Area Forecast Discussion
Issued by NWS Chicago, IL

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938
FXUS63 KLOT 280804
AFDLOT

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

.SHORT TERM...
303 AM CDT

Through tonight...

Focus is on increasing shower and thunderstorm chances toward
daybreak and spreading east through the morning, with several
periods/opportunities for scattered shower and storm development
through the day from west to east. We are starting to see increased
moist ascent in axis of elevated instability leading to convective
development along the Mississippi River. This should expand showers
and storms across NW and NC IL in the coming hours. 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 northern portions of the area. 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 across the Chicago area southward. Do
think the forcing is good enough for at least a period of showers,
with the previous mid morning to early afternoon time or concern
period. During the time of potential weakening of the southern end
of the line of showers late this morning, there appears to
additional storms that look to form ahead of the cold front. Thus
this areas of focus will be largely across east central Illinois
and into northwest Indiana. RAP progs 500+ J/kg of instability
and strong 40-45 kt of deep layer shear, so we have a marginally
severe storm potential as storms could quickly get organized with
a concern for hail or wind. The window of opportunity appears
limited to the early to mid afternoon before the instability axis
ahead of the front shifts into central Indiana.

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 across north central IL and
spreading east. Confidence is lower on coverage later this
afternoon but would be scattered at most. A dry slot will shift
overhead this evening and should really diminish precipitation
chances. Cold advection will continue behind the front this
evening and tonight, leading to lows in the low to middle 50s.

KMD

&&

.LONG TERM...
252 AM CDT

Monday through Saturday...

Closed upper low is progged to drift slowly east across the
northern Great Lakes region through mid-week, with the forecast
area within the southern periphery of the associated large scale
upper trough. A series of sheared, low-amplitude short wave
disturbances are depicted moving through the west-northwest flow
aloft, which will bring periodic isolated-scattered
shower/thunderstorms primarily during the diurnally-favored midday
through late afternoon hours. Monday appears to be likely to see
the greatest coverage, with a somewhat stronger wave apparent in
guidance as well as a surface trough/secondary cold front which
may provide some focus for showers and storms by afternoon.
Gradual cool-advection in the low-levels by Tuesday should provide
more of a mid-level (650 mb or so) cap/warm-nose, resulting in
weaker instability and lower coverage of weak convection. Guidance
generally indicates the last the series of waves rotating across
the area Tuesday night into early Wednesday.

Medium range model solutions indicate a brief period of rising
heights Wednesday, as the trough becomes elongated north of the
Lakes. This allows weak surface high pressure to slide southeast
across the Midwest through Wednesday night, after a period of
breezy west-northwest cyclonic low-level flow earlier in the
period. Guidance is in some agreement in dropping additional mid-
level energy across the upper Midwest and Great Lakes region late
in the period, though there are significant differences in timing
and placement as another closed upper low forms from the northern
Lakes to New England by Friday and Saturday. For the local area,
there is general consensus that the weak surface high will move
southeast of the forecast area Thursday, with modest south-
southwest return flow developing as the ridge moves away and
surface low pressure which passes north of the Lakes Thursday
night. A cold front trailing this low eventually sags into the
area by Friday or Friday night, though with the aforementioned
model differences there is spread in the timing and placement of
the surface front too. Overall, this scenario would bring the next
chance of organized precip to the region in that late Thursday-
Friday, and perhaps into Saturday given the ECMWF`s solution of
hanging the front up across the area.

Temperature wise, after highs in the mid-70`s Monday, slightly
lower than average temps are expected especially Tuesday-Wednesday
with highs in the upper 60`s to lower 70`s mid-week. West winds
should preclude any lake cooling however. Temps moderate back into
the low-mid 70`s across the cwa later in the week, and could be
warmer Friday if the cold front arrives later rather than sooner.

Ratzer

&&

.AVIATION...
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.

KMD

&&

.MARINE...
300 AM CDT

Only real marine forecast concern for the period is with a period
of modestly breezy west-southwest winds Monday into Tuesday, with
gusts around 25 kt possible.

Low pressure over northwest Ontario will gradually deepen as it
drifts slowly southeast and then east to near James Bay through
mid-week. A weaker low, currently over central IL, will lift
northeast into lower Michigan later today, and eventually will
merge into the deepening circulation north of the Lakes. This will
result in winds generally becoming west-southwesterly by tonight,
as some variability in direction today within a weaker gradient.
The gradient will tighten up Monday as the low deepens north of
the Lakes, with breezy/gust west-southwest winds spreading off of
the adjacent land areas of WI/IL. Relatively mild air over the
cool lake waters will make for stable conditions further out over
the lake, with the strongest gusts Monday afternoon largely along
the WI/IL shore areas. The gradient will continue to tighten
Monday night into Tuesday, particularly across the northern half
of the lake, increasing winds a bit even well away from shore. The
low over Ontario will eventually drift east across James Bay and
into northwestern Quebec by mid-late week, allowing the gradient
to ease and winds to diminish. Weak high pressure will pass south
of the lake Wednesday night, with winds eventually turning south
again Thursday and Thursday night. A cold front is then expected
to push south across the lake sometime Friday or Friday night.

Ratzer

&&

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

$$

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