Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
233 am CDT Wednesday Jul 29 2015
915 PM CDT
Main concern is with convection and associated heavy rain threat
late this evening and overnight. Dvn 00z radiosonde observation sampled very moist
airmass with precipitable water value of 2.02 inches. Storm Prediction Center mesoanalysis is
handling moist axis from southern half of Iowa to western Illinois
containing precipitable waters over 2 inches well and southwesterly 925-850 mb
low level jet will transport this moisture farther eastward.
Ascent ahead of midlevel vorticity maximum responsible for widespread
convection over southern Iowa and convergence on nose of low level
jet should result in increasing coverage of generally scattered
thunderstorms late this evening through the overnight from west
northwest to southeast.
In addition...there will be better low level moisture convergence
as cold front trailing from exceptionally deep low over southern
Manitoba approaches toward daybreak. Despite diurnally unfavorable
timing...afforementioned supporting factors mean that there should
be at least scattered coverage. However best focus for most
widespread rain showers/thunderstorms and rain coverage could be at and south of latitude of
current Iowa convection along axis of highest instability...in
southern half or third of County Warning Area. Have mentioned heavy rain with any
thunderstorms late this evening through the overnight given very
moist airmass. Have not issued Flash Flood Watch due to some
uncertainty on evolution but will likely need to monitor for
flooding...especially if any convection can train over the same
246 PM CDT
For the remainder of the afternoon...focus will be on temperatures
over the eastern portions of the County Warning Area and precipitation trends for the remainder
of the afternoon. A decaying mesoscale convective system continues to push through
northwestern/north central Illinois. The only active ts are approaching western La Salle and
Livingston counties with some trailing scattered rain showers to the north and
west. Latest radar trends indicate that the residual stratiform rain
over far northwestern Illinois is diminishing and even the leading rain showers/thunderstorms and rain are
on a waning trend as the activity moves to the southeast through the
favorable area of upper diffluence...but is at the same
time...moving away from the main shortwave which is lifting northeastward
into the upper Great Lakes. At this point...it is unlikely that any
residual showers will reach the Chicago metropolitan area. Temperature
trends are generally on track with temperatures in the upper 80s
inland of a pesky lake breeze northestern Illinois and away from the rain cooled
air over northwestern Illinois. South of the Illinois and Kankakee rivers...where
temperatures in the upper 80s and dewpoints in the middle 70s have combined to
produce heat indices at or slightly above 100f.
For tonight...concern will shift to thunderstorms expected to
develop in association with a cold front approaching from the west.
As of around 2pm CDT...the cold front extended from deep low pressure
over southern Saskatchewan...through the upper Missouri Valley and then
back into the Texas Panhandle. A quasi-stationary front waving across
central Iowa...through central Illinois/in is also acting as a lifting
mechanism for an east-west oriented band of ts over central Iowa...and
suspect that this boundary is helping to maintain some ts over central
Illinois...in the Bloomington area. While the environment over eastern
Iowa/northwestern Illinois is currently worked over from earlier convection...expect
that the environment will become more conducive to ts again through
the evening as the cold front begins to move from northwest to southeast across
the County Warning Area. While the region remains in the warm sector ahead of the
cold front...the region will remain under a very moist
airmass...with dewpoints remaining in the lower to middle 70s. The
quasi-stationary should be the initial focusing mechanism for some
elevated ts...but the cold front should quickly take over as the
main focusing mechanism for more surface based convection. The wild
card in the deck is the quasi-stationary front over central Illinois. The
interaction of this boundary with the cold front could add some
enhanced surface focusing to the convergence associated with the cold
front...leading to the potential for some stronger...and possible
severe thunderstorms over the far southern portions of the County Warning Area. The
other threat will be some locally heavy rainfall with the potential
for echo training along the east-west oriented boundary. Confidence
is not very high on the location of any potential echo
training...but should it develop...the potential will exist for some
localized flooding or flash flooding.
252 PM CDT
Wednesday through Tuesday...
