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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1258 am CDT Wednesday Oct 1 2014

908 PM CDT

No big changes to going forecast for tonight. Low level flow
should veer from northeast to southeast overnight probably causing
the southern extent of the stratus deck to lift slowly north. Far
northeast Illinois should spend most of the night socked in with clouds
and even have had some occasional reports of light drizzle or
have added that to the grids near the Illinois shore tonight. Only other
minor change to the forecast was to drop temperatures near the Illinois shore
Wednesday with marine air mass now in the low/middle 50s and unlikely
to rapidly recover think that even the short southeasterly fetch
tomorrow should keep Illinois shore from downtown northward struggling
to get into the 60s...with lake influence potentially even bending
winds more easterly near the lake.


//prev discussion...
238 PM CDT

Plenty to keep US busy on the forecast desk this afternoon...with
concerns being cloud cover and temperatures tonight followed by rain
and thunder potential Wednesday night through Thursday night...and
then finally much cooler air and shower chances this weekend.

this afternoon temperatures are ranging 20 to 30 degrees colder
across northern Illinois and Northwest Indiana than yesterday
afternoon. Northeast flow in the boundary layer has kept the
inversion strong especially in lake-bordering counties and stratus
has accordingly been slow to depart. Surface ridging extending
southward will move over the area tonight and weaken that flow but
also allowing for the inversion to remain and even strengthen.
Confidence is overall low on specifics on cloud cover with the
stratus being only about 1000 feet thick per amdar data. While there
may not be that much coverage of clouds by expect
thereafter to see an increase along with a westward expansion. Given
where clouds are now on satellite along with a reasonable 4km emc
WRF solution of column condensate...would envision clouds to become
the most widespread along and north of Interstate 80. Later tonight
as the 850-925mb flow turns southerly...the clearing line may creep
northward. Obviously temperatures will hinge on clouds quite a bit.
Outside of the immediate Lake Shore where lower 50s are
probable...40s are forecast but clouds cloud dictate whether near 40
to the lower 50s.

Wednesday through Thursday night...
a very active pattern is seen upstream with multiple upper cyclones
and troughs across the western half of the U.S. And Canada and into
the northern Pacific. One well-defined cyclone is moving nearly due
north across the Dakotas this afternoon. Its moist conveyor belt and
rain entering western Iowa this afternoon will eventually weaken as it
shifts eastward through Wednesday...but begin to re-enhance
Wednesday night as the next deeper and more progressive long wave
trough enters the plains. Embedded within this trough will be
multiple vorticity this a negative tilt over the region
Thursday night. Then into Friday is when a surface low will deepen
considerably as it moves northeast from the middle-Mississippi Valley
through the upper Great Lakes.

So after a quiet day Wednesday we continue to slowly spread chances
of rain eastward Wednesday night. Instability supportive of thunder
lags some and becomes better late in the night and through Thursday.
While it is likely there will be areas of rain with embedded storms
in the region Thursday...the much better support in upper
lift...deep moisture convergence...and even instability arrives
Thursday evening into overnight on the suite of guidance. This is
especially true considering the slowing trend that has been seen
over the past few model runs...something almost expected with such
an amplified and deepening system. So have likely probability of precipitation for most
areas Thursday and definite Thursday night. It is likely more
focused periods for the highest probability of precipitation will be refined in time.

Confidence is increasing for over an inch of rain in 18-24h with
this system /and maybe even in a much shorter time/ for most of the
area during Thursday and Thursday night...supported by cips and gefs
analog composites. It is possible for some localized flooding but
not expecting widespread issues. The severe potential continues to
look to be southwest of the area where a squall line is favored into
southern Illinois given the synoptics. The northern portion of
this...likely more loosely defined...could clip our
moderate lower and upper level wind shear shift over the region
ahead of the front. Overall the severe weather threat remains low.

