Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
634 PM CDT Wednesday may 27 2015
307 PM CDT
Main forecast concerns/challenges are with shower/thunderstorm
trends by the end of the work week into the weekend...and then
with cooler temperatures expected this weekend into the start of next
Cold front which pushed through the County Warning Area today continues off to
the east this afternoon...while middle/upper level trough over the
upper Midwest and Great Lakes lifts to the east/northeast.
Moderate instability which was in place today has weakened and
will further weaken this afternoon into the evening...as 50 degree
dewpoint air moves across the County Warning Area. Any more robust
shower/thunderstorm development this afternoon will remain east of
the area...with any isolated showery activity to diminish here in
the near term. Cloud cover will likewise diminish this
evening...with mostly clear skies likely tonight.
In the wake of the departing middle/upper level trough...rising
heights aloft will help set the stage for a dry/sunny day with
warm and above normal temperatures in the low to middle 80s on Thursday. The
exception will be for areas near the lake in Illinois where
cooler temperatures in the 70s will be observed with onshore flow. A
little uncertainty with how high the temperatures will get in this
location before the winds turn...but think temperatures will have a good
chance of rising quickly in the morning before this occurs. So did
bump up temperatures and even for areas near the lake in Indiana...where
southerly flow should allow less of an impact from the lake.
307 PM CDT
Thursday night through Wednesday...
With steering flow keeping most of the energy well north of the
County Warning Area Thursday night...expect dry conditions Thursday evening.
However...northeastward moving short wave energy and veering low level jet
could allow for some precipitation to move across northern Illinois. With
this possibility did leave chance probability of precipitation across north central
Illinois...but with most areas likely remaining dry during this
period. Instability does not look to be overly great during this
period and am not too concerned with thunder...but with at least
weak instability and a decent low level jet...did leave mention of thunder
in the grids. Upper level trough to the west/northwest will dip
across the region on Friday and allow for this more energetic flow
to draw closer to the County Warning Area. Although better low/middle level
convergence should remain to the north in Wisconsin through the
day Friday...increasing instability with little cin in the
presence of 60 degree dewpoints along with increasing large scale
lift should allow for scattered shower/thunderstorm development.
Initially this will be for areas across north central
Illinois...but do think that at least isolated to widely scattered
development will be possible for remaining areas in the afternoon.
Weaker middle and upper level flow should limit intensity of any
development...although cannot rule out isolated stronger storm
with strong gusts the main hazard. Also...increasing moisture will
once again help for heavier downpours during this time.
Friday night into Saturday morning appear to be the best periods
for widespread precipitation development and have increased to
categorical probability of precipitation during this period. This will occur as the middle
level trough and associated surface low and front push through.
With flow still remaining weak and with diminishing instability
throughout the night...not overly concerned with widespread severe
storms. However...stronger forcing could still allow for isolated
strong development. With an abundance of moisture still in
place...heavier rain would likely be the main hazard. Precipitation will
likely linger Saturday morning but pushing south during this time.
On the backside of this front/precip...stronger northerly winds
will allow much cooler air to move over the region...with temperatures
likely falling throughout the day.
//Ord and mdw concerns...updated 00z...
* easterly winds around 10 knots on Thursday afternoon.
Winds near the lake across northeastern Illinois are switching out
of the east-northeast as wind speeds drop in the area of a surface
ridge of high pressure. Wind directions may briefly switch at kord
and kmdw over the next hour. Confidence with this remains low...but
it appears that even if this does occur...wind speeds will remain
well below 10 knots...so operational impacts look unlikely. Otherwise
light and variable winds are expected the rest of tonight.
On Thursday the surface pressure gradient will support a
southeasterly wind component. However...the winds will likely become
more easterly off the lake during the afternoon...especially at the
eastern taf sites. Wind speeds are expected to be up around 10 knots.
//Ord and mdw confidence...updated 00z...
* high for all elements.
//outlook for Ord/mdw for 12z Friday-12z Wednesday...updated 00z...
Friday...periodic shra/tsra. MVFR possible. Southwest winds.
Saturday...chance rain showers. MVFR possible. Strong north-northeast
Sunday...VFR/wx none. Northeast winds.
Monday...VFR/wx none. East winds.
Tuesday...VFR/wx none. South winds.
225 PM CDT
Generally light to moderate southerly flow expected through tomorrow
night with southerly winds increasing some Friday in advance of
approaching cold front. Cold front will drop down the lake Friday
night with an unseasonably strong Canadian high building into the
region in its wake. A healthy late season shot of chilly air along
with the tight gradient should set the stage for a period of strong
full fetch north winds down the lake Saturday into Saturday night.
Forecast soundings suggest the mixed layer will be shallow but do
show gale force winds within the mixed layer that should be
transported to the surface...especially close to the shore where
water temperatures are warmer and conditions likely to be more unstable.
While wind and gust grids dont reflect this...the strongest winds
should occur within about 5-8mi of the shore over southern third of
the Lake. A little early to hoist a gale watch at this point...but
certainly something that may need to be considered soon.
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