Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260720

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
220 AM CDT Wed Apr 26 2017

933 PM CDT

For Evening Update:

Minor changes to going forecast mainly to bump min temps a bit
with readings slow to fall with modest but persistent south flow
and cloud cover. Also tweaked pops a bit, focused mainly on
western IL after midnight.

Elongated trough of low pressure was analyzed this evening from
West Texas to the Mid-Mississippi Valley. Low level south winds
in advance of this feature were tapping Gulf moisture along the
Texas and Louisiana coasts, and this moisture was beginning to
stream northward into the Midwest. A few high-based sprinkles have
already developed in an isolated fashion across parts of the
region this evening, though mosaic radar presentation is likely
over-representing the lower level returns and little precip is
reaching the ground. Warm/moist ascent will persist overnight
however, in association with a series of mid-level disturbances
ejecting from the base of an upper trough noted across the Rockies
and Plains. Moisture transport and advection of higher thete-E
air mass is progged to increase overnight with development of a
30-35 kt low level jet, beneath an elevated mixed layer (EML)
based around 700 mb as evident in 00Z DVN sounding. This increase
in low level moisture, and modest mid-level cooling associated
with a short wave feature currently lifting northeast across
eastern KS/OK, should tend to support elevated convective
development into parts of far western/northwest IL after midnight
tonight as depicted by several convective-allowing models. Capping
inversion looks to hold a bit stronger farther to the east per
forecast soundings though, and recent trends in CAM guidance is to
weaken/dissipate activity as it tries to spread east late
tonight. Guidance is also in good agreement however, in spreading
convection northeast across the area after sunrise Wednesday, with
model simulated reflectivity suggesting a remnant MCV (emanated
from current thunderstorms over southeastern KS) may lift across
the area during the morning. Otherwise, guidance appears to focus
on the very late afternoon and more so the early-mid evening hours
of Wednesday for the greatest potential for widespread
thunderstorms across the area. Deep south-southwesterly flow
develops across the region, resulting in sufficient speed shear
for strong to severe organized convection later in the day.

In the near term, have bumped min temps overnight based on
observed trends this evening, with temps slow to fall off under
cloud cover and with persistent south winds. Will likely trim pops
a bit in the east through Sunrise too based on recent high-res CAM
guidance as discussed above.



208 PM CDT

Through Wednesday...

For the remainder of the afternoon and overnight, conditions will be
relatively quiet.  Main forecast concern will be timing and
intensity of tsra expected to move into the region tomorrow.

With low pressure tracking north along the Atlantic coast, a ridge
of high pressure will remain parked from New England through the ern
Gulf Coast.  With deepening low pressure over the central plains and
a trough extending north into the upper Mississippi Valley, low level
flow will be off of the Gulf of Mexico and deep layer moisture will
be on the increase with sfc dewpoints climbing into the lower 60s,
temperatures in the mid to upper 70s and pwats approaching 1.5
inches. With the increasing moisture and instability as well as a
wind profile with a 50 kt low level jet extending into nrn IL ahead
of the approaching cold front, severe thunderstorms will be a
possibility and SPC has extended the Slight Risk for severe
thunderstorms through central IL into the far southern portions of
the Chicago Metro area with a Marginal Risk over the remainder of
the CWA, including the Rockford area and northwestern Indiana.  At
this point, the main severe threat should be for large hail and
strong straight line winds. Through the day, isolated severe storms
could evolve upscale into more linear bow echoes and strong winds
becoming the main severe threat.  However, the progression of the
pattern is looking to become less progressive through the day
tomorrow and heavy rainfall and flooding may become an increasing
concern, especially into the evening hours.


329 PM CDT

Wednesday night through Tuesday...

Main forecast concerns/issues are with a continued severe and
heavy rainfall threat Wednesday evening, as well as returning
chances for showers and thunderstorms over the weekend which could
provide periods of stronger development as well as periods of
heavy rainfall.

