Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 202100

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
300 PM CST Mon Feb 20 2017

147 PM CST

Through Tuesday...

Record breaking warmth continues today across northern Illinois
and northwest Indiana ahead of low pressure currently over
northern Minnesota and a trailing cold front that stretches south
across the mid Missouri Valley. Main forecast challenge for this
evening and overnight will be timing and strength of line of
showers and thunderstorms associated with the cold front.
Precipitation is ongoing across much of Iowa this afternoon within
a broad area of mid level WAA which is expected to lift NNE
across WI this evening. Meanwhile, an advancing cold front is
expected to kick off additional showers and possibly a few
thunderstorms this evening moving east across the Mississippi
Valley around 00Z and gradually spreading into the local forecast
area through the mid to late evening hours. Surface based
instability is expected to diminish after sunset, but forecast
soundings do indicate that very weak instability will persist
rooted just above the nocturnal boundary layer. Meanwhile, forcing
is also expected to weaken, especially after midnight as the
surface low lifts NNE and pre-frontal convergence weakens. All
this should amount to a decaying line of showers moving across
the area with precip chances winding down completely from the west
late tonight into tomorrow morning as the surface cold front
pushes across the area. Depending on the progress of the front,
there could be a few spotty showers that redevelop tomorrow
afternoon in the far southeastern portions of the CWA, but better
chances will be off to our east. Despite passage of a cold front,
temperatures behind the front quickly rebound. Clearing skies and
lower dewpoints will help surface temps respond to insolation
tomorrow, and expect highs once again well into the 60s with near
record warmth.



258 PM CST

Tuesday night through Monday...

A record warm stretch will start the extended and then a shift
will occur on Friday and Friday night, which given how anomalously
warm we have been, this windy transition will likely be
accompanied with active weather. A short-lived dose of winter is
then expected on Saturday with at least light snow looking more
favored for part of the area.

The weather pattern on Wednesday and the regime leading into it
continues to support the now likely of reaching or exceeding all
time February high temperatures during the afternoon. In the wake
of tonight`s cool front, the air mass will have dried out,
especially aloft limiting clouds. A quasi-zonal flow pattern
across northern states will send a 1003 mb low eastward across
Iowa into Wisconsin Wednesday afternoon and evening, providing
focused low-level warm advection into the inherent anomalously
warm air mass. The 925mb temperatures are forecast by almost all
available guidance to be 15C, which is basically what is occurring
today where sunny areas are reaching low 70s. Thus see no issues
with the entire CWA into the 70s, with plenty of mid and even some
upper 70s expected. Please see the climate discussion below for
more on the records presently forecast to be broken.

Regarding fire weather/dry concerns Wednesday afternoon, it
remains forecast below that, but will need to keep an eye on it.
Wind gusts are forecast to be a little above 20 mph Wednesday
afternoon. There has not been much afternoon mixing of dew points
during this warm stretch, but Wednesday may be a little more
favorable. At this time have afternoon humidity forecast between
30 and 35 percent. While not to what we consider red flag
concerns, it could be to still elevated fire risk levels.

The system to our north will steer an initial cool front
southward into the area Wednesday night. The air mass is forecast
to moisten some, but not enough for rain. Cooler air will
gradually ease into the area Thursday morning, especially near the
lake as winds turn onshore with little to no temperature
recovery. Where exactly the boundary stalls is a bit challenging
and becomes noteworthy for Friday. This stalling is most favored
across central Illinois on Thursday afternoon and retreating
slowly northward as a warm front Thursday night with strong lee
cyclogenesis across western Kansas.

Rain will expand quickly at some point late Thursday into
Thursday night across the region as forcing for broad ascent
increases as well as within a narrow corridor of strong isentropic
ascent and frontogenesis north of the warm front. Some bands of
moderate to heavy rain with totals in excess of one inch are
certainly possible with precipitable waters over an inch and the
focused forcing. Also the likelihood of scattered elevated thunder
should increase into the night. Overall model guidance is in good
agreement with Thursday night, so this portion of the forecast
has high confidence for three days out.

