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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Monday
• 
• High: 66 °
• Low: 52 °
• Partly Cloudy

• Tuesday
• 
• High: 68 °
• Low: 50 °
• Clear

• Wednesday
• 
• High: 77 °
• Low: 57 °
• Clear

• Thursday
• 
• High: 86 °
• Low: 64 °
• Partly Cloudy

• Friday
• 
• High: 95 °
• Low: 55 °
• Partly Cloudy

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
1105 am CDT Monday may 21 2012 


Previous discussion... 
345 am CDT 


08z surface analysis depicts low pressure northeast of the lakes 
across eastern Ontario...with its cold front trailing through 
Indiana and into the bootheel of Missouri. Rain continues to weaken 
and erode through the frontal zone...with just a few scattered 
showers lingering across the Illinois/ind state line into Northwest 
Indiana at this time. Short term high-res guidance indicates precipitation 
will continue to weaken and drift off to the east early this morning 
with clouds also slowly moving east. Thus will start of the forecast 
with a low pop across Northwest Indiana for lingering early showers 
and will have mostly cloudy this am/becoming mostly sunny later for 
most areas. After highs in the lower 90s again Sunday...today will 
be much cooler with temperatures a good 20-25 degrees lower. Winds turning 
north-northeast later this morning should limit areas along the lake 
to the low-middle 60s...with lower 70s across the far western County Warning Area. 


An elongated area of surface high pressure will spread slowly 
east-southeast from the upper Midwest across the western Great Lakes 
through Tuesday...providing dry/pleasant weather with cool 
nighttime temperatures in the middle 40s-lower 50s...and daytime highs in the 
low-middle 70s with 60s again along the lake. South-southeast winds 
will strengthen Tuesday night into Wednesday as the surface ridge 
moves off to the east and an area of low pressure lifts northeast 
across the northern plains. Breezy warm advection will push temperatures 
back into the upper 70s/low 80s Wednesday and into the middle-upper 80s 
Thursday as broad upper trough develops over the western Continental U.S. And 
develops deep south-southwest flow aloft. Both days will feature 
some modest lake cooling along the North Shore...as surface winds 
maintain a southeasterly component. 


Longer range models are in good agreement in depicting a fairly 
Stout short wave trough and associated surface low which emerge from 
the western trough and lift across the upper Midwest/northern lakes 
in the late Thursday/Friday period...and eventually trail a cold 
front into the forecast area Friday. There are some differences as 
to how quickly this occurs and just how far south the front settles 
before stalling out Friday night/Saturday. Overall have based 
forecast on slightly slower European model (ecmwf) rather than faster GFS solution 
and thus have generally low chance probability of precipitation Thursday night/Friday across 
the northern portions of the forecast area...and cooler temperatures mainly 
far north/along the lake Friday. Saturday features a relatively 
large thermal spread with northeast flow off the lake with front 
stalled out across central IL/ind...with temperatures ranging from near 90 
far south close to the boundary to the middle 60s right at the Lake 
Shore. 


Upper ridge then begins to build aloft as low level front stalls 
out...with forecast soundings indicating relatively strong capping 
with warming temperatures aloft. For this reason have followed the drier 
European model (ecmwf) with respect to precipitation/lack thereof in the Saturday/Sunday 
period. Warm front then lifts back north/northeast Sunday...though 
models differ in just how quickly this occurs. With somewhat low 
confidence at this great distance have used a blend of European model (ecmwf)-GFS 
frontal position Sunday which results in keeping a southeasterly 
component to surface winds along North Shore once again. Warm low level 
thermal fields support upper 80s/low 90s...with cooler temperatures along 
the Illinois Lake shore. 


Ratzer 


&& 


Aviation... 


//Ord and mdw concerns...updated 16z... 


* gusty northwest winds 
* morning MVFR ceilings 


Zebic 


//discussion...updated 12z... 


Surface cold front continues to shift through the region this 
evening...ending precipitation and Flipping to winds around to the 
north. MVFR ceilings are noted across most of the tracon this 
morning...and not expecting anyting to fall lower than the 023 to 
025 range. Dry air is quickly advancing...and despite a subsidence 
inversion...should see these quickly break up this morning in the 
14 to 15z hour. Winds are prognosticated to remain northerly and flop to 
the northeast. Climatologically speaking...winds typically wont 
stay out of the north...but shift east of north. Not the most 
confident it will happen at 15z...but more confident with winds 
turning towards 040 by 20z. Latest rap and hrrr data are 
indicating higher wind gusts and suspect that Ord will see low 20 
knots gusts this afternoon while gyy is flirting with 30 knots. 


Shea 


//Ord and mdw confidence...updated 16z... 


* moderate to high confidence on ceiling forecast 
* high confidence in wind forecast 
* high confidence in dry forecast 


Zebic 


//outlook for Ord/mdw for 00z Wednesday-12z Monday...updated 12z... 


Wednesday...VFR. 
Thursday...VFR. Slight chance of rain showers/thunderstorms and rain late. 
Friday...VFR. Slight chance of shra/tsra. 
Saturday and Sunday...VFR. 


Shea 


&& 


Marine... 
334 am CDT 


Low pressure continues to lift into Ontario this morning...and a 
cold front moved through. Showers and thunderstorms have come to 
an end and surface high pressure will be swinging overhead. The 
pressure gradient will remain high through the morning hours and 
allow wind gusts to approach 25 knots for most of the Indiana 
nearshore waters. Not fully confident that the Illinois zones will 
be spared and will include lmz742 /Northerly Island/ through 
Michigan City. Winds will abate this afternoon but waves are 
expected to remain above criteria through tomorrow morning. High 
pressure will then dominate the pattern through middle week...before 
strong southerly flow returns. 


Shea 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...nearshore northerly is to Michigan City in until 10 am 
Tuesday. 


Small Craft Advisory...nearshore Winthrop Harbor to northerly is until 11 PM Monday. 


&& 


$$