Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260208

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
908 PM CDT Thu May 25 2017


908 pm...No significant changes planned this evening. Lake breeze
boundary is dissipating with a thin band of stratocu along with
increasing high/mid clouds overnight. A weak ridge of high
pressure will move across the area tonight with light/variable or
calm winds at most locations. Have maintained patchy fog mention
but confidence is low. If any fog does form it will likely be
shallow and in the usual/favored locations.

Short term guidance continues to try to develop a few showers or
sprinkles overnight out of the approaching mid deck. There are a
few locations reporting sprinkles/light rain near the MN/IA
border. Maintained dry forecast for now and will monitored trends.


238 PM CDT

Through Friday...

The 19z experimental GOES-R imagery shows clearing skies
associated with a lake breeze across the WI/IL/IN shorelines of
Lake Michigan this afternoon. Thickness of the cloud cover is also
decreasing as the bases lift. Some peeks of sunshine will then be
likely through the evening with a scattering trend of the clouds.
Clouds are also taking on a more cumulus-nature as compared to
the stratus observed across the area earlier today. Churning low
pressure over the Ohio Valley will push further east through the
evening, allowing weak high pressure to influence the region. This
will bring lighter winds as the pressure gradient relaxes.
Moisture content remains quite low, but there will still be a
small chance for patchy fog in outlying rural areas tonight. This
is dependent on current cloud cover clearing out to allow for
radiational cooling and fog formation.

The 500mb pattern shows a departing trough in the eastern CONUS
as a ridge builds in from the Central Plains. This will allow for
warming temperatures the next couple days, and the flow will
become zonal for Friday. Satellite imagery is showing a southern
Pacific jet that continues to pump in moisture to the Southwest
US. A weak embedded wave near NM/TX will quickly shoot northeast
to help spark some showers for Friday. This wave will work in
association with developing low pressure and a warm front. There
is a marked increase in H85 theta-E and total totals index as
warmer and unstable air works into the region from the southwest.
While model guidance is quite variable with the probabilities for
convection or rain showers Friday, there remains a chance for
development from afternoon through evening.



253 PM CDT

Friday night through Thursday...

Zonal flow aloft early in the weekend will gradually become more
meridional through the period with longwave troughing expected to
result in active and seasonably cool weather much of the upcoming

Low amplitude shortwave and weak surface reflection will shift east
of the CWA after midnight Friday night with precipitation ending
behind the wave as mid level height rises overspread the region.
Most of the day Saturday should be dry under the influence of a
transient ridge building across the region. Northeast flow will keep
temperatures cooler along the lake front, in the upper 50s and low
60s, but inland expect the warmest day of the upcoming week with
highs in the upper 70s to around 80.

Attention will turn to the west later in the day as another
shortwave begins to carve out an increasingly amplified upper
trough over the northern Great Plains. At the surface, low
pressure will move across northern Missouri into central Illinois
Saturday evening. Showers and thunderstorms will increase in
advance of the low with the greatest focus along the warm front
which is expected to be draped across central Illinois. There
remains some uncertainty in the northward extent of precipitation
which may become cut off from the better moisture feed due to
ongoing convection to our south and for now have confined likely
PoPs to areas south of I-80 with just chance PoPs north.

There should be a lull in any precipitation starting Sunday morning
as the main 500mb vort passes overhead. Cold air advection into the
region will help steepen low level lapse rates during the day Sunday
which will help set the stage for isolated diurnal showers and
possibly a thunderstorm. Forcing on Sunday is fairly negligible
though there continue to be mid level height falls as an upper low
gradually meanders from the Canadian Prairies into the Upper
Midwest. Lack of any appreciable forcing or focus for ascent will
keep any precip coverage low on Sunday.

Upper low/trough axis will gradually shift east across the Great
Lakes early through the middle of next week which will keep the
local area under cooler northwest flow aloft. There will be periodic
chances for showers and possibly a thunderstorm or two as several
low amplitude waves rotate over the area. Steep low level lapse rate
and fairly minimal CIN will persist through the middle of next week
and precipitation will be diurnally favored.



For the 00Z TAFs...

628 pm...Main forecast concern is thunderstorm potential Friday
afternoon into early Friday evening.

A weak upper wave will move across IA this evening and into IL
overnight as it weakens. Models have been suggesting some light
showers or sprinkles are possible in the predawn hours of Friday
morning...mainly west of the Chicago terminals. Confidence is too
low to include any mention at rfd and duration would be short...if
anything did occur. A stronger upper wave will move across the
area Friday afternoon...though model guidance continues to differ
on timing and thunderstorm evolution with this wave. Latest
guidance would suggest showers with scattered thunderstorms may
develop over east/central IA late Friday morning and move across
northern IL during the mid/late afternoon. Confidence remains low
but instability begins to increase as this wave there
appears to be a high enough chance for some thunderstorms to
change current prob showers to prob thunder. Activity may continue
south of the terminals Friday evening and into northwest Indiana
Friday night.

East/northeast winds around 10kts behind a lake breeze will slowly
diminish this evening and winds are expected to become light and
variable at most locations as weak high pressure moves across the
area. Winds will shift light southeasterly by sunrise and then
increase into the 8-10kt range Friday morning. These southeasterly
winds may increase into the 10-15kt range by late morning and then
shift more easterly Friday afternoon as a weak area of low
pressure moves toward the area.

Sct/bkn 4-6kft cu/stratocu late this afternoon will dissipate this
evening with increasing high/mid clouds overnight...lowering to a
3-5kft cig Friday afternoon. Mvfr cigs are possible with precip
Friday afternoon and as the low arrives Friday evening...low mvfr
or ifr cigs are possible. cms


253 PM CDT

A moderate northerly gradient in place over Lake Michigan this
afternoon will gradually ease as low pressure over the Upper Ohio
Valley departs towards New England Friday. As winds ease, the
small craft advisory will be allowed to expire this afternoon for
the Illinois nearshore waters, but will continue a few more hours
into the evening as waves gradually diminish in the Indiana zones.
A transient ridge will pass over the lake Friday which will allow
for generally light flow across the lake and for afternoon lake
breezes to develop. Flow should remain relatively light through
the remainder of the weekend as the western lakes fall between two
areas of low pressure -- one moving east across the Canadian
Prairies and another lifting from the Southern Plains to the
central Great Lakes. Fresh west flow will develop early next week
as these two lows merge over Ontario and slowly lift to near James



IN...Beach Hazards Statement...INZ001-INZ002 until 10 PM Thursday.

LM...Small Craft Advisory...IN nearshore waters until 10 PM Thursday.




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