Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
410 am CDT Sat Aug 23 2014


327 am...forecast concerns include thunderstorm chances/timing
through the period along with high temperatures/heat indices
through Monday.

Areas of fog continue to develop with visibilities dropping under
1sm in a few locations. Not expecting widespread dense fog this
morning but will need to monitor observation over the next few hours. The
fog will lift this morning but with little change to the airmass
fog will be possible again tonight into Sunday morning.

A few showers developed across Northern Lake County in northeast
Illinois late Friday evening and persisted for a few hours before
dissipating. Otherwise...convection has been focused further west
early this morning in the vicinity of the low level jet. Some of
this activity over eastern Iowa may arrive across the western County Warning Area
later this morning...likely in a decaying form. But other than an
isolated shower...expect the next several hours to be mainly dry.

Confidence regarding thunderstorm chances this afternoon into
this evening is low though there is some consensus for development
by early afternoon across western/northwest Illinois from remnants of
convection over eastern Nebraska/western Iowa. How far it continues
east or northeast before weakening is another uncertainty. Made no
significant changes for today maintaining 30-40 probability of precipitation. Trends will
need to be monitored through the morning. Dewpoints in the 70s and
precipitable water values approaching 2 inches will lead to very
heavy rain with any thunderstorms that do develop and possibly
flash flooding...especially if the rain falls over harder hit
areas from Friday morning.

The upper ridge then builds across the area tonight and Sunday and
precipitation chances look below mentionable from late tonight through
Monday morning. Can/T completely rule out some isolated activity...
in particular on a lake breeze Sunday afternoon. A frontal
boundary will move across the upper Midwest Sunday night into
Monday with some differences as to how far south this boundary
will settle Monday afternoon into Monday night. This would likely
be the next chance of thunderstorms with the Gem/European model (ecmwf) both
pushing the front through much of the County Warning Area by Tuesday morning. This
boundary then meanders near the Illinois/WI border into Thursday with
the possibility of a prolonged unsettled pattern across the area.

Cloud cover/convection today will likely keep temperatures in the 80s...
though with the urban areas starting from Lower/Middle 70s...any
longer period of sunshine today could allow a few places to tag
90. Temperatures will warm a few degrees Sunday but southeasterly winds
will keep the Lakeshore cooler. Monday looks to be the best
chance for temperatures into the lower 90s County Warning Area wide but will be dependent
on cloud cover and timing/arrival of any precipitation. Dewpoints may
lower a few degrees...back into the middle/upper 60s across parts of
the area Sunday. Heat indices could range from Lower/Middle 90s to
100-105 far southwest. Dewpoints then back into the 70s everywhere
Monday with heat indices closer to 100-105 most areas with still
the potential to reach heat advisory criteria across the far
southern County Warning Area Monday afternoon. Cms



//Ord and mdw concerns...updated 09z...

* 200-400 feet ceilings at Ord expected to make it to mdw next 1-2
hours...then persist through daybreak before lifting/scattering
middle/late morning. Visibility may fall to around 2sm as well.

* East to northeast winds expected to be less than 10 knots today.

* Low chances for isolated-scattered thunderstorms and rain with best chance this
afternoon...but may remain to the west of Ord/mdw.


//discussion...updated 09z...

IFR ceilings and visibility have been gradually expanding south and
southwest over the last few hours and have reached Ord. Expect
that they will also move across mdw/dpa in the next 1-2 hours. Still
expect improvement/scattering but with light but persistent
northeast flow this may end up being a slow process and extend
beyond 14-15z. Rfd is more of a question mark and have further
slowed arrival of IFR but it is possible IFR ceilings do not make it
that far. Some patchy IFR visibility may be more likely as we get closer
to daybreak. Gyy remains in and out of IFR ceilings with IFR visibility more

From 06z...

Weak low level flow and humid conditions continue across the
region tonight with the main forecast challenge being potential
IFR ceilings and visibility. Latest ob trends show a reduction in earlier
lower cloud cover as an extensive shield of higher cloud cover
works in. Light northeast flow remains near the Lake Michigan
shoreline with Wisconsin shoreline areas continuing to report IFR
cigs/vsby. Have seen little southward advancement of this IFR into
NE Illinois thus far so have slowed the arrival in the tafs by several
hours...though ugn is now IFR. The higher cloud shield may be
preventing significant expansion for now. Conditions remain
favorable for an expansion of IFR but it may be that IFR only
develops for a few hours closer to daylight.

Beyond that easterly winds of 10 knots or less are expected during
the day and ease up this evening. Do not see much in the way of a
clear trigger for organized thunderstorms and rain development but thunderstorms and rain chances are
not zero. The most favored time should be this afternoon with maximum
heating. With flow off the lake any activity may remain west of
Ord/mdw/gyy at least initially but then drift to the east. Thunderstorms and rain
potential and confidence is too low to include any mention in the
tafs at this point.


//Ord and mdw confidence...updated 09z...

* medium-high confidence in IFR reaching mdw. Medium confidence in
timing of improvement.

* High confidence in wind forecast.

* Medium confidence that thunderstorms and rain chances are low. Low-medium
confidence in best timing being this afternoon.


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

Sunday...schc thunderstorms and rain in the afternoon. MVFR ceilings/visibility possible.

Monday through Wednesday...periodic chances for thunderstorms and rain/rain showers with
occasional MVFR cigs/vsby.

Thursday...schc rain showers.



340 am CDT

Broad low pressure remains across the middle Mississippi River valley
this morning while stronger low pressure organizes over the Central
High plains. An upper low spinning over the northwest U.S. Will help
kick the High Plains low northward into Sunday while high pressure
moves from Quebec to New England and the Middle-Atlantic States. East
to northeast winds will continue across Lake Michigan today but will
shift to the southeast into Sunday...with speeds increasing
modestly. The main track of low pressure will remain off to the
northwest of the lake into the middle of next week which will keep a
southerly wind going...though southern areas may see winds shift
onshore each afternoon where the pressure gradient is weaker. A
frontal boundary is expected to drop over northern parts of the lake
toward midweek which would lead to east or northeast winds across
the north with southerly winds continuing south. Medium range
guidance conflicts with how quickly this front moves across the lake
and the strength of cold air behind it during the middle of next week
leading to low forecast confidence by that time.



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