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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
625 PM CDT Thursday Sep 18 2014

Synopsis...
issued at 326 PM CDT Thursday Sep 18 2014

12z upper air analysis has an upper level high over the Great Lakes with
moisture associated with Odile in the southern rockies and Southern
Plains. Satellite trends through middle day Show Low clouds across
Wisconsin and the Central Plains slowly dissipating.

18z surface analysis has a dissipating cool front from Northern Ohio
into southeast Minnesota. Dew points behind this front were
generally in the 40s. Ahead of the front dew points were in the 50s
and 60s from the plains into the deep south.

&&

Short term...(tonight and friday)
issued at 326 PM CDT Thursday Sep 18 2014

Quiet and cool conditions will be seen through midnight as a weak
cool front slowly dissipates across the area. After midnight the
remnants of the weak front may provide the focus for patchy fog
development. Negatives against this are the slowly increasing wind
gradient toward sunrise and the arrival of high clouds. However...
if the convergence/moisture suggested by some of the models is
correct then patchy fog and low clouds is possible.

Going on the Assumption of patchy fog/low cloud development toward
sunrise...the fog/low clouds will slowly lift or burn off by the end
of the morning commute.

By middle to late morning...there is a respectable Theta-E gradient
associated with the leading edge of better moisture that begins
moving into the area. There is some forcing associated with this but
the support is weakening. Thus there should be only some passing
clouds. At worst...a few sprinkles might occur in the northwest
quarter of the area.

Long term...(friday night through next thursday)
issued at 326 PM CDT Thursday Sep 18 2014

Chance of showers and storms on Saturday with frontal passage followed
by generally dry conditions and seasonal temperatures.

Friday night...chance of showers and a few storms will exist across
the northern County Warning Area mainly well after midnight as front approaches.
Southerly winds ahead of front will result in milder night with lows
mainly in the 60s.

Saturday and Saturday night...any lingering precipitation north to largely
diminish by late am with weakening nocturnal low level jet. Front then
to begin moving into northwest County Warning Area by midday and continue shifting
across County Warning Area exiting the far southeast late Saturday evening. The front
will be accompanied by chance of showers and storms with areas south
and east of quadrant cities standing best chances. Cannot rule out a few
PM strong storms portions of southeast Iowa...northwest-west central Illinois
and northeast MO with sbcapes in excess of 3000 j/kg (based on temperatures
near 85 and dewpoints near 70) and 0-6km shear increasing to 30-35 kts.
Amount of instability will be dependent on extent of cloud cover with
impact on temperatures. Have continued for more middle Road approach of
around 80 to lower 80s in general for highs... but if any decent duration
of sunshine with southwest winds aiding pre-frontal compressional
heating then could not rule out some middle 80s portions of the southern
County Warning Area. Saturday night... chance of showers and storms will continue across
the south until frontal passage. Then...decreasing clouds and cooler
conditions with gusty northerly winds overnight. Lows mainly lower 50s
northwest to lower 60s southeast.

Sunday and Sunday night...secondary digging shortwave through the
Great Lakes and attendant front focus mainly clouds and showers
to our north/northeast from WI into Michigan. Incoming Canadian ridge will
be the dominant weather feature and provide dry and very pleasant
conditions. Gusty northwest winds and solar insolation to fuel deep
mixing around 850 mb per soundings to support highs mainly in the
lower to middle 70s. Clearing and cool Sunday night with relaxing
winds as high begins to settle over the area. Lows mainly ranging from
the middle 40s to around 50. However...if winds were to go calm for any
length then some drainage sites north could dip into the lower 40s based
on prognosticated 850 mb temperatures around 6c.

Monday through Thursday...large expanse of high pressure will dominate
providing generally dry and seasonal temperatures through midweek... with
just gradual moderation on temperatures. Toward the end of the period and
beyond the high shifts east and will see potential for some return flow
rain chances... but given strength of high and dry air typically models
too fast in bringing in by 24+ hours and thus may hold off any rain chances
until closer to Friday. Any rainfall amounts late in the period and just
beyond is looking rather light with surface ridging into the Gulf of
Mexico.

&&

Aviation...(for the 00z tafs through 00z Friday evening)
issued at 621 PM CDT Thursday Sep 18 2014

Another night tonight where we will begin with VFR
conditions...and unlimited visibility...and may end with dense
shallow ground fog. This is a low probability event tonight...as
there is less moisture than last night...and slightly stronger
winds. So...will add a period of MVFR fog to Cid and dbq...and a
temporary condition of IFR fog to dbq during the 09z-13z time
frame. As we go forward...we can more confidently resolve this
visibility issue for the predawn hours. Beyond...a VFR day with
south to southeast winds around 10 kts is expected Friday.

&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

Synopsis...08
short term...08
long term...05
aviation...Ervin

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