Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241922

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
222 PM CDT Sat Mar 24 2018

213 PM CDT

Through Sunday...

For headline purposes, we will likely drop the Winter Weather
Advisory for Ogle County in north central IL, Kankakee County of
east central IL, along with Newton and Jasper counties of NW
Indiana, as dry air is now winning outside of the moderate to
heavy snow band. Will leave the warning areas as is, and Benton
County, IN in an advisory with its close proximity to the moist
airmass. The the main impacts expected through the afternoon.

Fascinating dewpoint contrast is in place from area in the snow
to areas not very far upstream (Kankakee and Rensselaer in the
teens, with Lafayette, IN near 30 and same for Pontiac, IL). This
means the difference between 9-10" snow reports across Whiteside
and Carroll counties of northwest IL to little to nothing across
Winnebago and Ogle counties of north central IL.

Near whiteout conditions were been reported across portions of
Livingston, southern Lasalle, and Ford counties earlier, and now
winds are weakening some as the main surface low continues to
shift toward the Ohio valley and will continue even farther south
and west. That said, conditions are still very dangerous,
including along I-39, I-80, and I-55 in the warned areas. There
is still some lingering mesoscale frontogenetical forcing that
will remain in place across the I-24 corridor through the heart of
the warning area, and this combined with several more hours of
continued upper level support from the southeastward sinking upper
low will still maintain moderate to heavy snow potential. The
main instability axis is still just to our south and west, thus
think thunder chances are small, but elevated mid level lapse
rates will graze the area also (and suggested by the bumpy visible
satellite pics). Highest report of snow are in Chatsworth where
6" has been exceeded thus far.

RAP guidance does diminish the mesoscale forcing later this
afternoon and early evening. This along with the upper low
beginning to fill and a steady decrease in mid and lower level RH
(noted by the continued southwestward advance of the radar echoes
(that were attempting to shift NE) will suggest a rapid tapering
this evening.

The upper low will quickly sink south and be replaced the the high
pressure portion of the rex block in place, and there we will even
gradually thin out the higher moisture as well. Surface high
pressure will remain locked in place through the night over the
northern Great Lakes, and thus weaker but continued breezy NE
winds and chilly temperatures in the 20s. The pattern changes
little into Sunday, with continued cool east/northeast flow
suggesting highs in the lower 40s away from snow covered areas and
near the lakeshore where upper 30s may only be realized with the
March sunshine.



217 PM CDT

Sunday night through Saturday...

An increasingly meridional pattern will set up across the country
during the upcoming week setting the stage for periods of active
weather and temperature swings across northern Illinois and
northwest Indiana.

A deep upper trough is expected to develop over the Rockies while
a strong upper ridge will develop along the east coast.
Southwesterly jet stream will park between these two features over
the local area. A broad region of warm air advection will lift
across the region Monday and Monday night as this southwest flow
pattern develops while initial positively tilted shortwave lifts
across the Great Plains into the Upper Midwest. A weak surface
reflection will be present which will help transport surface warm
front north across the forecast area Monday afternoon and evening.
Showers and a few thunderstorms are expected as this occurs later
in the day Monday. A strengthening low level jet will help to
transport around 1.3 inch PWats into the area Monday night and may
serve as the focus for heavier showers overnight and into the day
Tuesday. Despite the expected precipitation and associated cloud
cover, warm advection will help drive a warming trend Monday and
Tuesday with afternoon highs in the upper 40s Monday and mid to
upper 50s expected by Tuesday.

Upper trough axis shifts into the northern Great Plains and Upper
Midwest midweek pushing the moisture axis off to our east. A
surface cold front will push across northern Illinois and
northwest Indiana Tuesday night into early Wednesday bringing
another chance for precip and result in a modest cooling trend
through the latter half of the week. A reinforcing cold front is
expected Thursday which will return us to below normal conditions
Friday into the weekend.



For the 18Z TAFs...

Forecast concerns for the terminals is mainly the winds. Heavy
snow has largely been confined south of I-80, and along the I-39
corridor south of I-88, where poor (VLIFR/LIFR) flying conditions
exist. Outside of the snow, dry and gusty ENE winds have limited
the eastward progression of snow and therefore VFR mid clouds are
in place and gusty east winds, though there is likely some virga
in the higher clouds. The strongest gusts up to 30-35 kt are south
and west of ORD/MDW, but still we should expect gusts at least
into the mid 20s for most areas. Low pressure will dip well south
of the region tonight and weaken, thus expect gusts to diminish
overnight. High pressure will shift into eastern Canada, which
will maintain a NE to E wind into Sunday, though of lower
magnitude on Sunday then today.



217 PM CDT

Easterly flow will prevail across the western Great Lakes through
the remainder of the weekend as a strong high slowly moves from
James Bay east across Quebec. A weakening area of low pressure
moving east across the mid Mississippi Valley today will continue
to cause near gale force gusts into the evening hours, but winds
should diminish gradually overnight into Sunday as the low fills.
Another low will form over the Colorado Front Range this weekend
and lift to Wisconsin early Tuesday then NNE across Lake Superior
through the day Tuesday. Out ahead of this feature, winds will
back to the southeast on Monday then to the SSW on Tuesday,
gusting to around 30 kt much of this time. There may be a brief
window of gales Monday afternoon into the evening, then the gale
threat should diminish Monday night as more stable conditions
develop as warmer and higher dewpoint air advects over the lake.
Will have to keep on some fog potential as this occurs. A cold
front will sweep across the lake Tuesday night with winds veering
to the NNW in its wake.



IL...Winter Storm Warning...ILZ010-ILZ019-ILZ021-ILZ032-ILZ033-ILZ039
     until 10 PM Saturday.

     Winter Weather Advisory...ILZ008-ILZ023 until 7 PM Saturday.

IN...Winter Weather Advisory...INZ010-INZ011 until 7 PM Saturday.

     Winter Weather Advisory...INZ019 until 10 PM Saturday.

LM...Small Craft Advisory...IL nearshore waters until 4 PM Sunday.

     Small Craft Advisory...IN nearshore waters until 10 AM Sunday.




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