Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
648 PM CDT Friday Aug 29 2014
411 PM CDT
Thunderstorms are generally isolated in northeast Illinois this
afternoon with scattered activity east of Interstate 57 in
illnois and along the Interstate 65 corridor in Northwest Indiana. Radar
estimates a quick three quarters of an inch to 1 inch has fallen
in the heavier downpours...with street flooding reported in
Lansing. Expect heavy rain and localized ponding of water to
remain a threat into the early evening...especially in Northwest
Indiana...though storms may increase as the warm front to the
south and deeper moisture spread northward.
221 PM CDT
Synopsis...unsettled weather continues through tomorrow with
scattered thunderstorms expected. Confidence remains low in when
and where thunderstorms will form. Sunday still looks dry but
multiple chances for storms return Monday through the end of the
week. Above normal temperatures are also expected through the period
//Near term forecast//
a large ridge is over the East Coast while a trough is over the
plains. The trough will weaken as it approaches our area and pass
overhead late Saturday night into Sunday morning. At the surface the
high has pushed east while a developing low pressure system is over
Minnesota. The lows warm front lies to our south across southern
Illinois...and it will lift north through the region this afternoon.
Have very low confidence in exact timing and location of
thunderstorms through this evening. There is no clear forcing
mechanism but with around 2000 j/kg of cape available...storms are
certainly possible. Precipitable water values are around 2 inches so the main
concern remains heavy downpours. The strongest storms may produce
wind gusts to 50 knots but no severe weather is expected as wind shear
is basically none.
For probability of precipitation used a blend of near term guidance which has been doing
reasonably well this morning. However...kept a chance of storms
across the region this afternoon and evening due to the scattered
nature of the storms that are expected. A wash out is not
expected. The best chance for widespread storms will be overnight
as the upper level trough approaches. In fact the storms associated
with the trough stretch from Minnesota through Arkansas. Models
differ in how well this area of precipitation will hold together so limited
probability of precipitation to the likely category.
Lows will be in the upper 60s to lower 70s tonight and fog is
possible. Given that we have not seen much rain today...have limited
confidence so left fog out of the forecast for now.
Saturday and Saturday night...
the surface low passes over Lake Michigan Saturday afternoon while
its cold front gets dragged through the area Saturday
afternoon/evening. Scattered storms are again possible in the
morning in the warm and moist environment with lingering 2 inch precipitable water
values. Storms will then form along the cold front south of a Gary
in to Pontiac Illinois line. Storms are possible north and west of this
line...but confidence is significantly lower.
Cape values will be around 1000 j/kg tomorrow with weak shear once
again. Therefore expecting storms to produce heavy downpours and the
strongest may be capable of gusty winds. Severe weather is not
Tomorrow looks like another warm and sticky day ahead of the cold
front. Lows will be in the 60s and fog is possible overnight.
expect a brief lull in the precipitation chances during much of the day
Sunday as the first cold front departs to the east and another low
takes shape over the northern plains. In between these two
systems...weak induced surface high should result in quiet weather
locally...with the 850 mb thermal ridge advecting overheard throughout
the day. Return flow will help push the dewpoints back up to around
70 with afternoon high temperatures expected to be in the middle 80s. Steep
low/middle level lapse rates will result in moderate instability
developing during the afternoon...but modest sbcin is prognosticated to
stay in place capping any surface based convection. In addition...
the area should be squarely under area of middle level height rises
which should help reinforce the inversion...and no significant
forcing noted over the area.
troughing currently moving onshore over the Pacific northwest today will
move across the Central Plains this weekend...then begin to lift
across the upper Midwest early next week. Surface low is prognosticated to
deepen from the northern plains Sunday as it lifts to near Hudson
Bay late on Monday. This will drag a cold front across the region on
Monday. With the low lifting away from the area...convergence along
the front is prognosticated to weaken as it comes across northern Illinois
early Monday with the better dynamics staying to our north.
Meanwhile...the local area appears to lie on the northern periphery
of the better instability to our south. GFS shows some hints that
particularly north central Illinois may be split by the
precipitation...especially considering the capping early in the day...but
will hold off on fine tuning this detail for now. In addition...the
region does fall within an overlapped area of favorable
shear/instability that could potentially support a severe weather
Tuesday through Friday...
frontal boundary is prognosticated to stall across/near central Illinois
Tuesday gradually lifting back north across the forecast area by the
end of the work week as a series of upper level disturbances move
across the region. This will result in periodic chances for showers
and thunderstorms across the area...though timing and location
details will be difficult to pin down this far out given the
pattern. The atmosphere remains quite soupy in the vicinity of the
front where precipitable waters remain around 2 inches. Thus the heavy rain and
flooding threat will have to be closely monitored through much of
the upcoming week.
//Ord and mdw concerns...updated 00z...
* slight chance of brief wind shift to more easterly with ts
outflow/lake breeze early this evening
* isolated thunderstorms and rain mostly just avoiding terminals through middle evening
* chance of rain showers/thunderstorms and rain late evening through midday Saturday
Isolated thunderstorms and rain have been dotting the area early this evening with a
couple cells east of Ord/mdw helping push the lake breeze west a
bit. Expect lake breeze/outflow to stay east of terminals...but it
is within a couple miles of Ord now and there is a slight chance
it could push a bit farther west resulting in wind shift to a
more easterly direction for a short time early this evening.
Isolated thunderstorms and rain likely to linger through early evening...but current
trends suggest the storms will probably stay out of the immediate
vicinity of the terminals. Band of rain showers/thunderstorms and rain toward the
Mississippi River will likely weaken and could even dissipate
before arriving into the region late this evening into the early
overnight hours. Plausible that there could be additional
development behind this band overnight...but with no obvious focus
for precipitation have gone dry in tafs overnight into early Saturday
morning. Weak trough will move through the terminal midday
Saturday and could see rain showers/thunderstorms and rain develop with this
feature...though better chances look to be southeast of the
terminals later in the afternoon.
//Ord and mdw confidence...updated 00z...
* low-medium confidence in precipitation trends
* medium-high confidence in winds
//outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z...
Monday...period of thunderstorms and rain likely.
Tuesday and Wednesday...predominantly VFR.
Thursday...chance thunderstorms and rain.
221 PM CDT
Moderate southerly winds are in place today ahead of low pressure
over the northern plains. South to southwest winds 20 to 25 knots
with an occasional 30 knots gusts possible will be in place through
tomorrow...then a cold front will start to push across the lake
turning winds northwesterly. High pressure will provide a brief
lull in the winds Sunday allowing lake breezes to develop...but
then southerly winds ramp back up to 25 to 30 knots Sunday night into
Monday ahead of another low. This low is prognosticated to deepen as it
lifts from the northern plains Sunday to Hudson Bay late Monday.
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