Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
307 PM CDT Wednesday may 22 2013
Discussion...
307 PM CDT
Synopsis...today and tomorrow morning look like the last of the
active weather through at least late this week as a blocking pattern
sets up aloft. Below normal temperatures are expected tomorrow as the upper
level trough passes overhead...but temperatures return to near normal for
Memorial Day.
Today through Thursday...
the surface low stretches from NE Iowa through Michigan this
afternoon while its cold front lies in western MO and eastern Kansas.
The low and the upper level trough will continue to weaken with the
low passing through northern Illinois tonight. Fog is expected across the
lake and may move onshore tonight as winds turn northeast. Not
expecting the fog to move too far inland...maybe an mile or two.
For precipitation...the majority of convection has moved out of the
County Warning Area...but another round of showers and possibly some thunder is
possible this afternoon and overnight. A break in the cloud cover
has allowed some isolated convective showers to form Mendota south
through southern Illinois. Low level lapse rates have increased to 7
degrees c per km or greater in the areas that have seen the sun.
Therefore expecting scattered showers and a few thunderstorms to
move into areas along and southeast of I-55 this afternoon.
Dewpoints along this line are in the middle to upper 50s...but thinking
these showers will also produce brief heavy downpours like the
showers and storms we experienced this morning.
Another axis of rain will stretch from northern Michigan through
southern WI and northeast Iowa on the backside of the low. The line
of showers will gradually sink south...but thinking the rain will
not infiltrate northern Illinois until after midnight tonight. The rain
will likely linger into Thursday morning and gradually exit to the
southeast as the low moves out of the region. Clouds will quickly
erode with it leading to a mostly sunny late afternoon/evening
Thursday.
For temperatures...much colder air funnels in tonight and tomorrow as the
upper level trough passes overhead. High temperatures tomorrow will be
much cooler in the 50s and 60s...and cooler by the lake due to
onshore flow. Temperatures Thursday night are another concern given the
light winds and clear skies. Went a degree or two cooler...with
onshore flow keeping downtown and areas along the Lake A bit
warmer. Patchy frost is certainly possible late Thursday night into
Friday morning...especially north of I-80...but thinking temperatures will
remain above freezing across the County Warning Area.
Friday and Saturday...
the blocking pattern begins to take shape Friday with a trough over
the northwest coast...the ridge grows across the Great Plains...and
the trough remains over the East Coast. The upper level jet is also
incredibly weak during this time frame...which helps the blocking
pattern remain in place. At the surface...a high pressure ridge
sinks south over the western Great Lakes. The ridge helps push some
of the cooler Canadian air out of the region...but temperatures look to
remain below normal through the end of this week.
Guidance suggests a weak upper level disturbance may travel around
the upper level ridge and lead to a chance of showers and
thunderstorms Saturday and Sunday.
Extended...Sunday through Tuesday...
the blocking pattern begins to break up Sunday night into Monday as
the East Coast trough finally begins to shift northeast. This
allows the ridge to open with short wave troughs passing through our
area. Guidance differs on where and when these disturbances will
pass...therefore went with a chance of showers and thunderstorms
throughout the extended. The best day for precipitation appears to be
Tuesday or Wednesday as an upper level passes through southern WI.
Next week begins with a warming trend. Near normal temperatures are
expected for Memorial Day and back into the upper 70s or low 80s by
middle week.
Jee
&&
Aviation...
//Ord and mdw concerns...updated 20z...
* variable ceilings...VFR...then possibly dropping to MVFR late this
afternoon...and IFR likely tonight.
* Additional rain showers early evening possible...low chance thunderstorms and rain.
* Wind shift with frontal passage this evening.
Bmd
//discussion...updated 18z...
First round of thunderstorms are passing over the terminals early
this afternoon...but should be out of the Chicago area around 19z.
Ceilings behind precipitation quickly rise back to VFR. Off to the west...cumulus
field is growing over the Mississippi River as upper level
disturbance lifts across the area. Forecast soundings show very
modest instability redeveloping middle to late this afternoon...with
additional showers possible...and cant rule out a thunderstorm.
Surface low currently over northeastern Iowa will translate east
today with winds becoming light and variable this evening...then
north to northeast behind the departing low late this evening and
tonight. Lower ceilings are likely behind the front as well as light
rain or drizzle. There is also potential for very low ceilings //less
than 005// or fog off the lake at the Chicago terminals. The
gradient tightens up early on Thursday and expect gusts to commence
early in the day...topping out potentially around 30 kts Thursday
late morning and afternoon.
Bmd
//Ord and mdw confidence...updated 20z...
* medium confidence in additional rain showers this evening but low
confidence on coverage. Low confidence in thunderstorms and rain.
* Low confidence in MVFR ceilings this afternoon/evening...medium to
high confidence in ceilings dropping to IFR tonight.
Bmd
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
Thursday night and Friday...VFR. Weather none
Saturday through Tuesday...VFR. Chance rain showers slight chance thunderstorms and rain.
Allsopp
&&
Marine...
1250 PM CDT
Surface low will move east of the lake tonight with strong high
pressure building into the region. Tight pressure gradient with
fairly strong late season high pressure will combine with a healthy
surge of cold air looks likely to result in a period of gale force
winds late tonight into Thursday...particularly across southern
portions of the lake. Will be upgrading gale watch to a warning with
the afternoon forecast and include the nsh in a warning...with
expectation that a small craft advection will be needed tomorrow night
into Friday for lingering waves. Beyond this wind event an extended
period of fairly benign conditions expected on the lake through the
weekend with high pressure over the area.
Izzi
&&
Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...dense fog advisory...lmz080-lmz366-lmz565-lmz567-lmz669-lmz671-
lmz673-lmz675-lmz777-lmz779-lmz868-lmz870-lmz872-lmz874-
lmz876-lmz878 until 3 am Thursday.
Gale Warning...lmz080-lmz671-lmz673-lmz675-lmz740-lmz741-lmz742-
lmz743-lmz744-lmz745-lmz777-lmz779-lmz872-lmz874-lmz876-
lmz878...7 am Thursday to 9 PM Thursday.
Gale Warning...lmz366-lmz565-lmz567-lmz669-lmz868-lmz870...4 am
Thursday to 3 PM Thursday.
Dense fog advisory...lmz261-lmz362-lmz364-lmz563 until 9 PM
Wednesday.
&&
$$
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