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fxus63 klot 201141 

Area forecast discussion
National Weather Service Chicago/Romeoville, Illinois
641 am CDT sun Oct 20 2019

Short term...
301 am CDT

Through Monday...

Early this morning: small T-TD spreads behind last evening's cold
front have set the stage for fog, some dense, upstream wherever a
trailing stratus deck has gaps or has shifted east. Obs in eastern
Iowa indicate that once the clouds clear a given location,
visibility falls rapidly, with the onset of fog within an hour. The
western edge of the stratus deck is currently on the doorstep of the
western County Warning Area. Its eastward motion has stalled as winds have veered
north within or just above the cloud deck per the dvn vwp. With this
slower motion, have delayed the onset and eastward extent of fog
through mid morning. The highest probability of dense fog remains
confined to areas west of I-39, though chances will begin to
diminish if the cloud deck does not begin any renewed eastward
motion in the next few hours.

Today and this evening: the evolution of the stratus clouds will be
the primary driver in the forecast today. Much of the area will
start the day under low clouds. Most guidance rids at least the
northern 2/3 of the County Warning Area of the clouds by this afternoon, which does
seem reasonable as the departure of deeper clouds per satellite
imagery will leave a remaining cloud thickness of around 1kft.
Daytime heating should mix out the remaining clouds, though how
quick that occurs late this morning into this afternoon remains in
question. There is some indication that low clouds may struggle to
erode over Southern Lake Michigan and keep the shoreline somewhat
cloud covered or advect the clouds onshore as winds become east this
afternoon. Any location away from the lake that clears by this
afternoon should see temps in the mid 60s.

Overnight and monday: much more unsettled weather will affect the
region beginning late tonight and especially Monday. A strengthening
mid-level low currently over Wyoming will shift eastward to southern
Minnesota by Monday morning and central Wisconsin by early evening.
A surface occlusion will shift NE across the County Warning Area Monday morning as
the strengthening parent surface low tracks from central Iowa to
northwest Wisconsin.

Broad warm air advection and moistening around 800 hpa late this evening may allow
for some spotty showers to develop across northern Illinois by
around midnight. This activity may gradually increase in coverage as
low-level moisture transport intensifies and warm air advection becomes more
pronounced. Poor lapse rates above this layer should preclude
thunderstorms chances for most of the night.

By Monday morning, significant moisture transport into the region,
bringing pwats up to 1.3-1.5", in conjunction with robust isentropic
ascent in the 295-315k layer will support a shield of moderate to
sometimes heavy rain spreading across the entire County Warning Area during the
morning and early afternoon. Steeping lapse rates resulting from the
warm air advection in the low-levels will support at least a slight chance of
embedded elevated thunderstorms with this activity. Meanwhile,
increasing south-southeast winds with gusts to around 30 mph by late morning
will result in a driving rain for much of the area. This shield of
rain will quickly exit to the northeast during the early afternoon,
but additional small vort maxes rotating around the mid-level low
should continue the chance for at least of isolated to scattered
light showers into the late afternoon. SW winds will also continue
to gust to around 30 mph.



Long term...
334 am CDT

Monday night through Saturday...

The surface low will continue to deepen Monday night before
the occlusion process begins later Tuesday. Therefore expect
conditions to remain fairly windy Monday night into Tuesday
with gusts of 35 to 40 mph likely attainable in the cold advection

There could be some light wrap around precip, though this does
appear to just graze the northern portions of the area. The main
upper cold core will be to our north, but energy wrapping around the
southern side of the low and 500 mb temps in the mid -20s should
support some clouds and some showers. This system will finally
pull away Tuesday night off to our north and east. Brief ridging
and warm air advection makes for dry and seasonal conditions into

Additional slightly lower amplitude shortwave energy will be
migrating toward the area on the back side of this departing system
accompanied by a quick reinforcing shot of cooler in the Thursday
time frame. With several waves expected through and a cold front
to maintain some rain shower chances Wednesday into Thursday, and
with some significant model differences in the progression of this
front, have had to hold onto model blended precipitation chances
into Thursday night/early Friday. It appears Friday into Saturday
will provide for dry conditions with some moderation in temps
Saturday, but still on the cooler side for highs.



for the 12z tafs...

Several lower cloud layers exist this morning. The first is a VFR
lowering to MVFR deck will continue to spread through areas along
and east of a kpnt-kjot-kmdw line. A brief clearing hole exists,
but then an IFR deck will fill back in at least in a broken nature
across the Chicago terminals for the next few hours. Rfd looks to
get brief LIFR, and possibly even some IFR vis as well. Its not
out the question at Ord/mdw, but feel that IFR at 700-800 feet
would be more likely before we are able clear some of this cloud
shield. Confidence on this cloud lifting is fairly high, but only
medium confidence on the timing. Mid morning seems reasonable.

A weak frontal boundary has also shifted winds to the northwest behind
it. Later this morning we will see winds flop over to NE. As this
occurs we could advect some lower cloud off the lake this evening.

There could also be some lighter precip late this evening and
overnight. Coverage appears low ahead of the main precip shield on
Monday morning. The main batch of showers Monday morning will be
accompanied by a fairly stiff southeast wind. Still some timing
differences on the leading arrival times of the showers, but most
guidance is pegging the morning to early afternoon for several
quick waves.

Instability with the morning wave of locally modest rain is too
limited for taf inclusion at this distance.



Lot watches/warnings/advisories...


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