Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
324 PM CDT Tuesday Sep 2 2014

324 PM CDT

Cold front which moved through the forecast area last night has
settled into the Ohio River valley this afternoon...with weak high
pressure center developing eastward from the lower Missouri Valley.
Modest west-southwest low level winds were noted across the County Warning Area...
several weak convergent zones including an outflow boundary from
morning thunderstorms over northwest Illinois...the lake breeze boundary
along the Illinois shore and extending east across northern
Indiana...and a subtle trough axis/moisture gradient across our
southeastern counties. Visible satellite imagery has shown cumulus/towering cumulus
development along these boundaries at times with a few isolated
showers having formed along the boundaries across far east central
Illinois and Northwest Indiana. Another weak convergence axis was from
near Milwaukee back into northwest Illinois...also with some cumulus
development along it. While an isolated shower will be possible
along any of these boundaries during the remainder of the afternoon
hours...the higher surface dew point and MLCAPE axis over the far
southeastern County Warning Area will have the best chance (and only isolated
coverage at that) of any more significant showers or isolated
thunder. Loss of diurnal instability should end isolated precipitation
threat by early evening. Weak surface pressure gradient north of
surface high pressure center will allow for light winds overnight...
and with mainly clear skies should see some patchy fog develop
especially across areas south of Chicago where dew point temperatures will
be a little higher and where ground likely remains moist from recent
heavy rains.

Attention then turns toward the west surface low
pressure development takes place across the High Plains in response
to strong middle-level short wave crossing the northern rockies.
Guidance has trended deeper with the middle-level wave and the
resulting surface low...with southerly return flow developing and
strengthening across the Mississippi Valley and western lakes region
by later Wednesday and Wednesday night. This will result in the cold
front to our south returning north as a warm front Wednesday
night...and bringing very warm/hot and humid conditions back for
Thursday and Friday. Thunderstorm potential increases Wednesday
night and early Thursday as the warm front pushes north...before
capping inversion develops in 900-700 mb layer within warm sector
Thursday. Hot and humid conditions expected Thursday...with 850 mb
temperatures in excess of +20 c and 950 mb temperatures approaching +28 c.
Provided that any morning clouds/precipitation with early morning warm
frontal passage scatter...and no significant convective outflow
remains...afternoon highs around 90/lower 90s look reasonable.
Combined with dew point temperatures climbing into the 70s... heat index
values of 95-105 would make for uncomfortable conditions.

Deep surface low continues to deepen as it moves east across Ontario
Thursday night...with a trailing cold front pushing into northern
Illinois pre-dawn Friday. Shower/thunderstorm potential increases
early Friday with arrival of front...and continues through Friday
night as front sags south into downstate IL/in. With front
bisecting County Warning Area during the day...a fairly large temperature spread is
anticipated with upper 70s/low 80s along the Wisconsin border...and
upper 80s/near 90 again across the southern counties of the forecast
area with most of the cloud cover and precipitation expected along/north of
the advancing front as capping remains in place south of the

Broad surface high pressure then spreads across the upper Midwest
and Great Lakes region Saturday and Sunday...with dry and much
cooler conditions with easterly low level flow and lake breezes.
Next chance of organized precipitation appears to arrive at the end of the
forecast period on Tuesday.




//Ord and mdw concerns...updated 20z...

* none.


//discussion...updated 18z...

The showers and thunderstorms associated with a old middle level
impulse have dissipated and no precipitation is now expected in vicinity of the
terminals. Latest radar imagery shows an outflow boundary
extending from karr to kvys and tracking to the southeast. Some
northwesterly wind gusts follow the passage of the outflow boundary...but
do not expect much...if any...impact at the terminals. Also...a
lake breeze boundary is forming along the lake front. With surface
winds approaching 10kt and winds just off the deck in excess of not expect the lake breeze boundary to push inland much
and should remain east of Ord/mdw. Under generally clear
skies...expect the surface layer to decouple from winds aloft and
winds should becm light/variable overnight. There is a chance for some
patchy ground fog overnight tonight. There is a chance it could
impact dpa/rfd...but is unlikely at Ord/mdw/gyy. Will leave
mention out of the tafs for now since it should be confined to
sheltered...low lying areas.


//Ord and mdw confidence...updated 20z...

* medium to high confidence in Ord/mdw remaining dry this

* High confidence in all other forecast elements.


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

Wednesday night and Thursday...predominantly VFR...though there
is a slight chance of a thunderstorm.

Friday...pretty good chance of a period or two of thunderstorms and rain.

Saturday through Monday...VFR. Northeast winds likely.



251 PM CDT

Main forecast concern is with strong southerly winds ahead of
deepening low pressure moving from the northern plains on Thursday
to James Bay by Friday morning. Tightening pressure gradient and
pressure falls will result in strengthening southerly flow during
the day Wednesday and especially Wednesday night. Sustained winds
will peak on Thursday in the lower 20 knots range...but have lower
confidence in gust speeds due warm air mass overspreading the
region. Will continue to mention 30 knots gusts in the Gulf. Its
possible that near shore waters could see more frequent gusts up to
30 knots in strong offshore flow...with forecast soundings indicating
that potential. Small Craft Advisory winds are likely Thursday
morning into Thursday evening for the entire nearshore.

Then a strong cold front trailing from the low pressure area will
sweep down the lake Thursday night through Friday afternoon. A
period of decent northerly winds could set up especially on the
southern half of the lake...but high pressure will quickly spread
eastward and reduce speeds. is uncertain whether
duration of northerly winds will be sufficient to result in waves
building to Small Craft Advisory criteria. For now do have waves
above criteria in grids. Winds will become relatively light over
the weekend with expansive high pressure slowly sliding
eastward...though northerly.



Lot watches/warnings/advisories...


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

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

National Weather Service Glossary of Abbreviations