Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
325 PM CDT Wednesday Aug 20 2014
320 PM CDT
Main forecast concerns are with persistent chances for showers and
thunderstorms over the next several days...with some potential for
strong/severe storms but with higher confidence for heavy
rainfall. With confidence still lower on coverage and placement
of storms tonight...have not issued a Flood Watch at this time
but did issue an Special Weather Statement to raise awareness for this potential.
In the near term...quieter conditions ongoing this afternoon with
the bulk of the County Warning Area dry. Latest radar imagery depicting steady
stream of showers and thunderstorms still moving southeast through
eastern Iowa and west central Illinois...but with coverage and
intensity diminishing. Short term guidance still trying to bring
precipitation across north/northeast Illinois late this afternoon into
the early evening but dont really agree. Feel that conditions will
remain dry through early this evening for most locations with the
exception for areas along and southwest of line from Oregon to
Watseka...where steering flow could drift a stray shower or
thunderstorm briefly over that area. Elsewhere...brief and light
returns continue to show on radar over northeast Illinois in and
around weak convergence along lake breeze in far southeast
Wisconsin and far northeast Illinois. A lack of any inhibition
this afternoon and this weak convergence could allow for continued
development of these light showers this afternoon.
Tonight...increasing chances for showers/thunderstorms will occur
throughout the night as upstream upper level ridge continues to
build across the region with the ridge axis shifting east towards
the County Warning Area...and while surface warm front situated over the southern
County Warning Area lifts north. Upstream shortwave energy noted on current water
vapor imagery will ride along this ridge and move across the County Warning Area
tonight into early Thursday morning. Meanwhile...strengthening low level jet
will begin veering this evening and then be oriented towards the
County Warning Area later in the night. This will all provide focus for
thunderstorm development overnight especially as warm air advection ramps up with
the veering/strengthening low level jet. Weak to moderate instability axis
will be in place tonight with potential for some steeper middle lapse
rates...currently observed just south of the County Warning Area...to linger
tonight. Confidence is not overly great with regards to the
intensity of storms tonight but with the previously mentioned
forcing mechanisms coming together overhead tonight...would expect
the possibility for at least an isolated stronger storm to provide
a hail/wind threat. Feel that the best chances for this
possibility could occur over the south/southwest half of the
County Warning Area...where the steeper middle level lapse rates will possibly reside
per current analysis and sref guidance.
A growing concern will be for the potential of heavy rainfall and
localized flooding with these storms tonight into early Thursday.
Guidance continues to vary with regard to overall coverage and
exact placement of best development tonight...and with my
confidence not the greatest with regards to residence time of any
storm. Nonetheless...observed high precipitable water axis of 1.5 to 2 inches
currently observed just to the southwest of the County Warning Area will shift
northeast over the County Warning Area tonight...and with this highest axis
expected to be centered overhead tonight. With these high precipitable waters in
place and with surface dewpoints around 70 degree and 850mb dewpoints
approaching 15-16c...any storm will easily have the potential to
produce heavy downpours. Despite these concerns...I once again
dont have the greatest confidence of coverage/placement and have
decided to hold off on any Flood Watch at this time. This will
need to continue to be closely monitored this evening but in the
near term have decided to issue an Special Weather Statement for this possibility. Still
think that what occurred just to the west/southwest today could
easily occur tonight into Thursday over the County Warning Area...although with
the possibility for a slightly different setup providing
better/stronger thunderstorm development.
Any lingering storms Thursday morning will shift east and
northeast of the County Warning Area through middle day with a possibility for
continued heavy rainfall. These departing storms could allow for
clearing skies and the environment to destabilize once again
during middle day and early afternoon...with the possibility for
thunderstorms to redevelop along lifting frontal boundary.
Clearing will be key with potential for intensity of these storms
on Thursday but with strong to severe storms possible...especially
if a good amount of heating could occur. Focus for thunderstorms
will continue Thursday night as well...with high moisture content
air still in place. This will also continue concerns for heavy
rainfall across the area.
//Ord and mdw concerns...updated 20z...
* lake breeze turning winds east/northeast this evening.
* Low chance for thunderstorms and rain this afternoon/evening...then better chance
for thunderstorms and rain overnight.
A frontal boundary straddles central Illinois this afternoon with
convection ongoing near the front. SW to west-southwest winds are in place
across the terminals this afternoon with a few sporadic gusts to
around 20 knots. Winds will slowly veer to the west-northwest through the
afternoon and a Late Lake breeze is expected to push across the
terminals this evening turning winds to the northeast/east. A
cumulus field has developed across far northern and northeast
Illinois...north of the cirrus shield to our south and is showing
some signs of vertical growth. Several higher res models do hint at
isolated to widely scattered thunderstorms this afternoon where the
cumulus field is currently in place. Coverage should remain low enough to
preclude any mention in the taf at this time. Better chances for
thunderstorms will develop tonight as the front starts to lift
north. A low level jet will develop to our west with resulting
convection tracking east towards the terminals late tonight.
Thunderstorm probabilities will continue Thursday morning into early
afternoon with the frontal boundary over northern Illinois and
models suggesting that the area will remain unstable. Coverage is a
concern so will maintain a prob30 for now.
//Ord and mdw confidence...updated 20z...
* high confidence in winds through early this evening...then low
confidence in wind direction this evening and overnight.
* Low confidence in thunderstorms and rain this afternoon/evening...then medium-high
confidence in thunderstorms and rain late tonight into Thursday morning.
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
* Thursday night through Tuesday...periodic chances for thunderstorms and rain.
158 PM CDT
Fairly light wind regime is expected through the weekend with weak
pressure gradient expected with high pressure over the St Lawrence
valleys region and a trough of low pressure over the northern and
Central Plains. A quasi-stationary warm front will extend east from
the low into the lower Great Lakes region...with the position of the
warm front likely being modulated at times by convective complexes.
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