Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
317 PM CDT Thursday may 28 2015
225 PM CDT
The forecast area finds itself between two stationary fronts
this afternoon with a weak ridge of high pressure sandwiched in
between directly over the area. This ridge has allowed a lake
breeze with Lakeside cooling at the Chicago shore about 14
degrees cooler than inland at Ord/mdw as of 200 PM. The front of
interest for our area tonight will be the one to the south which
will lift north as a warm front...bringing a return to muggy air
by daybreak...as well as keep nighttime temperatures mild...even
increasing them late tonight along the Lake Shore areas.
Multiple middle-level disturbances...some likely convectively
enhanced...are noted on water vapor and radar mosaic imagery across
the Central Plains this afternoon. One of these just north of Kansas
City as of 200 PM is well analyzed by the latest rap and this is
expected to propagate into Wisconsin overnight...with one or two
weaker impulses to its south approaching the area. With a 25-30 knots
low-level jet ahead of these and increasing moisture at that
height...there will likely be scattered showers or storms from
southwest Wisconsin into northwest and possibly north central
314 PM CDT
Friday through Thursday...
Main forecast concerns/challenges during the period are with
shower/thunderstorm chances Friday into Saturday...and then with
much cooler air to push across the area Saturday.
Some uncertainty as to how the period will begin...due to how
precipitation development will evolve late tonight into Friday morning.
The current upstream middle level trough over the Central Plains will
continue moving northeast through the County Warning Area Friday morning...with
lingering precipitation from the overnight still likely to affect
primarily the western half of the County Warning Area in north central Illinois.
Likely scenario will be for this development to continue east
through the morning. Confidence with overall coverage during
Friday morning is low...and have thus capped probability of precipitation at likelys.
Morning precipitation and even cloud cover should exit through late
morning and midday...with skies becoming more partly cloudy/partly
sunny. This will help with early destabilization with little to no
inhibition and with middle level energy returning during this
time...think development will once again be possible in the
afternoon. With a lack of good focus at the surface...think best
development will still be across north central Illinois where
better large scale ascent will reside. Weak middle/upper level flow
will be a limiting factor with how organized any storm will be and
although I dont think there will be any widespread severe
threat...an isolated strong to severe storm will still be possible
with strong winds and heavy rainfall the main hazards due to
increasing instability and moisture. 80 degree weather will be
likely Friday...but with temperatures likely being limited in any area
that observes more precipitation/cloud cover.
Still think that the best chances for widespread
shower/thunderstorm development will be Friday night into Saturday
morning. However...guidance is varying with regards to how the
night will evolve. This is likely due to larger trough over
southern Canada and northern plains moving east/southeast...while
a separate trough and several impulses works east through the
plains. Nonetheless...best low/middle level coverage will push
through the County Warning Area during this time while large scale forcing also
increases. So do think precipitation will push through the County Warning Area...but with
coverage and duration possibly lower in some locations. Thunder
should be decreasing in coverage throughout the night and
especially into Saturday morning as the instability axis shifts
south...with heavy rainfall the main hazard during this time. Any
lingering precipitation will continue south of the County Warning Area through midday
Saturday as surface trough and front push through the region.
Cloud cover will likely linger through much of Saturday while
winds/gusts increase throughout the day. High temperatures will likely
occur early Saturday morning with temperatures falling through the
day...with 40 degree weather likely for most locations in far
northeast Illinois and Northwest Indiana for much of the day.
//Ord and mdw concerns...updated 20z...
* lake breeze winds varying from 080-120 at 8-12kt gusting near
18kt this afternoon
* -shra becoming thunderstorms and rain Friday afternoon
Weak high pressure drifting eastward has influenced a southeast
flow. Lake Michigan has taken hold of the winds at Ord/mdw/gyy
with winds now easterly over 10kt. Observations have shown some
gusts to 15kt as well from the east to southeast. After 01z...the
winds will relax some to become less than 10kt. Overall flow will
also then become southerly as low pressure approaches from the
west early Friday. Some rain showers are expected and a canopy of
bkn100 ceilings are expected at most sites.
Timed out best chance for thunderstorms and rain based on instability increasing in
the 15-19z timeframe...along with the potential for a wave to
help lift things off during that time. Looking forward...a better
chance for widespread precipitation exists early Saturday.
//Ord and mdw confidence...updated 20z...
* high confidence in east winds through 01z with medium
confidence in wind speeds.
* Medium confidence in rain showers and thunderstorms and rain occurring Friday...with low
confidence on timing.
//outlook for Ord/mdw for 00z Saturday-12z Thursday...updated 12z...
Friday night...periodic shra/tsra. MVFR possible. Southwest winds.
Saturday....chance rain showers early. MVFR possible. Strong north-
Sunday...VFR/wx none. Northeast winds.
Monday...VFR/wx none. East winds.
Tuesday...VFR/wx none. Southeast winds.
Wednesday...slight chance rain showers. South winds.
225 PM CDT
A surface cold front draped from west-to-east across the northern
part of the lake will shift northward this evening. This will be a
source for fog along it and this has been seen on visible satellite
at times during Thursday. Even higher moisture will move northward
across the lake on Friday allowing for the possibility of fog as
well...especially where showers are not prevalent.
Low pressure will move east into Quebec on Friday night and steer
a cold front southeastward across the lake. As high pressure
builds southward from Ontario on Saturday...it will drive a strong
northerly winds for late may across the central and southern parts
of the lake. This will result in Small Craft Advisory conditions
for the Illinois and Indiana nearshore areas Saturday afternoon
and night with lingering high waves on Sunday.
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