000 
fxus63 klot 201510 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville, Illinois 
1010 am CDT sun may 20 2018 


Update... 
1009 am CDT 


An area of showers with embedded thunder in response to a mid- 
level jet Max and possibly a subtle short-wave has been rather 
persistent this morning across the far northern County Warning Area. Model 
guidance continues to struggle with this evolution, but the 12z 
suite appears to have a somewhat better handle on it. Expectation 
is for showers to continue to drift eastward and possibly expand 
in coverage farther south to near I-80 into early afternoon. The 
12z dvn radiosonde observation showed nearly 800j/kg of MUCAPE that, while 
diminishing with eastward extent, will continue to support an 
isolated thunder threat. Precip looks to gradually end by late 
afternoon. Additionally, with the extensive cloud cover and 
blustery NE winds, have dropped Max temps a degree or two for 
northern portions of the County Warning Area. 


Kluber 


&& 


Short term... 
220 am CDT 


Through tonight... 


Early this morning, high pressure is building into the upper Midwest 
and has helped push cooler air south into northern Illinois. A cold 
front has surged down the lake moving across northeastern Illinois 
early today where temperatures have fallen into the 40s. Southwest 
flow aloft overriding this colder air is kicking off a few scattered 
showers this morning over portions of north central Illinois. Expect 
this to continue across the northern tier of counties in Illinois 
into southern Wisconsin through the morning. Temperatures will 
remain on the cooler side today topping out in the 50s and low 60s 
north of I-80 and eventually warming into the 70s south. 


A pair of shortwave troughs are expected to lift across the region 
later today bringing additional chances for thunderstorms. The lead 
wave lifting from the Southern Plains will move across central 
Illinois early to mid this afternoon. There remains relatively poor 
model agreement on location and coverage of any potential storms, 
but the best chances will likely be confined to areas south of the 
Illinois/Kankakee River valleys this afternoon, with a decent 
likelihood it stays south of the County Warning Area altogether. Modest instability 
coupled with moderately strong shear across the southern counties in 
the County Warning Area would support a conditional severe threat if storms were to 
be in the area. 


A second wave lifts across the region overnight. Given the fairly 
modest forcing for ascent and unfavorable diurnal timing of the 
wave, opted to lower pops from going forecast for tonight. Think 
that widely scattered showers/isolated thunderstorms that form 
upstream may be able to persist for a while across the County Warning Area 
overnight, but likely in a weakening state. 


Deubelbeiss 


&& 


Long term... 
220 am CDT 


Monday through Saturday... 


Showers and a few thunderstorms are possible again on Monday. GFS 
and ecm are in good agreement with right entrance region to upper 
jet translating overhead coincident with a damping 500mb vort Max. 
Soundings show very modest instability over northern Illinois and 
Northwest Indiana, though enough to support at least a low end 
thunder threat. Temperatures continue to run on the cooler side 
Monday under the influence of the broad surface ridge over the 
Great Lakes. Onshore flow will hold highs in the 50s and low 60s 
near the lake while inland 60s and 70s are expected. 


Tuesday through Thursday, upper low is expected to close off 
across socal while downstream amplification of upper ridge is 
expected across the mid section of the country. Lee side of the 
upper ridge begins to build across the western Great Lakes Tuesday 
while surface high pressure builds into portions of the upper 
Midwest and western Great Lakes. This provides the best chance for 
a couple days of primarily dry weather, though guidance does show 
convection all around the cwa, and couldn't rule out a stray 
shower moving into the area. Expect some sunshine on Tuesday, 
though would also expect cirrus from upstream convection to stream 
overhead at times. Soundings show more mid/high level moisture on 
Wednesday as upstream convection moves even closer to the County Warning Area. 
Despite increasing cloud cover, guidance does indicate we will see 
a gradual warming trend as the upper ridge builds overhead. By 
Wednesday expect widespread upper 70s with some 80s possible, with 
the exception again being areas along the lake which will be held 
in the 60s. Warming trend should continue into Thursday as the 
500mb ridge axis passes overhead. Convection won't be too far off 
to our west by this time, focused along the Mississippi Valley. 


Friday and Saturday, buckle develops within the upper flow pattern 
and shower and thunderstorm chances return in earnest to the 
forecast area. Global models in reasonable agreement showing a broad 
surface low developing over the Canadian prairies and upper 
Midwest Friday and shifting east across the Great Lakes over the 
weekend. Initial warm advection Wing brings precip chances on 
Friday and a trailing cold front brings additional chances on 
Saturday. 


Deubelbeiss 


&& 


Aviation... 
for the 12z tafs... 


Mainly MVFR ceilings in place this morning, with the exception of 
gyy where LIFR/IFR ceilings remain. In this location, a slow rise 
is likely, however, it may take longer than previously thought. 
Elsewhere, ceilings may bounce around low end MVFR and IFR for a 
couple hours before a slow trend upward occurs. MVFR will then 
likely continue for much of today, and will once again monitor a 
period of lower ceilings later tonight into Monday morning. Fog 
and lower vis will be limited this morning, with gyy the only site 
to likely observe this vis restriction. No real big change in 
winds expected, with an east northeast wind likely to persist. 
Precip trends will be challenging today, but in the near term, it 
does appear that showers with isolated thunder will be possible. 
Highest chances will be across the rfd terminal but latest radar 
imagery is depicting scattered development further to the east. 
This precip may persist for the next several hours, but with a dry 
trend expected later this afternoon. Confidence lowers with 
precip chances later tonight, but with the pattern supportive 
another chance of showers and even possibly thunder late tonight 
into Monday morning. 


Rodriguez 


&& 


Marine... 
248 am CDT 


A surface trough/front has pushed south through much of the lake 
early this morning, with the northerly winds observing an increase 
to the 15 to 25 kt range over much of the lake. Although winds up 
to 30 kt are not expected, a few gusts approaching these speeds 
may still occur early this morning. While northerly winds will be 
present today across the lake, for the nearshore waters, highest 
speeds this morning should remain mainly along the Illinois nearshore 
with speeds and waves slightly lower over the in nearshore. Winds 
are then expected to turn more to the east tonight, while speeds 
and waves lower. 


Rodriguez 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...Winthrop Harbor to Burns Harbor until 
9 PM Sunday. 


&& 


$$ 


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