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Area Forecast Discussion (AFD)

Issued by Chicago, IL (LOT)

FXUS63 KLOT 260603

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
103 AM CDT Tue Mar 26 2019

207 PM CDT

Through Tuesday night...

Overall, it will be a fairly quiet period of weather through the

Surface high pressure will settle over the area tonight into
Tuesday. With clear skies and light winds, expect a chilly night
tonight, with lows dropping into the 20s. Light easterly winds
will continue on Tuesday with the surface high nearly overhead.
However, wind speeds will likely increase a bit in the afternoon
near the lake as a lake breeze push occurs. Highs on Tuesday will
be a bit warmer than today, especially for inland areas where 
temperatures will warm into the mid to upper 40s. Colder
conditions in the 30s are expected near the lake. 



255 PM CDT

Wednesday through Monday...

Wednesday: Dry weather will continue as high pressure exits to the 
east. Increasing SW winds will advect in a milder air mass. Expect 
high in the mid 50s amidst breezy conditions and mostly sunny skies.

Wednesday night through Thursday night: A deep trough over southern 
Canada and an associated surface low will push a cold front into the 
western Great Lakes by Wednesday night. The front will ultimately 
stall somewhere across central Illinois by Thursday night. A series 
of weak impulses traversing over this boundary will support sporadic 
chances of showers with an outside chance of an isolated embedded 
thunderstorm. Overall, most of the period will be dry across the 
area, though the highest chances of showers will be later Thursday 

Friday through Saturday: Focus then shifts to the potential for a 
couple rounds of potentially moderate to heavy rain across the CWA. 
The focus for heavier rain will be dependent on the location of the 
stalled surface/low-level boundary. This boundary should remain 
across the southern CWA or south of the CWA Friday before shifting 
northward into the central CWA as the surface low tracks NE across 
the area Friday night into Saturday. Heavy rain potential will be 
enhanced by a few factors:

1) High PWAT anomalies for late March of around two standard 
deviations and averaging near the climatological maximum values 
based off sounding climatology at DVN and ILX.

2) Stalled frontal boundary draped west to east across the CWA.

3) Notable low-level moisture transport in response to increasing
SW flow ahead of low pressure in the Central Great Plains, 
particularly Friday afternoon and night.

Given the recent river flooding across the NW half of the CWA, there 
is increased concern of heavy rainfall occurring across those river 
basins. Long-range guidance, including forecast ensembles, continue 
to shift the heavy rain axis north and south from run to run. 
However, it is becoming more evident that the axis of heavy rain 
will affect at least portions of the CWA, with the period of 
heaviest rain with possible a few embedded thunderstorms occurring 
Friday afternoon into Saturday morning. Consensus forecasts produce 
widespread 1" to locally 2"+ within the CWA. Continue to monitor the 
latest forecasts and statements for the latest details.

Saturday night through Monday: High pressure will settle behind the 
late-week system, resulting in dry conditions into early next week. 
Unseasonably cold conditions can also be expected during this time, 
with highs in the upper 30s to mid 40s on Sunday, coldest near Lake 




A front will stall over the region late in the week, which will 
result in a couple bouts of moderate to heavy rainfall from 
Thursday night through Saturday. Isolated thunderstorms are also
possible. Significant rainfall totals of one inch to in excess of
two inches are possible. Despite it being about five days from the
start of this potential event, model guidance is in very good
agreement on area wide rainfall totals of at least one inch. Given
that the air mass over the region will be unseasonably moist and
supportive of efficient rainfall rates, pattern recognition of
past similar heavy rainfall setups suggest that event totals of 
2.5" or higher will be possible in spots. 

It's too early to be able to pinpoint specifics for which areas 
will receive the highest rainfall totals, but even the ensemble 
baseline of 1"+ is concerning for renewed sharp river and stream 
rises and lowland flooding. This includes the most impacted 
basins the past few weeks, the Rock, Pecatonica and Fox, with 
moderate flooding still ongoing along portions of these rivers.
Soil moisture remains above normal for late March, so increased
runoff will be a concern with respect to river flooding area wide. 
An ESF/Hydrologic Outlook has been issued to highlight the 
concerns for this potentially significant rainfall event. 



For the 06Z TAFs...

High pressure will remain centered over the lake today, thus
expect after the overnight land breeze influence that winds will
shift to the northeast this morning. The lake influence will aid 
in an increase in wind speeds this afternoon, eventually shifting 
to a southeast direction and easing tonight. VFR conditions with 
passing high clouds today. 



LM...Small Craft Advisory...nearshore waters 
     until 10 AM Tuesday.