Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221808

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1208 PM CST Thu Feb 22 2018


Temperatures have warmed above freezing in most areas outside of
the I-39 corridor north of I-88 across north central Illinois.
Upstream observations in northwest Illinois have reported brief
snow or sleet, and expect this batch of precip to pass through
these areas through early afternoon. Impacts should be fairly low
and precip would be short lived as the low levels are pretty dry.
Water vapor imagery indicates continued though brief drying
behind this wave of precip.

Two areas of focus will be in place for increasing rain chances
tonight, both along the elevated warm front across far northern
Illinois and with a better band of moisture across central
Illinois. Broad synoptic lift from low pressure tonight will
promote fairly widespread chances of rain, though generally light
as much of the area will be in between slightly better forcing to
our north and better moisture to the south. Forecast profiles
suggest the p-type will be rain.



324 AM CST

Today through Friday...

Main near term concerns focus on light precip trends today,
particularly with type over the southern cwa early this morning
where light rain/freezing rain is possible. A more substantial
wave brings the potential for moderate QPF amounts late tonight,
with 0.20-0.30" of unneeded rainfall possible across the
southeastern part of the cwa.

Surface cold front which moved across the area Tuesday evening
has become stationary south of the Ohio River, though the 925-850
mb baroclinic zone associated with the elevated frontal zone
lingers across MO-IL-IN. Very light precipitation has been
occurring just south/southeast of the LOT cwa so far overnight,
though an approaching mid-level short wave from the south-central
Plains will result in veering flow, and increasing ascent across
the frontal zone this morning. Recent regional radar mosaic
imagery depicts an increase in returns into western/central IL in
association, and high-res guidance suggests this should continue
to expand into the far southern parts of the cwa over the next few
hours. Warm air aloft, as indicated by 850 mb temps near +2C.
This would support a liquid precipitation type per forecast
soundings, though surface temps across the southern tier of
counties is currently 31-34 degrees which would present a freezing
rain threat. Fortunately, clouds and breezy northeast winds have
held temps fairly steady, even rising a degree or two overnight,
which should help limit the impact of any freezing precip. Diurnal
temp rise after sunrise should bring an end to the freezing rain
threat fairly quickly. With limited areal coverage and marginal
temps, unless conditions change significantly am planning on going
without an advisory headline, and will handle with a special
weather statement and graphical messaging. Farther to the west,
precipitation across Nebraska/Iowa and spreading into southwestern
Wisconsin in regional radar mosaic is progged to fill in a bit
across northwestern IL this morning, with high-res guidance
brining some of this lighter precip into north central IL. Cooler
thermal column in that area should support some light snow
initially, but quickly changing over to light rain as surface
temps rise. None of the precip during the day today is expected to
amount to much, with model QPF mainly just trace amounts to a
couple of hundredths of an inch.

Energy currently rounding the base of the western CONUS upper
trough is then progged to lift northeast across the Plains
tonight, and into the western Great Lakes area by Friday morning.
Surface low pressure associated with this wave tracks into the
Upper Mississippi Valley by 6 am Friday, with low level warm/moist
advection developing into the forecast area as low level winds
turn southeasterly ahead of the low and occluding cold front. This
looks to be a more significant precip producer, with several
models developing a swath of 0.75-1.00"+ rainfall amounts across
central IL and IN. Two to three tenths of rain are possible across
the southeastern WFO LOT cwa, which obviously needs no more
rainfall after impressively heavy totals earlier in the week. The
good news is that it looks like temps will generally be above
freezing for the duration of the event, with the possible
exception of this evening over northwest IL where readings around
32-33F are possible. Thus the threat of freezing rain appears
pretty low.

Rain is expected to end quickly Friday morning behind the cold
front, with weak high pressure ridging across the Upper Midwest by
Friday afternoon/evening. Southern counties may tag 50 degrees,
while the remainder of the area still above average with highs in
the low to mid 40`s.



341 AM CST

Friday night through Wednesday...

Main focus in the longer term portion of the forecast is on the
quick return of rain to the forecast area Friday night through
Saturday night. Another swath of heavy rainfall is possible across
central IL/IN during this period, with the southeastern counties
of the LOT cwa again picking up 0.50-1.00"+ inch rainfall amounts
by the time precip ends just before sunrise Sunday morning.

Brief lull in precipitation should be ongoing Friday evening,
with weak surface high pressure ridging centered across the Upper
Mississippi Valley. Precipitation threat returns overnight
however, as the remnants of the western CONUS trough kicks out
across into the Plains as a strong mid-level trough. Resulting
return flow again develops warm/moist advection across the stalled
frontal zone to our south, with ascent increasing during the day
Saturday as the wave approaches and surface low pressure deepens
to our west. Guidance depicts 1.00"+ precipitable water being
pulled north across the warm front, setting the stage for
additional moderate/heavy rainfall. The surface low passes just
northwest of the forecast area during the pre-dawn hours of Sunday
morning, with the cold front quickly moving through the forecast
area and cutting off precipitation. Guidance varies a bit with
rainfall totals at this distance, though general amounts of a
quarter to a half inch are depicted over the northern parts of the
FA, while a half to in excess of an inch are produced across the
southeast. Combined with rain from Thursday night into early
Friday, this will likely at least slow the fall of river levels
which are already in flood, and could result in new rises if the
heavier end of QPF amounts are realized.

Things look to quiet down a bit Sunday through Tuesday, with a
welcome period of dry, mild weather expected, as the weekend
system finally pushes the frontal zone well south of the area.
Upper flow remains generally west-southwesterly through the period
however, with another chance for precip toward mid-week.



For the 18Z TAFs...

Concerns center on occasionally gusty NE winds, then lowering cigs
with rain overnight.

An elevated warm front south of the terminals will lift through
the area this eveing and tonight. This will bring lowering
ceilings through the afternoon. Confidence on timing is only
medium as low level easterly flow is slowing down some of the
lower cloud arrival. Expect MVFR at first. Once low pressure
arrives overnight, expect some rain showers to develop later this
evening, and then a period of more widespread ligther to briefly
moderate rain overnight. Confidence on coverage this evening is
not that high, thus some timning concerns with initial onset of
rain and IFR cigs, but have higher confidence in at least
occasional LIFR overnight at least IFR. Precip will end Friday
morning from west to east, with a rapid shift in winds to SW.
Winds may be initially gusty in the morning. Some patchy fog may
linger before ceilings rise through the morning. Winds will be
westerly in the afternoon.



304 AM CST

High pressure lies over northern Lake Michigan early this morning
with relatively light flow in place across the northern portions
of the lake, and a fresh northeast to easterly breeze in place
across the south end of the lake. A broad and disorganized low
will lift across the Upper Midwest this evening and tonight
resulting in winds veering to the southeast and increasing to
around 30 kt, strongest over the south end of the lake. Winds
continue to veer around the dial, westerly on Friday, then
easterly again Saturday as another low begins to deepen over the
Central Plains. This deeper low will lift across the Upper Midwest
Saturday night when south gales are expected. Gales continue out
of the west on Sunday as the low lifts to James Bay.






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