Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1227 PM CDT Friday may 24 2013
Discussion...
256 am CDT
For the remainder of the night...the main concern will continue to
focus on temperatures. Under clear skies with a ridge of high
pressure nosing through Wisconsin and Illinois...clear skies
prevail. However...a relatively strong pressure gradient remains in
place to the east...so...in spite of clear skies...temperatures have
only been able to drop to 40-45f with a few isolated 39f readings.
With winds beginning to weaken over northcentral Illinois...expect that
there may still be some opportunity for temperatures to radiate into the
middle to upper 30s in the low-lying and sheltered areas of northern
portions of the County Warning Area...away from the lake. So...will maintain the
mention of patchy frost for the pre-dawn hours...for locations
generally west of the Fox Valley where surface dewpoints are in the
lower to middle 30s.
The rest of Friday should be generally sunny...but on the cooler
side as high pressure continues to build southward across the area and
into the Ohio Valley. Relatively light flow off of Lake Michigan
will keep the Lakefront locations the coolest with highs only in the
middle 50s...with temperatures quickly increasing into the lower to middle
60s away from the lake under sunny skies. As the ridge axis of the
high pressure slides off to the southeast and cloud cover begins to
increase in advance of the next upstream system...do not expect
temperatures to drop off quite so much into Saturday morning...though temperatures
should remain a bit below normal with lows int he lower to middle
40s.
For the remainder of the weekend and into early next week...the
focus of the forecast will shift to precipitation chances. The latest short
range model guidance remains relatively consistent in keeping the
upper level pattern relatively slowly progressive...mainly in
deepening the upper trough over the eastern Continental U.S. Into a closed low over
New England while keeping the upper ridge west of the Mississippi
River. This will keep the local area under northwesterly flow
aloft...so...do not expect any significant change to the below
normal temperatures under persistent deep layer cold advection.
Meanwhile...farther west...a weak shortwave is expected to ride up
the western side of the upper ridge as a weak warm front develops at
the surface. With the upper low remaining off the New England coast
through the weekend...the system to the west will have trouble
making significant progression to the east. So...have confined probability of precipitation
generally to the far western and southwestern portions of the County Warning Area as
the warm front slowly tries to lift northward. Expect that much of the
County Warning Area will remain dry through the weekend...with the precipitation not
beginning to spread northward until Sunday night and Monday as the upper
low finally lifts off to the northeast and the warm front can then
surge more northward. With a weak warm advection/overrunning
pattern setting up over the area...expect that there will be some
chance for some elevated convection...but anticipate that any
thunderstorm activity will be more isolated/embedded within the precipitation
area.
For the longer term...from Tuesday through Thursday...the main issue
will be temperatures. As the upper ridge will finally be able to
progress eastward across the Mississippi Valley...rising heights aloft
and persistent southwesterly flow at the surface will signal a significant
warming trend. Have not made a significant change to the going
longer term forecast...with temperatures forecast to reach into the
lower 80s for Tuesday and Wednesday and middle 80s for Thursday.
However...there is a good chance that this warming trend may be a
bit conservative with GFS/European model (ecmwf) both bringing 850mb temperatures of 17-18c
over the area for Tuesday through Thursday. There is the chance
that the area could see the middle 80s temperatures as early as Tuesday.
With warm/moist low level flow from an open gulfmex...there will be
some chances for rain showers/thunderstorms and rain through the extended forecast
period...but have kept probability of precipitation in the slight chance to lower chance
range as the earlier warm front will have lifted well north of the
area by then and there is no significant signal to latch on to
which would justify higher probability of precipitation at this time.
Krein
&&
Aviation...
//Ord and mdw concerns...updated 18z...
* northeast winds fluctuating in speed but averaging around 10 knots
with periodic gusts of 13-16kt through middle to possibly late
afternoon.
* Lake breeze Saturday afternoon increasing wind speeds to near 10
knots.
* A small chance for light rain Saturday afternoon.
//Discussion...updated 18z...
While northeast winds continue across Chicago area taf sites early
this afternoon...high pressure is building over the immediate
region. This will lead to a rapid decrease in wind speed this
evening...but beforehand there will be continued fluctuations in speed
and gusts...all averaging near 10 knots. Saturday morning will see
light southeast winds as the ridge passes. The setup should favor
a lake breeze development at least into northeast Illinois on Sat
afternoon...so expecting winds to become more easterly at Ord and
mdw and approach if not reach 10 knots sustained. Clouds will thicken
tonight and lower to 7k-10k feet Saturday morning. There will be
lift aloft likely to generate virga...but the dry low-levels are
expected to keep it only at that across Chicago sites. Would not
be surprised if rfd sees some temporary VFR light rain late in the
morning into the afternoon.
//Ord and mdw confidence...updated 18z...
* medium confidence in wind speed and gusts this afternoon...with
high confidence in quite a bit of variability in speed.
* Medium confidence in east winds nearing 10 knots Saturday
afternoon.
* High confidence that if light rain were to move as far east as
Chicago Sat afternoon it would be VFR.
//Outlook for Ord/mdw for 00z Sunday-12z Friday...updated 12z...
Saturday night...VFR.
Sunday...VFR...slight chance rain showers/thunderstorms and rain Sunday night.
Monday through Thursday...VFR...low chances for shra/tsra. Breezy
south winds Wednesday.
Mtf/mdb
&&
Marine...
237 am CDT
Northerly winds continue to diminish across Lake Michigan early this
morning and will continue on this trend into the afternoon. High
pressure spread across western Ontario and down into the lower Ohio
Valley will move right over the lake today resulting in much lighter
winds by afternoon across most of the lake. The exception may be the
eastern shoreline areas where the gradient will hang on the longest.
Otherwise expect more of a northeast direction to develop this
afternoon but there will probably end up being some light and
variable areas as well. The high will be overhead tonight keeping
light winds in place and allow for the development of onshore flow
Saturday. The high will then shift to the east by Sunday allowing
a southeasterly wind to develop. Low pressure developing out to
the west will tighten the pressure gradient allowing wind speeds
to come up...especially Tuesday. Warmer air will be moving in so a
sharp inversion is expected to keep wind gusts in check next week.
Lows will pass to the west and north of the lake but the gradient
will come around enough to allow a more southwesterly direction by
middle week.
Mdb
&&
Lot watches/warnings/advisories...
Illinois...beach hazards statement...ilz006-ilz014 until 7 PM Friday.
In...beach hazards statement...inz001-inz002 until 10 PM Friday.
Lm...Small Craft Advisory...Illinois nearshore waters until 4 PM Friday.
Small Craft Advisory...Indiana nearshore waters until 7 PM Friday.
&&
$$
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