Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 130921

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
321 AM CST Fri Jan 13 2017

321 AM CST

Through Monday...

Strong Canadian high pressure will move across the Great Lakes
today, providing for a seasonably cold temperatures and dry
weather across the area through this evening. Modest isentropic
ascent coupled with upper level divergence in the right entrance
region of 140kt+ 250mb jet will support the development of
precipitation across MO and southern IL with this activity likely
to slowly develop northward through tonight as weak elevated warm
front makes some northward progress. Majority of available model
guidance now suggests that dry air advection should limit the
northward extent of this precip, with model models suggesting our
southern CWA narrowly dodges a bullet. Still some model support
for light wintry precip making into Ford, Iroquois, and Benton
counties late tonight and Sat morning, so opted to maintain
chances pops as a nod to previous forecast. If any light wintry
precip does fall it could cause problems and would likely require
an advisory, but given pops solidly below 50% for now and the
model trends to shift QPF axis farther south, no headlines planned
at this time.

This lead wave should move east and leave conditions dry the
majority of the weekend across our CWA as strong cut off low over
CA digs south to central Baja California by Saturday afternoon
before rounding the bend and moving northeast into southern High
Plains region Sunday. Impressive dynamics and moisture
availability should lead to a quickly blossoming area of
significant precipitation in association with this system over
the southern plains Saturday night into Sunday. The precip shield
with this cyclone looks to overspread our area Sunday night and
continuing into Monday.

P-type issues certainly look to exist with potential for a period
of accumulating sleet and freezing rain before warmer air floods
into the area eventually on Monday changing precip to all rain.
Exact thermal profiles do vary some by model, but all generally
support a period of freezing rain/sleet, perhaps starting as some
snow Sunday night. Synoptically, the set-up does not look to be
one supportive of a major ice storm in our area as weakening
Canadian high will be quickly departing the area and we lose our
feed of cold air to maintain a longer duration icing event. In
addition, lack of snow cover certainly doesn`t bode well for
maintaining sub-freezing temps for a long duration. Having said
that, dry low level air should result in wet-bulb cooling and
likely provide enough cooling for potentially a 6+ hour long
window of periodic, mostly light, freezing rain and sleet. P-type
will be very dependent on precise temps through various layers of
the atmosphere and even a 1F error in forecast thermal profiles
could mean the difference between snow, freezing rain, sleet or
snow, so a little soon to start getting too specific with p-types
or amounts. Certainly looks like a strong candidate for winter
weather advisory type event with combo of freezing rain and sleet
Sunday night into early Monday before temps warm safely above
freezing at the sfc leading to precip changing to all rain.



321 AM CST

Monday night through Thursday...

In the wake of this system, guidance strongly suggests a quick
transition to a more zonal flow with above normal heights and
temps over the eastern 2/3rd of the nation. Later next week,
strong Pacific jet is forecast to carve out another deep western
trough leading to downstream amplification of a very impressive
ridge over the eastern half of N America. In fact, medium range
models with strong support from their ensembles are developing a
pattern that could send temperatures into near record warm
territory by late next week. Have nudged temps upward from the
blended model forecast initialization cocktail, but these temps
appear too cool (possibly significantly so). The forecast 925mb
temps on the operational GFS/ECMWF both support potential for
highs to reach 60F by Friday and possibly into the weekend, but
forecasting record highs this far out is risky with so many
potential things to go wrong (like cloud cover), so conservatively
forecasting 50s for now for Friday.



For the 06Z TAFs...

Strong high pressure will move across the upper midwest and
western Great Lakes Friday and Friday night. This will allow
west/northwest winds to turn more northerly overnight and then to
the northeast Friday morning. Speeds will generally be 10-12kts
but may drop below 10kts Friday morning. Winds will remain
northeast or easterly into Friday evening with speeds diminish
under 10kts.

Lake effect mvfr clouds are expected Friday morning into Friday
afternoon. Still some uncertainty regarding if prevailing mvfr
cigs develop and confidence remains low enough to keep sct mention
for now. cms


317 AM CST

A strong area of high pressure is centered over the Upper Midwest
early this morning and will spread east across the Great Lakes
today and persist through the weekend. Moderately strong winds,
especially across the north half of the lake where winds are
gusting to around 30 kt early this morning, will continue to
diminish through the day today as the high spreads across the lake
allowing the gradient to relax. A small craft advisory will remain
in effect for the Indiana nearshore waters through this afternoon
when waves should finally subside below criteria. Early next week,
low pressure is expected to lift from the Southern Plains Monday
into the western Great Lakes Tuesday. Southeast to south flow will
strengthen to 15 to 25 kt monday night into Tuesday then will
veer to the southwest through the day Tuesday as the high starts
to shift east of the lake.







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