Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220214

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
914 PM CDT Fri Apr 21 2017

914 PM CDT

For Evening Update:

Minor adjustments to going forecast this evening, mainly to
increase cloud cover a bit for the dense cirrus overcast which is
expected to persist much of the night. In turn, also nudged min
temps up a degree or two farther north overnight, and also removed
mention of frost given mid-upper 30`s temps, cloud cover and 8-9
kts of wind.

Surface high pressure was analyzed across the Northern Plains and
Upper Mississippi Valley this evening, with persistent north-
northeast winds across the forecast area. To our south, low level
baroclinic zone stretches from Texas to south of the Ohio Valley,
over which a series of mid-level impulses were noted. Extensive
mid and high clouds extended north of this zone, across most of
Iowa, southern Wisconsin and northern Illinois and Indiana. Model
forecast soundings indicate high-level RH remains in place across
the area into early Saturday morning, thus expect broken-overcast
high clouds to persist overnight. Given the high cloud cover,
favor forecast min temps tonight closer to the slightly warmer MAV
guidance numbers - generally mid-upper 30`s in the coolest spots
over far northern IL and parts of northwest IN. Patchy frost,
while not impossible toward the WI border where thinner cloud
cover and lightest winds will generally be juxtaposed, doesn`t
appear likely to be widespread if it occurs.

Updated digital/text forecast updates available.



243 PM CDT

Through Saturday...

High pressure continues to work its way across the region this
afternoon, with quiet and dry conditions in place. Expect this to
persist through the period, while upstream system stays just to
the south of the CWA tonight through Saturday. Outside of stratocu
in place, still feeling the effects of this upstream system
through rather dense cirrus overhead. The more dense cirrus is
sliding to the east at this time, but satellite imagery depicting
additional thicker cirrus approaching from Iowa. Did increase
cloud cover through early this evening to keep some mention of
cloud cover over northern Illinois, while the southern CWA
observers more of this cloud cover. Should see a trend for
clearing skies from north to south tonight, however, its quite
possible that the entire CWA observes additional cloud cover from
this system overnight. Cooler temps around 40 expected tonight,
with upper 30s still possible over north central Illinois. Cloud
cover and slightly elevated winds tonight may impact night time
lows, including the extent of any frost tonight. Did maintain
patchy frost mention over north central Illinois, but with this
cloud cover and steady winds, there is a strong possibility that
this front remains really limited.



248 PM CDT

Saturday Night through Friday...

High pressure will build across the region Sunday into Monday
resulting in fair and mild conditions across the region. Synoptic
flow remains out of the north-northeast albeit light Sunday which
will result in continued cooler temperatures near the lake front.
Expect an afternoon lake breeze to develop and locally enhance the
winds and help to spread those cooler temperatures farther
inland. Winds will swing around to the south-southeast Monday
behind the ridge axis and an afternoon lake breeze appears
favorable once again for areas along the Illinois lake front,
especially Lake County and northern Cook County.

More active weather is expected to develop as we approach the
middle of next week as a series of upper level disturbances and
an increasingly amplified pattern develops across the mid section
of the country. Broad corridor of deep southerly flow will promote
good moisture transport from the Gulf into the Midwest with PWats
increasing to just over and inch Tuesday as shortwave trough
lifts across portions of the Upper Midwest. Models are in decent
agreement handling the low amplitude wave though forecast
soundings do show dry low levels slow to saturate through the day
Tuesday so expect highest PoPs late afternoon into the evening.

Wednesday through the end of the week, a strong upper wave is
expected to carve out a deep trough over the Intermountain West
which will set the stage for a classic severe weather setup over
portions of the Great Plains and potentially into portions of the
Midwest. A deep upper trough axis centered over the Desert
Southwest and a Bermuda High will allow a wide open gulf and good
moisture transport into the region while aloft strong flow is
expected as a negatively tilted shortwave trough lifts across the
region. Way to early to get into specific location and timing
details, but initial indications are for the possibility of
several rounds of showers and thunderstorms locally Thursday
through the weekend along with much warmer and potentially summer-
like temperatures starting Friday. Warm front lifting across the
region Thursday will bring a shot of showers and thunderstorms
with temperatures possibly well into the 80s as we get into the
warm sector Friday and/or Saturday. As models begin to hone in on
the timing and track of the surface low and attendant front over
the next few days, any local severe thunderstorm threat should
become more clear, but in the meantime just bears close watching
until the details come into better focus.



For the 00Z TAFs...

A storm system tracking south of the area tonight into Saturday
will help maintain a northeasterly wind across the region through
the period as high pressure continues to dominate across the
upper Midwest. Otherwise, VFR conditions are expected through the
period with only high cloud cover.



248 PM CDT

Modest north to northeast flow will continue over Lake Michigan
through midday Saturday as low pressure continues to depart east
towards the New England coast and as high pressure builds into the
western Great Lakes. This ridge axis will begin to settle south
across Lake Michigan through the day Saturday with winds turning
southerly north of the ridge axis. Any southerly flow looks to be
short lived as another low moves east across Ontario into Quebec
over the weekend with a trailing cold front moving south across
Lake Michigan Sunday. Low pressure will deepen over the Plains
early next week then lift across the western Great Lakes Monday
night into Tuesday with winds swinging around to the south across
the entire lake and peaking around 30 kt during this time frame.
An active weather patten is expected from the middle of next week
into next weekend as a series of lows move across the region.



LM...Small Craft Advisory...nearshore waters
     until 10 AM Saturday.




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