Area Forecast Discussion
Issued by NWS Chicago, IL
FXUS63 KLOT 201900
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
200 PM CDT FRI MAY 20 2016
1119 AM CDT
No significant changes to going forecast. Pretty impressive
looking mid/upper low on w/v imagery this morning tracking toward
The area. Normally in late May, this would be a cause for concern
but 12z raobs and forecast soundings depict quite the dry layer
around 700 mb with dewpoints colder than -40F in addition to low
dewpoints and lack of instability. Main sensible weather impact is
the mid level overcast that will cap highs around 70/a few low 70s
inland today. Onshore flow will keep lakeshore in 50s to low 60s.
Have not made changes to low (~20%) POPs into southeast CWA late
this afternoon and evening, but having some doubts about whether
anything other than a few very light sprinkles or even just virga
will be able to occur. It`s possible that the CWA gets split with
isolated showers into west central IL under upper low this
afternoon and better coverage of showers moving into the Ohio
Valley, with dry conditions in between. Late morning regional
radar does seem to be hinting at this and to note, 12z 12km NAM
does keep the whole area dry through this evening. Will monitor
trends and update as needed this afternoon. Even if it does rain
in east central IL and NW Indiana south of Kankakee River, it will
be light and spotty and cause no impacts.
Today through Saturday night...
300 am...A weak area of low pressure will move northeast across
the Ohio Valley today and tonight with the back edge of its
showers moving across eastern IL and northwest Indiana. Initially
any precip will be falling into a fairly dry easterly flow but by
late this afternoon into this evening...there is some potential
for a few showers across the far southeast cwa and introduced low
chance pops. For the rest of the area...increasing high/mid clouds
from this system will make for mostly cloudy or cloudy skies by
midday with the clouds exiting later tonight. Temps into the upper
40s in some locations this morning but with no precip today...
guidance highs back in the upper 60s/near 70 seem ok but cooler
near the lake.
High pressure will build south across MN/IA/MO on Saturday and a
few models squeeze out some light qpf Saturday afternoon mainly
north of the cwa as a weak trough passes across the northern
lakes. Stayed dry with this feature but will need to be monitored
with later forecasts. North/northeasterly winds will keep the lake
shore areas cooler as the rest of the area should see highs well
into the 70s. cms
158 PM CDT
Saturday night through Monday...
High pressure continues this weekend with an upper level ridge also
moving overhead Sunday night. Looking at continued dry conditions
with temps around 80 away from the lake. The high shifts east Monday
but rain should hold off until after midnight Monday night for areas
along and west of I-39. Monday will be similar to Sunday except for
increasing cloud cover...southerly winds...and no lakeside cooling.
Monday night through Friday...
The stage is still set for a more active weather pattern to be in
place much of next week. Upper level ridge over the Upper
Mississippi Valley early in the week will continue to translate
eastward with the local area placed in the lee of a broad upper
trough across the western two thirds of the country. Southwest flow
aloft will help transport unseasonably warm air into the area and
expect temps near or into the 80s from Tuesday on. At the
surface...Bermuda high and troughing across the plains will allow
gulf moisture to spread north into the midwest. Guidance suggests
dewpoints into the 60s from Tuesday on which combined with steep mid-
level lapse rates associated with the elevated mixed layer will
allow moderately strong instability to develop.
With moisture and instability in place...the focus will be on timing
of several low amplitude waves as they lift across the region for
specific timing of thunderstorms. Guidance continues to show several
waves though at this distance honing in on timing is difficult so
continue to broad brush extended pops with chance/high chance pops.
Will also have to keep a close eye on deep layer shear and the
chance for severe weather. In general...expect increasing shear
Tuesday through Thursday which should also correspond to an
increasing severe weather threat providing the other ingredients
remain in place. On Tuesday...the jet is still focused well to our
southwest and 500 mb flow of only around 25 kt locally appears for
now at least to be a significant limiting factor in the severe
weather threat for northern Illinois and northwest Indiana. Guidance
suggests by Thursday though that we may see the left exit region of
the jet overhead with stronger flow aloft resulting in more
supportive shear for severe weather. Not a sure thing of course at
this range but bears close watching as we get closer in time.
FOR THE 18Z TAFS...
VFR conditions will continue through the period for the TAF sites,
so no major forecast concerns. East and southeast winds as of mid
day will become east to northeast this afternoon, with speeds of
10-12 kt at the near lake terminals. As low pressure passes
through the Ohio Valley tonight, 10+ kt speeds at these sites may
linger a bit longer into the evening, then drop off and become
light northerly. Expecting north-northeast winds to increase by
late morning or mid day on Saturday, with 10-12 kt speeds again
favored at ORD/MDW/GYY.
158 PM CDT
High pressure controls the pattern through the weekend with winds
less than 20 kt expected. On shore flow or lake breezes are
expected every day through Sunday. Winds become south as the high
shifts east Sunday night and increase to 20 kt Monday. Guidance
differs on the strength and path of the next low mid week One model
has the low go over the lake Wednesday night with another model has
a weaker low staying west of the lake before dissipating over the
northern Great Lakes late in the week. Have medium confidence in
light winds mid to late next week...but low confidence in wind
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