Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231804

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
104 PM CDT Tue May 23 2017


SPC has included a small portion of the area in a marginal risk
for severe thunderstorms today in the 13z Day 1 outlook. The
large upper low with a very nice circulation over southwest
Minnesota will dig south today. As this occurs cooler temperatures
aloft will promote increasing lapse rates aloft which will
overspread a weak cold frontal boundary. While looking outside
with cool and cloudy conditions outside does not scream severe
weather, the front and deep circulation to support ambient
vorticity remaining elevated and with the slow moving low allow
any storms that develop could linger on the increased vorticity
presence on the boundary. If we get some weak surface heating,
which is the caveat here, a weak tornado or damaging wind threat
will exist in the presence of steeper lapse rates on the boundary
as the vort max at the base of the trough rotates northeastward
later this afternoon.

In addition, this vort max will steer a moisture axis (PW > 1")
northeastward across the area today which will limit instability
but will likely lead to occasional to steady rain through the day
and evening that will expand in coverage.



304 AM CDT

Through tonight...

Short term part of the forecast can pretty much be summed up as
unsettled, with at least a low end/conditional tornado threat
potentially developing this afternoon.

GOES-16 mid level water vapor imagery (6.95 micron) shows a large
trough extending from the northern high plains east to the Upper
Mississippi Valley with multiple pronounced closed circulations
embedded within the large scale trough. The most vigorous of the
embedded circulations is seen digging south across MN early this
morning with numerical guidance suggesting that this feature will
become the dominant upper low that will meander southeast across
the region the next few days.

Another noteworthy shortwave trough was moving east across
southern Nebraska early this morning with a fairly well defined
speed max on the southern flank of this shortwave zipping
eastward. This shortwave is forecast to continue shearing east
across IA today, approaching the MS later this afternoon. At the
surface, broad low pressure near the WI/MN/IA border is forecast
to move open up into a trough, merging with another weaker low
over MO. By late afternoon, a N-S oriented sfc trough/convergence
axis is forecast to bisect the state.

Bit of a tricky forecast today as area of rain over central IL
likely to persist into the morning hours, likely shifting north
and east a bit with time. In the southeast quadrant of the upper
low, models are forecasting a corridor of wind profiles that
feature significant veering from the surface to 6km across
portions of central IL, including our southern CWA. Potential
exists for some breaks in the OVC this afternoon, which given the
cold temperature aloft, it wouldn`t require much heating for
SBCAPE values of 500+ j/kg to be realized. Should a pocket of
instability become juxtaposed beneath the favorably veering wind
profile, it would be a pretty classic low topped/mini supercell
set up. Given the very moist low levels an ambient synoptic
vorticity with the nearby upper low, some potential for brief
tornadoes would exist. This is all contingent on sufficient
instability developing, but should it develop, the presence of
nearby sfc trough and favorably timed shortwave approaching the MS
River during peak heating suggests that the potential is non-zero
and worthy of keeping an eye on this afternoon.

Regardless if any low end conditional severe threat materializes
or not, am expecting an increasing coverage of showers and
thunderstorms ahead of the surface trough later this afternoon as
shortwave approaches. This axis of showers and t-storms, with
some risk for healthy downpours, will likely spread gradually east
across central and into eastern portions of our CWA this evening.
Behind this band, could see precip end altogether or certainly
significantly decrease in coverage from the west overnight.

- Izzi


304 AM CDT

Wednesday through Monday...

By Wednesday, upper low is forecast to continue moving southward,
but models continue to vary some run to run wrt potential
development of and subsequent location of TROWAL-like feature
Wednesday into Wednesday night. Where any potential TROWAL sets
up, threat would exist for steady, soaking rainfall, but given the
complex interaction of several vorts pivoting around this upper
low, guidance continues to struggle with the exact details
regarding where any such feature will develop. Strongest signal
looks to be over our eastern CWA or points east Wednesday into
Wednesday evening, which is where highest pops have been painted
in the grids.

Upper low will begin to move out of the area Thursday, though
lingering low level cyclonic flow on the back side and lingering
moisture could result in only slow clearing in its wake. Yet
another closed upper low is progged to dig into the northern
plains by Friday, with shortwave energy riding the jet south of
this upper low into the midwest Friday into the weekend at times,
bringing at least periodic chances for showers and t-storms.
Holiday weekend looks to be far from a washout at this point,
despite precip chances in the forecast each period. In fact,
some indication exists that the first shortwave could move
through Fri night and push the effective baroclinic zone far
enough south to leave us mild and dry the remainder of the
weekend. Too many uncertainties at this distance to justify any
changes from the blended model solution, but its worth stressing
that there is no indication at this point of any day over the
holiday weekend looking like a rain out.

Into early next week, the upper low could meander farther south
and bring another bout of seasonably cool temperatures and perhaps
some instability showers, but that is getting pretty far out
there into lower forecast confidence time range.

- Izzi


For the 18Z TAFs...

-Expanding showers embedded thunder this afternoon
-Brief IFR vsby in moderate rain or t-storms
-MVFR cigs lowering to IFR or possibly LIFR
-Continued showers and lower cigs into Wednesday
-Winds becoming erratic this afternoon and settling northeast

Deep low pressure will continue to sink south from the corn belt
to the lower Mississippi valley Thursday. The first in a series of
waves ahead of the low will shift northeast through the flying
area this afternoon. Expect and increase in shower coverage, with
MVFR vsby, possibly briefly IFR. There is a small pocket of
instability across north Central Illinois into adjacent Wisconsin
where the best near term chances for embedded thunderstorms
exists. With the upper low shifting farther east, cold
temperatures aloft will allow some of this weak instability to
shift over the terminals this afternoon. This combined with the
vort max still warrants the VCTS mention, through coverage may not
be that large. Largely VFR cigs are expected but occasional MVFR
is possible.

Expect continued waves of showers through much of the period as
additional energy spreads northward ahead of the low. Precipitable
water values in excess of 1 inch suggest occasionally moderate
rain with reduced visibilities. The other concern is with cooler
air spilling southward as the surface low will shift to south
central Indiana, model guidance paints IFR cigs, possibly even
LIFR cigs or vsbys late tonight as light winds from the northeast
and moist conditions linger. Confidence is not high on the extent
or coverage, but it is a favorable setup for low cigs overnight
into early Wednesday, possibly alleviated by periods of rain.



428 AM CDT

While low pressure is situated just to the west of the lake this
morning, associated boundary is bisecting the lake. Northerly
winds are occurring over the north half while southerly winds are
in place over the south half due to this boundary. Expect these
winds to continue for much of the day, until winds turn northerly
over the entire lake tonight. Monitoring some patchy fog across
the north part of the lake and at this time, it looks to be mainly
land fog. However, did add in fog mention in the forecast and will
continue to monitor trends this morning. Will also need to monitor
all areas of the lake later today into this evening for
additional fog development. Expect the northerly winds tonight to
increase, with waves increasing tonight into Wednesday. These
higher waves and winds will then likely remain through Thursday,
with hazardous conditions for small craft returning Wednesday
through Thursday.






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