Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152035

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
235 PM CST Wed Feb 15 2017

235 PM CST

Through Thursday...

Winter lovers, enjoy what`s left of today, because this is as
winter-like as our weather will be for the foreseeable future. No
weather concerns in the short term, with seasonable temps today
and tonight. Significant warming will take place above the
boundary layer tonight as upper trough moves out, though at the
surface high pressure will move in and should allow winds to go
light/variable tonight. Mid-high level cloudiness associated with
the WAA aloft should increase tonight and likely keep low temps
from getting too low despite the proximity of the surface ridge.
Warm air advection gets under at the surface Thursday and look for
a healthy rebound in temps, with 40s to lower 50s expected for
highs. Gradient will favor southeast winds, which should allow
for healthy lake breeze to form and keep temps along the IL
shoreline only in the upper 30s.



235 PM CST

Thursday night through Wednesday...

For all intents and purposes, meteorologically, we`re about to
flip the calendar forward 2 months as our forecast through the
long term period looks far more like April than February.

Have sided with the ECMWF/GFS solution for Friday into Saturday
largely disregarding the NAM which has a weak surface low
developing over the MO and tracking into southern WI, slowing the
northward progress of the warm front. No hint of such a surface
wave in the global models which send warm front careening
northward into WI/MI Friday morning placing our CWA solidly in the
warm sector Friday. Have generally trended temps in the grids
toward the warmer side of the guidance envelop Friday and Saturday
with the expectation that gradient will be strong enough to
prevent a lake breeze from forming. Highs Friday will likely range
from mid-upper 50s NE CWA to low-mid 60s SW areas with Saturday
likely topping out in the low-mid 60s. Will probably fall a
couple/few degrees shy of record highs Friday at RFD/ORD, but
Saturday`s record highs look to be in serious jeopardy.

Weak (and weakening) cold front looks to settle southward into our
CWA Sat night and likely stall out/wash out over southern CWA
Sunday. As surface flow turns light northerly behind the weak
boundary, seems quite plausible the cold marine layer could lead
to a lake enhancement to the front with a sharp temperature drop
Saturday night adjacent to Lake Michigan. No real synoptic push of
cooler air behind the front Sunday, so Sunday will likely be
another unseasonably warm day, though lake cooling will likely
play a prominent role in high temps near near the lake.

Anomalously warm pattern will likely persist into and through
much of next week. Highs will likely top out at least in the 50s
and probably in the 60s at least a couple times next week. Monday
and Tuesday looks to be the strongest candidates for 60F+, though
a distinct possibility almost any day next week depending on
timing of systems. There are some differences in the operational
medium range models this morning in the details of timing the
front and resultant surface flow. Latest ECMWF and GEM would favor
a wind off the lake now Monday, which could keep immediate
Chicago area only in the 50s with 40s along the shore. Conversely
GFS has stronger SW surface flow and would favor 60s area wide.

Small scale details in the wind regimes and timing of weak cold
fronts and resultant slight cool downs will probably vary a bit
from run to run, but confidence in an overall regime of much above
average to near record warmth continues for next week. As is
typically the case in April, any days with light winds or any
slight easterly component to the surface pressure gradient will
likely feature fairly dramatic lake cooling, but pinning down
those days is tricky at this distance. Only other weather of note
is a weak cold front sometime late Monday or Tuesday bringing
some shower and thunderstorm threat.



For the 18Z TAFs...

No significant aviation concerns through the next 24 to 30 hours.
Main areas of focus are potential for a period of MVFR CIGs this
evening and then southeast to east-southeast or east winds on
Thursday. Northwest winds with relatively frequent gusts will
diminish shortly after sunset as a weak high pressure ridge
settles over the area. Some guidance brings patchy MVFR CIGs
currently over northern WI to the eastern terminals this evening,
with SCT MVFR cloud mention due to lower confidence.

After a brief period of VRB/calm winds early Thursday morning,
winds will turn southeast by the mid to late morning. Setup
appears favorable for lake influence to shift winds more easterly
in the afternoon at ORD/MDW, though guidance suggests this shift
would be more likely to occur in the late afternoon. Speeds in the
afternoon could approach or slightly exceed 10 kt. Conditions
will remain VFR throughout Thursday.



235 PM CST

After the brisk northwesterly flow subsides tonight with a weak
high pressure ridge crossing the lake, there are no periods of
significant wind concern until next week. Moderate east-southeast
to southeast winds will focus on the south and central portion of
the lake late Thursday into Thursday night ahead of a warm front.
The front should largely clear the lake to the north on Friday,
yielding moderate south-southwest winds. Low pressure north of the
lakes region will drag a trailing cold front across the lake on
Saturday, followed by fairly light winds on Sunday with another
weak high pressure moving over the lake. There is the potential
direction to vary from the current forecast. Uncertainty in the
wind direction increases significantly Monday into Tuesday,
though there is decent agreement in the guidance for stronger
speeds during this period. Gales are not currently favored.








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