Friday through early next week...
this remains a challenging period but confidence has increased that
much cooler than normal temperatures will be prevalent through much
of this period. The upper level low/trough which becomes negatively
tilted at the beginning of this period will be further strengthened
by multiple upper jet maximums. As this occurs...cooler than normal
low-level air will be replenished. Upper level disturbances...namely
sheared but still each with lapse rates and moisture supportive of
scattered showers...will be supported in this pattern. If these pass
over the area they will allow for some wet/cloudy/even cooler
weather. Have trended cooler in multiple periods but confidence is
generally low on specifics. It is possible that patchy frost may
need to be added to the forecast in time depending on cloud cover
and winds.




//Ord and mdw concerns...updated 06z...

* MVFR ceilings staying overhead through early morning.

* East southeast winds through period...around 10 knots Wednesday.

* Increasing chances for precipitation very late in the period.


//discussion...updated 06z...

Satellite imagery showing MVFR ceilings slowly departing to the
north...while an extension of these clouds also pushing towards
the terminals. With the possibility for a longer duration of these
ceilings based on latest trends...did push back timing of
scattering 1-2 hours. These ceilings around 2kft may fall some
over the next couple of hours but expect them to stay above
1500ft...before they continue north of all the terminals towards
the 11-12z time frame. VFR conditions will then prevail on
Wednesday as the current east winds turn more to the
southeast...while increasing to around 10 knots later this morning.
There could be some lake influence later this afternoon...but this
would be no more to take them to more of an east southeast
component. Diminishing winds and increasing VFR ceilings will
occur Wednesday the next system draws near and helps
steer precipitation towards the terminals late in the forecast period.


//Ord and mdw confidence...updated 06z...

* high confidence with MVFR ceilings and trends...medium
confidence of exact timing of scattering.

* High confidence with wind speed/direction.

* Medium confidence with precipitation trends.


//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...

* Thursday...thunderstorms and rain likely. MVFR possible. Gusty southerly winds
* Thursday night...periods of thunderstorms and rain. MVFR likely. IFR possible.
Gusty southerly winds possible.
* Friday...slight chance of PM rain showers. Strong/gusty west winds.
* Friday night...chance of rain showers. MVFR possible. Gusty westerly
* Saturday...dry/VFR. Westerly winds.
* Sunday...slight chance of rain showers. Westerly winds.
* Monday...dry/VFR. Westerly winds.


215 PM CDT

High pressure stretching through southern Ontario and Quebec will
continue drifting east tonight as low pressure near the eastern
Great Lakes weakens. While winds are abating somewhat across Lake
Michigan this afternoon as the pressure gradient relaxes...the
northerly fetch is holding waves along the southern third of the
lake at 4-6 feet or higher. Tonight the ridge axis associated with
the departing high will pass over the lake then push to the
eastern lakes Thursday. Expect a more significant easing of the
waves as this occurs.

Meanwhile...several areas of low pressure in the plains will allow
winds to veer to southeastlerly. The first low in western Nebraska
will lift into Manitoba Wednesday then deepen further as it continues
toward Hudson Bay. A secondary low will develop across the
southern/Central Plains Wednesday night. It is this low that will
eject northwestward toward the western Great Lakes later in the
week. This low is expected to deepen as it passes through Wisconsin
Friday than north into Ontario on Friday. Expect southeast winds to
increase in response to the strengthening low...with winds to 30 knots
possible. At this time it appears that the strongest winds aloft
ahead of the low on Thursday will have a harder time surfacing
due to this westerly track...but as the low lifts north there may
be a brief period of southerly gales Thursday night. While the
winds aloft show support for gales...there is still quite a bit of
uncertainty with how deep the low will be and how far east it will
be during this period.

On the back side of the low which should still be strengthening
north of the Lake...A sharp cold front will bring another chance for
gales...this time with west winds. The wind support aloft would
suggest just borderline gale force gusts...but it will be a pretty
strong front and unstable conditions should allow for better
mixing. Following the front a modest gradient remains in place
through the weekend ahead of a clipper-like low for Monday. Kmd


Lot watches/warnings/advisories...


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