Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230804

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

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 06Z TAFs...

VFR conditions in place for most of the terminals at this time,
with the exception of RFD where MVFR ceilings are overhead. While
RFD stays MVFR do think the other terminals will remain VFR for most
of the early morning hours. However, as the surface low to the
northwest slowly drops southeast and precip increases across the
area, MVFR will likely be observed for all terminals later this
morning. These ceilings should remain in place for most of the day
and although some improvement may occur later this afternoon,
thinking MVFR and quite possibly lower may stick around into
tonight. Have not completely gone this way, but may need to lower
ceilings later in the period with later forecast updates. Precip
axis staying primarily just to the south at this time. GYY and MDW
may see some brief and light showers over the next couple of hours,
but mainly dry conditions should continue early this morning.
Then anticipate this precip axis to shift overhead later in the
morning and likely continue for much of the day. Have continued
VCTS wording for RFD as the threat of scattered thunder still
remains and although I have not included for the remaining
terminals, could see adding it for at least DPA and ORD for a time
this afternoon. Periodic light showers will then likely continue
for most of tonight. Southwest winds expected today, until winds
turn more easterly off the lake later this afternoon.



205 PM CDT

Breezy southwest winds courtesy of high pressure across
the Ohio valley and Low pressure north of Lake Superior, most
notably across the nearshore waters, will ease as a secondary area
of low pressure across the corn belt will shift to the south and
west of the southern tip of Lake Michigan through Tuesday. Weak
high pressure will build across north central Ontario, which will
allow a weak cold front to shift south across Lake Michigan
beginning tonight across the north half, and eventually across the
south half Tuesday night. The low south of Lake Michigan will
strengthen through mid week which will lead to a several day
period of increasing northerly winds across the entire lake. Gales
appear unlikely though expect winds to pick back up to 25 kt or
briefly higher especially Wednesday, which will build modest
waves across southern Lake Michigan and lead to hazardous
conditions for small craft.






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