Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
516 am CST Wednesday Nov 25 2015
322 am CST
High pressure is over New England and the next low is taking shape
over the northern plains. The main low will deepen over Ontario
later today and continue to the northeast. South winds increase
today with gusts up to 25 miles per hour. A shortwave trough currently over
Missouri passes over central and southern Illinois today with weaker
energy over the warning area. Forcing from the weak trough and warm
air advection should lead to light rain mainly northwest of a
McHenry to Dixon line across north central Illinois this afternoon. Not
expecting any significant rain through this afternoon with most areas
remaining dry throughout the day. Cloud cover will increase and warm air advection
will lead to further melting of the snowpack. High temperatures will be
even warmer than yesterday ranging from the middle 40s along the Illinois/WI
state line to middle 50s across central Illinois.
Increased moisture along with warm air advection will provide enough forcing for
light rain across the warning area tonight. Damp conditions will
also lead to drizzle and possibly areas of fog. Have more confidence
in drizzle than fog as south winds will still be 10-15 miles per hour
overnight. Expecting temperatures to rise overnight due to continued warm air advection.
The warmest temperatures in the forecast arrive on Thanksgiving with highs
in the middle to upper 50s. Drizzle...fog...and light rain are
expected Thanksgiving morning. Expecting persistent damp conditions
instead of periods of rain for those with Thanksgiving morning
activities planned south and east of ohare to La Salle Illinois line.
South winds increase with gusts in the 20-30 miles per hour range. Guidance
keeps the majority of precipitation tied to the cold front that stays just
north and west of the warning area through the afternoon. Forcing
focuses ahead of the cold front Thursday afternoon leading to higher
confidence in precipitation occurring northwest of a Gary Indiana to
Pontiac Illinois line. The heavier rain totals will be northwest of a
ohare to Mendota line with up to 0.3 inches of rain possible.
322 am CST
Thursday night through Tuesday...
Rain infiltrates the warning area ahead of the cold front Thursday
night with winds becoming north behind the front. Guidance has
backed off on precipitation amounts a bit but 0.5-1.0 inches of rain are
expected with higher totals north of I-80. Temperatures fall into the upper
30s across north central Illinois but hold steady in the upper 40s south
Friday is looking like a raw and damp day. Temperatures continue to fall
with highs in the 40s. Precipitation slides southeast with the GFS still
faster than the European model (ecmwf). An additional 0.05 to 0.5 inches are expected
with higher totals south of I-80. North winds will gust to 25
miles per hour...except for gusts to 35 miles per hour along the Lake Shore. Storm total
rainfall amounts are a bit lower and will be 1-1.3 inches with
totals up to 1.5-2 inches possible in some areas.
Have low confidence in precipitation chances Friday night as guidance
differs on how far south the cold front will settle as high pressure
spreads to our north. Kept slight chance to chance probability of precipitation in south of
I-80 but there is a good chance no precipitation will fall. The precipitation may
also transition to snow at the tail end. Only minor amounts of
precipitation are expected.
Saturday may also be dry with high pressure over Wisconsin...but it
once again depends on where the cold front stalls. Kept a slight
chance to low end chance of precipitation south of I-80 but have low
confidence in accumulating precipitation occurring. Cooler and less windy
conditions are expected with high temperatures around 40. Once again raised
Saturday night temperatures to above freezing. The next low shifts north
with better chances of rain south of I-80.
Guidance differs on the path of the low early next week with the GFS
taking the low into central Kentucky while the European model (ecmwf) has more easterly
track that would lead to less precipitation locally. Kept chance probability of precipitation in
the forecast Sunday but have medium-low confidence. Guidance
continues to feature major pattern differences so have low
confidence in precipitation chances Monday and Tuesday.
324 am CST
A rain on snow melt scenario is shaping up over the region
tonight 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 Tuesday with milder conditions and sunshine. Weather
parameters favorable for rapid snow melt are set to move across
the region later today 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 up to 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 creates 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 12z...
* deteriorating conditions late tonight into Thursday...with IFR
ceilings and MVFR visibilities in br and dz.
The main forecast concerns continue to focus on later tonight
into Thursday...when IFR ceilings and lower visibilities develop over
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 12z...
* high with wind dir/speed.
* Medium with timing of IFR ceilings late tonight.
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
Thursday night...IFR likely LIFR possible with periods of rain.
Friday...MVFR likely with rain. Gusty north-northeasterly winds.
Saturday...VFR. East-northeasterly winds.
Sunday through Tuesday...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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