Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 201950

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
150 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.



405 AM CST

Tuesday through Sunday...

The main story for Tuesday, but especially Wednesday, will be the
continued near record to record warmth across the area. While
temperatures could be a touch cooler across the area on Tuesday
following the Monday night Pacific cold frontal passage, the
airmass over the area will still remain very mild for late
February. As a result, with clearing skies expected during the day
high temperatures will again likely top out in the 60s. We may,
however, fall just short of the record high on Tuesday. This is
mainly because the record is a bit higher, than the previous few
days. By Wednesday we may see our warmest temperatures of the
entire stretch. Another Pacific system is forecast to shift
eastward across the Upper Great Lakes during the day, and this
should set up a return to dry and breezy southwest winds. Current
forecast guidance suggests that 925 MB temperatures up around 15C
will overspread the area, which given the warmth of late would
support temperatures into the low 70s, though given the dry
southwest winds, even warmer temperatures into the mid 70s may not
be out of the question. The current record warmest high for the
entire month of February is 75 degrees for Chicago. Gusty
southwest winds and warm and dry conditions on Wednesday could
also result in a high fire danger across the area.

Another cold front, associated with the Upper Great Lakes storm
system, is expected to shift southward over the area for Thursday.
This will likely set up cooler temperatures on Thursday. Onshore
flow could easily keep temperatures in the lower 40s near Lake
Michigan, with 50s farther inland.

The main forecast focus later in the week then quickly turns to
the likelihood of a strong storm system expected to impact the
central CONUS late Thursday through Friday. This storm system
could have a significant impact on the area in terms of a threat
of severe thunderstorms Friday, along with a period of very strong
synoptic winds Friday and Friday night.

While there area still some uncertainties in the exact track and
timing of this storm system on Friday, there is moderate to high
confidence in a significant storm system impacting the region
Friday. The current thinking is that a ~990 MB surface low will
track northeastward across southern Wisconsin on Friday. This low
will likely drive a warm front back north across the area late
Thursday and into Thursday night. Expect a period of showers and
storms develop over the area during this time as the nose of a
low-level jet sets up over the area into early Friday. Another
round of storms then looks like a good likelihood on Friday as the
strong dynamic system drives a cold front eastward across the
area. Given that the storm system could produce a strong kinematic
field over the area in combination with a warm and unstable
prefrontal airmass, severe weather will be possible across the
area Friday. As a result, this potential will have to be watched
closely in the coming days.

Strong synoptic winds, potentially gusting to or above 50 MPH are
also possible with this storm system late Friday and Friday night.
Colder air will move in over the area Friday night into Saturday,
and as this occurs, the precipitation could end as a little snow
into early Saturday.



350 PM CST

A highly anomalous warm stretch that has already broke daily
records will continue to threaten records this week. Here are
daily records.

Chicago:                         Rockford:
      High:      High Min:       High:      High Min:
2/20: 64 (1930)  49 (1930)       61 (1983)  46 (1930)
2/21: 67 (1930)  47 (1930)       64 (1930)  42 (1930)
2/22: 68 (1922)  52 (1930)       61 (1922)  48 (1930)

For more, including all-time February warmth and consecutive
days of such warmth this time of year, please see our web top
news headline or Public Information Statement.


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.



203 AM CST

Headlines...Issued a small craft advisory for this afternoon and
evening for hazardous wind gusts across all of the nearshore waters.
Low end gales may occur over the northern half of the lake this
evening, but do not have enough confidence to issue a gale warning.

The pressure gradient tightens as the high currently over the Ohio
Valley reaches the east coast, and the low over the Dakotas moves
over Manitoba this afternoon. Southeast to south winds increase and
become hazardous to small craft. Low end gales are possible over the
northern half of the lake, but unsure how stable the surface layer
will be. If the surface layer is weaker than currently forecast,
gales will be more prevalent and a gale warning will be needed.
Winds weaken and become west behind the low`s cold front Tuesday.

The next low passes over the northern end of the lake Wednesday
evening and winds become northwest behind it.  The bigger story is
the strong low that will impact the midwest later this week.
Guidance features a 990-992mb low passing over the lake Friday
night. Have medium confidence in gales with an easterly component
ahead of the low Friday, but high confidence in northwest gales
behind the low.  It`s still too early for exact details and a
headline. The low continues to Quebec Saturday while a surface high
passes south of the lake over the weekend.



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




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