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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
718 am CDT Thursday Aug 28 2014

Update...
issued at 718 am CDT Thursday Aug 28 2014

Have updated the forecast to tweak timing and probability of precipitation for the leading
edge of the large mesoscale convective system lifting northeast out of central and western
Iowa and far eastern Nebraska. Overall...the system continues to
demonstrate stronger convection along its south and southeast axis
than both short term convective models and synoptic models have
advertised. The primary shortwave and mesoscale convective vortex is evident on radar
loops over far western Iowa...with a smaller mesoscale convective vortex in S central Iowa...
which will likely help maintain the convection over the next
several hours even as the nocturnal jet veers and diminishes.
Radar rainfall estimates have shown a slow decreasing trend in
the S central Iowa convection...suggesting rates as high as .25
inches per hour on average likely as it reaches into the ffl...Cid
and ind corridor between 9 and 10 am.

&&

Synopsis...
issued at 337 am CDT Thursday Aug 28 2014

High pressure centered over the western Great Lakes was channeling
a relatively cool...dry easterly flow into the area early this
morning. More active weather was not far to the west...as a large
mesoscale convective system was over southeast Nebraska into SW Iowa...occurring in an area of strong
thetae advection and convergence along a 40 knots low level jet. This
jet and deep moisture return was being fed by an upper low over
northwest Colorado. A weak shortwave trough...associated with the mesoscale convective system was
lifting northeast along the edge of an upper level ridge axis
reaching from eastern OK north-NE into eastern Iowa. At the
surface...msas pressure fields indicated the surface boundary from
near stl west-northwest across MO into northern Kansas. Moisture pooling along
this boundary was resulting in dewpoints in the upper 60s to lower
70s...while to the north...readings were primarily in the lower
60s over the local forecast area.

&&

Short term...(today and tonight)
issued at 337 am CDT Thursday Aug 28 2014

The primary focus is on rain and thunderstorm chances as the
shortwave and mesoscale convective system continue to advance northeast. At the
surface...this will drag the warm front northward into the
forecast area tonight...allowing a slightly more humid airmass to
build into the region along with potential showers and
thunderstorms

Today...near consensus of the 00z models has the mesoscale convective system complex lifting
northeast through north central Iowa into southern Minnesota by evening along
the northwest periphery of the upper ridge axis. Model forcing and quantitative precipitation forecast
fields now focus the heavier rainfall to the northwest of
the forecast area...possibly brushing our far northwest counties from late
morning through early afternoon where highest probability of precipitation are maintained.
Have gone with forecast quantitative precipitation forecast of one half to nearly one inch for today
in this area...with generally only a quarter inch or less elsewhere
and lower probability of precipitation from the previous forecast. Outside the far northwest...showers
will be the primary Mode with isolated to scattered thunderstorms
due to relatively stable middle level lapse rates and widespread cloud
cover limiting the low level instability. The clouds and
precipitation will limit highs to the 70s central and north...with
the south pushing into the lower 80s as the surface front pushes
through in the afternoon.

Tonight...the main focus for organized mesoscale convective system activity will be further
north across WI into the western Great Lakes. However...with low
level moisture convergence along the surface warm front that may
still be lingering over the central or northern portions of the
forecast area...and possible weak impulses riding the upper
ridge...cannot rule out showers and a few thunderstorms and will
continue chance probability of precipitation. Limited instability and shear values should
preclude any risk of severe storms. Mins will likely be about 3 to 5
degrees warmer than the previous night.

Long term...(friday through wednesday)
issued at 337 am CDT Thursday Aug 28 2014

The question on Friday is the overall rain chances. The better
forcing/moisture remains west of the area during the morning hours.
Thus there is a very real possibility that the morning hours could
be dry. As such...probability of precipitation for the morning has been lowered to reflect a
more isolated to low end scattered nature for any convection.

Better moisture starts arriving Friday afternoon along with some
forcing. However...the strongest forcing is still west of the area.
What moisture/forcing that is present should combine with daytime
heating to produce isolated to scattered convection.

Friday night/Saturday much stronger forcing moves through the area
with a weak front. Isolated to scattered convection may continue
Friday evening but the better chances of rain will be late Friday
night into Saturday morning. Rain will end from west to east during
the day Saturday.

Saturday night...there may or may not be some very light rain
lingering in the far east up to sunset. After sunset...high pressure
will begin building into the upper Midwest resulting in dry
conditions overnight.

Sunday on...
Sunday now looks to be dry. There has been a reasonable signal
developing in the models of high pressure over the Great Lakes.

Sunday night through Monday night the model consensus has chance to
likely probability of precipitation for Sunday night/Monday and slight chance to chance probability of precipitation
for Monday night. Although there are still differences in overall
timing...the models are now looking at another cold front to move
through the area in the Sunday night/Monday time frame.

The model consensus has dry conditions Tuesday with slight chance
probability of precipitation Tuesday night. The models indicate a weak disturbance aloft
moving through the area but differ on its timing. Moisture
availability is also in question given the recent passage of a cold
front.

&&

Aviation...(for the 12z tafs through 12z Friday morning)
issued at 718 am CDT Thursday Aug 28 2014

A large complex of thunderstorms over western Iowa will move into
the forecast terminals from middle to late morning. This will bring
widespread MVFR to local IFR conditions. There is somewhat low
confidence in how long thunderstorms will continue into the
afternoon as the primary upper level disturbance lifts northeast
out of the area. Although not mentioned in the tafs at this time...
additional scattered thunderstorms will be possible overnight as a
surface warm front slowly advances from the south. Associated
moisture will likely result in MVFR visibilities and possible low
clouds.

&&

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

$$

Update...sheets
synopsis...sheets
short term...sheets
long term...08
aviation...sheets

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