625 
fxus63 klot 180032 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
732 PM CDT Wednesday Mar 17 2010 


Discussion... 
330 PM CDT 


A period of relatively pleasant weather for the next few days...but 
significant change is on tap for the weekend. 


For the short term...the main concern will be for the possibility 
of fog again tonight. Will little significant change to the 
general weather pattern over the area tonight...feel that another 
round of fog is likely. Weak surface ridging will remain in place 
overnight with winds becoming light and variable. Clear skies has 
allowed the land areas to warm up nicely...but after 
sunset...temperatures should drop off rapidly. An area of higher 
dewpoints will also remain in place over the western and southern sections of 
the County Warning Area...with slightly drier air in vicinity of Lake Michigan and out to 
the Fox and Kankakee valley. Temperatures should drop off quick 
enough to allow for fog development after midnight tonight and 
persist through the late morning hours. 


After the fog Burns off...the remainder of the day and into Friday 
will be quiet. Have made little other change to the going short 
term forecast with pleasant late winter temperatures in the low 
60s for Thursday and the upper 50s to low 60s for Friday. The one 
possible exception to the pleasant temperatures will be along the 
lake where onshore flow will keep conditions a little cooler. 


The next feature of concern with be the next upstream system which 
will crash into the Pacific northwest tonight. The models are in 
good overall agreement on handling this system into Friday 
night...tracking a strong short wave trough out of the northern rockies 
and into the plains Friday night. The models begin to diverge by 
Saturday morning with the GFS/shrf trending faster than the 
NAM/ECMWF. This trend continues through the weekend...with the 
European model (ecmwf) trending the slowest and even closing off an upper low over 
southeastern Kansas by Saturday night. The GFS is much quicker and 
maintains an open trough...and even shows some indications of 
phasing with some southern stream energy as well. At this 
juncture...given the spread of model solution...have gone with 
more of a blended approach...but with a greater emphasis on the 
GFS solution rather than the European model (ecmwf) solution. Currently feel that 
the GFS might be a little too fast...but given the time of 
year...the upper levels still have a high amplitude...but strongly 
progressive appearance...driven by strong middle latitude and 
subtropical jet streams. 


So...the screaming message is that winter is not over just yet. 
The high amplitude pattern will be strong enough to tap cold 
Arctic air...with 850mb temperatures dropping to -9c for the weekend. 
While surface temperatures are looking a bit marginal...deep layer cold air 
will be adequate for much of the precipitation to be snow...following an 
initial shot of rain. Hydrologic factors will also become a 
significant concern given the expected quantitative precipitation forecast. The current forecast 
has in 1 to 1.5 inches of liquid equivalent quantitative precipitation forecast for the 36 hour 
period from 00z Saturday to 12z Sunday. If this pans out river and 
areal flooding will be a major concern as the ground is still 
saturated and area rivers are still high from recent snowmelt and 
rain. 


Krein 


&& 


Aviation... 
715 PM CDT 


0000 UTC tafs...potential for fog tonight across mainly north 
central Illinois and winds trends will continue to be the main 
forecast concerns with otherwise VFR conditions prevailing. 


Lake breeze boundary evident with mesonet observation and radar imagery is 
beginning to lose its definition with sunset this evening. 
Expecting perhaps another hour or two of east southeast winds at 
Ord/mdw terminal and east northeast at gyy. Winds are then 
expected to become light and variable for remainder of overnight 
hours...or perhaps light westerly as surface high pressure across the 
Central Plains builds southeast. With light winds and clear skies 
tonight will have to consider fog potential once again. Main 
difference from yesterday...is that surface dew points have been able 
to mix out much more significantly this afternoon with dew points 
even at rfd down into the upper teens. More impressive low level 
moisture not that far off to the west however with surface dew points 
still in the middle to upper 30s back closer to the MS river. With 
loss of daytime mixing...do feel dew points will steadily creep back 
up at rfd this evening and with good radiational cooling 
conditions could have some fog form again. With drier low level 
air initially this evening...did keep idea of previous taf in 
later mention of fog at rfd compared to yesterday with MVFR visibilities 
by the 09z-10z timeframe. Confidence is rather low on how to go 
given drier low level air currently in place but NAM buffer 
soundings continue to indicate mixing ratios increasing with 
height and a favorable setup for fog formation. With the low 
confidence...did maintain tempo rfd to 2sm in the 09z to 13z time 
frame and will wait to see temperature/dew pt trends this evening before 
going any lower. Quiet conditions tomorrow with southwesterly surface 
gradient strengthening and gusts to around 20 knots possible mainly 
during the afternoon. This should keep any weak lake breeze east 
of the terminals. 


Marsili 


&& 


Marine... 
249 PM CDT 


Main marine forecast concern is with development of low pressure 
across the Midwest Saturday...and strength of resulting pressure 
gradient and winds. 


In near term...weak eastern extension of plains high pressure 
ridge will drift south tonight...allowing winds to shift to the 
west/southwest and increase as gradient tightens in response to 
low pressure moving across Ontario. As this low passes east into 
Quebec early Friday...it will trail a cold front across Lake 
Michigan which will shift winds to the north/northeast. Gradient 
initially not expected to be too strong behind front...as main 
high pressure well to the west across northern High Plains. 
Questions begin to arise however with development of low pressure 
along southern part of front Friday night. Various numerical 
models differing with handling low pressure development into the 
day Saturday...with varying speed...placement and strength as this 
feature tracks across the region over the weekend. At this time 
have relied on blend of model solutions...which suggests 
tightening of gradient and intrusion of colder air down the lake 
late Friday night/Saturday. Forecast soundings from WRF/GFS would 
support north winds gusting to 30 knots range...if not marginal gale 
force...though have kept gales out of forecast for now given 
somewhat low confidence on details of low pressure development. 
Should be a lull in the stronger winds late Saturday/Sunday as low 
pulls off and weak high pressure ridge sinks across the lake. 
Extended models suggest another low will move across Ontario and 
northern lakes later Sunday however...which looks to again 
increase gradient and raise winds into 15-25 knots range late in the 
weekend. 


Ratzer 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...none. 
&& 


$$ 

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