Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 010549

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1249 AM CDT Mon May 1 2017

303 PM CDT

Through Tonight...

Showers are spreading back north over the area this mid-afternoon
with scattered thunderstorms favored after 7 pm. The flooding
threat will inch up within the Flash Flood Watch area with the
shower and storms this evening. The severe threat continues to be
minimal with any storms as instability remains very limited within
an otherwise robust environment.

Moist ascent through the lower and mid-levels is gradually
spreading back north over northern Illinois and northwest
Indiana per satellite and VWP trends. Rain will continue to
advance back north with this as well, with mainly light rates
observed in central Illinois at present. As forcing for ascent
increases owing to the approach of the large, mature cyclone
centered in Kansas, arcs of showers are likely to increase into
the evening.

The 997 mb surface low in located near the KS/MO border with the
eastward extension/triple point into north central/northeast MO.
The north end of the warm sector to its east remains shrouded in
clouds and clusters of showers, keeping the effective low-level
instability well to the south (nose is near the Ohio River as of 3
pm CDT). The initial warm front is in the far southern CWA, with
mid 60s up to Rensselaer, IN, and this should continue to slowly
advance north bringing warming temperatures across the
southern/central CWA into this evening. While there will be a
north to northeastward advance of low-level buoyancy into central
Illinois during early to mid-evening as indicated by strengthening
pressure falls, the axis for thunderstorms with any residence
time near the effective warm frontal boundary should be minimal,
and most favored to the south of the area. This could be close to
the southeast CWA though, where storms will have to monitored
east of I-55 and south of I-80 between 7-11 pm.

Individual showers and storms during the evening period will be
moving at a quick pace (50+ mph). While this will be the time
when they tap into the highest PWAT air (1.50"+), the quick
motion and semi-discrete modes may limit flash flood potential.
The metro area southwest across the Illinois River Valley that was
hard hit yesterday will be the most prone to some flooding if
multiple rounds of showers/storms were to pass over.

As the dry slot moves over late evening into overnight, shower
coverage will greatly diminish from west to east. Wind speeds
will actually drop off for a period owing to the bagginess of the
pressure field near the low center, and there could be some
temporary fog development.



228 PM CDT

Monday through Sunday...

The surface and upper level low shift east over the region Monday
bringing one more day of light showers. High pressure arrives
Tuesday and helps keep the next surface low to our southeast.
Temperatures slowly warm through the extended and mid 60s are
possible late this week.

Outside of the NAM, guidance has the surface low traveling over WI
and the northern end of the lake through Monday night. Light to
moderate showers are expected Monday with better coverage northwest
of I-55 closer to the low`s main vorticity wave. Cooler air will
move in with the trough leading to low to mid 50s across the region.
Breezy conditions are also still expected with southwest winds
gusting to around 35 MPH. If we realize maximum mixing, gusts could
be pushing 40 MPH.

Leaning toward the ECMWF which has the trough shifting NE quicker
than the GFS. Thinking Tuesday will be dry and some areas may see
peaks of sun in the afternoon. High pressure builds over the region
Tuesday night and persists into Wednesday. An upper level wave sinks
southeast over the plains and helps the next surface low form over
the Gulf coast Wednesday night. The northern end of the rain shield
may clip the southeast corner of the forecast area Wednesday
evening. Precip may cover points south of a Pontiac, IL to
Valparaiso, IN Thursday.  The system shifts east, but guidance
differs on how long the cooler air associated with the trough will
hang out over the region.  The GFS warms the upper levels quicker
than the ECMWF.

Both models feature a secondary wave that dives southeast across the
lake Saturday.  A quick shot of rain is possible and lake cooling
due to onshore winds. Mid 60s look probable away from the lake.



For the 06Z TAFs...

A large and slow moving storm system will continue to impact the
weather across the region through the period. While this will be
the case, the thunderstorm threat is quickly coming to an end in
response to a mid-level dry slot beginning to shift in over
northern Illinois. In the next hour there will continue to be a
period or two of showers, especially over the eastern terminal
sites, as activity currently to the south of the terminals shifts
northward. Thereafter, it appears a period of VFR CIGs will
persist into the early to mid morning hours, before some MVFR CIGs
move back across the area and continue through the day. Winds
will also continue to be light out of a north-northeasterly
direction for the next 1 to 2 hours, before a warm front moves
northward over the eastern terminals. This will result in a short
period of southerly winds overnight before they shift southwest
around daybreak following a cold frontal passage. Expect gusty
southwest winds through the day Monday, along with additional
periods of light afternoon showers.



228 PM CDT

Headlines...Still looking at a busy period over the lake. The Small
Craft Advisory continues through Tuesday afternoon as does the Gale
Warning for the northern end of the lake this afternoon into
tonight.  Gales look possible again over the southern half Monday
afternoon into Tuesday morning so went with a Gale Watch.

The low over Kansas will deepen as it reaches Iowa late tonight,
and then it will continue across the northern end of the lake to
Quebec by Tuesday afternoon.  As the low approaches and then passes
over the northern end of the lake Monday and Monday night, winds
will become south to southwest and increase to low end gales over
the southern half. Winds become west behind the low Tuesday, but
continue to diminish and veer to northwest as high pressure builds
over the lake Tuesday night into Wednesday.  Light and variable
winds are expected under the high. The next low forms over the
southern Mississippi Valley late Wednesday night and deepens as it
travels up the Ohio Valley to Quebec through the end of the week.
High pressure will lie to the east of the low, and some models
suggest the pressure gradient would be strong enough to support
gales. Other models are not as supportive, but kept north gales as a
possibility in the forecast Friday afternoon.



IL...Flash Flood Watch...ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 until 7 AM Monday.

IN...Flash Flood Watch...INZ001-INZ002-INZ010-INZ011-INZ019 until 7
     AM Monday.

LM...Small Craft Advisory...nearshore waters
     until 4 PM Tuesday.

     Gale Watch...LMZ777-LMZ779...4 PM Monday to 10 AM Tuesday.




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