546 
fxus63 klot 180821 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
321 am CDT Thursday Mar 18 2010 


Discussion... 
321 am CDT 


Not much change in thinking from last night...which unfortunately 
also means not particularly confident on chances for accumulating 
snow this weekend. 


However...the first course on our forecast plate this morning is 
temperatures today. Objective guidance remains surprisingly and 
given warming temperatures/increasing low level thickness today when 
compared to Tuesday and Wednesday I am reluctant to believe that we wont be at 
least a couple degrees warmer today. There is a batch of high 
cloudiness upstream heading for the area...but last GOES visible 
images of the day indicated that this cirrus was fairly translucent. 
In addition...models and simple extrapolation would suggest the bulk 
of the cirrus will be departing with high cloudiness decreasing a 
bit this afternoon. Gusty west-southwest wind and at least partial sunshine 
should be enough to push temperatures to the middle/upper 60s...even up to the 
Lakefront. 


Guidance is all a bit slower with the movement of the northern 
stream shortwave across southern Canada tonight into Friday which 
should result in slower progression of the southward moving cold 
front. In fact...it now appears at least half the day Friday will be 
spent in the warm sector over most of the County Warning Area with enough of a SW 
wind to hold any lake cooling off until the front accelerates down 
the lake late in the afternoon. Front back dooring into the area 
Friday afternoon should result in falling temperatures northern County Warning Area and 
most noticeably along the lake with large 20f+ gradient in temperatures 
likely across the County Warning Area by late afternoon with 40s probable North 
Shore and well into the 60s southern County Warning Area. 


Front should continue to inch slowly southward across the County Warning Area Friday 
night into Saturday with impressive low level frontogenesis likely 
supporting a band of mainly Post frontal precipitation spreading 
south across the County Warning Area Friday night into Saturday. Models continue to 
trend toward the European model (ecmwf) solution of less phasing of northern/southern 
stream systems with the southern stream vorticity now prognosticated to evolve 
into a slower moving cut off low...tracking much farther south than 
previously forecast. 


This latest development in model trends would suggest lighter precipitation 
totals in general across the area as well as significantly reduced 
(although not eliminated) chances of significant snow accumulations. 
Could still be a narrow band of light to moderate snows driven by 
the low level frontogenesis over the area...mainly Saturday into 
Saturday night. However...relatively light quantitative precipitation forecast...the likelihood of 
fairly low snow to liquid ratios (wetter snow)...initially warm (and 
warming soil temperatures near/above 50f)...and insolation through the 
cloud cover during the day Saturday all would argue for tempering 
snow amounts from the raw model output. And with chances of 
heavier/convective snows now looking to remain well south of 
our County Warning Area closer to the closed middle level low I am less concerned about 
truly significant snows in our County Warning Area. While there is a decent chance 
that much of the County Warning Area could see some accumulation I am really not 
comfortable throwing numbers out there yet given the variability in 
the past couple days of models handling this system(s). 


Trend is certainly toward a slower departing southern system and 
have hung onto probability of precipitation/clouds a bit longer than previous forecast. 
Other change was to begin to trend upward with temperatures by Tuesday and Wednesday 
as models are all digging a western trough which should induce down 
stream ridging over our area. While there could be some snow that 
will need to be melted Monday into early Tuesday...as long as we dont 
end up with more snow than expected suspect that this will melting 
with be a quick process with sunshine and high sun angle this time 
of year. Once snow is gone...thermal profiles in the latest runs of 
medium range models would support 60s again possibly by Tuesday but 
certainly by Wednesday. 


Izzi 


&& 


Aviation... 


0600 UTC tafs...main forecast challenge is with fog 
potential...mainly near the rfd terminal. 


The residual moisture from the previous night in western Illinois 
took some time to mix out on Wednesday...but when it did mix out 
dewpoints went plummeting into the teens. Although dewpoints have 
risen some this evening...main moisture axis remains further west 
tonight. With these thoughts in mind...dont feel as if the rfd 
terminal will witness another morning like Wednesday in terms of 
such thick fog and low stratus. Have started to trend more 
optimistically tonight...only forecasting MVFR conditions for 
early this morning through the middle part of the morning. 
Elsewhere...fog should not be an issue this morning for the 
terminals near the lake as drier air is once again in place. 


Sky clear skies will begin to be replaced with some high level cirrus 
this morning as latest satellite imagery showing some middle to upper 
level moisture upstream moving southeast out of Minnesota and 
Wisconsin. Expect this to once again be the predominant cloud 
cover for the remainder of the forecast period with upper level 
ridge overhead on Thursday. Middle level wave which moisture is 
riding along will help give more structure to the surface pressure 
field this morning which will help winds to become oriented out 
of the southwest. As daytime mixing occurs Thursday 
morning...winds will be on the increase with gusts up to 20 knots out 
of the southwest. This will be ongoing until atmosphere decouples 
once again Thursday evening...and winds quickly diminish. 


Rodriguez 


&& 


Marine... 
319 am CDT 


West winds will become more southwesterly across the lake as a 
ridge of high pressure stretched across the Central Plains 
continues to drop south. Meanwhile weak low pressure to the north 
will quickly move east today into Ontario and then to Quebec early 
Friday morning. As this pushes through...pressure gradient across 
the northern half of the lake will remain more organized with 
stronger winds of 10 to 20 knots observed as opposed to lighter 
winds over the southern half. With the passage of this low to the 
northeast...winds will turn back to west and then to northwest 
late in the day on Friday as a cold front moves through the 
region. This colder air over the lake will increase the likelihood 
of stronger gusts occurring. This will occur late Friday night 
through Saturday as winds turn to the north and increase to 30 knots 
while waves also build with the highest waves of 8 feet over the 
southern half of the lake expected. Winds and waves begin to 
diminish/subside Saturday night...but still remain in the 15 to 25 
knots range through early Monday. 


Rodriguez 


&& 


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


$$ 

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