Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 191739

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1139 AM CST Thu Jan 19 2017

1051 AM CST

Morning Update...

Updates to going forecast primarily for coverage of fog/patchy
dense across far northern Illinois through midday, and to lower
temps through the day as area remains socked in with stratus and
shallow but strong inversion.

Had updated for fog earlier generally across northern IL along and
north of the I-88 corridor where visibility trended down after
sunrise. Visibility has shown a slow improving trend over the past
1-2 hours and expect this to continue, with true <1/4SM dense fog
limited. Visibility will likely linger in the 2-5SM range into
the afternoon however as temps and dew points slowly rise with
warm/moist advection from the south-southeast. 12Z raob from DVN
shows shallow but strong inversion which in concert with low
stratus and fog has been holding temps down in the low/mid 30`s
across much of the area. None of the available guidance has this
handled well, and have generally taken some of the cooler hourly
guidance and lowered it 3-5 degrees F with highs from the lower
40`s expected south/southeast counties and upper 30`s farther

Area of rain/convective showers currently noted in regional radar
mosaic from central Iowa into the lower Ohio Valley will spread
north and increase in coverage this afternoon. Expect this to fill
in across our south/southwest counties over the next few hours, so
no big changes made to pops/timing for now.

Updated grids/zfp already available.



255 AM CST

Through tonight...

Early morning water vapor imagery reveals a highly amplified
pattern across the country with the jet stream originating in
central Mexico and oriented north-northeast into the Midwest.
While this is resulting in an unseasonably warm airmass over the
region, it will also be transporting an unusual amount of moisture
into the area through the day today. PWats are expected to tag 1.0
inch across portions of the forecast area this afternoon and
evening which would represent near max values per observed upper
air climatology from ILX/DVN and around 300 percent of the normal
value for this time of year. As a closed upper low currently over
the Oklahoma/Texas panhandle region lifts towards the Upper
Midwest this evening and tonight, expect precip chances to
increase from the south through the afternoon and should peak
across northern Illinois and northwest Indiana this evening and
overnight. Storm total QPF amounts of half to three quarters of an
inch are generally expected, with the highest amounts across the
eastern half of the CWA. In addition, models continue to advertise
100-200 J/kg MUCAPE overspreading the CWA which will support
widely scattered embedded thunderstorms and locally higher
rainfall totals. With recent rainfall already causing flooding
issues, ice jams on portions of the Rock River, and numerous area
streams and rivers either already in flood or near bankfull, there
is obvious concern for additional flooding to occur as we receive
new rainfall on top of still frozen sub-soil conditions. For now,
have opted to issue an SPS to highlight the threat, but those with
hydro interests should keep a close eye on updates to river
forecasts over the next couple days.



255 AM CST

Friday through Wednesday...

Active weather continues throughout the extended forecast with a
series of waves moving across the country resulting in periodic
chances for precipitation. We should see a brief lull in rain
during the day Friday as our upper low departs northeast of the
area and weak mid/upper ridging builds overhead. Meanwhile,
another wave is progged to round the base of the longwave trough
over the Central Plains early Friday and lift into the Upper
Midwest Friday night into Saturday. Continue to carry a chance for
rain with this wave but the better chances and amounts are still
focused to our west. 850 mb temps are still expected to peak in
the 6-8C range early Saturday out ahead of the trough and
afternoon highs are still expected to top out in the mid 50s. A
stronger upper wave/cutoff low is progged to undercut the region
late in the weekend and lift across the Appalachians early next
week. Temperatures are expected to moderate some as northwest flow
develops behind this low and there will be periodic chances for
precip as an inverted trough slowly rotates through the area.
Models seem to be in better agreement at least on the track of the
500 mb low through Monday, though timing differences persist, and
solutions begin to diverge more significantly beyond Monday.



For the 18Z TAFs...

Main TAF concerns are timing of IFR/LIFR cig/vis trends through
the period, primarily early this afternoon for KRFD, then tonight
through Friday morning for all sites. Showers/isolated embedded
thunder expected to spread across from the south area late this
afternoon, continuing past midnight CST before diminishing to
drizzle which will linger Friday morning.

Weak surface high pressure ridge across the forecast area this
morning has allowed extensive low stratus/fog to develop, though
obs trends over the past few hours continue to indicate a gradual
improvement in conditions with lower cigs scattering across
portions of the Chicago metro area. Low level moisture remains
trapped beneath shallow (but fairly strong) inversion north of an
area of weak low pressure and a warm front currently over the
lower mid-Mississippi and lower Ohio river valleys. A band of
showers from eastern Iowa to downstate IL/lower Ohio Valley will
spread northeast and increase in coverage across the terminals
later this afternoon, with a period of rain/showers expected to
persist through the evening hours in advance of a mid-level
disturbance approaching from the southwest. Forecast soundings and
RAP model analysis does depict an axis of weak elevated
instability coincident with this band of precipitation, which
would support the potential for isolated embedded thunderstorms
especially late this evening through midnight when strongest large
scale ascent is present. Though greatest instability and thunder
threat appears to remain just south of the terminals, did include
a VCTS mention late this evening to express the overall trend.

LIFR/VLIFR conditions are then expected after midnight through
Friday morning, as surface low pressure trough lifts across the
region. Forecast models are in good agreement in depicting
saturated boundary layer up through about 850 mb, which will
likely support an extended period of low cigs/fog and drizzle.
LIFR conditions could persist into Friday afternoon.



401 AM CST

Generally quiet conditions expected across the lake at this time.
High pressure will exit to the east today, and allow for a weak
low to lift up the Mississippi river today. However, it is
expected to dissipate as it moves north. Another low is forecast
to form over the Plains Friday night and reach Minnesota Saturday
night, with a stronger low expected to form over the southern
Plains Saturday night. However, neither systems expected to bring
any significant winds across the lake during this time.







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