing uncertainty with increasing slope late in the 
period. Felt the best option was to stick closer to ensemble 
means...shying away from deterministic GFS/Gem/ECMWF 
runs...adjusting climatology numbers to ensemble trends. Did feel chance 
probability of precipitation beyond sun are warranted even though timing and storm tracks 
are still uncertain...given the pattern in place with a 2 day 
clipper frequency. 


Friday into sun...model trends in this period are for a deeper surface low 
reflection with a descent warm up Friday night ahead of the surface cold 
front. Have leaned toward the stronger surface reflection depicted in 
GFS and NAM forecasts given typical intense nature of 
northern stream upper trough in a strong baroclinic env. Given 
anticipated warm nose aloft with a cold surface layer in 
place...expecting another mix precipitation event especially in the 
north...with a period of freezing rain and/or snow Friday night. 
Freezing drizzle would also be expected given surface low track near or 
over the forecast area with a period of middle level drying over ll saturation. 
Have opted for mix wording...and trended toward the faster system 
speed of the GFS...which is typical for northwest flow surface lows. Strong cold air advection 
coupled with a tight pressure gradient in place and steep ll lapse 
rates will support gusty winds Saturday...with periods of lake 
effect snow possible in the northwest. Have opted to end precipitation slightly 
faster sun given fast progressive flow in place...supporting middle 
level riding by sun. 


Sun night-tues...active northwest flow continues with 2 clipper 
systems expected to impact the region. One Sun night into Monday and a 
second system that is expected to support an Arctic intrusion given 
850 mb temperatures down to -20 to -25c by Tuesday. Very cold air and steep ll 
lapse rates would likely support lake snow showers. In 
addition...dangerously cold temperatures below zero would be a possibility. 


&& 


Iwx watches/warnings/advisories... 
in...none. 
Michigan...none. 
Ohio...none. 
Lm...Small Craft Advisory until 7 PM EST Thursday for lmz043-046. 


&& 


$$ 


Short term...Lashley 
long term...jc 
aviation...Arnott 

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