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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
616 am CST Friday Jan 9 2009 


Discussion... 


346 am CST 


Short term (today through saturday)... 


Forecast concerns in short term are on two pronged snow event to 
affect the County Warning Area into Saturday. Have made a few minor changes to 
southern border of Winter Weather Advisory with addition of Lee 
and Kendall counties. 36 hour forecast snow totals through 
Saturday remain nearly unchanged...ranging from 5 to 8 inches 
across northern Illinois roughly north of I-90...to 3 to 5 inches down 
to I-80 corridor...and 1-2 inches southern tier of County Warning Area counties. 


Regional radar mosaic depicts initial burst of snow moving into 
northwest/north central Illinois early this morning...associated with 
isentropic ascent on nose of 30-40 knot 850 hpa jet oriented into 
the middle/upper Mississippi Valley per profiler wind data. Surface 
observation across eastern Iowa have indicated visibility down to 1/2-3/4sm 
beneath stronger radar reflectivities last few hours...and this 
band expected to sweep east/northeast across northern Illinois portions 
of County Warning Area early this morning. Short term guidance then in pretty good 
agreement in taking strongest forcing east/northeast of area this 
afternoon...though persistent but weaker isentropic ascent 
expected to keep periods of lighter snow continuing across 
especially far northern parts of the forecast area along Illinois/WI border region 
this afternoon. Forecast soundings across southern parts of forecast area 
depict temperatures rising to +1/+3 c at/above 800 hpa this 
afternoon with surface temperatures rising above freezing...though little 
precipitation expected there. Despite relatively warm temperatures above 
boundary layer... dryness in low levels would support snow/sleet 
given evaporative cooling likely if precipitation occurred. 


Second round of heavier snowfall expected tonight as main upper 
level trough approaches and moves across forecast area. NAM/GFS 
both indicate relatively focused east/west frontogenetic forcing 
which sags across the forecast area overnight...though model time 
sections of potential temperature profile suggest fairly high 
static stability and thus banded precipitation should not be 
overly narrow/intense. Southern counties may see a brief period of 
sleet/snow mix as evaporation Alabama cooling quickly erodes previously 
mentioned warm layer aloft. 


Middle level trough axis moves east of the area Saturday morning...as 
low begins to deepen across Ohio Valley. Period of lighter 
deformation zone snowfall expected to continue into afternoon 
especially across southeast 1/2 of County Warning Area as system continues to pull 
away to the east. 


Ratzer 


Long term (saturday night onward)... 


Pattern through this entire period should be dominated by a strong 
western ridge and an eastern North American trough...with medium 
range models in excellent agreement on further amplification of 
both the trough and ridge by middle/late next week. All indications 
are that this could effectively dislodge the coldest airmass this 
season from the frigid...dark...depths of the Arctic. 


Before the brutal cold airmass arrives...several northwest flow 
disturbances (alberta clippers) will pivot around the Hudson 
vortex. Given limited moisture...snowfall will be derived entirely 
through lift which tends to be confined to the north of the track 
of these surface lows. In all reality...cannot with any degree of 
certainty pinpoint the tracks of these systems more than a couple 
days out. Based on some reasonable degree of model agreement have 
introduced some chance probability of precipitation Sunday...Monday...then again late 
Tuesday with each fast moving wave. Kept probability of precipitation modest for now...but 
in all likelihood there will be swaths of a couple/few inches of 
accumulating fluffy snow with each clipped either in or near our 
County Warning Area. No doubt later shifts will be able to better fine tune the 
details. 


Despite the chances for occasional bouts of snow...the bigger 
story by far will likely be the brutally cold air. Does appear as 
though there will be one quick hitting shot of Arctic air arriving 
Monday night into Tuesday...followed by some moderation in temperatures 
Tuesday night into Wednesday morning as warm air advection with the 
next clipper arrives. It is behind this clipper that medium range 
models send temperatures falling fast and hard. As would be expected this 
far out...there are some slight disagreements in timing of 
arrival of the Arctic air between the 00z runs of the 
ECMWF/Gem/NOGAPS/GFS...though all generally agree that the bottom 
will have fallen out by Wednesday night. The intensity of the 
Arctic airmass varies a bit among the models as well...with the 
GFS the most Bone chilling with a 1058mb high/850mb temperatures near 
-25c building into the area. Though even the most temperate 
European model (ecmwf)/NOGAPS drop 850mb temperatures to -20 to -22c...so fairly high 
confidence that temperatures will go well below zero during the Wednesday 
night/Thursday time frame. Biggest change to going forecast was to 
bump up the arrival time of the bitter cold air by about 12-18 
hours and also nudge lows colder Wednesday night. 


