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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
955 PM CDT sun Apr 20 2014

Discussion...
259 PM CDT

This afternoon through tonight...
skies have remained mostly sunny this afternoon...and with
southerly flow has allowed surface temperatures to quickly push well into the
70s across the bulk of the forecast area with a handful of points
at 80 degrees. Dew points were able to rise to around 40...however
with full sunshine the mixing has been effective at producing some
subsidence and pushed dew points back into the upper 30s. While the
anti-cyclonic feature has become centered well east of the
region...enough of an influence remains across the western Ohio
Valley and the forecast area that dry weather will prevail through
the evening and likely through the overnight.

There is some concern that the isolated showers/thunderstorms that
developed this afternoon across northern Iowa may creep
east...however feel that with the southerly flow this area of precipitation
should be able to lift northeast and remain west/northwest of the County warning forecast area
this evening/overnight. Tonight will remain mild with temperatures holding
in the low/middle 50s. The coolest locations will likely be focused
near Lake Michigan.

Confidence in dry weather through overnight...medium/high.
Confidence in temperatures...high.

Monday through Tuesday...
middle-level heights begin to lower early Monday...however low/middle levels
remain moisture starved. The middle-level weak vorticity is prognosticated to slide
east into the Ozarks early Monday morning...and with the southerly flow
out ahead of this feature...llvls should slowly moisten. Guidance
has been steadily backing off on precipitation chances arriving across the
forecast area...and have trimmed back probability of precipitation accordingly. Instability
does bump up closer to midday Monday...however the peak instability
timeframe appears to be focused between 21-00z.

Northern stream middle-level vorticity pivots east across northern wisc Monday
afternoon...which may provide some added forcing as the frontal boundary
nears the County warning forecast area. Expect precipitation coverage to remain low. Given the
instability prognosticated to be nearing 400-600j/kg based off of local
WRF...this continues to support thinking of chance thunder. Storm Prediction Center
continues to have the area highlighted for general thunder. Temperatures
will once again steadily warm into the middle/upper 70s. Could see a few
points still touch 80 degrees...although with slightly higher low level
moisture the rate of climb may be diminished and temperatures could hold
generally in the middle 70s.

The frontal boundary should near the northwest County warning forecast area around 00z...and
will steadily bring drier air in and shut-off precipitation from northwest
to southeast. Have pushed up the ending time of probability of precipitation closer to 6z
Tuesday...as winds turn northwest.

Middle-level trough pivots overhead midday Tuesday. The surface ridge center will
be centered northwest of the County warning forecast area Tuesday...this will keep temperatures held
down in the middle/upper 50s and likely upper 40s for locations adjacent to
Lake Michigan. The far south/southwest County warning forecast area should be able to warm
back to around 60.

Confidence in precipitation Monday...medium/high.
Confidence in temperatures...high.

Wednesday through Sunday...
anti-cyclonic flow will setup shop across the region Wednesday. With light
east/northeast flow off of Lake Michigan...temperatures will struggle to
warm beyond the upper 40s/low 50s for areas adjacent to the lake.
Inland areas will warm into the upper 50s to low 60s Wednesday.

Then the focus turns towards the next weather system poised to take
shape across the Central Plains Wednesday night. Guidance has been rather
consistent with the surface ridge sliding east of the forecast area late
Wednesday night...with a warm frontal boundary lifting north from the surface
low and bringing precipitation chances back to the area. Guidance forecasts the
track of the surface low to be lifting northeast into the upper Midwest
Thursday morning. Instability does appear likely to develop and increase
for Thursday...thus have held onto the mention of thunder. Frontal
boundary should arrive over the area Friday...which will bring an end to
the convective chances. Guidance indicates a dry wedge lifting
northeast into the forecast area Friday.

Depending on cloud cover Friday...temperatures may be able to warm back into
the middle/upper 60s or possibly low 70s across the southern County warning forecast area. Then
for the upcoming weekend temperatures are back into the middle/upper 50s.

Confidence...medium/high.

Beachler

&&

Aviation...

//Ord and mdw concerns...updated 03z...

* sct'd rain showers/thunderstorms and rain likely developing Monday afternoon

Izzi

//discussion...updated 00z...

No significant weather expected until Monday afternoon when sct'd
rain showers and thunderstorms and rain expected to develop over northern Illinois. At this
point it is not looking like a high coverage/long duration event
but probability of a brief period or two of rain showers/thunderstorms and rain looks fairly
high. Winds shift to northwest Monday evening...precipitation should move
out around sunset and then VFR conditions expected again.

Izzi

//Ord and mdw confidence...updated 03z...

* medium confidence in coverage/timing of rain showers/thunderstorms and rain Monday
* high confidence in remaining forecast elements

Izzi

//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...

Tuesday and Wednesday...VFR.

Thursday...VFR early...then a period of rain showers/thunderstorms and rain likely late.

Friday and Saturday...MVFR ceilings possible.

Izzi

&&

Marine...
125 PM CDT

Low pressure tracking across northern Ontario...will push a
frontal boundary across the northern portion of the lake tonight
into Monday. This will set up a period of light and variable
winds there through Monday. A second area of low pressure will
move along this front late Monday...and once this low moves east
of the Lake...A period of stronger northerly winds will set up
across the lake Monday night into Tuesday...before high pressure
builds over the lake Wednesday night. These stronger northerly
winds could result in Small Craft Advisory conditions with high
waves into the near shore waters for Tuesday.

The later portion of the week appears to remain active
across the Great Lakes region. It appears that another significant
area of low pressure will impact the upper Midwest and the Great
Lakes region late in the week and into early next weekend. This
system could produce periods of thunderstorms across the lake and
also additional periods of significant winds.

Kjb

&&

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

$$

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