Area Forecast Discussion
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FXUS63 KLOT 291128

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

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.



255 AM CDT

Tuesday through Sunday...

Our friendly upper level low across the northern
Great Lakes will only slowly pull northeast of the region over the
course of the week. While the low will still partially play a role
in our local weather through a good chunk of the week, we really
only feel its influence from a potential precipitation perspective
for one more day on Tuesday. This is due to upper level energy
still rotating on the back side of the low and  as upper level
heights remain relatively low at the base of the trough axis. One
such shortwave will lead to some showers on Tuesday. NAM forecast
soundings show some capping in the morning thus possibly a diurnal
uptick would be expected in this pattern. While the GFS shows
limited instability, the NAM shows some weak instability (up to
around 500 J/KG) and around 35 kt of bulk shear thus some chance
of thunder.

And while we do still remain in northwest flow Wednesday, upper
heights will be rising and thus lapse rates weaken. Also surface
high pressure will push southeast, thus we remain largely dry, but
seasonally a tad cool with highs in the upper 60s to low 70s.

The low level frontal boundary to our south will lift northward on
Thursday as a warm front ahead of southwest lower level flow
beneath a ridge across the high plains states. There appears a
hint of a convective complex developing along the nose of the low
level jet across the plains Wednesday night that will likely be in
somewhat of decaying state on Thursday to our west. Thursday night
the low level jet will veer a bit more westerly and point toward
our region as the warm front will be lifting through northeast
Illinois. This would be a favored period for showers and
thunderstorms. Shear is not that great, but slow storm motion
would suggest some good rain could fall.

With high pressure to the north, a back door cold front is
advertised by the GFS/EC. After some decay in the activity, the
front could get active again Friday afternoon with warm moist
southwest flow. After that the pattern suggests the front lingers
in the region with active northwest flow to our north. The
position of the front will play a big role on whether we continue
showers and storms Saturday or if we remain stable to the north
of the front with its position being across central Illinois. Most
guidance has the front through Saturday night/Sunday.



For the 12Z TAFs...

Primary terminal concerns remain isolated/scattered shower
potential today, and gusty west winds.

Low pressure was north of the Lakes this morning, with a secondary
cold front trailing into northern IL. This front will become
oriented from southwest to northeast to the south of the terminals
by this afternoon. Mid-level cloud deck was noted from eastern IA
into southern WI, with radar indicating some isolated light shra
with these. Expect only RFD will be affected by these early this
morning, with high-res models consistent in dissipating precip by
14-15Z. Isolated to scattered showers and perhaps a thunderstorm
or two are expected to redevelop across a similar area later this
afternoon, in response to daytime heating and a series of minor
mid level disturbances. The frontal boundary south of the
terminals may also serve as a focus for showers and isolated
thunderstorms later this afternoon. Precipitation will dissipate
with sunset this evening. VFR conditions expected to persist.

Winds were in the process of shifting from southwest to west
across the area at this hour with the weak cold frontal passage.
Heating will deepen the mixed layer, producing gusty west winds
25-30 kts by midday/afternoon. Winds will also subside with loss
of heating this evening.



255 AM CDT

Expect another breezy day with west winds approaching 25
kt as low pressure lingers across western Ontario while high
pressure across west central Canada expands southward across the
plains. This pattern continues into Tuesday though with some
slight weakening to the pressure gradient. The high will then
spread to the southern tip of the lake late Wendesday. The high
will spread to the mid atlantic coast Thursday as a warm front
lifts northeast across at least a portion of the lake Thursday
night into early Friday. Late in the week a backdoor cold front
will shift south across Lake Michigan with a quick shift to
northeast winds.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...10 AM
     Monday to 7 PM Monday.




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