Izzi 


&& 


Aviation... 


610 am CST 


1200 UTC tafs...periods of snow and impacts on ceilings/visibilities will 
continue to be the main aviation forecast issues over the next 24 
to 36 hours. 


Strong low level warm advection has led to widespread light snow 
across northern Illinois. Slater Iowa profiler capturing this low 
level warm advection nicely with a 60 knot 850 hpa southwest wind. 
As this low level jet axis shifts eastward into northern Illinois 
this morning into early afternoon...heavier snow should also skirt 
Illinois/Wisconsin Stateline and then eventually into Southern Lower 
Michigan this afternoon. Will maintain mention of some visibilities down 
to around 1 mile this morning with the snow trending to MVFR visibilities by 
middle morning. Best chance of continued light snow this afternoon will be 
across rfd/dpa/Ord where low level saturation at northern extent 
of warm advection forcing will be more favorable. Chances for snow 
this afternoon will be a bit more questionable at 
mdw/gyy...although given strength of low level isentropic lift and 
strong upper jet streak moving across the western Great 
Lakes...will keep a mention of light snow through the afternoon. 


After initial round of accumulating snow this morning...next round 
of significant snow appears to be in store for tonight as a 
stronger middle level trough approaches from the northern plains. 
Model guidance has been fairly consistent in developing a middle 
level frontogenesis snow band across northern Illinois later this 
evening...and dropping south of terminals late tonight. Will 
continue idea of previous taf of mentioning 1/2sm visibilities with the 
expected period of steadiest snow tonight...roughly in the 04z- 
08z time frame. Some light snow or flurries will likely persist 
into Saturday morning with the NAM/WRF suggesting possibility of a 
weak lake enhanced convergence band setting up. 


In terms of winds...initial light and variable winds this morning 
will become east by late morning and increase to 10 to 15 knots 
with some gusts to 15 to 20 knots this afternoon as low pressure 
across the Central Plains tracks southeast. This low will move 
into southern Illinois Illinois late tonight...and winds will back 
to the northeast in response with speeds continuing at 10 to 15 
knots and gusts to 20 knots. 


Marsili 


&& 


Marine... 


346 am CST 


A ridge of high pressure extending from The Arrowhead of Minnesota 
into Kentucky will slide into the eastern Great Lakes today. An area 
of low pressure across the Central Plains will move southeast into 
southern Illinois tonight...and then into northern Kentucky by 
midday Saturday. Winds over Lake Michigan east of the ridge axis 
remain from the northwest early this morning...but will become east 
across the south half of Lake Michigan this afternoon with some 
gusts to 20 knots possible. As the low tracks well to the south 
tonight...winds will back to the northeast in response with speeds 
expected to continue in the 15 to 20 knot range with perhaps some 
gusts around 25 knots across southern half of the lake including the 
nearshore waters. Will hold off on the issuance of any nearshore 
headlines at this time given existence of ice across nearshore 
waters. However...may need to issue a Small Craft Advisory later 
today for possibility of some northeast wind gusts to 25 knots this 
evening. The next ridge of high pressure will move into the western 
Great Lakes Saturday night...and will be progressive through the 
region which will establish southwest winds again for the latter 
part of the weekend. A surface trough will track across Lake Michigan 
Sunday night...followed by a stronger cold front on Monday. 
Northwest gales will be possible across the open waters again for 
Tuesday behind this front. 


Marsili 


&& 


Lot watches/warnings/advisories... 
Illinois...Winter Weather Advisory...ilz003-ilz004-ilz005-ilz006-ilz008- 
ilz010-ilz011-ilz012-ilz013-ilz014-ilz020-ilz022 until 6 am 
Saturday. 


In...Winter Weather Advisory...inz001-inz002 until 6 am Saturday. 


Lm...none. 
&& 


$$ 

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