Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
233 am CST Wednesday Nov 25 2015
230 PM CST
Through Wednesday night...
We continue to focus on the start time of fog and light
precipitation potential on one of if not the highest travel days
of the year on Wednesday. Continue to have a little more
confidence of the primary chances being after dark on Wednesday
more so than much of the day...although high dew point air over
snow melt can be a scenario where fog and low clouds develop much
quicker than it would appear and have attempted to account for
The 2 PM surface map shows the boundary to the south having weakened
and starting to lift northward as a warm front. While somewhat weak
convergence along this feature...dew points are pooled around 40 at
some central Illinois sites...a possible boundary layer artifact of
snow melt yesterday and today. Snow melt is also hastened across our
area with somewhat impressive temperatures over the snow cover with
a handful of locations up to the middle 40s.
As the boundary eases northward tonight...winds will turn southerly
and likely slowly increase in speed. This will be a different
scenario with more of an advection fog as opposed to radiation fog.
Do not feel that great about the chances for fog tonight at this
time...but am concerned at these high dew points nearby. Continue
with the patchy fog wording after midnight through early morning for
As we go into Wednesday it looks like clouds should spill into the
area from the southwest. Upstream visible satellite imagery this
afternoon indicates some clouds streaming northward into Oklahoma
and while NAM parcel trajectories keep most of that west of the
forecast area this afternoon...have guided more toward the low-level
isentropic surfaces. These indicate ascent with gradually
diminishing condensation pressure deficits through the day...so have
clouds spreading in during the late morning and afternoon. Dew
points are expected to be reaching the 40s by afternoon which will
really hasten the snow melt and keep near-surface T/dew point spreads low.
Advection fog will remain possible during the day and bring in areas
of fog from west to east during the evening. Some of this could be
dense in this setup...although confidence on that at this point is
low...as temperatures are likely to inch upward and may keep slight
enough T/dew point spread to prevent dense fog. The rain chances after dark
appear light and patchy...with drizzle being equally likely.
221 PM CST
Thursday night through Tuesday...
Long term focus is on heavy rainfall potential...largely occurring
during the evening hours Thanksgiving night into Friday
morning...then on what appears to be an active pattern into much of
early next week.
The Holiday appears to start of murky with some lingering fog and
drizzle from the overnight hours as warm/moist advection occurs over
the cool surface/boundary layer. Low level status and fog look to
remain in place all day long. This combined with dewpoints in the
upper 40s and steady temperatures in the low 50s creates ideal
conditions for eating away at the lingering snowpack. Low level
isentropic surfaces suggest more focused low level lift Wednesday
night will become more broad during the day as we become firmly
supporting some very light rain/drizzle. SW flow aloft will promote
a multitude of shortwave progressing through the area...but the flow
aloft will be parallel to a SW moving cold front pushing southeast out of
the upper Midwest. With this pattern...the boundary will not
cruise along and likely strengthen in intensity. Meanwhile...the
warm frontal boundary may tighten as well and act as a focus for
rain Wednesday afternoon...but there is model disagreement as to
whether this boundary will be over our area or still north in
Wisconsin. Still the message remains damp and mild. There is
strong high off the Atlantic coast that looks to also keep the
heavy rainfall axis off to our west most of the day in Iowa and
western Illinois where some very heavy rain is expected...and
certainly out of Northwest Indiana during the daytime hours.
But there is also rather strong high pressure area moving out of the
plains which will move the cold front southeastward. Meanwhile the
high off the Atlantic will act to pinch off a portion of the Gulf
moisture. During Thanksgiving evening...the front will undergo some
strengthening and slide into northwest/north central Illinois.
Still strong frontal forcing Thursday night with the deep layered
moisture and high precipitable water air should result in some moderate to heavy
rainfall. The heaviest rain looks to be in north central Illinois...but
moderate fgen with even some elevated conditional instability will
lead to some bands of rain in northeast Illinois...with possibly the
lighter numbers in northwest indy due to a weakening front and slightly
less moisture. Deep layer moisture really weakens. There could be
some shallow moisture remaining. There may be a period of snow or
mixed precipitation as temperatures take a tumble mainly along the
Wisconsin. It looks like temperatures in the main precipitation axis hang
on enough for all liquid but thermal profiles are awfully close.
More significant drying occurs on Friday afternoon...with possibly
some lingering low level cloudiness with winds off the lake
potentially trapped below an inversion as the elevated frontal
boundary never really clears the area.
Without a strong upper level system moving in from the
northwest...the baroclinic zone will lay out over the County Warning
Area on Friday. A northern stream shortwave will move east Friday
night...and we quickly find ourselves back in SW flow aloft ahead of
cut off low pressure over the Great Basin/Middle Mountain west and
on the back side a strong upper ridge in the Gulf of Mexico. The
850/700 mb baroclinic lifts back north. The high precipitable water air never
truly pushes back to the Gulf...and a period of isentropic
lift...most focused in central Illinois into Northwest Indiana will
lead to a period of rain later Saturday into Sunday morning.
Again...the baroclinic zone still lingers in the area.
