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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
1132 PM CDT Thursday Aug 27 2015

Synopsis...
issued at 330 PM CDT Thursday Aug 27 2015

As of 3 PM CDT...mid-level cirrus from decaying convection across
central Iowa has resulted in filtered sunshine locally which has
kept temperatures down into the lower to middle 70s. Although we should
still warm a few more degrees through the late afternoon. Dvn radar was
detecting some elevated echoes over central Iowa but do not expect
precipitation to reach the surface due to very dry atmospheric column.

Main area of surface low pressure was still located over SW Nebraska
and northwest Kansas. The associated thetae plume extended from central
Oklahoma northward through East Kansas into southeast Nebraska/west Iowa.

&&

Short term...(this evening through friday)
issued at 330 PM CDT Thursday Aug 27 2015

Tonight...500mb vorticity. Maximum and surface low are forecast to enter into
western Iowa. Atmosphere preceding these two features is rather
dry. 12z/8.27 dvn sounding sampled a very dry layer around 800mb
where relative humidity values were near 15 percent. Thus it will likely take
some time to saturate the column and produce rain locally. Best
chance for showers prior to 7 am Friday is across the western tier of
counties where probability of precipitation increase to 50-60 percent.

Friday...periods of rain are likely over the northwest one-third of the dvn
County Warning Area...roughly along and north of a Cedar Rapids to Dubuque line.
Along this line...0.50 to 1.00 inches are possible...further northwest 1.00
to 1.50 inches are possible. Amounts drop off substantially to
the S/se...and any precipitation that occurs will be more showery in
nature. Since the upper disturbance has incorporated monsoonal
/sub-tropical moisture...precipitable waters will be high near 1.75 inches which
will make for moderate/heavy rain rates at times.

Model consensus is to have upper ageo. Divergence maximized along
the track of the 850mb low which favors northern Iowa/southeast
Minnesota/S Wisconsin to receive the heaviest rainfall. 0-1 km
convergence will develop downstream of the 850mb low and 850-700mb
warm air advection is forecast to be maximized in the same vicinity. The gefs raw
ensemble mean precipitation forecast has the quantitative precipitation forecast bullseye slightly
further south than model consensus which is more in line with the
European model (ecmwf). Going forecast puts a little more weight on these two
solutions.

Long term...(friday night through thursday)
issued at 330 PM CDT Thursday Aug 27 2015

Main challenge centers on rain and temperatures with the exiting
system early in the weekend. The remainder of forecast features a
warming trend supporting much above normal temperatures by the middle
to late portions of next week.

Friday night...model consensus has the upper level shortwave passing
through the area with its attendant surface low weakening and
transitioning across eastern Iowa or far northwest Illinois by Saturday morning.
With better continuity and consistency now for several runs...have
increased probability of precipitation to categorical along and north of the Highway 20
corridor with likelies over all but the south. The potential for
thunderstorms still looks low due to the lacking instability...thus
will continue the slight chance thunderstorm wording...which is more
favored in the evening. Largely stratiform type rain should be in
the range of a quarter to half inch north to a few hundredths in the
south.

Saturday...the surface low washes out to our east with lingering
clouds with some model disagreement in how fast the upper level wave
departs. Weak cyclonic flow and lingering moisture will keep a
threat of showers and isolated thunderstorms going across the
east...while most of eastern Iowa should be dry with decreasing cloud
cover in the afternoon. MOS guidance highs from the upper 70s north
to lower 80s south appear too warm considering the departing clouds
and precipitation...along with weak cold air advection on light north-northwest
surface winds. Have thus gone closer to the European model (ecmwf)/S highs ranging
from the middle to upper 70s.

Saturday night into Sunday...the incoming upper level ridge and weak
flow under high pressure at the surface may lead to a favorable
setup for late night/early morning fog...which may need to be added
in the forecast in latter updates. Slightly drier air moving in
should allow lows to return to a range from the upper 50s to lower
60s...while highs may recover to the lower 80s. The middle and upper
80s advertised by the GFS would require deep mixing...not supported
by this setup.

Next week the upper level flow transitions to a SW flow...followed
by a ridge axis centered nearly overhead by late in the week...
allowing much warmer air to envelope the region. Guidance is
consistent with highs in the middle and upper 80s from Tuesday through Thursday
with mins in the 60s...which would be roughly 4 to 8 degrees above
early Sep norms.

&&

Aviation...(for the 06z tafs through 06z Friday night)
issued at 1130 PM CDT Thursday Aug 27 2015

06z tafs updated for slightly slower timing of rain onset...from
late morning northwest to late afternoon southeast. MVFR ceilings
and visibilities in rain will become common through the afternoon
and evening.

&&

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

$$

Synopsis...uttech
short term...uttech
long term...sheets
aviation...rp Kinney

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