Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 172340

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
540 PM CST Tue Jan 17 2017

158 PM CST

Through Wednesday...

Damp and cloudy conditions continue overnight.  An upper level wave
currently over Iowa will pass overhead through this evening. Light
rain showers and drizzle will form ahead of the wave late this
afternoon. Rainfall amounts will be very minor at around or less
0.05 inches, and nothing like the rainfall we received Monday night.
The majority of rain will fall north and east of a Rochelle, IL to
Rensselaer, IN line.  Rain and drizzle come to an end around
midnight. Fog may redevelop especially across north central IL, but
have low confidence in how dense fog will be.  Due to my lack of
confidence, kept did not include dense fog in the forecast.

High pressure spreads east Wednesday and we should be on an
improving trend. Winds become southwest during the day and usher in
warmer air. Models differ on how quickly clouds will clear out and
there is a chance we may not see the sun tomorrow.  Decided to lean
on clearing in the afternoon and raised high temps into the low to
mid 40s. Local climatology would suggest we could see slightly
higher temps if clouds clear quicker, but on the other hand, temps
could be cooler if clouds linger.



232 PM CST

Wednesday night through Tuesday...

The weather pattern in the extended favors above normal
temperatures but also quite a bit of cloud cover and potentially
a couple wet periods. Confidence in the forecast specifics and
in rain chances really drops for the weekend into early next

The next in a series of upper closed lows is over the desert
southwest this afternoon, and this will meander its way northeast
during midweek. As an upper jet rotates around this, model
guidance deepens it on Thursday, with a moisture draw ahead of
this. PWATs and surface dew points are already running high for
January over the mid and lower Mississippi Valleys, so this moist
air mass and clouds should swiftly return sometime on Thursday,
and light rain appears likely in at least southern areas by
Thursday evening. While it is always a challenge with a closed low
in terms of speed and how expansive its precipitation will be,
the deepening forecast should support rain and probably drizzle
at times to blossom over the area Thursday night and Friday
morning. Forcing is not as stout as this past event, however still
0.25" to 0.75" of rain is forecast. There is potential again for
fog. Depending on how quickly clouds move in Thursday, highs
should reach into the mid 40s...about 10 to 15 degrees above

A more pronounced and broad trough will build over the western
U.S. Friday, with multiple cut off lows forecast to eject into the
Midwest during the weekend as a blocking ridge is in place across
eastern NOAM. All in all this is pretty bizarre pattern forecast,
and is one reason confidence is low. Certainly temperatures will
be above normal and because of lingering clouds and possibly
rain early on Friday, the day with the best and maybe only chance
to see 50s areawide is Saturday, as more of a true warm sector is
forecast. The 925mb temperatures are forecast to be 9C to 11C
across the area. In the reanalysis dataset, there`s only about a
half dozen days with 925mb temperatures that warm in the latter
half of January over the area, and ORD has averaged a high of 56
on those days. There is some uncertainty on how much lake cooling
component we may receive, but right now have low to mid 50s
forecast on Saturday.

East winds and lake cooling are presently forecast to become
larger players Sunday and Monday, with a strong low pressure
indicated to our south. If this pattern pans out, it could keep
temperatures in the 30s near the lake, including in downtown
Chicago. Inland, Sunday could see the 50 degree mark reached, but
chances for rain are also forecast to quickly rise. Needless to
say, confidence is not the highest. Being on the northwest side of
this system, its possible enough cool air wraps in that before it
departs...tentatively Monday/Monday night right could
have some wet snow on its backside. Not too bad for January.



For the 00Z TAFs...

Concerns for the evening and much of the overnight period include
low ceilings and visibilities, with drizzle and possibly a shower
this evening. Expecting trends to generally move toward IFR
despite slightly better conditions at time of TAF issuance, thus
the decision to start a bit below present observations. Added a
SCT003 to hint at the possibility of something a bit lower than
OVC300 since that can be an important threshold, but would not
expect that ceiling to be more than a very temporary condition.
Also added a bit lower vsby in the tempo group with showers given
some upstream obs that are trending toward 1 mile in some

Elected not to make any big changes to timing of end of precip
overnight, and to end of IFR in the morning. Latest forecast
guidance products continue to support existing trends shown in
TAF. The loss of IFR and improvement to VFR and likely clear in
the morning could occur pretty quickly, over just an hour or two,
rather than trending slowly upward.



232 PM CST

Fog across the lake should be slow to improve tonight and more
swiftly show improvement Wednesday morning. Northwest winds
tonight will become southwest Wednesday morning and remain as such
through the day Thursday before turning east or northeast. All in
all, speeds remain in check through midweek. Higher dew point air
creeps northward Friday and Saturday, and may support some fog,
especially later Saturday. Confidence in wind direction drops
during the weekend into early next week, as the weather pattern
has multiple cutoff lows across the central U.S. Presently,
increasing northeast winds are forecast Sunday and Monday that
would support Small Craft Advisory criteria in the Illinois and
Indiana nearshores.






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