Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
315 am CST Thursday Nov 26 2015
314 am CST
Through Friday night...
The main low is over James Bay and its cold front stretches through
central Iowa into south central Kansas. South winds and warm air
advection have helped temperatures rise overnight. Temperatures are rising at
about the same rate as the increase in moisture...so that combined
with decent mixing has severely limited fog development. Light rain
coverage will increase through the morning with areas of drizzle
also possible. Drizzle and fog are less likely this afternoon as
south winds begin gusting 30-35 miles per hour. The more steady rain will fall
north of I-80 this afternoon with rain shifting southeast this
evening. Gusty south winds also continue overnight but the low
levels will saturate and an inversion is expected to develop. Areas
of fog are expected this evening with the fog dissipating when
heavier rain moves in overnight. The cold front makes it halfway
through the warning area tonight with gusty north winds behind it.
Temperatures will also fall with lows getting into the upper 30s along the
Illinois/WI state line...while areas south of I-80 will remain around 50.
Rain shifts southeast Friday with rain mainly falling south of I-80
by Friday evening. The NAM is now the fastest model with the European model (ecmwf)
still holding up the rear...so went somewhere in the middle. Have
high confidence in the trend...but medium confidence in exact timing
of when rain will end north of I-80. High temperatures fall through the
day Friday. Gusty north winds continue with gusts up to 35 miles per hour along
the lake. Storm total precipitation will range from 1-1.5 inches north of I-
80 and 0.75-1.25 inches south of I-80.
Have even less confidence in when precipitation ends Friday night as the
European model (ecmwf) still wants to keep precipitation south of I-80. The GFS and NAM
slowly shift precipitation south of the warning area. Due to a lack of
confidence kept a slight chance to chance of rain turning over to
rain or snow after midnight south of I-80. Temperatures continue to fall
with middle 20s along the Illinois/WI state line. Have medium confidence in
min temperatures south of I-80 as they could remain above freezing.
Forecast soundings feature a Stout melting layer with a less
impressive freezing layer just above the surface. Precipitation type will
likely vary from rain to ice pellets since the ice crystals will
fully melt aloft.
314 am CST
Saturday through Wednesday...
High pressure is centered over Iowa Saturday...but it/S eastward
extent is over the warning area. The European model (ecmwf) suggests more rain south
of I-80...but thinking it is overdone with its precipitation and went dry
except for a slight chance of precipitation south of a Pontiac to
Rensselaer line. Saturday night looks dry with temperatures at or below
freezing away from the lake. Onshore flow will lead to temperatures right
around freezing especially downtown.
The models differ on the northern push and speed of the next wave as
the emcwf is more aggressive than the GFS. Kept probability of precipitation in the low
chance range or lower south of I-80 Sunday and Sunday night with
greater confidence in probability of precipitation Sunday night. Also increased Sunday night
low temperatures due to the push of warmer air north in the models. This
allowed ME to keep any precipitation that may occur as rain especially
given a deep melting layer aloft in the model soundings.
The models handle the low early next week very differently despite
agreeing that more precipitation is in store. Have low confidence in exact
details...but medium confidence in probability of precipitation and temperatures. Periods of rain
are possible Monday. Colder air moves in Monday night with rain
possibly mixing with or becoming snow behind the low Monday night
into Tuesday. The colder air leads to a decline in temperatures through middle
week with highs in the middle to upper 30s and lows in the 20s.
315 am CST
Rapid snowmelt continues. Much of the snow should be melted by
the time that steady rain begins to overspread the region
overnight tonight. The latest guidance suggests an areal average
total quantitative precipitation forecast of up to 1.5 inches...with locally higher amounts very
likely...across region following the melting of snow with water
equivalent values of slightly in excess of 1 inch across far
northern Illinois where the deepest snow cover had been. The
frozen soil depth is very shallow and the melting snow saturating
the ground...additional infiltration will be difficult and much of
the rainfall should go to runoff. With streamflow values running
higher than normal for some streams and rivers due a period of wet
weather the past few 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 09z...
* low level wind shear continuing into this morning.
* Gusty southerly winds today.
* Ceilings steady then gradually lowering below 2500 feet overnight and to
around 1000 feet by middle morning.
* MVFR visibilities at times today with periods of showers.
* More steady rain tonight...with IFR ceilings/visby likely.
* Cold frontal passage by early Friday morning...with gusty
northerly winds expected in its wake.
Rain showers are beginning to develop over the region...and will
likely be impacting the terminals at times through the morning
hours. Overall...expect ceilings to be on a downward trend through the
night with ceilings around 1000 feet above ground level expected by middle morning today. It
appears visibilities will not drop too much outside of any rain
showers. Therefore...it appears the 4-5 mile+ visibility will
persist through much of the day.
Strong and gusty southerly flow (around 200 degrees) is also
expected through much of the day. Wind gusts may increase a bit this
afternoon and evening though as surface temperatures warm to near
60. This looks to allow for slightly deeper atmospheric
mixing...which could help transport some of the higher winds aloft
to the surface. This could result in periods of gusts up around or
just above 30 knots at times this afternoon...as well as help produce a
temporary increase in ceilings.
The main rain event now looks like it will not arrive until this
evening at krfd...and possibly not until late evening or after
midnight tonight at the eastern terminals. Ceilings and visibilities
will likely deteriorate again during this period. The cold front will
then shift southward over the region overnight and into early Friday
morning. Expect winds to shift northerly and become gusty in its
wake for Friday.
//Ord and mdw confidence...updated 09z...
* high with low level wind shear.
* High with wind speed/direction.
* Medium with ceilings and visibilities forecasts and timing.
//outlook for Ord/mdw for 12z Friday-12z Wednesday...updated 00z...
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.
200 am CST
A cold front is expected to begin shifting southward across the
northern portion of Lake Michigan this afternoon. The front will
then continue southward over the lake tonight...and is expected to
clear the southern tip of the lake either very late tonight or very
early Friday morning. Ahead of this front expect south-southwesterly
winds to remain up around 30 knots...especially over the southern
end of the lake...where these winds will likely continue through
this evening. We could even get a few gale force gusts across the
Illinois and Indiana near shores this afternoon and evening.
In the wake of this cold front expect solid northerly winds up
around 30 knots through the day Friday. A few low end gales could
materialize...but it appears that the the mention of occasional
gale force gusts is the way to go at this time. These northerly
winds will result in waves up around 10 feet across Southern Lake
Michigan...so the Small Craft Advisory will need to be extended to
cover this period of high winds and waves. A ridge of surface high
pressure will begin to build over portions of the lake Friday night
and Saturday. This should be a favorable setup for enhanced
northeasterly winds over the southern end of the lake into Saturday.
Therefore...high waves above 4 feet could persist into Saturday
afternoon or evening before the winds and waves subside.
Another storm system looks like it will impact the Great Lakes
region for at least the first half of next week. This system could
produce a period of strong easterly winds Monday...followed by
strong west-southwesterly...and eventually northwesterly winds on
Tuesday into Wednesday. There is still a good amount of uncertainty
with the timing and strength of this system...however...but there
are good signs that it will have an impact on the region.
Lm...Small Craft Advisory...nearshore waters
until 10 am Saturday.
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