Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Chicago/Romeoville Illinois
1109 am CDT Wednesday Sep 17 2014
231 am CDT
Overarching theme of the forecast for the next seven days is fairly
nice early Fall weather with a dash of unseasonable warmth and
splash of thunderstorms.
Today looks to be another glorious day...likely a couple/few degrees
warmer than Tuesday. Guidance was a couple degrees too cool with
highs Tuesday and with 925mb and 850mb temperatures prognosticated to be 2-3c
warmer than yesterday have opted to go a couple degrees above
guidance with highs near or into the lower 70s most areas. Some
altocumulus castellanus and middle level cloudiness upstream associated with middle-level
warm air advection could lead to a bit more cloudiness than the
nearly cloud free skies Tuesday but still look for plenty of
Weakening backdoor cold front moves in off the lake early Thursday
morning resulting in some slightly cooler temperatures Thursday...especially
downwind of the lake where northeast winds will have a better umph
behind the front and result in a greater lake influence to temperatures.
Winds begin to veer and should becoming southeasterly Thursday night
with warm air advection getting underway. Warm air advection gets
going into full gear by late Friday and given the later arrival of
the toastier temperatures its unclear how much warming will be realized
with highs Friday. Have conservatively bumped temperatures up into the 70s
area wide...but prognosticated 00z 850mb/925mb temperatures would actually support
temperatures around 80f. Friday night looks to be fairly warm with
southerly winds continuing...in fact many areas will see low temperatures
Friday night warmer than high temperatures were last week! Guidance continues
to advertise 850mb temperatures in the middle-upper teens with 925mb temperatures
around 20c...which climatology suggests highs well into the 80s. Have
trended temperatures upward some into the low to middle 80s and with enough
sunshine even those forecast highs could end up being a bit too low!
Cold front still looks slated to arrive late Saturday with showers
and thunderstorms certainly looking like a decent bet along the
front. Medium range guidance has trended toward less phasing with
the northern stream trough and the remnants of Odile...so not
looking like quite as big of a rain producer as previous runs
suggested. The push of colder air behind the front Saturday looks to
come in phases...with the first phase initially looking like more of
a push of Pacific air Sunday before secondary push of more
substantial cold air arrives early next week sending temperatures dropping
below average again.
//Ord and mdw concerns...updated 16z...
* lake breeze this afternoon.
* Cold front passage early Thursday with possibility of lower
High pressure will be in place today with continued light winds and
primarily VFR conditions outside of early morning fog. Confidence in
wind direction is lower mainly due to the overall light
magnitude...but expect a generally southwest wind early this morning
to gradually veer to northwest by early afternoon...then a lake
breeze will turn winds easterly middle to late this afternoon.
Throughout winds should remain less than 10 knots. Mostly sunny skies
will be in place again today with a few more middle and high clouds as
compared to yesterday. Winds are expected to go light and variable
overnight...then late tonight into early Thursday a cold front will
drop across the region turning winds back to the northeast. Some
models hint at the possibility of some MVFR stratus coming in behind
//Ord and mdw confidence...updated 16z...
* medium-high confidence in lake breeze timing to within an hour
* Low confidence in ceilings Thursday morning.
* High confidence in VFR conditions today and most of tonight.
* High confidence in winds less than 10 knots.
//outlook for Ord/mdw for 00z Friday-12z Wednesday...updated 12z...
Thursday night through Friday...VFR
Saturday...chance of thunderstorms.
Sunday through Tuesday...VFR.
222 am CDT
Areas of high pressure lie across the Ohio Valley and the Canadian
prairies early this morning with cold front in between these two
features just north of Lake Michigan. This front will move south
across the lake this evening and tonight with winds turning
northeast and increasing sharply into the 20-25 knots range
particularly over the north half of the lake. Winds diminish
Thursday..then turn back to the southeast and increase again
Thursday night into Friday. By Friday...south winds to 30 knots are
expected over the north half of the lake and a brief period of south
gales is not out of the question. Winds will remain fresh through
the weekend...turning northwest on Sunday as another cold front
moves across the lake.
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