Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251046

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
546 AM CDT Sun Jun 25 2017

226 AM CDT

Today and Tonight...

Near term weather focus is on cooler than average temps, breezy west
to northwest winds and the potential for isolated late afternoon
showers and thunderstorms.

Fairly strong mid-level vort circulation is noted moving east across
central Lake Michigan early this morning in GOES-16 water vapor
imagery. Patchy mid-level cloud cover lingers across the southeast
half or so of the cwa within weak isentropic upglide region ahead of
the mid-level trough axis, though isolated showers have dissipated
and/or moved east of the area. Guidance indicates a period of
subsidence across the western Lakes region in the wake of this
feature this morning, indicated by 50-60 meter 500 mb height rises.
Gradual cool-advection does persist today, with 850 mb temps
decreasing from about +8 to +6C through this afternoon. These cooler
temps aloft should again support development of a broken stratocu
deck by mid-day or early afternoon, as diurnal heating produces
steepening low-level lapse rates and a deepening boundary layer.
Despite relatively cool air aloft, deeper layer lapse rates are not
that impressive in various forecast soundings today, with generally
less than 65-75 J/kg MLCAPES. In addition, water vapor imagery,
RAP analysis fields and guidance progs indicate weaker mid-level
forcing today with a generally sheared vorticity channel devoid of
sharper features such as the one moving across the region early
this morning. Not surprisingly, much of the high-res convection
allowing model guidance depicts fairly low coverage/intensity of
any isolated shower/thunderstorms across the cwa this afternoon.
Based on this have backed off on pops to just a slight
chance/isolated mention of less than 20 percent across the
northern third or so of the cwa during the late afternoon hours
today, and suspect many areas will remain dry.

Cooler air aloft looks to keep afternoon highs a little cooler as
well today. Model 925 mb temps range from around +12C north to near
+15C south by afternoon, which allowing for slightly deeper mixing
(as has been the case the past couple of days) yields highs from the
lower 70`s near the IL/WI border to the mid 70`s farther to the
south. Mixing to near 800 mb per forecast soundings will also
support breezy conditions with west-northwest winds expected to
become gusty at 20-25 kts (23-29 mph). Winds will diminish with
sunset this evening and cooling/stabilization of the boundary layer.
Another stronger short wave rotates southeastward across the Upper
Midwest later tonight, though much of the forcing remains north and
northwest of the forecast area. Some guidance does develop some
light precip back to the west across IA where modest frontogenesis
is noted along a tightening baroclinic zone there. Dry weather
expected locally, with partly cloudy skies and cool temps in the



226 AM CDT

Monday through Saturday...

An anomalous mid/upper trough will be over the region to start
the period, favoring comfortably cooler than average temperatures
and an overall dry air mass. Despite the overall dry air mass on
Monday (dew points in the 40s), the mid day through afternoon
timeframe will offer some isolated to widely scattered shower
potential as short-wave disturbances pivot around the trough over
the Great Lakes. Steep low and mid level lapse rates in cool
profile (highs in upper 60s-low 70s) will yield a few hundred
j/kg of skinny CAPE, so isolated gusty thunderstorms are also
possible in the afternoon. A weak surface low will be tracking
across the northern Lakes, with trailing cold front moving across
the area and shifting breezy west to west-northwest winds more
northwesterly by late day/early evening. It`s possible that a few
isolated showers could linger into the evening across portions of
northeast Illinois and northwest Indiana with final wave wrapping

The cool and breezy conditions on Monday will be followed by a
fairly quick transition to a more zonal pattern by the middle of
the work week as the Great Lakes trough exits east. After a chilly
Monday night-early Tuesday with lows in the upper 40s to low 50s
outside Chicago, Tuesday will be the transition day. Surface
ridging and a still very dry low-level air mass will favor a
pleasant summer day with highs well into the 70s under plenty of
sunshine. Weak flow aloft on northwest side of the surface high
and land/lake thermal differential could enable a lake breeze in
the afternoon, but do not yet have enough confidence to include it
in the grids.

