Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
1002 am CDT sun may 3 2015

1002 am CDT

Well defined mesoscale convective vortex over eastern Iowa is move steadily eastward and
starting to spread some showers east into our western County Warning Area and also
bringing with it a good deal of cloud cover. Given the lack of
organized deep convection and downstream air mass being slightly
drier and a bit more stable...would anticipate some weakening of
the vorticity as it moves east into northern Illinois this afternoon.
Despite this...the mesoscale convective vortex looks very wound up and impressive on
satellite and radar imagery this morning and seems unlikely that
it would completely dissipate before bringing at least a threat of
showers and perhaps isolated thunderstorms this afternoon to the
area. Have made some adjustments to first period grids to beef up
probability of precipitation and cloud cover. Except where pockets of sunshine allow temperatures
to would seem that our going high temperatures are too warm and
need to be lowered. Northwestern County Warning Area seems most prone to being
socked in with cloud cover and for now just going to
lower temperatures there but may need to eventually follow suit
elsewhere. Updated grids have been sent and text products will be
updated shortly.



Short term...
327 am CDT

Through Monday night...

A potent shortwave is over northwest Illinois this morning and it will
move over central Indiana this afternoon. Scattered showers are
aligned with the wave...while a couple of convective complexes lag
behind the wave over Iowa. Thinking isolated showers are possible
through 7am across the County Warning Area. Near term guidance agrees that the best
forcing for additional isolated showers will be along and west of
the I-55 corridor through noon.

Early morning cloud cover should dissipate this afternoon which
would allow temperatures to climb into the low 80s. Have medium-high
confidence in highs today as a change in cloud cover will lead to
either warmer or cooler temperatures. Wind gusts will also be
dependent on cloud cover and mixing. Backed off on gusts slightly
but still expecting a few gusts up to 30 miles per hour this afternoon.

The surface low that will bring thunderstorms to the region is
moving over Hudson Bay this morning. Its cold front will slide
through the County Warning Area Monday morning but thunderstorms ahead of it will
infiltrate the County Warning Area from north to south beginning late this evening.
Have medium confidence in the timing of thunderstorms as guidance
typically is a bit fast with cold fronts. Have high confidence in
thunderstorms as there is enough cape for thunderstorms and steep
lapse rates. Severe weather is not expected as the other parameters
are not supportive of severe storms. Heavy downpours are possible
with the storms however as precipitable water values will increase to 1-1.2 inches

Showers and storms continue throughout the day Monday as the front
stalls at or south of I-80. Models show better consistency
today...but still have subtle differences as to where the front will
stall out. Convection will fire along the front Monday and the only
concern I see is possible flooding. Precipitable water values increase up to 1.5
inches Monday evening and night which could lead to some locations
receiving over a half inch of rain in a 24 hour period.

Highs Monday will vary from the low to middle 70s north of the the 60s along the the middle 70s south of the



Long term...
327 am CDT

Tuesday through Saturday...

The front remains south of I-80 Tuesday and looking at another day
with scattered to numerous thunderstorms. Guidance suggests we may
see a few dry periods but have low confidence in when and where
those would occur so kept at least a chance of thunder across the
County Warning Area. Temperatures on Tuesday will once again vary depending on which side
of the front you are on. Areas north of I-80 will be in the 60s with
middle to upper 50s along the lake thanks to onshore flow. South of I-
80 and the front...middle to upper 70s.

The front lifts north as a warm front Tuesday night into
Wednesday morning pushing the convection north with it leading to
what could be a dry Wednesday. Guidance suggests isolated to
scattered warm sector convection is possible Wednesday. Model
soundings feature a few hundred j/kg of cape with no cap so went
with a slight chance to chance of thunder. Shear looks weak at best
so not expecting severe storms at this time. Warmer conditions are
expected since we will be in the warm sector. Cloud cover will once
again possibly bust the temperature forecast so went with conservative
numbers. Looking at middle 70s to around 80 away from the lake.

The next surface low passes over Ontario Thursday night through
Friday morning and its cold front stalls out near the Illinois/WI state
line Saturday morning. Additional thunderstorms are possible from
Friday Onward as the front hangs around the region. Only have medium
confidence in thunderstorm coverage at this time. Temperatures late this
week look to be around 80.




//Ord and mdw concerns...updated 14z...

* gusty southwest winds late morning through early this evening.
* Chance of rain showers this afternoon.
* Rain showers/thunderstorms and rain developing middle/late evening and continuing into Monday
* Wind shift to the north/northeast Monday afternoon. Cms

//discussion...updated 12z...

Broad warm advection continues over the region this morning with a
speckling of showers. Instability is still quite minimal east of
the Mississippi these have mainly been light across
northern Illinois. As an upper disturbance moves from south central Illinois
early this morning across northern Illinois/far southern WI this
afternoon it could help to focus an uptick in shower activity.
Confidence on this is low though as this feature is convectively-
enhanced from storms last night. Slightly better chances for
shower activity will be seen at rfd...but these could reach
Chicago area airports as well.

Winds will be on the increase from the south-southwest today as
the pressure gradient tightens. High convective cloud
debris...along with possibly more cloud development...could limit
the gust magnitude some from what is in current tafs. Later this
evening...a low-level jet will develop over the area with 35-45 knots
flow above 1000 feet. With surface winds remaining around 12kt or so
tonight...true low-level wind shear is not forecast at this time.

With the development of the low-level jet will come the
possibility of scattered rain showers/thunderstorms and rain overnight across northern Illinois
and northwest in. As a cold front approaches from the northwest
and a series of upper disturbances pass over the area Monday
morning...the rain coverage is expected to be higher. Embedded
thunder is likely and thunderstorms and rain very well may be added into future tafs
for part of that time period. When the cold front passes early
Monday afternoon or so...winds will abruptly shift to the north-

//Ord and mdw confidence...updated 14z...

* medium for winds through the period.
* Low for rain showers this afternoon.
* Medium for rain showers/thunderstorms and rain this evening through Monday morning.
* High for wind shift Monday afternoon...medium for timing. Cms

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

Monday night-Tuesday...rain showers with periods of thunderstorms and rain likely. MVFR
likely at times. East-northeast winds.
Wednesday...chance of thunderstorms and rain. Southeast winds.
Thursday...VFR. South winds.
Friday-Saturday...chance of thunderstorms and rain. South winds.



135 am CDT

South-southwest winds will increase across the Great Lakes region
today as the pressure gradient tightens in advance of an
approaching cold front. The offshore component along the western
shore of the lake will be enough to warrant a Small Craft
Advisory from late morning through the afternoon.

Low pressure far to the north of the Great Lakes will usher the
cold front into the western lakes region later tonight. This front
will ease its way southeastward across Lake Michigan by early
afternoon Monday turning winds to the north in its wake. With the
front forecast to stall just south of Lake Michigan...winds across
the lake will turn easterly later Monday night and through
Tuesday. The front will then return northward as a warm front
Tuesday night and Wednesday. Southerly winds are then expected for
much of the rest of the week.



Lot watches/warnings/advisories...
Lm...Small Craft Advisory...Illinois nearshore waters until 7 PM Sunday.



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

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

National Weather Service Glossary of Abbreviations