Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280609

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
109 AM CDT Sun May 28 2017


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

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.



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.



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.



For the 06Z TAFs...

Focus is on increasing shower and thunderstorm chances toward
daybreak and spreading east through the morning. Fairly quiet
conditions for the next few hours, but increased moist ascent
in axis of elevated instability should lead to scattered showers
and storms across NW and NC IL after 10z. The initial forcing is
coming out of the southwest so activity will be INITIALLY slower
to spread east, but there are several waves upstream of the low.
The strong compact one over NW MN will stay north but allow the
elongated trough axis to shift the precip east through the
Chicagoland terminals. Have kept the Prob30 mention of TSRA given
the fact that some guidance shifts this axis right on through, but
others such as the RAP and HRRR are somewhat less aggressive with
the coverage an intensity on the southern end of the line near
ORD/MDW. Do think the forcing is good enough for at least a period
of showers, with the previous 16-19z time or possibly a window a
bit sooner would be the highest ORD/MDW concern period. Otherwise
additional NW flow energy on the back side of the low and the
continued cold advection behind a cold front (which will shift
winds to WNW this afternoon) may lead to a few additional showers
in the later afternoon. Confidence is lower on converge of these
and thus kept TAFs dry for now.



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.






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