Broad lower than average heights will be in place over the
Intermountain West and Plains by Wednesday, with subtropical
ridging setting up off the Southeast and Mid Atlantic Coast on
Thursday and Friday. Ensembles are in decent agreement in the
large scale pattern, so confidence is decent in this. The
zonal/westerly to west-southwest mid-upper level flow pattern will
favor multiple convectively modified short-wave disturbances
traversing the region Wednesday-Friday and a return to more
seasonable temperatures and moist dewpoints. Therefore, the mid
to late week period appears to be fairly active with periodic
precipitation/thunder potential. The synoptic set-up could also be
one to monitor for a strong/severe thunderstorm potential, as
guidance is indicating relatively strong wind fields supportive of
favorable bulk shear.

Wednesday afternoon and evening will be the first period to watch
as a warm front lifts north across the region and surface low
pressure passes to our north, possibly followed by sometime
Thursday-Friday as well. Any strong/severe risk will be pending
evolution of surface pattern due to influence of ongoing
convection as well as influence of upstream convection/MCS
activity. Models are also indicating the potential for
anomalously high precipitable water values around 2" at times late
Wednesday through Friday, so a risk for heavy downpours and at
least localized flooding may also evolve. Finally, depending on
the track and strength of the surface low on Wednesday, it could
be quite windy as well with south-southwest winds gusting to 30-35

It appears that a cold front trailing from low pressure somewhere
over the region may move across the area on Friday, the timing of
which would also affect any strong/severe threat. While zonal
mid-upper flow will remain into next weekend, Friday`s possible
cold front passage could set the stage for a drier/quieter Friday
night and Saturday. While confidence in medium-high in large scale
pattern as mentioned previously, confidence in the day to day
sensible weather details is low as is common this time of year
with mesoscale convective uncertainties that will need to be
sorted out.



For the 12Z TAFs...

Breezy west-northwest winds and VFR conditions expected again
today...with low probability potential for a late afternoon
isolated shower or thunderstorm.

Area remains in modestly tight west-northwesterly pressure
gradient today between low pressure over the northern Lakes and
high pressure well to the west across the Plains. Diurnal heating
and mixing of the boundary layer will bring winds up again...with
gusts 20-25 kt by midday/early afternoon. Cool air aloft and
development of steep low level lapse rates will again result in
sct-bkn VFR stratocu...and while an isolated shower or
thunderstorm cannot be ruled out later this appears
that forcing will be weaker aloft than that which brought decaying
showers to the area last evening. Thus TAF`s are dry...with probs
less than 30 percent of precip at a given TAF site.



323 AM CDT

A few periods of stronger winds are expected through mid week.
Current indications favor tonight into Monday for 25 to 30 kt
west-northwest winds on the central and southern portion of the
lake. A weak surface low will move across the northern Lakes
tonight into Monday, with its trailing cold front shifting winds
to northerly up to 25 kt on the northern portion of the lake.
Winds will remain northerly on the north half Monday afternoon
and shift to northwest on the southern and central portion of the
lake behind the cold front passage. Hazardous conditions are
possible for small craft on Monday, especially for the Indiana
nearshore where waves could build to 3 to 6 feet.

The next period of interest for strong winds is on Wednesday. An
unseasonably deep surface low will move near or over northern
Lake Michigan Wednesday night or Thursday. Very strong southerly
winds are likely on Wednesday and Wednesday evening, shifting to
southwest after midnight Wednesday night. Forecast models are
exhibiting some variance on the strength of the surface low and
also its track and timing. This will affect the magnitude of top
end wind speeds/gusts on Wednesday and Wednesday evening. The
current forecast indicates gale potential on the northern portion
of the lake, with up to 30 kt on the south half. Wind speeds will
likely be hazardous for small craft in the Illinois and Indiana
nearshore. The cold front trailing from the surface low will move
down the lake on Thursday, with uncertainty regarding whether the
front clears the lake by Thursday evening or stalls over the
southern portion of the lake.






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