Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1003 PM CDT Sat may 18 2013
Discussion...
252 PM...multiple forecast concerns and challenges continue
through the middle of next week including severe potential Sunday
afternoon through Monday night...heavy rain potential especially
Monday night through Tuesday night along with associated cloud
cover...winds off the Lake/Lake breezes and their affects on
temperatures.
In the short term...low levels are now quite moist with dewpoints
generally in the lower 60s west of the lake breeze. Broken to at
times overcast cumulus field developed earlier this morning which
helped but the brakes on temperatures making it into the lower 80s...
though still a few locations tagging 80. Lack of instability
however has limited shower development with isolated showers now
developing south of the Kankakee River. Other than an isolated
sprinkle/shower...dry weather expected through Sunday morning.
With continued light winds and clearing skies as the cumulus dissipate
this evening...expect at least some patchy fog development as
temperatures cool into the lower 60s dewpoints. Though confidence
regarding fog development is low.
Convective trends/potential from Sunday afternoon through Monday
night are fairly uncertain. However there appears to be increasing
consensus...both with timing and location...of a shortwave lifting
northeast across Iowa into southern Wisconsin Sunday afternoon
into Sunday evening with increasing chances for thunderstorms
Sunday afternoon/evening. How far east this activity may spread
appears to be the most uncertain and probability of precipitation may need to be adjusted
as well as increased as trends emerge. With the upper ridge
building across the region late tonight...expect high temperatures
Sunday to be well into the middle/upper 80s with perhaps a few
locations tagging 90. These temperatures combined with dewpoints creeping
into the upper 60s...perhaps even 70 in some locations...will
provide plenty of instability if thunderstorms are able to spread
into the County Warning Area.
As time GOES on...confidence decreases regarding convective
trends as timing differences and locations of various waves then
impact future potential convective time periods. With enough
sunshine and enough of a dry period midday...expect Monday could
be as warm perhaps a few degrees warmer than Sunday with highs
back in the middle/upper 80s. Also still looks breezy which could
lower dewpoints some in the afternoon. Should convection be able
to fire...atmosphere remains unstable and shear would support
continued severe threat.
As the upper low over the plains slowly spins east into the
western lakes Tuesday and Wednesday...this seems to be the best
timing for a more prolonged/continued heavy rain threat. Any of
the storms that form through the middle of next week will have the
potential to produce heavy rain...but waves of more widespread
storms with heavy rain appear to be possible late Monday night
into Tuesday and then again Tuesday night into Wednesday.
Confidence regarding rainfall amounts remains low however with
model quantitative precipitation forecast storm total amounts lower than 24 hours ago. Of
course...this doesn/T downplay the potential for what could end up
being several inches of rain in parts of the area by midweek...
just adds to low confidence regarding location/amounts.
Low temperatures appear to be rather warm in the 60s into Wednesday
morning with highs potentially in the upper 70s/lower 80s both
Tuesday and Wednesday though cloud cover/precipitation duration/amounts
will determine just how warm temperatures are able to go. Lake breezes
and outflows from convective waves will also make temperature
forecasts difficult. As the main surface low passes east of the
area Thursday/Thursday night...a cold front will pass through the
region...ending the precipitation with much cooler and drier air expected
to spread across the Great Lakes region into the beginning of the
Holiday weekend. Cms
&&
Aviation...
//Ord and mdw concerns...updated 03z...
* MVFR fog potential late tonight/early Sun morning.
* South-southeast winds gusting to 15-20 knots Sun afternoon.
* A chance for thunderstorms and rain sun evening.
//Discussion...updated 00z...
Low pressure over the Central Plains will develop slowly east and
north through the first half of the week. A remnant stationary
boundary across the area...coupled with the lake breeze
initially...will keep winds east-southeast over Ord and mdw through tonight.
Speeds should diminish quickly by middle-evening which coupled with clear
skies and elevated low-level moisture will aid in another night
of likely areas of MVFR fog. While some wind component generally
does not favor fog this time of year at Ord and mdw
climatologically...the temperatures should near the dew points regardless
by overnight given the high dew points moving in.
Sunday will see southeast to south-southeast winds increase with some gustiness in
the afternoon. Modest to high instability will develop and should
lead to a fairly widespread cumulus field. Focus for any storms looks to
be to the west of the taf sites through middle afternoon...but by
late in the day into the evening there is a general trend in model
guidance to show steadily improving chances across northern Illinois.
Any convection is favored to be mainly scattered tomorrow late
afternoon into the evening. Could be a few gusty storms toward rfd.
//Ord and mdw confidence...updated 03z...
* medium confidence on MVFR visby late tonight but high confidence
on timing if it occurs.
* High confidence on wind trends on Sunday...as lake breeze should
remain east during most and likely all of the afternoon.
* Low confidence on thunder chances sun evening.
//Outlook for Ord/mdw for 12z Monday-12z Saturday...updated 00z...
Monday-Tuesday...likely periods of thunderstorms and rain in the region but with some
if not more breaks as well. Still too early
to refine more precise timing. Gusty south-southwest winds
on Monday.
Wednesday...likely thunderstorms and rain during the day with a chance of rain showers
during the night.
Thursday...chance of rain showers.
Friday...VFR likely.
Mtf
&&
Marine...
151 PM CDT
A surface ridge of high pressure continues to build
eastward across eastern Quebec and the northeastern Continental U.S. This
afternoon. However...attention is quickly turning out west across
the Central High plains...where an area of low pressure is taking
shape this afternoon. This storm system is expected to move
east-northeastward towards the middle Missouri Valley by Sunday
evening. An associated surface warm front...extending eastward
across the middle Mississippi Valley...will move northward across at
least the southern half to two thirds of the lake Sunday night and
Monday. Behind this front winds looks to increase out
of the south in the 15 to 25 knots range Sunday night into Monday.
However...to the north of this front...winds will primarily be
more easterly. There is some uncertainty in how far north up the lake
this warm front will reach. It is possible that it never reaches the
northern third of the lake...which would result in easterly winds
remaining in place there during the period.
The other concern will be the threat for periods of dense marine fog
Sunday night and into at least the first part of the coming week. As
this warm front lifts over the lake...much higher dew points will
move over the relatively cold water...possibly resulting in some
fog development.
It appears that the surface low will slowly move eastward across the
middle Mississippi Valley through middle week and then across the southern
lakes and the Ohio Valley later in the week. This should allow warm
front to shift back southward with time during the week. During this
time...the wind directions will primarily become east northeastward
15 to 25 knots north of the front. Later in the week...as the surface
low passes east of Lake Michigan...the winds look to be rather Stout
up to 25 to possibly as high as 30 knots out of the north for a period
as a good area of high pressure builds eastward across Ontario and
the upper Great Lakes region.
Kjb
&&
Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...none.
&&
$$
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