Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 261958
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
258 PM CDT THU MAY 26 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 18Z TAFS...
Earlier this morning a couple areas of showers and thunderstorms
were approaching the TAF sites...however one area of thunderstorms
lifted Northwest and dissipated and the other went South and has
since developed into a line of strong thunderstorms stretching
across Southeast Indiana stretching West through Southeast
Missouri. In the next several hours across Northern Il/Northwest
IN quiet weather looks to persist...with visible imagery
indicating a steady erosion to the lower clouds with continued VFR
conds. Most forecast guidance continues to lean towards a dry
afternoon and evening timeframe...with the next potential for
showers and thunderstorms nearing the TAF sites around 9Z Fri at
the earliest...and perhaps around daybreak across the ORD/MDW
sites. This continues to be a low confidence scenario though...and
may end up remaining dry through this period as well. Winds
throughout the TAF time will be generally from a 220 degree
direction trending to 190 with speeds between 7-12kt.
Thunderstorms will increase by late Friday morning through the
afternoon hours...and could easily see the VCSH trending towards
either a PROB30 or likely a TEMPO for thunderstorms Fri aftn at
ORD. This has increased confidence in impacting the TAF sites.
257 PM CDT
Despite active weather in regards to thunderstorms and fog...not
expecting any wind related headlines. Fog persists over the
northern end of the lake...and it may persist into Friday. Fog has
mostly burned off over the southern end of the lake. Going to leave
the marine fog advisory as is given a lack of confidence in how much
fog will redevelop tonight and Friday. Surface conditions are
certainly damp and forecast soundings continue to feature a very
strong low level inversion that would help fog form. However...ample
sunshine will limit fog development.
The low over southern Manitoba will pass north of the lake tonight
into Friday. Another low will pass west of the lake Friday and
Saturday and then north of the lake Saturday night into Sunday.
South to southwest winds are expected through Sunday and then winds
become west to northwest behind the low. High pressure moves over
the lake briefly Monday and winds become east by Tuesday morning as
the high shifts east and a low moves over the northern plains.
Guidance differs on the path of the low mid to late next week...so
while on shore flow is currently forecast...have low confidence in
LM...Dense Fog Advisory...LMZ777-LMZ779 UNTIL 9 AM Friday.
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