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Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois
135 am CDT Thursday Aug 21 2014

819 PM CDT

For evening update...

Have made some fairly significant updates to shower/thunderstorm
probability of precipitation and timing overnight...based on observational data and high
res model trends over the past several hours. Overall...have
trimmed back probability of precipitation to isolated/scattered showers through the
remainder of this evening...with gradual increase in scattered
shower/thunderstorm coverage after midnight...but with greatest
potential looking like Thursday morning with expected mesoscale convective system remnants
arriving from Minnesota.

A stationary front stretches from South Dakota southeast across
northern Missouri and into the lower Ohio Valley this evening.
North of the surface area of clouds and isolated
showers has persisted within the elevated baroclinic zone...with a
couple of weak short waves noted topping middle-level short wave
ridge axis which was roughly from St Louis northwest into
southwest Minnesota. Model guidance has been very insistent in
developing strong convection along this elevated baroclinic zone
this evening into early Thursday morning...though comparison to
observed radar/satellite data suggests guidance has been over
forecasting extensive coverage of deep convection so far.

With the best low level jet flow and short wave forcing currently
across the plains and oriented into the middle-Missouri Valley
region...would anticipate the best area for strong convection to
develop over the next few hours would be from northeast
Nebraska/southeast South Dakota into western Minnesota/northwest
Iowa. Weaker forcing and weaker west-southwest low level flow into
baroclinic zone east of the Mississippi...which suggests less
intensive development here. Development still expected across the
area during the night as the low level west-southwesterly flow
strengthens a bit...though confidence is fairly low on amount of
coverage. Hrrr has consistently shown the scenario of an mesoscale convective system
developing out of anticipated convection over the previously
mentioned middle-upper Missouri Valley region overnight...which would
approach the forecast area from the northwest toward morning. With
all of this in mind...have trimmed back probability of precipitation overnight and
especially this evening...gradually increasing chances after
midnight. Maintained likely probability of precipitation Thursday morning...with mesoscale convective system
remnants expected to perhaps present the greatest rainfall
probability. 00z soundings upstream of our forecast area depict
precipitable waters in the 1.5-1.6 inch range so still a locally heavy rainfall
threat from stronger convection especially during the pre-dawn
through middle-morning Thursday period.

Also...with clouds and light west-southwest flow bumped min temperatures
just a bit overnight.


//prev discussion...
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 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 06z...

* periodic chances for thunderstorms through the period.

* Southwesterly winds gusting up to 20kt in the afternoon.


//discussion...updated 06z..

Primary forecast concern will remain the chances for thunderstorms
over the area. At issuance time...a line of showers and a few
thunderstorms was developing along a narrow corridor from around
rfd to just south of mdw and into northwestern Indiana. The precipitation is
developing along an elevated warm front...with the surface warm front
still to the south...extending from low pressuure over South
Dakota southeast to southern Illinois. Individual cells are tracking to the
southeast...with northwesterly flow aloft...but the area of precipitation is slowly
lifting to the north. Given the somewhat scattered nature of the
precipitation area...have added vcsh mention back to the tafs through 12z.
Have resisted the temptation to call is thunderstorms in the vicinity as there is little
lightning activity. However...ts should become more of an issue
for the terminals through the day tomorrow as the warm front
slowly lifts to the north. The warm front should lift through northern
Illinois/in through the day tomorrow...putting the region in the warm
sector of the system...with persistent warm/moist advection. The
environment should become increasingly unstable through the day
and additional thunderstorm activity is likely over the area...but
confidence remains low in the exact timing and coverage of the
impacts at the terminals. So...have only gone with a thunderstorms in the vicinity for the
morning hours...though confidence is higher that there should be
some ts around the area. Have kept the tafs dry for the afternoon
hours...with the surface warm front pushing into Wisconsin and an
absense of upper level forcing...though there still could be some
isolated-scattered shra/tsra. There should be a better chance for more
organized convection thusday night as the models are indicating a
rather vigorous shortwave overtopping the upper ridge over the
Mississippi Valley and turning southeast across northern Illinois. This
shortwave...coincident with the warm front which should be close
to the Wisconsin border should provide both surface and upper level
forcing in an already moist and unstable environment. So...have
introduced tempo ts for the evening hours and a prob30 for the
late night hours. As the forecast picture become a bit more
clear...prevailing ts timing will likely be added in the updates
later today.


//Ord and mdw confidence...updated 06z...

* medium to high confidence that there be periods of showers and
thunderstorms through the forecast period...but low in the
timing and coverage.

//Outlook for Ord/mdw for 12z Friday-12z Wednesday...updated 00z...

* Friday 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.



Lot watches/warnings/advisories...


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