Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1154 am CST Thursday Nov 26 2015
931 am CST
Main forecast update this morning was to increase wind
speeds/gusts into this afternoon especially for areas south of
Interstate 80...and to include slight chance of thunder for north
Southwest flow aloft still ushering in a steady stream of middle
level energy across the region while warm advection persists this
morning. Precipitation activity has remained scattered for most of this
morning...however...a larger area of precipitation has recently moved
into areas in northern Illinois mainly west of i39. Although
focus for best widespread development will be across northern
Illinois...mainly north of i88 this morning...do expect widely
scattered shower development to continue across other areas in
northeast Illinois and Northwest Indiana. Isolated lightning
strikes have been observed with this better precipitation development as
its moved into northwest Illinois and with short term guidance
indicating at least some very weak instability lingering across
the Rockford area over the next couple of hours...have included
slight chance thunder for this area this morning as this stronger
forcing offsets any lacking instability. This better development
across northern Illinois will begin to slide south/southeast this
afternoon...with this trend continuing into this evening.
Did increase wind speeds and gusts today...particularly south of
i80...as am a little concerned guidance may be under doing how
strong the winds could be. Forecast soundings would suggest
lower ceiling in place than what is currently occurring with VFR
conditions in place and with these better conditions...mixing may
be more efficient than what is advertised at this time. With both
observed and forecast sounding indicating rather strong speeds
just off the deck...did trend higher but did not go too high at
this time with some uncertainty remaining. Will need to monitor
speeds potentially approaching Wind Advisory criteria south of i80
briefly this afternoon.
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 18z...
* gusty winds.
* MVFR/IFR ceiling and visibility.
* Showers this afternoon...with more steady rain tonight.
* North northeast winds/gusts Friday.
Precipitation continues to move across all of the terminals early this
afternoon...with more widespread and steady rain falling at rfd.
This trend will likely continue for most of the afternoon as
additional upstream development moves overhead. Ceilings and
visibility in the near term have bounced around and expect this
to likely continue this afternoon...with periodic MVFR
ceilings/visible likely under the better precipitation. Steady rain will
continue this evening while ceilings really begin to fall as a
surface trough and front approach the terminals. Still think that
low end MVFR and then IFR will be likely tonight into Friday
morning...and have made little adjustments during that time. Sharp
change in wind direction will likely occur along this frontal passage
tonight as winds quickly turn to the west/northwest and then to
the north. These northerly winds will continue Friday while any
remaining precipitation drifts south of the terminals and while ceilings
slowly improve through midday.
//Ord and mdw confidence...updated 18z...
* medium high with wind speed/direction.
* Medium with ceiling/visibility trends and timing.
* Medium high with showers/rain.
//outlook for Ord/mdw for 00z Saturday-12z Thursday...updated 12z...
Friday night...VFR. North-northeasterly winds.
Saturday...VFR. East-northeasterly winds.
Sunday...VFR. Northeasterly winds.
Monday...MVFR possible. Slight chance of rain. East winds.
Tuesday...MVFR possible. Slight chance of rain/snow. West winds.
Wednesday...MVFR ceilings possible. Northwest winds.
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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