Attention then focuses on the bowling Ball of a low pressure area
still out west and migrating into the plains. A strong upper jet on
the south side of the low will shove it eastward. The low level
front lifts north on Monday possibly leading to a period of light
rain as moisture is somewhat shallow. The upper low will pass
through the region Tuesday. Ahead of this low a cold front will
move in Monday night. The strong upper jet will progress east and
contribute to significant cyclogenesis of a surface low that will
pass to our west than north. The air mass here looks like it is at
least possible for a mix of rain/snow. The weather will be pretty
active at this point across the region...but differences in
moisture availability/dry slotting create some uncertainty on
impacts locally...but at least some precipitation mix and some
winds as well.
Temperatures after Friday will fall back to highs in the 40s and
low in the 30s...which is still slightly above normal for this
time of year. Cooler conditions look on tap for middle of next
week behind the upper low.
A rain on snow melt scenario is shaping up over the region
Wednesday night through early Friday. Observers on Monday morning
recorded snow water equivalent values of mainly 0.9 to 1.3 across
far northern Illinois within the deepest snow cover. Some of this
has melted today with milder conditions and sunshine. Weather
parameters favorable for rapid snow melt are set to move across
the region later Wednesday with rain further speeding melt up on
Thanksgiving. Rainfall amounts of an inch to 1.5 inches are likely
across especially northern Illinois...and it is not impossible to
have two inch amounts given the parameters. The frozen soil depth
is very shallow and would lean toward that not really impacting
percolation of the rain/melt. With streamflow values running
higher than normal for some streams and rivers thanks to the wet
past four weeks...there is concern for some streams to potentially
rise into minor flood stage.
Current forecasts highlight that this is possible across portions
of the Des Plaines...Fox...and Illinois River basins. The North
Branch of the Chicago river may also reach near 5 feet as
well...which createst some urban concerns in the city of Chicago.
Other basins will have elevated streamflows. Expect ponding of water on
area roadways may be possible as well. Have continued esf at this
time with details in specific rainfall amounts and timing to be
refined as the event approaches.
//Ord and mdw concerns...updated 09z...
* stronger winds aloft around 2kft above ground level this morning with speeds
between 35-40kt from 200-220 degree direction.
* Deteriorating conditions late tonight into Thursday...with IFR
ceilings and MVFR to IFR visibilities in br and dz.
The main forecast concerns will be later in the forecast
period...Wednesday night into Thursday...when IFR to LIFR ceilings and
lower visibilities develop over the area.
Surface low pressure over the eastern Dakotas...will rapidly shift
northeastward over Ontario later today...then into northern Quebec
by Thursday morning. A cold front associated with this low...will
then slowly shift eastward towards the area by Thanksgiving night.
Persistent southerly flow across much of the central Continental U.S. Ahead of
this feature will continue to transport warmer and more moist air
northward over of the area today and tonight. Overall...this will
result in deteriorating conditions over the region...especially
later tonight into Thanksgiving. Ceilings will gradually develop around
5,000 feet above ground level over the area this afternoon...then begin to lower into
through the MVFR range tonight. IFR ceilings could develop as soon as
later tonight over north central Illinois...and then over the
eastern taf sites by early Thursday morning. Periods of drizzle and
areas of MVFR to IFR visibilities also appear to be decent bet late
tonight into Thursday before the main rain event arrives by Thursday
Winds will be southerly 10-15 knots through the period.
//Ord and mdw confidence...updated 09z...
* high with wind dir/speed.
* Medium with timing of IFR ceilings late tonight.
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
Thursday...IFR likely/LIFR possible fog with rain and drizzle
Friday...MVFR likely with rain...possibly mixing with snow before
ending by evening.
Sunday and Monday...MVFR possible. Slight chance of rain.
215 am CST
The main forecast concern continues to be with the increasing
potential for a period of southerly gales over the northern half
of Lake Michigan today and into this evening...then the potential
for a period of northerly Gales Lake-wide on Friday.
Low pressure is expected to shift rapidly to the northeast from
the eastern Dakotas this morning into Ontario today...then to near
James Bay by this evening. Strong pressure falls ahead of this
feature will likely lead to a quick uptick in southerly winds
over the lake during the middle to late morning hours...with gales
of 35 to possibly briefly to 40 knots expected over northern half of
the lake. Slightly lighter winds around 30 knots are expected over
the south half of the lake. Although the gale force winds should
ease later this evening...it appears that southerly winds will
remain up in the 25 to 30 knots into Thursday before a cold front
begins to gradually shift southward down the lake into Friday
morning. Therefore...it appears that over the Illinois and Indiana
near shore waters...that small craft conditions...due mainly to
winds around 25 knots...with continue through Thursday evening...and
I have therefore extended the advisory to cover this period.
The cold front is expected to begin shifting over the northern
portion of the lake Thursday afternoon...then clear the southern
end of the lake by daybreak Friday. Expect winds to increase up to
30 to 35 knots out of the north in its wake. A gale headline may be
needed for the lake for Friday...but I have opted to hold off on a
watch at this time. At the very least...the Small Craft Advisory
will either have to be extended again to cover the Friday and
Friday night timeframe...or a new advisory will need to be issued.
The northerly winds will likely lead to around 10 foot waves
impacting the Southern Shores of Lake Michigan Friday into Friday
Lm...Small Craft Advisory...Illinois nearshore waters until 10 PM Thursday.
Small Craft Advisory...lmz743-lmz744-lmz745...9 am Wednesday to
10 PM Thursday.
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