Area Forecast Discussion
Issued by NWS Chicago, IL

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598
FXUS63 KLOT 280110 AAA
AFDLOT

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
810 PM CDT Sat May 27 2017

.UPDATE...
810 PM CDT

Changes to the forecast for the rest of tonight into Sunday
morning have been minor with just a tad higher chance for some
showers and possibly storms mentioned across north central
Illinois, with probably the best chance still looking to be very
late tonight/Sunday morning.

Across far eastern Iowa early this evening there is a short wave
impulse that was seen on earlier satellite imagery before high
clouds shrouded the area. Some light rain with isolated convection
has spread into southern Wisconsin, with one shower across Boone
County. A few showers could continue to spring up on the
eastern/southern trajectory of this short wave, so across mainly
north central Illinois and the IL/WI border region of northeast
Illinois. The 00Z DVN sounding indicated around 800 J/kg of
MUCAPE, and enough effective shear that if a storm were to pop
across north central Illinois it could have some organization to
briefly be strong. All in all though, a pretty quiet weather
evening.

A few showers, possibly a storm or two, could fester in these same
areas into overnight. In addition, it continues to look like
elevated return flow and instability also creep into the southern
CWA late tonight in response to cyclogenesis across
western/southwestern Illinois. So could also see a few storms pop
up in that area. Overall this was well covered by the forecast.

With some of the better upper level forcing ahead of the primary
trough moving over the area early Sunday, would expect then to
see an uptick in shower coverage. While a decent spread in 18Z
guidance on the rate of speed of the front Sunday afternoon, it
still looks like by peak afternoon heating it should be east of
the area, keeping the main potential for any severe storms east.

MTF

&&

.SHORT TERM...
222 PM CDT

Through Sunday...

Light winds and mostly sunny skies will continue into this evening,
except for lingering cloud cover east of I-55. The two big
factors in the forecast are the convective complexes over Iowa and
Missouri. Models have struggled to capture convection today, but
the latest run of the RAP and HRRR seem to have a better handle on
the situation. The latest RAP analysis features minimal CAPE
values and lingering CIN across the region. Ample sunshine will
certainly help erode CIN, but dewpoint depressions are mostly 10
degrees or more.

Therefore, expecting the storm complex over Iowa to dissipate before
it reaches north central IL.  Lightning strikes are rapidly
decreasing in the complex, which supports the idea that the system
is weakening.  In addition, a new and much more impressive
convective complex is taking shape over Missouri. The southern
convection should rob us of CAPE and moisture.

Overall have high confidence in a dry and really nice afternoon, but
confidence lowers to medium-high for overnight.  Thinking the storms
over Iowa will continue northeast to Wisconsin this evening. Areas
northwest of a McHenry to DeKalb to Peru line may see showers this
evening, but only have a slight chance of showers and thunderstorms
in the forecast. There is a small chance that the southern storm
complex clips the far southern part of the forecast area after
midnight. The remainder of the night should be dry with lows
around 60.

Showers and storms may fester just west of the forecast area Sunday
morning, and expecting showers and storms to move into the Rockford
area around sunrise Sunday.  Guidance differs in how quickly showers
and storms will form and move through northern IL and northwest
Indiana. The NAM is by far the slowest solution, and opted to go
with a quicker evolution ahead of the cold front. Models suggest
another band of light rain may form as the upper level vort streamer
moves through.  Kept a chance of showers and storms, but have low
confidence in additional precip behind the first wave given very dry
soundings behind the front.  Kept high temps lower than climo would
suggest given thick cloud cover. Highs will be in the low 70s.

JEE

&&

.LONG TERM...
222 PM CDT

Sunday night through Saturday...

An upper low is expected to be in place centered just north of the
Great Lakes through much of the upcoming week resulting in active
weather and below normal temperatures.

A blocky weather pattern will be in place over North America much
of the upcoming week as a strong ridge remains anchored over the
central and western half of the country while an upper low
meanders over the Canadian Prairies and northern Great Lakes
region with pin wheeling vort lobes rotating across the local
area. First, and one of the stronger in a series of waves, will
drop across the region on Monday with a modest surface
reflection/cold front moving across northern Illinois and
northwest Indiana primarily during the afternoon. Overall forcing
is not impressive with only modest low level convergence noted
along the front and a narrow corridor of DCVA associated with the
shortwave, but steep low to mid level lapse rates will be in
place with no CIN noted on GFS forecast soundings which should
allow for some scattered showers and possibly a thunderstorm or
two to develop.

Slight chances for precip continue Tuesday as lower amplitude
shortwaves continue to rotate through the region. Steep low level
lapse rates persist and weak diurnal instability is expected to
develop. Showers will be a possibility especially if shortwave
timing falls within peak heating. The GFS strengthens a mid level
capping inversion Wednesday as the upper low inches east and
moderately strong mid level height rises build into the Upper
Midwest resulting in subsidence aloft which should keep the area
dry midweek. This will continue on Thursday as the surface high
moves overhead. Expect a warming trend to occur late in the week
into next weekend as the upper low slowly departs east, but with
warmer temperatures will also come additional chances for showers
and thunderstorms.

Deubelbeiss

&&

.AVIATION...
For the 00Z TAFs...

642 pm...Main forecast concern is potential for thunderstorms
Sunday morning into early Sunday afternoon.

Current showers and isolated thunderstorms over northeast IA are
expected to continue to weaken/dissipate this evening. Some light
showers or sprinkles may reach rfd in the next few hours. There
remains quite a bit of uncertainty regarding precip chances late
this evening and overnight. Most recent short term guidance
suggests isolated/scattered activity may develop by 06z southwest
of the terminals. This is a change to previous runs and confidence
remains low so will maintain dry forecast overnight and monitor
trends. A cold front will move across the area Sunday and
scattered showers and thunderstorms are expected along/ahead of
this front. Timing looks on track if they occur but also low
confidence regarding coverage.

A lake breeze has moved west of ord/mdw with easterly winds
continuing tonight...diminishing to 5 kts or less. Many areas may
become light and variable. Winds will shift northwesterly behind
the cold front Sunday morning and speeds are likely to increase
to 10-15 kts with some higher gusts possible...especially by mid/
late afternoon.

Increasing mid clouds this evening will lower by morning and
possibly into high mvfr...especially at rfd. Fairly low confidence
for how prevailing mvfr cigs will become...especially after the
front passes when cigs should become vfr. cms

&&

.MARINE...
222 PM CDT

Relatively light flow will continue across Lake Michigan through
the remainder of the weekend as the Great Lakes fall between two
areas of low pressure. A northern low will meander over Manitoba
and Ontario through the weekend while a southern low will lift
from the southern Great Plains to the central Great Lakes Sunday.
As these lows merge over Ontario Sunday night into Monday, a fresh
westerly to southwest wind will develop early next week and
continue through midweek.

BMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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