Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1009 PM CDT sun Aug 2 2015
915 PM CDT
Line segment of storms moving into Northeast McHenry and Northern
Lake County has pockets of stronger winds on kmkx and tmke radar.
Overall reflectivity signature does not look the greatest...but
these pockets of winds...along with transient Couplets at times in
far southern Wisconsin...are enough to keep a close eye on this
area. Low-level wind shear remains 20-30 knots in the 0-1km layer based
on mkx VAD wind profiler and rap analysis. In addition...the
boundary remains mixed given surface gusts to 25 to 30 miles per hour...so
areas of wind damage will continue to be a threat and certainly a
non-zero tornado threat too. The earlier Tornado Warning in Lake
County was for a discrete supercell...which was different Mode
than the storms moving in...however indicates the potential for at
least a brief threat of this amongst the higher wind threat.
Storms should move through downtown Chicago between 1015-1130 PM
151 PM CDT
A few isolated showers/storms are working there way eastward across the
area. These storms are holding together to some degree as
instability is favorable. The storm in DeKalb County has produced
some hail of nickel to quarter size. This storm will continue to
propagate southeastward along the Interstate 88 corridor. Shear is
weaker closer to the lake...so a downward trend is possible...but
it will likely hold its own for a bit longer. There is quite a bit
of inter-cloud lightning with this storm...with occasional/frequent
cloud to ground lightning as well.
We do still expect afternoon activity to remain isolated as water vapor
imagery suggests the better shortwave energy is passing through
central Illinois...and even more so across Wisconsin. Still...weak
energy in the northwest flow pattern keeps the isolated/widely
scattered storm chance in place in an uncapped atmosphere. Shear
is still low this afternoon...so the severe threat at least
through middle afternoon appears on the low side...though these
storms could locally enhance the already synoptically modest wind
field and may have some marginally severe hail.
For the rest of the area...just warm and breezy conditions remain
in place across the area and readings in upper 80s/low 90s.
Afternoon mixing is keeping dewpoints from getting out of the low
to middle 60s...except in favored outlying areas where aided
evapotranspiration locally enhances the moisture readings.
More organized thunderstorms are expected to spread south and eastward
toward the area ahead of better height falls and a modest cold
front. Still some concerns over how widespread the convective
coverage will be this evening. Best forcing is tied to the cold
front which will be coming through the area just after peak
heating time. For storms that do form...ample shear and
instability will be present for storms to quickly become
severe...and outflow boundaries from storms to our north coupled
with the cold frontal push will complicate matters. Aided
convergence/lift can potentially be enhanced closer to the lake
where additional momentum may keep the higher chances for storms
north and east closer to the lake...and potentially in our
southwest zones where dewpoints/instability may be a little
higher. Convective allowing models painting quite a range of
solutions from a forward propagating mesoscale convective system in our southwest areas
and very little closer to Chicago...with alternate solutions
suggesting the former scenario with lake adjacent counties
favored. The cold front will push southeastward through the area
tonight...with thunderstorms exiting to the southeast.
344 PM CDT
Monday through Sunday...
Period will begin with surface trough/frontal boundary departing
to the east...with associated precipitation likewise departing to the
east. Cooler and drier conditions still appear likely for the
beginning of this upcoming week as large upper level low settles
into southern Canada and the northeast Continental U.S.. no precipitation expected
Monday into Tuesday...with temperatures in the low 80s likely both days.
Models quickly diverge with approaching system by midweek...with
the GFS the outlier bringing stronger/more defined low closer to
the County Warning Area. This would provide a further northward push of precipitation
axis especially during the Wednesday time frame. With uncertainty
in place and with some guidance keeping the County Warning Area dry...have made
little changes to the going forecast and kept low chance probability of precipitation for
the southern half of the County Warning Area. After this possible precipitation...a drier
trend is looking likely towards the end of the work week while
below normal temperatures occur.
//Ord and mdw concerns...updated 03z...
* thunderstorms and rain impacting the local terminals over the next couple of
Forecast for the evening remains a bit uncertain even at this
late hour. Satellite pics show a line of clouds developing along a
cold front across southern WI and dropping south. Considerable
instability ahead of this front across northern Illinois would be
supportive of widespread thunderstorms and rain activity if and when storms do
manage to pop through a weak cap. Motion of the front would carry
the activity through fairly rapidly...with VFR and west to northwest winds
in its wake. Timing around 02z into the terminals still looks
reasonable...and allows for possible isolated activity beforehand
and lingering showers for a short while after.
//Ord and mdw confidence...updated 03z...
* medium to high confidence in timing and duration of thunderstorms and rain. Lower
confidence regarding ceilings and visibilities.
* Higher confidence for all elements after evening convection.
//outlook for Ord/mdw for 12z Tuesday-12z Sunday...updated 00z...
Tuesday...VFR. Northwest winds.
Wednesday...mainly VFR. Chance of showers late.
Thursday...VFR. Chance of shra/tsra.
Friday and Saturday...VFR.
440 PM CDT
Strong southwest winds will continue late this afternoon into the
evening across much of the lake including the nearshore areas...as
strong low pressure moves through the Great Lakes region. Highest
speeds are occurring across the southern half of the lake and
especially along the nearshore where a few gale force gusts are
also possible. This will persist before winds diminish late this
evening and into the overnight hours as this low exits to the
east...with winds becoming more west northwest.
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