Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
624 PM CDT Wednesday may 6 2015

Short term...
224 PM CDT

Through Thursday...

Much of the area is getting out of the haze/fog early this
afternoon...with fog even lifting in downtown Chicago. At 2
PM...there was a 29 degree difference in temperature between
Midway Airport (81) and Northerly Island (52). The stationary
front that was parked over central Illinois yesterday is marching
northward and all but far northeast Illinois...including northern
Cook and lake counties...is feeling the warmth behind this front
as temperatures quickly warmed to around 80 elsewhere. An
extensive cumulus field is filling in in the warm sector. In
addition to this fun...the lake breeze is now moving through Cook
County which will limit the northeastward movement of the front.
Meanwhile aloft...shortwave energy is rotating northward ahead of
an upper low in the plains. Showers and thunderstorms remain
confined east of the Mississippi.

For the rest of this afternoon/early evening...weak shortwave energy
continues north with the warm front. High res guidance suggests that
any showers/thunderstorms that form this far east would be isolated in
nature at best as the upper level height falls are mainly off into
Iowa. Models suggest the warm front will stall its northward
movement...and therefore with the moist airmass in place...expect
some fog/haze to reform this evening. Not quite sure how thick or
widespread it will be...but not like it was this morning. Expect
fog to reform over the lake...so it could become dense again along
the Lakeshore areas. Through the night we will also see some high
cloud out ahead of convection to our west.

On Thursday...pronounced southerly flow should finally allow the
entire area to have a taste of Summer warmth. Slight southeast flow at the
surface at the lake will provide some cooling from Chicago northward
into areas near the lake in Lake County Illinois after warming
through early afternoon. Widespread 80s are still anticipated with
850/925 temperatures a touch higher than today where we were easily able to
quickly reach the upper 70s to low 80s already today. The upper low
in the plains only inches eastward...but some weak height falls in
the afternoon may allow a few showers/T-storms to push into areas
along and east of I-39...but more organized activity during the day
will remain closer to a cold front extending from the upper Midwest
into the Central Plains.

Kmd

&&

Long term...
320 PM CDT

Thursday night through Wednesday...

The extended period looks to become rather active...especially by
the weekend. Overall...a digging northern stream jet over the
western coast of the country will set up a broad trough with an
embedded closed low across the western Continental U.S. Late this week. As
southwesterly flow aloft continues to increase downstream of this
feature in the coming days...numerous smaller scale middle-level short
wave disturbances are likely to eject out of the main southwest
trough. Meanwhile...in the lower levels...an increasing lower level
wind field will allow a plumb of subtropic moisture to be
transported northward across the central Continental U.S. Into the upcoming
weekend. This ultimately looks to result in several periods of
showers and thunderstorms during the later half of the period.

The first of several of these small scale disturbances look to move
northeastward across the upper Midwest Thursday night. Moisture
transport and warm air advection looks to begin to increase across
Iowa and northwestern Illinois by Thursday evening. This could
result in some scattered shower and storm development across
portions of the area Thursday night. However...it appears the better
focus for activity may remain to the west and north of most of my
area...so I have only low end probability of precipitation across my north central counties.

Model guidance is in good agreement that another disturbance will
shift across northern Illinois sometime during the day/evening on
Friday...leading to good chances for thunderstorm development across
northern Illinois out ahead of a slowly southeastward advancing
surface cold front. This looks to produce our best chance of showers
and thunderstorms over the region through at least later in the
weekend. This disturbance is expected to induce an uptick in the
moisture transport over the area later Friday and this should allow
some healthy precipitable water values around 1.5" to set up over
the area. Although no severe thunderstorms are expected with this
round of storms...it appears that some heavy rainfall could occur
given the higher precipitable waters and hence more effect rainfall potential
setting up.

The surface cold frontal boundary is expected shift over far
northern Illinois by Saturday afternoon...then become quasi-
stationary near the Interstate 80 corridor through Sunday. This will
again result in much cooler conditions across northern sections of
the area...with onshore flow likely to result in very cool
conditions near the lake again...similar to what we saw yesterday.
A sharp temperature gradient is again likely over the area...with
temperatures remaining near 80 across my southern counties on the
warm side of the front. Areas of fog are also possible again over
northern Illinois...but I will leave out of the forecast until we
get a better idea on timing.

