Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231756

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1256 PM CDT MON MAY 23 2016

250 AM CDT

Through Tuesday...

Today will be the tenth consecutive day without measurable
precipitation for Chicago...Rockford...and many CWA communities.
This is the longest such dry stretch in three to five months for
our area. As for the forecast for today...have increased high
temperatures several degrees areawide with mid 80s more prevalent.
Have not made any large changes otherwise to today through
Tuesday...with Tuesday looking to have scattered showers and some
storms with a low threat for severe weather.

Morning water vapor imagery and last evening soundings indicates
strong drying and suppression aloft associated with a narrow but
stout upper level ridge. Dew points at the surface remain mainly
in the 40s as return flow has just begun. With a clear sky today
it is a favorable air mass for temperatures to outperform model
guidance. The 850mb temperatures are forecast to remain at 13C to
15C like yesterday which brought a handful of mid 80s in our
area. Have thus went with fairly widespread mid 80s in the
Illinois portion of the forecast area away from the lake
shore...with lower 80s in northwest Indiana. Southeast winds will
keep lake adjacent areas of Cook and Lake Counties in northeast IL
likely in the 70s.

Deeper moisture and elevated instability across the Plains will
advance its leading edge into northern Illinois late tonight. A
short wave disturbance or two...possibly with some convective
enhancement from afternoon/evening convection to our
forecast to be oriented from southern Wisconsin through western
Illinois by daybreak Tuesday. Lift from this working along a 30 kt
southwesterly low-level jet should continue some showers/storms in
possibly an ill-defined MCS form into western and maybe north
central Illinois Tuesday morning. Given the stoutness of the ridge
and the inherent drier air mass...along with the weakening of the
LLJ as the morning expect shower/storm coverage to
drop off by early Tuesday afternoon which is supported by a
majority of guidance. Any redeveloping or festering convection
into the afternoon would have a limited potential to become strong
but the focus for most if not all convection very well could pass
with the short wave by midday Tuesday. So again not seeing much
for severe potential on Tuesday at this time.

While the air mass on Tuesday would support highs like today...the
morning cloud cover and some showers should keep temperatures at a
slower rate to climb. Have upper 70s to lower 80s forecast from
north to south.



250 AM CDT

Tuesday Night through Sunday...

The pattern remains active during mid to late week and possibly
into the beginning of the holiday weekend. The better chances for
storms appear centered on Wednesday and Wednesday night and then
again on Friday. The potential for scattered severe storms does
exist on Wednesday and Wednesday night.

The surface cool front is forecast to advance south into far
northern Illinois late Tuesday night and likely have some lake
enhancement to it. This front and potentially low clouds
along/north of it will keep northern locations cooler...though
confidence is low on just how much so. Moist advection aloft and
potential propagation of upstream convection will bring increasing
storm chances by Wednesday morning and growing into the afternoon
and evening as instability south of the boundary increases. Dew
points are forecast to reach the mid to upper 60s south of the
boundary providing modest instability. In addition...the boundary
looks to become more warm frontal by afternoon and begin to lift
north in response to a 1006 mb surface low lifting northeast over
the Upper Midwest. Mid-level winds of 30-40 kt are expected to
bring decent deep layer shear that working with the instability
could bring scattered severe storms in an environment that should
be favoring at least storms. This is most true for along/south of
the warm the southern CWA. So have likely
thunderstorms mentioned Wednesday into Wednesday night and more
precise timing will be refined as we draw closer.

Presently Thursday looks like a day in-between with less
convective chances though still quite mild with southwest winds
capable of bringing 80s all the way to the lakeshore. The upper
low off the northern CA shore this morning is forecast by the GFS
and EC to track northeast across the Midwest as a fairly strong
wave on Friday and Friday Night. Some slowing potential exists
with this into Saturday given the recent pattern. Above normal
heights and southerly low-level flow look to continue supporting
80s into the holiday weekend.




Main concern through this evening remains wind direction and the
impact of a lake breeze. Guidance has been very inconsistent on
the development of a lake breeze and the resultant wind shift.
Much of the guidance is backing off on the idea that winds will
turn ELY and is settling into a more SELY direction and that wind
speeds may be a bit lower. Latest radar imagery and coastal obs
showing the lake breeze just inland into far NERN IL amd making
very slow progress. So, will continue to monitor lake breeze
progress, but currently thinking that there is less of a chance
for winds to shift ely. Winds should diminish to arnd 5 kt at

Thunderstorms are developing across parts of IA and MO this
afternoon and are expected to slowly move east this evening and
tonight. There is still a chance of this activity reaching RFD
shortly before 12z tomorrow morning and then possibly across the
rest of the terminals mid/late morning. Given the continued
uncertainty...with some guidance suggesting that the activity
could diminish in the morning hours and then refiring in the
afternoon, have opted to just go with a prob30 mention.




110 am...High pressure centered over the eastern lakes this
morning will move southeast to the Carolinas Tuesday morning. The
gradient will tighten this afternoon into tonight ahead of cold
front with southerly winds increasing to 15 to 25 kt. Speeds will
begin to diminish Tuesday afternoon with the cold front moving
across northern parts of Lake Michigan Tuesday evening and then
moving to the southern part of Lake Michigan by Wednesday morning.
Winds will shift northerly behind the front but the gradient is
not particularly strong so not expecting more then 10-20
most. As low pressure moves across the northern plains Wednesday
night into Thursday...this frontal boundary will lift back north
across the lake as a warm front with southerly winds then expected
into this weekend. cms





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