Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1103 am CDT Thursday may 23 2013
Discussion...
240 am CDT
Today...
forecast challenges for today reside with high temperatures...and precipitation
chances this morning.
Infrared imagery early this morning shows considerable cloud cover
remaining across the western Great Lakes...extending south through
the middle-Mississippi Valley. Along the western periphery of the cloud
cover lies the leading edge of the strong surface ridge...beginning to
push into the upper Midwest/northern plains. Moisture has been
consistently peeling off to the northeast...noted in the steadily
lowering dew points. Dew points across central wisc have already
fallen into the upper 40s...and expect this trend to eventually arrive
across northern Illinois/northwest in. A broad surface trough was analyzed
across northeast Illinois stretching into southcentral lower Michigan. As a
result surface winds across the County warning forecast area have mainly been from the
west/southwest...however just north of the County warning forecast area into wisc winds have
already shifted to the northwest/north. Locations adjacent to Lake
Michigan will see winds flop over to the northeast this
morning...allowing the much cooler marine environment to readily
flow across the land.
Regional radar early this morning shows an area of showers across
central wisc...slowly sliding southeast. Hi-res guidance suggests
this channel of precipitation will eventually arrive across northern Illinois
just before daybreak. As a result of the increased confidence in
precipitation...have nudged the 12-15z timeframe to likely probability of precipitation for far
northeast Illinois.
Dry air continues to pour into the region mid-morning...and should
bring an end to the precipitation from northwest to southeast across the
County warning forecast area by early this afternoon. The steady push of dry air will also
aid in eroding the cloud cover...and by late this afternoon expect
mostly sunny skies to develop. Cold air advection will dominate the
day...coupled precipitation and clouds...expect much cooler temperatures for this
afternoon. Guidance has continued to suggest temperatures will struggle to
warm beyond the low 60s for the far western County warning forecast area...meanwhile further
east temperatures will only warm a few degrees from their starting point.
This will most likely lead to areas east of I-39 only warming into
the middle/upper 50s. Locations immediately adjacent to Lake Michigan
could see temperatures struggle to warm beyond 50 degrees.
Confidence in precipitation ending time...medium/high.
Confidence in temperatures...high.
Tonight...
anti-cyclonic flow will continue to build overhead early this
evening...and erode any lingering clouds by midnight. Dew points as
a result will continue to dry...bottoming out in the middle 30s in many
locations. The combination of near cloudless skies...light north to
at times calm winds...and recent rainfall...will set the stage for
not only strong radiational cooling but patchy frost away from Lake
Michigan. Guidance continues to forecast temperatures falling overnight into
the middle/upper 30s for areas west of Waukegan to Kankakee. Far southern
portions of the County warning forecast area should have temperatures hold around 40 degrees
overnight...however a few low-lying areas could see spotty frost. At
this point it does not appear to be a widespread frost setup...so
will hold off on any headlines at this point.
Confidence...high.
Friday...
surface high pressure will dominate the region for the end of the
week...and more than likely only allow a few diurnal clouds to
develop. A light northeast flow off of Lake Michigan will continue
to keep adjacent areas much cooler in the middle/upper 50s...meanwhile
inland locations will begin to warm into the low/middle 60s.
With high pressure still in control Friday night...expect another
night of strong radiational cooling. The only difference for Friday
night will be the ridge axis will have shifted east. This will allow
the light winds to begin turning southeasterly after midnight...and
could slow the rate of cooling. Have maintained temperatures in the low 40s
for many areas. Could see a few of the traditional cool spots dip to
the upper 30s early Sat morning...however at this point not
anticipating frost formation.
Confidence in temperatures...high.
Saturday...
discrepancies begin to emerge amongst solutions for Sat...with
respect to placement/timing of a moist boundary. GFS has continued
to be the more progressive solution...bringing this channel of
moisture/precipitation into the region and stretches from the northern
plains southeast through the Ohio Valley. Meanwhile the ec has
maintained the surface ridge...and suppressing the moist channel to the
southwest of the County warning forecast area.
The largest weather player is the downstream 500mb ridge located
across the western Atlantic. This will act as a block...slowing the
departure of the middle-level trough across the middle-Atlantic region. Then
further upstream yet another 500mb ridge was prognosticated to arrive
across the central Continental U.S....and does show some signs of
amplification. Based on this setup it is becoming more evident that
Sat could end up being dry...and possibly extend into sun. At this
point have only adjusted probability of precipitation Sat marginally...to slight chance
wording.
Surface temperatures will remain unseasonably cool in the low/middle 60s...with
areas adjacent to Lake Michigan being even cooler than this. Despite
the expected southeast component to the surface winds...enough heating
could allow a lake breeze to form and allow winds to become more
easterly for locations adjacent to Lake Michigan. This will most
likely result in those areas immediately next to the lake remaining
in the 50s Sat afternoon.