A southern portion of the broad western NOAM upper trough will
shift eastward Friday, with global guidance trending the past 24
hours in closing off the mid-level circulation by Friday
afternoon, resulting in a slower but still strong surface low. The
general slowing over the past 48 hours in guidance solutions
supports very warm temperatures across much of if not the entire
CWA. However, east winds north of the warm front may keep some
lakeside areas much cooler. The effective warm sector...that is
where the highest dew points and low-level instability will
be...varies some in placement on guidance. The southern and
eastern CWA remain most likely on GEFS spreads and deterministic
solutions to be into this, but this low-level instability of
500-1000 J/kg could be all the way up to the warm front. Given
the kinematics with a near 990 mb low in February, this continues
concerns for some strong to severe convection Friday afternoon as
the dry slot moves in. Synoptically this too continues to show
some similarities to cool season severe weather events. Will have
to watch if model guidance is a little generous in veering the
surface winds in eastern areas, as they could be more backed than
forecast. Even without that, the potential for isolated strong to
severe winds looks reasonable with any quick-moving convection if
the evolution pans out as presently shown.

The surface low should progress northeast across the area with
some signs it may be a double-barreled low on the 20.12 GFS and
GEM, which both provide better wraparound potential for rain
showers transitioning to snow late Friday night and Saturday
morning. This however will depend on the low track and speed, as
will how long duration the gusty winds are. There is potential
for gusts in excess of 40 mph, including during Saturday morning
when it very well could be snowing too. At the least this would
provide a completely different picture outside than what we have
been seeing this past week. Light accumulation is possible,
especially if this double-barreled signature pans out, slowing the
system and providing a more southerly surge to the system TROWAL.
We are not expecting surface temperatures to climb much if at all
on Saturday with them holding steady in the lower to mid 30s with
a little warmer on Sunday.

Confidence in specifics really drops off in the forecast after
that time period. The primary low-level baroclinic zone will be
located to our south with the active jet pattern that has been
pummeling California to then be across a good part of the
southern/central U.S. latitudes. So it certainly looks active
into the last days of February into the first week of March, and
CPC 6-10 day outlook of better chances for above normal
precipitation is reasonable.



258 PM CST

A highly anomalous warm stretch that has already broke daily
records will continue to threaten records this week, including the
all time February warm records for any time of the month on
Wednesday. Here are daily records.

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/21: 67 (1930)  47 (1930)       64 (1930)  42 (1930)
2/22: 68 (1922)  52 (1930)       61 (1922)  48 (1930)

All-time February Record Highs:
Chicago:  75  2/27/1976
Rockford: 70  2/25/2000

For more, including consecutive days of such warmth this time of
year, please see our web top news headline.


For the 18Z TAFs...

Low pressure will lift from northern Minnesota late this morning
to Hudson Bay early tomorrow with a trailing cold front sweeping
across northern Illinois and northwest Indiana late tonight into
Tuesday. For the rest of today, expect fairly steady state
conditions out ahead of the front with SE/SSE winds gusting into
the low to mid 20 kt range. VFR conditions will prevail through
late this evening with mid and high clouds moving across the
region. Winds are expected to veer to the southwest late this
evening and overnight. Confidence in wind speeds is low tonight
with strong winds just off the deck but low level lapse rates
becoming increasingly prohibitive for mixing. Suspect there will
be sporadic gusts through the overnight, but not a prevailing
condition. Cloud cover is expected to increase and lower out ahead
of the front and precipitation is expected to move across the
area. MVFR is likely overnight, though timing is still a bit
uncertain. Also cannot rule out IFR cigs late tonight into
tomorrow morning. A north-south oriented line of showers and
thunderstorms will move east across the Mississippi Valley this
evening and across the terminals overnight, though the line should
be in a weakening phase at that time. Will hold off on any
thunder mention but do expect convective showers. Southwest winds
will veer to the west/northwest tomorrow behind the cold front and
expect any lower cigs to scour out through the morning and
afternoon hours as drier air advects into the region.



258 PM CST

A more active period of the lake is likely, especially late week.
An approaching cold front will help tighten the pressure gradient
with increasing southerly winds this evening. The stable air over
the lake may limit full gust potential, though could be some
occasional gale force gusts in the northern to central part of the
open waters. Conditions will be near small craft advisory criteria
in the Illinois and Indiana nearshore zones.

The cold front will pass across the lake by Tuesday morning with a
limited cold push behind it. Thus winds should not increase too
much. Another low will scoot across the lake on Wednesday night,
with a much deeper low at the end of the week. This deep low still
looks to favor a period of gales, possibly both ahead and behind
the low passage. Behind the low is most favored as the cold
advection sweeps over the lake.



LM...Small Craft Advisory...NEARSHORE WATERS
     UNTIL 3 AM Tuesday.




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