Showers and storms will again be possible on Saturday...though the
best chances look to remain on the warm side of the frontal boundary.
This boundary may then shift back northward as a warm front at some
point later in the weekend as the main southwestern Continental U.S. System
shifts over the plains...and drives a surface low eastward over the
middle-Missouri Valley by Sunday evening. Increasing middle-level flow and
falling heights aloft atop a warm and moist warm sector could set
the stage for a period of strong or even severe thunderstorms later
Sunday/Sunday evening ahead of an eastward advancing cold frontal
boundary.

Much cooler weather looks on tap for a day or two on Tuesday and
possibly Wednesday of next week as a cool area of high pressure
shifts over the area in the wake of the cold frontal passage. The
cooler airmass may only support lower 60s for daytime
highs...though the magnitude of this colder airmass is still a
bit uncertain at this range.

Kjb

&&

Aviation...

//Ord and mdw concerns...updated 00z...

* easterly winds under 10kt this evening.
* MVFR visible tonight.
* Strong/gusty southerly winds Thursday. Cms

//discussion...updated 00z...

Lake breeze well west of Ord/mdw late this afternoon with easterly
winds generally under 10kts which will turn more southeast this
evening and then more southerly overnight. Quite a bit of fog
still over Lake Michigan and after sunset with some easterly
component to the wind expected...this fog and perhaps lower ceilings
may move inland but how far and how low is uncertain. Short range
guidance suggests it will remain east of Ord/mdw and have
continued with that trend with MVFR visible this evening. If winds do
turn more southerly overnight as expected...could still be some
light fog/haze...but would not expecting anything too low.

Southerly winds begin to increase during Thursday morning and as
the low levels continue to mix...gusts into middle perhaps upper 20
knots range look possible. Cumulus will dissipate early this evening with
generally just high clouds tonight...possibly some middle clouds
across northwest IL/rfd. Then additional cumulus development Thursday.

Mainly dry weather expected through the period. But there is the
chance of showers or a few thunderstorms across northwest Illinois
Thursday afternoon/evening...which could move as far east at rfd.
Confidence is fairly low regarding coverage and will stay dry with
this forecast. Cms

//Ord and mdw confidence...updated 00z...

* high for winds through the period.
* Low for MVFR visible tonight. Cms

//outlook for Ord/mdw for 12z Friday-12z Wednesday...updated 00z...

* Friday...shra/tsra likely. MVFR possible. South winds.
* Saturday...chance of shra/tsra. MVFR likely. IFR possible. North
winds.
* Sunday...chance of shra/tsra. MVFR possible. Northeast winds.
* Monday...shra/tsra likely. MVFR likely. Gusty southwest winds.
* Tuesday...chance of rain showers. Strong/gusty westerly winds.

&&

Marine...
332 PM CDT

Areas of dense fog continue over mainly the western half of
Southern Lake Michigan and this looks to continue into Thursday.
Therefore the dense fog advisory has been extended into Thursday
morning. Areas of fog may begin to shift farther north over
northern sections of the lake tonight into tomorrow as a much
warmer and moist airmass begins to shift northward over the lake
as the winds become southerly. Dense fog is possible in these
areas...but we will hold off an advisory at this time.

Southerly winds up around 20 knots look to continue through at least
later Friday. Thereafter an area of low pressure shifting
northeastward towards the James Bay will shift a cold front over
the lake into Saturday morning. Winds in the wake of this front
will become northerly for a period through the weekend. By Sunday
night and Monday another area of low pressure is expected to move
over the middle Missouri Valley...then somewhere across the Great
Lakes by Monday night. This system could produce a period of
stronger east-northeasterly winds early next week...especially
over northern portions of the lake.

Kjb

&&

Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...dense fog advisory...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745-
lmz777-lmz779 until 11 am Thursday.

&&

$$

Visit US at http://weather.Gov/Chicago (all lowercase)

Follow US on facebook...twitter...and youtube at:
www.Facebook.Com/nwschicago
www.Twitter.Com/nwschicago
www.Youtube.Com/nwschicago

National Weather Service Glossary of Abbreviations