Confidence...medium.
Sunday through Wednesday...
for the extended periods guidance has continued to demonstrate
minimal changes from previous solutions...however operational
European model (ecmwf)/GFS continue to differ on the placement/timing of precipitation. The
European model (ecmwf) continues to suggest the surface ridge from late in the week will
hold tough and keep northern Illinois/northwest in dry through the weekend
and into early next week. Meanwhile the GFS is still more
progressive and brings the boundary overhead and becomes
quasi-stationary through the first half of next week.
Ensembles continue to indicate the western Atlantic middle-level ridge
will weaken...allowing the 500mb trough to finally eject east. In
addition the upstream ridge will begin to flatten...leading to a
somewhat progressive flow of systems from west to east towards the
middle of next week. Thus precipitation chance will be on the increase towards
the final few periods of the current extended forecast.
By Tuesday/Wednesday of next week...temperatures could easily be back into the upper
70s to low 80s.
Confidence in precipitation...low.
Confidence in temperatures...high.
Beachler
&&
Aviation...
//Ord and mdw concerns...updated 16z...
* variable MVFR/IFR ceiling slowly improving at or above 1500 feet around 17z.
VFR/clearing by middle afternoon.
* North wind with gusts to around 30 knots continuing through afternoon.
Allsopp/bmd
//discussion...updated 12z...
Main concern this morning is ceiling trends...then this afternoon
focus will be on strong gusty north-northeast winds. Low pressure was over
the eastern Great Lakes with a cold front trailing westward into
northern Illinois. At 11z the front was from about ikk-vys. Based on
NAM and hrrr model soundings...ceilings should slowly improve to
around 1000-1200 feet in the next couple hours. This seems
reasonable with stronger winds mixing colder and slightly drier
air down from the north. Also spotty light rain/drizzle will be
diminishing/moving out and that may also help improve ceiling and visibility
a bit this morning. As of 11z the back edge of the main band of
rain associated with the cold front was lined up from dkb-dpa-
mdw-gyy and sinking very slowly S. There were some visibility
restrictions down to 2-3 miles in this band of rain...but it has
pushed south of the terminals. Further north into southern WI
there were some patches of light rain/drizzle..but this was very
light. Short range/hi res models suggest this spotty light rain
will be drying up and moving southeast out of the area by middle to late
morning. Therefore dont expect any significant visibility restrictions
below 5 or 6 miles the rest of the morning. As strong north winds
continue to push colder drier air down...expect clouds to lift to
1500 feet or better by noon and break up this afternoon with
clearing middle to late afternoon.
North winds have picked up and winds will continue to increase to
20 to 30 knots by middle morning. Direction may vary some from 350-030
but will be predominantly just east of north today. Pressure gradient
slowly begins to relax late this afternoon and winds may diminish
slightly during the late afternoon. Atmosphere should begin to decouple
this evening as pressure gradient continues to relax.
Outlook for Friday is VFR with light northeast winds.
Allsopp
//Ord and mdw confidence...updated 16z...
* medium-high confidence in ceiling trends this morning.
* Medium confidence in timing of clearing this afternoon.
* High confidence in wind forecast. Direction may vary from
350-030.
Allsopp/bmd
//outlook for Ord/mdw for 00z Saturday-12z Thursday...updated 12z...
Friday night and Saturday...VFR. Weather none.
Sunday and Monday...VFR. Chance rain showers slight chance thunderstorms and rain.
Tuesday and Wednesday...chance thunderstorms and rain.
Allsopp
&&
Marine...
147 am CDT
Early Thursday morning an elongated area of low pressure was from
the eastern Great Lakes back to northern Illinois...while a strong
area of high pressure was centered over northern Manitoba with a
ridge extending southward into the northern plains. As the trough
swings east and the ridge builds into the western Great Lakes
today...gale force north winds will develop over Lake Michigan.
The gales will develop over Central Lake Michigan early this
morning and spread to the south end by middle or late morning. Winds
will diminish overnight as the high settles over the western Great
Lakes and the low moves up the East Coast. The high will then be
nearly stationary over the Great Lakes through the weekend with
light east to southeast flow over the lake. Low pressure will move
from the Central Plains to the upper Mississippi Valley Monday and
Tuesday which will cause winds to become southerly over Lake
Michigan.
Allsopp
&&
Lot watches/warnings/advisories...
Illinois...none.
In...none.
Lm...Gale Warning...lmz080-lmz671-lmz673-lmz675-lmz740-lmz741-lmz742-
lmz743-lmz744-lmz745-lmz777-lmz779-lmz872-lmz874-lmz876-
lmz878 until 9 PM Thursday.
Gale Warning...lmz366-lmz565-lmz567-lmz669-lmz868-lmz870 until 3
PM Thursday.
&&
$$
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