As the deep summertime low progresses across Ontario on Wednesday
the front will advance through the forecast area in the morning. At
least ongoing scattered showers and possibly some storms are
expected at daybreak within the pre-trough deep moist plume and
glancing upper forcing. That said...the trend last night was for
regional convection to be further south than high-res guidance had
indicated and already seeing that trend today...so it is certainly
possible than any morning activity is mainly south of I-80.
Confidence remains high that the boundary will be south of the area
by peak heating and convective regeneration time on Wednesday
afternoon...so continue to drop probability of precipitation off fairly quickly during the
late morning...keeping only a small chance in the far southeast in
With the parent low well to the north on Wednesday...the 850-925mb
thermal ridge becomes elongated over the area and not shunted
southward at as great of speed as the surface front. Given clearing
by early Wednesday afternoon and very mild starting point to
temperatures to start the day...highs should reach middle to upper 80s
with 90 possible...especially if dew points can drop some.
High pressure from the northern plains through the lower Ohio Valley
on Thursday should provide quiet weather with a west-northwest wind
continuing to not allow much of or any lake breeze. A weak northern
stream cool front is forecast by the suite of available guidance to
inch southward through the Great Lakes region on Friday...with a
slightly better defined frontal boundary later in the weekend or
early next week. Dew points should gradually climb through the 60s
into the weekend and may allow that second boundary a better chance
to generate some storms. Temperatures look to remain slightly above
normal late this week and more at normal this weekend.
//Ord and mdw concerns...updated 06z...
* scattered thunderstorms and rain early this morning.
* Possible MVFR ceilings this morning.
* Gusty west/northwest winds today. Cms
A line of thunderstorms extends from near dkb southwest to vys and
just west of pia. Short term/hi res guidance seems to have this
handled fairly well and continues to move it across the area early
this morning. How far north this activity will spread/develop is
still uncertain but close enough to include tempo thunderstorms and rain at all the
Chicago area terminals for a few hours this morning. High moisture
air will allow outflow/gusts from these storms to make it to the
surface...so some gusty winds will be possible with the strongest
storms...along with torrential rainfall which may briefly reduce
ceilings/visible to IFR.
Further west...a large area of rain with embedded thunderstorms
looks like it will mainly move southeast and slowly weaken so not
completely sure how much of this will impact the terminals around
and after sunrise. Maintained vicinity showers for now but may
end up being dry.
Patchy MVFR ceilings have already developed west of this convection
and extend back into Iowa. There will likely be a period of MVFR
ceilings later this morning...but should be fairly brief in
duration...perhaps only a few hours and confidence is low
regarding scattered vs. Broken.
Light southerly winds to start will shift more westerly toward
daybreak with speeds slowly increasing but as clouds begin to
scatter this morning and drier air spreads across the region...
low levels will mix and west/northwest winds will increase with
gusts into the upper teens...perhaps low 20kt range...which will
then diminish with sunset this evening. Cms
//Ord and mdw confidence...updated 06z...
* high for thunderstorms and rain early this morning...medium for coverage/timing.
* Medium for MVFR ceilings this morning.
* High for winds through the period. Cms
//outlook for Ord/mdw for 12z Thursday-12z Tuesday...updated 00z...
* Thursday through Saturday...dry/VFR. Westerly winds.
* Sunday...chance of thunderstorms and rain. Southwest winds.
* Monday...chance of thunderstorms and Rain. Lake breeze possible.
233 am...deep low pressure over Manitoba will move northeast to
Hudson Bay Thursday as a trailing cold front moves across the lake
today. Southerly winds 15-25 kts over the northern portion of the
lake will remain at those speeds but only gradually turn westerly
by this evening. This low will remain over Hudson Bay through the
end of the week...while slowly weakening. High pressure will build
across the Central Plains today and tonight and gradually spread
into the Ohio Valley Thursday and Friday. This will maintain a
westerly flow across Lake Michigan with periods of 15-25 kts
possible into Thursday. The gradient will then relax with speeds
diminishing. A weak trough/front will move across the lake Friday
night...briefly turning winds to the northwest before turning back
to the southwest Saturday. Cms
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