Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
315 PM CDT Tuesday may 21 2013
Discussion...
1228 PM CDT
Morning severe weather potential statement update was sent after the 1630z Storm Prediction Center day one outlook was
received...in anticipation that the local area would be pulled
from the slight risk. This is in fact what occurred...as models
are showing less potential for the areas of greatest instability
to be aligned with deep layer shear that would be supportive of
organized convection.
Latest visible satellite imagery is still showing quite a wide area
with little cumulus development across Illinois as you get farther away from
the upper low positioned over the eastern Dakotas. Model forecast
soundings are similar to latest acars profiles in showing a weak
cap still in place between 850-800 mb or so...and then another
warm layer above 500 mb that might limit vertical development.
Latest infrared/WV image suggests the presence of a weak shortwave
lifting across eastern Iowa but also points to the best moisture
convergence being focused across the southern Midwest where
thunderstorm clusters have already developed. By tonight...diurnal
heating will be less favorable but better shortwave energy...surface
and midlevel moisture...and cooler temperatures aloft will support
an increased chance of thunderstorms and rain ahead of an approaching cold front.
Lenning
//prev 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 20z...
* strong gusty south to southwest winds through 00z.
* Slight chance thunderstorms and rain this afternoon and evening.
* Thunderstorms and rain likely after midnight.
Paw
//discussion...updated 18z...
In the wake of last nights showers and storms...things have
stabilized for now. Expect mostly clear skies through the morning
hours. By middle day expect atmosphere to begin to destabilize again
but dont see any real focus for thunderstorms to develop.
Scattered thunderstorms are possible in the afternoon and evening
but dont have enough confidence in timing or location to put
thunder in the tafs at this time. Wave rounding the base of the
plains trough over eastern nm this morning will lift northeast
toward Illinois late tonight. This should be the focus for more
widespread showers and thunderstorms late tonight into Wednesday
morning.
South winds will be increasing this morning and turning SW this
afternoon. Winds will diminish and gustiness will end this
evening. Winds will become light and variable late tonight as weak
low pressure moves overhead.
Allsopp
//Ord and mdw confidence...updated 20z...
* high confidence in wind trend.
* Low confidence in thunderstorms this afternoon and early
evening.
* Medium-high confidence in thunderstorms after midnight.
Paw
//outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z...
Wednesday night...chance rain showers. Possible MVFR.
Thursday...chance rain showers in the morning. VFR afternoon.
Friday...VFR.
Saturday - Monday...slight chance thunderstorms and rain.
Allsopp
&&
Marine...
314 PM CDT
Low pressure currently over southern Minnesota and central Wisconsin
will weaken and drift over the western Great Lakes tonight. The
lows cold front will pass south of the lake tonight into tomorrow.
Winds will diminish this evening therefore thinking the small craft
will be allowed to expire this evening as well.
As the low shifts east over the New England coast...northwest to
north gales will spread south across the lake. Thinking gales will
develop over the northern end of the lake late Wednesday
night...spread across down the lake on Thursday...then diminish
Thursday evening. Issued a gale watch as the exact timing and
strength of the gales is uncertain at this time. Model soundings
indicate there will be a strong cap over the lake which may limit
mixing. That being said guidance indicates 40+ knots winds will be
only 1200 feet above the surface so thinking gales are certainly
possible.
A ridge of high pressure spreads over the lake Friday morning with
north winds diminishing and becoming east on Saturday.
Jee
&&
Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Small Craft Advisory...nearshore waters
until 10 PM Tuesday.
Gale watch...lmz740-lmz741-lmz742-lmz743-lmz744-lmz745...10 am
Thursday to 9 PM Thursday.
Gale watch...lmz080-lmz669-lmz671-lmz673-lmz675-lmz777-lmz779-
lmz870-lmz872-lmz874-lmz876-lmz878...3 am Thursday to 9 PM
Thursday.
Gale watch...lmz261-lmz362-lmz364-lmz366-lmz563-lmz565-lmz567-
lmz868...3 am Thursday to 7 PM Thursday.
&&
$$
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