Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 291929

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
229 PM CDT Mon May 29 2017


1157 AM CDT

No big changes to the forecast for this afternoon, with scattered
showers and isolated thunderstorms still possible across the
area. Early morning isolated sprinkles and cloud cover have
diminished, with most locations still either sunny or partly
cloudy. With the arrival of large scale ascent from the north,
will see current cloud cover spread south into northern Illinois
by mid afternoon. Showery development will accompany these clouds,
with locations near the WI/IL border having the highest chances
of observing any precip today. Drier air in place has limited
instability, but steepening low level lapse rates will support
some isolated thunderstorms this afternoon, before any precip
diminishes this evening. Temps will steadily warm today with
slightly warmer temps around 80 now expected for locations in the
eastern half of the CWA.



252 AM CDT

Today and Tonight...

Main forecast concerns in the short term are with the potential
for isolated to scattered showers and thunderstorms later this
afternoon, and breezy west winds.

Early morning surface analysis depicts low pressure north of the
Great Lakes, across central Ontario. A secondary cold front
trailed from this low, bisecting the state of Iowa from northeast
to southwest. Aloft, a series of minor-amplitude short waves were
evident within northwest flow aloft, along a sheared mid-level
vorticity axis in association with an upper level jet streak which
will round the southwest and southern peripheries of the trough
and closed low north of the Lakes. Several models indicate the
potential for some elevated shower development ahead of the trough
axis and low level frontal zone across northwest IL by sunrise,
where a patch of mid-level clouds currently exists northwest of
DBQ. High-res CAM guidance generally limits this to northwest/far
north central IL along the WI border, with activity dissipating or
moving east-northeast north of the WI border by 14-15Z this
morning. Otherwise, it appears most areas will remain dry through
the morning hours, until diurnal heating eventually results in the
development of some weak surface-based instability this afternoon.
The greatest focus for shower/thunderstorm development is expected
to be across far northern IL (and northern IA into WI) beneath
slightly cooler mid-level temps, and across the far southeast or
off to the southeast of the cwa where the secondary cold front
ends up later today. Weakly unstable conditions and lack of low
level convergent focus should result in convection being fairly
disorganized and relatively low coverage, with many dry hours in
most areas. Loss of diurnal instability should allow for
dissipation of any lingering showers with sunset this evening.

Forecast soundings do indicate fairly deeply mixed boundary layer
conditions today, and while the surface pressure gradient is not
overly strong, 25-30 kt winds within the top of the mixed layer
will make for some breezy/gusty west winds to around 30 mph. Low
level thermal progs support temps ranging from the low-mid 70`s
north, to the upper 70`s to near 80 along our ILX/IND borders.



227 PM CDT

Tuesday night through Monday...

Showers diminish and push east Tuesday night as a secondary cold
front and the upper level vort streamer move through.  High pressure
slides over or just south of the forecast area Wednesday. With ample
sunshine, raised high temps a couple of degrees Wednesday.

Active weather returns Thursday and appears to last through the
weekend. Temps and dewpoints increase Thursday. Thursday could be
warmer than currently forecast as local guidance suggests 80s are
possible, but with clouds and precip in the forecast, unsure on how
well we will warm.  The GFS and NAM both have a weakening MCS
reaching the forecast area Thursday afternoon. The GFS is much more
aggressive with precip than the other models, but a lingering MCV
could lead to new convection in the region. As such, left a chance
for showers and storms Thursday afternoon. Precip should become more
widespread Thursday night as a low level jet points into the region.
As the previous discussion suggested, main threat will probably be
heavy rainfall as PWAT values are around 1.25 inches, which is above
average. In addition, steering flow is weak so storms will likely be
slow moving.

Showers and storm persist through Friday. A backdoor cold front
slides down the lake and will provide additional forcing. Forecast
soundings feature 1000-1500 J/kg of CAPE, but shear is weak. PWAT
values remain above average so main threats will likely be heavy
rain and maybe hail.

Initially thought Saturday could be dry like the GFS suggests, but
the 12Z ECMWF came in with a stalled front and precip. Given
northwest flow aloft, kept slight chance precip chances in the
forecast.  The same is true for Sunday, models differ on if there
will be precip, but with northwest flow, I do not feel comfortable
going dry in the extended forecast. Models agree on high pressure
early next week so went dry for Monday.



For the 18Z TAFs...

VFR, dry, and with gust westerly winds in place across the
terminals at this time. Similar conditions expected for the
remainder of the afternoon, but with an increase in cloud cover
along with developing scattered showers likely. Anticipating the
bulk of any precip this afternoon to stay closer to the WI/IL
border, but can`t rule out an isolated shower across all of the
terminals. An isolated thunderstorm is also possible this
afternoon, but this development should rather limited. Both cloud
cover and any precip diminish this evening, with clear/VFR
conditions expected tonight. Similar pattern will provide
additional cloud cover and precip chances on Tuesday, but with
slightly higher chances. Anticipate winds to remain
west/southwest through the period, with gusts possible once again



227 PM CDT

Gusty west winds diminish and will no longer pose a threat to small
craft this evening. Low pressure will slowly shift northeast from
central Ontario to James Bay through Wednesday morning, and a high
will build over the plains through Tuesday night. As such, an
extended period of westerly winds is expected through Wednesday
night.  The high passes south of the lake Wednesday night. A trough
of low pressure shifts from western Ontario Thursday morning to the
northern Great Lakes Friday morning. A cold front will shift south
down the lake Friday and winds become north behind it.  High
pressure then builds over the western Great Lakes early next week.



LM...Small Craft Advisory...Winthrop Harbor to Gary IN until 7 PM




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