Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

000
FXUS63 KLOT 212028
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
328 PM CDT Sun May 21 2017

.SHORT TERM...
127 PM CDT

Through Monday...

Deep upper level low pressure across the upper midwest will
continue to stratocumulus deck across the region through the
evening and early overnight before diminishing. With cold
advection that will continue through the night and the lingering cloud
cover, expect temperatures at this point to largely hold steady
or even fall some through the afternoon. At the surface, high
pressure across the southern plains will shift to the lower
Mississippi Valley tonight while low pressure lingers across the
northern Great Lakes. This will maintain an occasionally gusty SW
wind through Monday especially during the daytime hours, though
cannot rule out occasional gusts overnight.

Clouds should thin by daybreak Monday which like today should
start off on the sunnier side. Several waves will rotate around
the back side of the low beginning Monday which will increase
clouds and bring the chance for showers and even a few
thunderstorms Monday afternoon. Confidence is high that there will
be at least scattered rain showers in northeast Illinois, and
possibly thunderstorms during this time frame, but uncertainty on
how much coverage precludes likely pops just yet. The
west/southwest wind on Monday will be a warmer wind then today,
thus expect temps to climb above today`s lower 60s readings.

KMD

&&

.LONG TERM...
328 PM CDT

Monday night through Sunday...

A fairly active pattern will remain in place into Memorial Day
weekend with multiple opportunities for periodic showers and at
times thunderstorms. The mid week period will be marked by cooler
than normal temperatures for late May as a large upper low
traverses the region, followed by a return closer to seasonable
temperatures Friday into the holiday weekend.

On Monday evening and night, weak broad surface low embedded in an
elongated trough stretching across the Great Lakes will be near or
just northwest of the area. Multiple mid-level shortwaves in
southwest flow aloft will keep a threat for scattered showers
going. Thunder potential should be largely diurnally driven and
focused into Illinois so should wane later in the evening and
especially overnight.

Looking ahead into Tuesday, the closed mid-upper low will move
slowly southward along with associated surface low. The surface
wind pattern will be tricky and based off position of the surface
low which is exhibiting variance in operational guidance. This
will affect temperature trends in the afternoon, especially near
Lake Michigan. Temperatures will range from the lower to mid 60s
northwest of I-55 and upper 60s to around 70 southeast, though
again an earlier onset of onshore winds shown on some guidance
will result in temperatures several degrees cooler near Lake
Michigan.

More importantly, short-wave energy rotating around the
upper low will keep forcing going for shower activity with an
assist from lower-level convergence. With the cold mid-level
temperatures associated with the upper low over the area featuring
freezing levels below 10kft AGL and providing minor instability,
isolated/slight chance thunder potential is evident. This setup
may be one to watch for small hail/graupel given such low freezing
levels. Moisture levels will not be all that high/around 1" PWAT
values/but relatively slow storm motions could result in decent
downpours, particularly in any thunderstorms. Since instability
will already be limited to begin with, isolated thunder potential
will again likely be diurnally driven and unlikely to linger into
Tuesday evening.

The track and intensity of the upper level low into Wednesday and
Thursday will drive the mid-week pattern and our sensible weather.
Model guidance often struggles with these features, decreasing
confidence in trends. Among the 12z guidance, GFS and GEFS are on
the deeper and farther west side of the envelope for surface low
associated with the upper low, though ECMWF and its ensembles did
trend a bit westward and stronger vs. 00z suite. The difference in
handling of the synoptic system will dictate whether the weather
is merely showery and cool (especially lakeside) or downright raw
with steady rain and strong north winds.

These trends will affect Wednesday night into Thursday as well and
how quickly the cloudy showery and cool conditions exit east.
Short-wave ridging looks to slide east on Friday to return
seasonable temperatures, but looks to be accompanied by a threat
for showers and possibly thunder from a ridge rider disturbance.
There is plenty of uncertainty regarding the extent of
precipitation potential over the Memorial Day weekend, so do not
cancel outdoor plans yet. Temperatures appear they will be
seasonable in the 70s, though likely affected by cloud and shower
coverage, with Saturday currently favored to have lake cooling.

Castro

&&

.AVIATION...
For the 18Z TAFs...

Deep upper level low pressure across the upper midwest will
continue to steer low end VFR or higher end MVFR stratocumulus
deck across the region through the evening and early overnight
before diminishing. At the surface, high pressure across the
southern plains will shift to the lower Mississippi Valley tonight
while low pressure lingers across the northern Great Lakes. This
will maintain an occasionally gusty SW wind through most of the
period, especially during the daytime hours, though cannot rule
out occasional gusts overnight.

Several waves will rotate around the back side of the low and
bring the chance for showers and even a few thunderstorms Monday
afternoon. Confidence is high that there will be at least
scattered rain showers in northeast Illinois, but low on coverage
and medium on timing (per the ORD 30 hr TAF). Thunder chances are
too low to include at this distance but forecast soundings to
suggest the afternoon period could have a few thunderstorms.

KMD

&&

.MARINE...
259 AM CDT

Occluded front will sweep across the lake this morning shifting
winds to the southwest. Fairly stable conditions this afternoon
should keep wind somewhat decoupled over the open waters with a
somewhat lighter southerly wind expected when compared to the
stronger southwesterly offshore winds near the southern/western
nearshore. Stability decreases some tonight as cooler air filters
in, so expected stronger winds to develop over the open waters
tonight, probably peaking in the 25-30kt range for gusts after
midnight. Generally west/southwesterly winds likely until cold
front moves across the lake Tuesday shifting winds to the north in
its wake. Medium range models are showing some variability in
solutions with respect to the midweek storm system, but does look
like a potential for a period of fairly stout northerly winds in
the Tuesday night through Wednesday night time frame.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters
     until 9 PM Sunday.

&&

$$

VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE)

FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSCHICAGO
WWW.TWITTER.COM/NWSCHICAGO
WWW.YOUTUBE.COM/NWSCHICAGO


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.