As was mentioned in the short term AFD, the severe threat may
transition over to heavy rainfall and possible flooding threats in
the evening. The severe threat will likely continue for a time in
the evening though, especially across the eastern CWA, far
northeast Illinois and northwest Indiana. This is as a front
pushes through the remaining CWA, but with a weakening trend of
the overall instability while this occurs. Once again, showers
likely to continue for much of the night. Strong ascent owing to
negatively tilted trough overhead, lingering instability, as well
as high PWAT axis still in place will support this setup. Have
increased pops during the Wednesday night time frame, along with
QPF. Given the convective nature of the anticipated precip, likely
did not increase enough. Most locations will likely get into this
continued precip axis, however, it does appear that northeast
Illinois and northwest Indiana will be a continued favored
location. System will finally kick out of the region Thursday with
any lingering showers in the morning exiting to the east.

After somewhat of a lull period, active weather returns on Friday
with showers returning in the afternoon and evening. At this
time, not overly concerned for hazardous weather, either stronger
storms or additional heavy rainfall during this period. However,
this begins to change on Saturday as attention turns towards
expected large system to slowly move through the central CONUS
through the weekend. Increasing moisture and instability will
assist in returning thunderstorm chances across the area Saturday
and Saturday night, and then likely continuing Sunday and quite
possibly Sunday night. Details with exact solution/evolution still
to come, but still monitoring for possible periods of stronger
thunderstorms and heavy rainfall that could produce flooding



For the 06Z TAFs...

1235 am...Forecast concerns include thunderstorm potential later
this morning and again this afternoon into this evening...wind
directions this afternoon into this evening and cigs this
afternoon through tonight.

A fairly complex pattern is setting up across the region for the
next 36 hours or so. Low pressure over eastern IA will lift into
WI early this morning. A band of showers and possibly some thunder
is expected to move across the terminals after sunrise.
Instability is limited during this time period and confidence on
thunder coverage is low. A second low pressure over OK early
this morning will move to northeast IL this evening. Additional
showers and thunderstorms are expected to develop ahead of this
approaching low during the mid/late afternoon hours. While
thunderstorms are expected...only medium confidence on both timing
and location. Storms could be later than current 22z timing and
potentially be a bit further east...perhaps gyy/ikk/pnt. Trends
will need to be monitored later this morning. As the low
approaches a period of moderate to perhaps heavy rain is expected
during the mid/late evening...with lingering showers into early
Thursday morning.

South/Southeasterly winds around 10kts overnight are expected to
turn more to the south/southwest by mid morning. Confidence is low
regarding how much speeds/gusts will increase this morning with
cloudy skies potentially limiting mixing. Gusts into the mid to
perhaps upper 20kt range will be possible. Wind directions from
late this afternoon through mid evening will be problematic since
the low is going to move right over the Chicago terminals. Some
shift to southeast or east seems plausible but if the low were to
track just a little further east...winds would quickly turn
northerly and then northwesterly. Changes...perhaps large changes
to the wind forecast are possible with later forecasts. Eventually
winds will turn more to the northwest late this evening into early
Thursday morning.

Cigs will gradually lower through vfr today with a period of mvfr
possible with showers this morning. The mvfr cigs may continue
across northwest IL/rfd today. Cigs will then lower through mvfr
late this afternoon and into ifr this evening or overnight as the
low approaches. Only medium confidence on cig trends. cms



220 am...One area of low pressure over eastern IA will lift
northeast into WI this morning as it slowly weakens. A trailing
cold front extends to a second area of low pressure over the
southern plains. This low will move northeast over Lake Michigan
tonight. Southerly winds today will shift more southeasterly this
evening and then west/southwest Thursday as the low continues
moving into Ontario. Small changes to the track of the low will
have large impacts on wind directions. Thus...changes to wind
directions are possible with later forecasts.

A weak ridge of high pressure will move across the lakes region
Friday and remain nearly stationary across the northern lakes high pressure then builds across Ontario. Low
pressure will develop over the southern plains and eventually
lift northeast to the western Great lakes Sunday night. The
gradient between the high to the north and this approaching low
will slowly tighten with northeasterly 30kts expected and the
potential for some low end gales...especially over northern Lake
Michigan by Sunday evening. cms


LM...Small Craft Advisory...nearshore waters until 11 PM




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