Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 270659
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
159 AM CDT FRI MAY 27 2016
212 PM CDT
Showers and thunderstorms from this morning have moved East...with
diffluent atmosphere left in the wake this afternoon with becoming
mostly sunny skies. Clouds have been slow to erode...but expect by
late afternoon mainly just a lingering thin cirrus shield
overhead. Upstream across Northern Missouri an area of
thunderstorms was developing...however guidance does not indicate
that this will lift towards Northern Illinois this evening. Have
trended towards a dry late afternoon persisting...and likely into
the evening hours as well. Some of the hi-res forecast guidance
continues to indicate a few showers/thunderstorms developing as a
weak lobe of vorticity lifts North after 4z...however with 500mb
heights likely to hold through this evening the potential for any
organized convection looks very low through at least 6z...then
have trimmed back on pops through the remainder of the overnight
with just a slight chance increasing towards chance by daybreak.
Winds should diminish with the lack of a gradient in the near
surface environment later this evening as well...and with dew
points in the middle 60s progged to advect North/Northeast pushing
PWAT values towards 1.5 to 2 inches...it is conceivable that some
patchy fog may develop. At this time have held it out of the
current forecast as there may be just enough wind at 1000-1500ft
to keep some mixing. With the high water content in the lowest
layers of the atmosphere...temps will be slow to cool tonight and
likely remain mild. Have adjusted temps to the mid/upr 60s...but
could easily see temps not dropping below 70 closer to downtown
Chicago and perhaps a few other locations. Cloud cover tonight
will also play a role in lows.
For Friday a deep trough remains anchored across the Central
Plains with the continued Southwest to Northeast feed of moisture
towards the Western Great Lakes/Ohio Valley. There is a weakly
defined surface low that will remain near the upper level
trough...but a weak mid-lvl vort wave will lift Northeast across
the forecast area later Friday morning through the afternoon. At
this time not anticipating any severe storms for Friday with the
continued lack of shear in the lowest 0-3km along with weak
forcing...but if any storms develop Fri aftn the greater concern
will be on brief heavy downpours given the anomalous PWAT that is
progged for the area.
With anticipated considerable cloud cover for Fri...temps will
likely hold in the lower 80s with some locations nearing the mid
80s mainly in the Southeast CWFA.
257 PM CDT
Friday night through Thursday...
The extended continues to look busy with a chance of thunderstorms
every day. High temperatures will be in the 80s each day...but lake
side cooling develops next week. Could see even warmer high temps
if we get more sun than forecast.
For thunderstorms...forcing will be the big question each day.
Thinking we will have leftover convection spread along and west of I-
55 Friday night and then dissipate Saturday morning. A weak vort
streamer swings through the region Saturday while the main short
wave stays well west of the region over SD and NE. Cannot rule out
a few thunderstorms so kept a chance of thunderstorms going in the
forecast. Forecasted CAPE looks skinny and we are also lacking
shear. A few storms may become strong...but not expecting any severe
storms. Pwat values will still be around 1.5 inches and storm
motion will be around 20-30 kt. Storms will be probably produce
heavy rain but should be moving quick enough to avoid flooding.
However...a few training storms could produce localized flooding
like we saw yesterday.
The story looks similar for next week with no clear forcing but
ample CAPE and moisture. Lingering convection weakens as it spreads
over the region Saturday night and once again expecting better
thunderstorm coverage along and west of I-55. The big questions
Sunday will be how quickly we recover from the morning convection
and how much forcing we will get from the shortwave/vort streamer
that rotates through. Forecast soundings feature a decent cap and
little to no shear again. If we recover...could see better
thunderstorm coverage than what is currently forecast. Still only
expecting a few stronger storms due to the lack of shear. Thinking
there will be better thunderstorm coverage over WI closer to the
Thinking Monday will be dry so backed off on precip chances. A
better chance of thunderstorms arrives ahead of a cold front Tuesday
and more likely Tuesday evening/night. Long range guidance does
differ with the exact path of the low next week so have low
confidence in timing and details of storms from Tuesday onward.
For the 06Z TAFs...
The main forecast concern will be with the potential and timing
of any thunderstorms over the area today.
An atmospheric disturbance is currently driving a cluster of showers
and storms to our southwest over northeastern MO and into portions
of west central IL. This activity is expected to shift to the
northeast through the night...but will likely do so in a weakening
fashion. Remnants of this activity could move over the terminals
after daybreak today...but at this time...I only prefer to mention a
The convective picture looks a bit murky through this afternoon and
evening. The only real feature of interest...which could drive a
period of thunderstorms over the area later this afternoon...is an
upper level disturbance currently noted over the ARKLATEX region.
Forecast guidance suggests this disturbance will shift
northeastward over northeastern IL later in the day during peak
heating. As such...this could be enough to result in thunderstorm
development over the area after 20Z this afternoon. Overall
confidence is low to medium at best at this juncture. However...I
felt justified to include a PROB30 mention for thunder at the
159 AM CDT
The main weather concerns over Lake Michigan will be the continued
dense fog today...especially over the northern half of the
lake...and also the potential for more thunderstorms.
Light winds and high a dew point air mass over Lake Michigan is
resulting in dense fog...especially over the northern half of the
lake. It appears that this fog will persist through the day
today...and possibly well into tonight as well. As a result...we
have extended the dense fog advisory through late tonight for the
north end of the lake. It appears that although some fog could
persist over the south end of the lake...that the fog advisory there
may be allowed to expire later this morning.
An area of low pressure over the plains...is expected to shift
northeastward over the upper Midwest late Saturday. This storm
system will result in the possibility of a few waves of showers and
storms over the next couple of days. Additionally...it appears that
south winds will be on the increase into the 15 to 20 KT range for
Saturday and Sunday as the low lifts into Ontario.
LM...Dense Fog Advisory...LMZ777-LMZ779 UNTIL 9 AM Friday.
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