FXUS63 KLOT 261121

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
621 AM CDT Wed Sep 26 2018

251 AM CDT

Through tonight...

Cold front has pushed east of our CWA with northwest winds locked
in behind the front ushering in a much cooler air mass. Despite a
good deal of sunshine today (especially this morning), temps
should only top out in the 60s most areas.

Mostly clear skies and light winds should set the stage for strong
radiational cooling tonight. Likely going to be one of those
nights with large temp spread from downtown Chicago to the coldest
outlying areas. Actually wouldn't be surprising to see a couple of
the typical cold spots come in with lows tagging the upper 30s
tonight. Meanwhile, the urban heat island should keep downtown
lows in the low to mid 50s.

- Izzi


251 AM CDT

Thursday through Tuesday...

Southwesterly winds will increase Thursday ahead of another cold
front slated to move across the area Thursday night. The southwest
winds should help temps moderate a bit over Wednesday's highs.
Cold front Thursday night should pass through largely precip
free. Air mass behind this front will be coldest of this still
very young fall season. With 850mb temps forecast to fall within a
couple/few degrees of 0C, which should keep highs in the 50s,
especially north of I-80.

Confidence in the forecast Friday through Saturday is fairly low.
Still seeing some models, including the normally very solid ECMWF,
break out some F-gen driven rain to the north of the boundary.
Timing is all over the place with the subtle wave that is progged
to enhance the mid level f-gen circulation, so confidence remains
extremely low in whether precip even occurs and if so when.

The GFS and NAM suggest Friday night will be clear and with sfc
high overhead stage would be set for another night of strong
radiational cooling and given the cooler air mass in place and
cooler start to the night, we could be looking at a frost threat
in the coldest outlying areas over northern IL. The latest 00z
ECMWF would have skies mainly cloudy with a chance of rain Friday
night in association with the aforementioned f-gen band. Given the
uncertainty and how far from climo it is, opted to hold off on
introducing frost in the grids for now, but if EC trends drier and
clearer Friday night it is something later shifts will need to

Heading into early next week, the GFS, EC, and GEM all suggest the
western trough will reload with downstream amplification of the
central and eastern U.S. upper ridge. This would likely set the
stage for front to lift back north as a warm front, with some
potential for warm air advection driven convection in advance of
the front. In the wake sector, temperatures should rebound back to
above to much above average levels and current blended model
forecast could end up being to conservative withe the warm up.

- Izzi


For the 12Z TAFs...

High pressure will build across the region today providing VFR
conditions and modest west winds. There may be a few gusts into
the mid to high teens this afternoon, then winds should diminish
again this evening and overnight. Winds may become light and
variable overnight as the ridge axis moves over head, then
southwest flow should set up behind the ridge axis Thursday



IN...Beach Hazards Statement...INZ002 until 4 PM Wednesday.

LM...Small Craft Advisory...Winthrop Harbor to Gary IN until 10 AM

Small Craft Advisory...Gary to Michigan City IN until 1 PM Wednesday.




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