Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
324 am CDT Tuesday may 21 2013
Discussion...
321 am CDT
Today through tonight...
main concern for today revolves around the potential for severe
weather...mainly along and east of a Pontiac to Chicago line.
Line of showers and thunderstorms from early this morning continues
to push east...stretching from lower Michigan south through southern
in. This area of convection had brought severe weather to portions
of northern Illinois last evening...associated with a tongue of
anomalously high moisture. Precipitable water values within this channel of
moisture hovered between 1.2-1.6" as some drying was noted across
northern Missouri/southern Iowa in the wake of the evenings
convection. Temperatures should hold up early this morning in the middle/upper
60s. Then the focus turns towards timing of convection and the
intensify...possibly severe weather again.
Guidance indicates another cap will develop and hold through most of
the morning timeframe...then the question is when this cap will
dissolve. Local guidance suggests the more favored zone for the
strongest instability will be along and east of a Pontiac to Gary
line...where lapse rates will steepen quickly by midday/afternoon.
The overall system that has produced the widespread severe weather
remains negatively tilted...which this type of a setup coupled with
instability and abnormally high moisture...has a tendency to lead to
severe weather occurrence. The challenge is trying to pin down the
timing/placement...and once again the lack of a trigger. A weak lobe
of vorticity does appear to be prognosticated to slide north through
southcentral Illinois during the afternoon hours...which may aid in initiation
of convection across the southeastern County warning forecast area during the afternoon/evening
hours. Prognosticated wind fields are more uniform...which should limit The
Hazards to a wind/hail threat. Expect any convection to form into a
line segments shortly after generation.
Temperatures will be difficult to pindown for this afternoon as well. Given the
recent rainfall...coupled with slightly cooler air aloft to keep
temperatures closer to the low 80s for much of the County warning forecast area. The far southeast
County warning forecast area may touch the middle 80s.
The Channel of moisture will continue to hover across the County warning forecast area
through the overnight hours...as dew points remain in the low/middle
60s. Have maintained likely probability of precipitation through the overnight hours...with a
continued thunderstorms mentioned. Feel that as time moves forward
this will be able to be fine tuned...and possibly limit convection
to
the eastern County warning forecast area.
Confidence in severe weather initiation...low/medium.
Wednesday through Thursday...
500mb trough axis finally arrives across overhead...however remains
in a negatively tilted state. It appears instability will be waining
a bit throughout the day...so have trimmed back to a slight chance of
thunder. This seems reasonable given the abundance of clouds. Temperatures
will be much cooler Wednesday...mainly in the middle 70s. For Thursday the 500mb
vorticity slows across the County warning forecast area...with cooler air slowly filtering into
the near surface environment. Winds will slowly turn northerly behind a
frontal boundary...then northeasterly early Thursday. This will allow
the marine environment to filter into far northeast Illinois...keeping
temperatures in the upper 50s Thursday. Anti-cyclonic flow is prognosticated to arrive
across the region Thursday aftn/eve...bringing an end to the precipitation.
Confidence...medium/high.
Friday through Monday...
ensembles have continued to suggest the departing 500mb trough will
bump up against a western Atlantic ridge. This will slow the
eastward progression of the trough...which should hold the
anti-cyclonic flow across the Great Lakes region through Friday night with
dry weather. Considerable differences on precipitation returning to the
region amongst guidance for Sat/Sat night...which is most likely a
reflection of the slowing trend and the middle-level ridge that is
prognosticated to build across the central Continental U.S. Into the Canadian prairies
Sat/sun. Much cooler air will remain in place for the start of the
extended...as temperatures struggle to reach the middle/upper 60s...and for
areas adjacent to Lake Michigan...temperatures may actually struggle to
reach the upper 50s as the cooler marine environment flows inland from
a northeast wind. Then temperatures will begin to warm for the weekend into
the upper 60s to near 70 degree. Then by early next week temperatures should
steadily warm into the low 70s. Have held onto mention of slight chance
probability of precipitation in the later periods of the extended...however it is possible
that as heights increase aloft that diffluent flow will inhibit the
precipitation potential. Could see things trending drier for the final few
periods of the extended.
Confidence...medium/high.
Beachler
&&
Aviation...
//Ord and mdw concerns...updated 06z...
* showers and a few embedded thunderstorms next few hours.
* Strong gusty south-southwest winds developing during the day Tuesday.
* Another round of thunderstorms likely Tuesday night.
Allsopp
//discussion...updated 06z...
Main line of thunderstorms has moved east into Northwest Indiana
and east central Illinois. Lingering showers back through central
Illinois will continue to spread north-northeast through the Chicago metropolitan area
for next few hours with some embedded thunder. MVFR ceilings/visibilities at
times in heavier showers but expect VFR conditions in breaks and
lighter showers. This entire area should dissipate and move out of
the area before daybreak. A second narrow line of showers and
thunderstorms over eastern Iowa appears to be weakening and don't
expect this to hold together long enough to impact Chicago.
Winds may flop around a bit next hour or two in the wake of the
main band of thunderstorms but expect prevailing winds to be south
around 10 kts through the overnight hours.
In the wake of these showers and thunderstorms...expect
subsidence/drying through the morning. Scattered thunderstorms are
not out of the question during the day Tuesday but expect the
main action to be associated with a short wave trough moving through
New Mexico...this will be rounding the base of the large plains
trough Tuesday afternoon and moving rapidly northeast Tuesday night.
Have maintained tempo thunder after 03z Wednesday.
Allsopp
//Ord and mdw confidence...updated 06z...
* medium-low confidence in ceiling/visibility next few hours in showers.
* Medium-high confidence in VFR conditions prevailing through the day Tuesday.
* Medium confidence in wind forecast next few hours.
* High confidence in wind forecast Tuesday.
Allsopp
//outlook for Ord/mdw for 12z Wednesday-12z Monday...updated 00z...
Wednesday...scattered rain showers/thunderstorms and rain and possible MVFR conditions.
Thursday...chance of rain showers. Gusty NE winds.
Friday...VFR likely.
Saturday...slt chance of thunderstorms and rain.
Sunday...chance of thunderstorms and rain.
Mtf
&&
Marine...
144 am CDT
Low pressure was over the ND/Minnesota border with a warm front extending
east across Northern Lake Michigan early this morning. This
will maintain south winds over all but the far north end of the
lake today. The low will move slowly east across the lake tonight
through Wednesday. In the wake of the low Wednesday night north
winds will increase over the Lake. A period of gale force north
winds is possible Thursday before winds diminish Thursday night.
High pressure will build into the upper Mississippi Valley and
western lakes Thursday night and Friday.
Allsopp
&&
Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&
$$
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