Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
616 am CDT Wednesday Oct 7 2015

Short term...
334 am CDT

Through Thursday...

Areas of dense fog have formed over southern and far eastern portion
of County Warning Area and given earlier quick drops in visibility at ASOS/AWOS sites
opted to hoist a dense fog advisory in coordination with neighboring
offices. Could see visibility improve in spots as middle level cloudiness
moves in...but areas of dense fog likely to persist until a couple
hours past sunrise.

Weak surface trough will move south into the area this morning
before largely washing out and becoming nearly indistinguishable
this afternoon. Primary effect will be encouraging a synoptically
enhanced lake breeze today...which given air temperatures over the lake
should keep lake adjacent areas pretty chilly this afternoon with
temperatures likely in the lower to middle 60s at best. Air mass outside the
influences of the lake will be milder than Tuesday with highs likely
climbing into the 70s.

Large region of middle-upper 50 degree dewpoints reside to our
southwest across the lower Missouri Valley. As a quick moving
shortwave and associated surface low move into the western lakes region
Thursday look for increasing southwesterly flow to transport those
higher dewpoints northeastward into our County Warning Area...with some guidance
suggesting moisture pooling could allow for dew points peaking around 60
Thursday afternoon.

Showers and thunderstorms are likely along and just ahead of a cold
front dropping in from the northwest Thursday afternoon spreading
southeast across the County Warning Area Thursday evening. Instability does not look
all that impressive thanks to fairly weak middle level lapse
rates...but still ncar WRF ensemble puts mean MLCAPES around 500
j/kg over our northwest County Warning Area later Thursday afternoon ahead of the front.
The modest cape combined with seasonably strong flow aloft could
allow for isolated to widely scattered stronger storms...especially
northwest County Warning Area where frontal timing will be better aligned with peak
heating. Showers and thunderstorms will end from northwest to southeast later
Thursday evening.



Long term...
334 am CDT

Friday through Tuesday...

Brief but healthy shot of colder air arrives Friday on somewhat
brisk northerly flow. Despite afternoon sunshine look for highs to
struggle to break the 60 degree mark...especially near the lake.
Slight moderation in temperatures expected Saturday as light southwesterly
winds develop and weak warm air advection commences.

Medium range guidance has sped up the progression of the system
early next week and its associated cold front. Quickly
intensifying central Pacific hurricane oho is forecast to
accelerate north into the middle latitudes by later Thursday and
Friday and its not uncommon for medium range models to struggle a
bit handling pattern evolutions when stronger hurricanes reach middle probably not safe to lock in an early Monday
frontal timing just yet. Still looks like a shot of unseasonably
warm air will surge into the region ahead of the front...though
current timing suggests the thermal ridge will be in the area
Sunday night possibly lessening our chances of hitting 80 a bit.
Still depending on how quickly warm air surges in on Sunday there
is a chance of 80 then...especially southwestern County Warning Area. Cooler air
looks to lag behind the front despite an early frontal passage
highs well into the 70s still look like a good bet. Brief cool
shot behind the front before another surge of unseasonable warmth
is prognosticated to arrive toward middle of next week...though
confidence is obviously lower in details at that distance.




//Ord and mdw concerns...updated 12z...

* MVFR fog briefly this morning.
* Northeast winds near 10 knots this afternoon
* MVFR/IFR late this afternoon/evening


//discussion...updated 12z...middle level cloud deck is shifting
southeastward/thinning out. Moist low level air mass and weak winds
will allow some fog to continue for a few hours ahead of a weak
cold front. Winds shift to the NE quickly this morning...and then
pickup in speed throughout the afternoon...reaching around 10 knots
as winds veer more easterly...with some high res guidance
suggesting 10 knots could be reach later this afternoon as high
pressure builds over the lake and increases the synoptic push of
winds off the lake. Some guidance suggests low clouds late this
afternoon and evening off the lake and advecting inland...but low
confidence at this point as to the extent of coverage and most
only suggesting few to scattered coverage. Winds slacken and turn
southeast overnight...then increase out of the southwest on
Thursday morning ahead of an approaching cold front. Rain showers will
hold off until the afternoon.


//Ord and mdw confidence...updated 12z...

* medium that speeds stay under 10 knots. Could see speeds pickup to
10 knots or so right with the frontal passage this afternoon.
Medium on timing of the wind increase.
* Low for MVFR/IFR clouds behind the front.


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

Thursday night...rain showers likely with possible thunderstorms and rain. MVFR/IFR possible. SW wind.
Friday...improving to VFR. Gusty north wind becomng NE. Saturday and
Sunday... VFR. Breezy SW wind. Monday...schc rain showers. Winds quickly
turning gusty out of the northwest.
Tuesday...VFR. West wind.



334 am CDT

A weak cold front will move down the lake through early morning.
High pressure over the northern plains will quickly slide east over
the northern lakes then to the Atlantic Seaboard Friday.
Meanwhile...low pressure that will emerge from the Canadian rockies
today will pass southeast across the central part of Lake Michigan
Thursday afternoon/evening. South winds will increase to 30 knots ahead
of the low...then north winds in the unstable regime behind the
front will build to 30 knots Thursday night into early Friday. High
pressure quickly returns Friday night but shifts east Saturday as a
series of low presssure systems move thorugh Canada. The stronger of
these will reach Hudson Bay Monday. A period of gusty south winds
will build waves on the northern half of the lake especially. This
stronger low will steer send a stronger cold front across the lake
Sunday night into Monday.



Lot watches/warnings/advisories...
Illinois...dense fog advisory...ilz023-ilz032-ilz033-ilz039 until 9 am

In...dense fog advisory...inz001-inz002-inz010-inz011-inz019 until 9
am Wednesday.



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

Follow US on facebook...twitter...and youtube at:

National Weather Service Glossary of Abbreviations