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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
329 am CDT Sat Aug 23 2014

issued at 327 am CDT Sat Aug 23 2014

Highly amplified upper ridge dominates the Mississippi Valley while
an upper low digs into the Pacific northwest. At the surface...low
over Nebraska with a diffuse stationary front bisecting Iowa from the
southwest corner to near the quadrant cities. Environment remains
largely the same as recent mornings with weak low level
convergence...weak flow aloft...and weak shear...but abundant
moisture and instability.


Short term...(today and tonight)
issued at 327 am CDT Sat Aug 23 2014

Scattered convection is expected to continue to fester this
morning. High precipitable waters around 2 inches will yield high
rainfall rates...which will be aggravated by slow storm movement due
to the weak flow aloft. Wet and unstable environment also suggests
downburst near-severe winds are a possibility...and especially early
in the storm evolution when updrafts are rapidly strengthening.

Meanwhile...area of storms over SW Iowa is expected to expand southward as
it lifts NE due to increasing low level jet feeding into it from the
SW. If this in fact evolves as prognosticated...then as convection moves
into eastern Iowa later this would be in a weakening
phase as it moves away from out the low level jet but should still hold
together into eastern Iowa and northwest Illinois.

Temperatures today will once again near 90. With dew points in the
70s...this will mean heat index readings near or just above
100...especially in the southern third of the County warning forecast area. One fly in the
ointment could be precipitation and convective cloud debris. The longer
the clouds linger...the greater the potential for heat indices not
to reach 100. Either way it will be warm and muggy.

Long term...(sunday through friday)
issued at 327 am CDT Sat Aug 23 2014

This stretch of the warmest weather of the season is likely to
persist well into next week with continued nearly daily chances for
thunderstorms. While details and timing remain uncertain...a pattern
shift is likely to take place by late in the week...providing a
return to below normal temperatures and drier conditions.

Sunday...a strengthening south to southwest low level flow resulting
from the plains low lifting north-NE into Canada will wash out the low
level convergence and provide deeper mixing. 00z models have
converged to better agreement showing a general range of 21 to 23
degrees celsius at 850 mb over the area...which would generally
suggest highs in the lower to middle 90s. However...with continued high
precipitable water over especially the east and possibility of occasional cloud
cover...have trended highs a bit lower and closer to
guidance...which has trended too warm over the past several days.
The combination of highs from the upper 80s north and northeast to
the lower 90s central and south and dew points holding in the lower
70s would push heat index values into a 100 to near 105 range mainly
south of I-80 in the afternoon and a heat advisory may be needed.
Forecast soundings suggest a strong enough cap to prevent daytime
thunderstorms and kept probability of precipitation below slight chances.

Sunday night through next week...models continue to vary
considerably with the timing of a weak cold front Monday...which
largely washes out to keep very warm and humid conditions in place
through at least Tuesday. The GFS remains a more progressive outlier
by middle to late week with a faster breakdown of the upper level ridge
and the ensuing onset of an upper trough and surface high by Thursday and
Friday. At the same time...the European model (ecmwf) and Canadian have a less phased
configuration of the upper level flow by Thursday and Friday...showing a
surface low moving out of the Central Plains spreading precipitation
into the region. Thus...a low confidence extended period continues.
Near advisory heat and humidity is maintained for Monday and
Tuesday...with highs trending back to only the 70s for Thursday and
Friday. Thunderstorm chances are greatest Tuesday night and
Wednesday with low confidence...slight chance to chance probability of precipitation nearly
every period from Sunday night through Friday.


Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1119 PM CDT Friday Aug 22 2014

Overnight thunderstorms most likely at kcid based on recent radar
trends. Confidence low in timing/coverage at other taf sites.
Hi-res models suggest potential for a complex moving into the
forecast area from the west after 12z...and have this covered with
morning thunderstorms in the vicinity groups at kbrl/kcid/kmli. IFR fog/stratus favored at
kdbq during the early morning provided there is enough clearing.


Dvn watches/warnings/advisories...


short term...dmd
long term...sheets
aviation...rp Kinney

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