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Area forecast discussion 
National Weather Service Des Moines Iowa
1214 am CDT Sat Aug 30 2014

Short term /tonight/...
issued at 315 PM CDT Friday Aug 29 2014

Another active night is in store as the deep layered trough feature
that has been lingering to our west the last day or two finally
progresses eastward across Iowa. Regional radar imagery from the
last couple of hours shows an increase in coverage of showers and
thunderstorms around the broad circulation of the trough coincident
with daytime heating maximum...but expect scattered to numerous
showers/storms to persist through the evening and into tonight
before finally fading out and moving off to the east early Saturday
morning. Have maintained mostly 50 to 80 probability of precipitation through midnight
accordingly. A heavy rain threat remains particularly closer to the
trough axis where cell motion is quite slow...however this threat
will be isolated and no widespread flooding is anticipated. Likewise
diurnal heating in the sunnier areas across our eastern sections
this afternoon has allowed for impressive destabilization and there
is some potential here for isolated severe wind gusts...but again no
widespread or significant severe weather is anticipated.

Long term /Saturday through Friday/...
issued at 315 PM CDT Friday Aug 29 2014

Long awaited short break from thunderstorms to arrive early this weeks persistent upper level wave finally moves
off to the northeast. A weak wave behind this system may bring some
residual clouds over northern sections tomorrow...but overall a nice
change from the threat for thunderstorms everyday. A slightly cooler
airmass will arrive as well with highs in the upper 70s to lower
80s...though humidity levels will not change all that much. A weak
ridge of high pressure will cross the region Saturday night into
early Sunday morning.

As the ridge recedes to the east...stronger southwest flow will
develop both at the surface and aloft in response to a rather strong
h500 short wave approaching from the west. Ahead of the system...
dew points will quickly return to the 70s with highs in the upper
80s to near 90 across the south. With the quick warm up...
instability...precipitable water...and warm cloud depths will again
rise to levels of concern...with severe storm potential increasing
across the west along the boundary from the middle to late afternoon
hours northwest to central Iowa in the evening. Though wind fields
aloft are not significant...there has been a consistent signal for
40 to 50kt winds through the column by late day Sunday and Sunday
evening. Precipitable waters are expected to once again pool along the boundary
and increase to 1.75 to 2.0 inches by late day and evening. Warm
cloud depths again are maximized to nearly 13kft by late afternoon
west and central. Current bufr soundings across the region for 21z
Sunday show a dry layer through the lower to middle levels with some
veering winds with height...though generally unidirectional. Looking
at the GFS 12z model data...0-3km bulk shear vectors become more
perpendicular to the expected line of storm development Sunday
evening...suggesting that bowing linear segments and a faster
moving line of storms with a greater potential of wind damage may
develop after some initial supercells and any linear segments
merge. Currently the area is outlooked for a slight risk for
severe weather. There will be a good chance early in the event of
heavy rainfall of 1 to +2 inches nearer the cool frontal boundary
in the far west/northwest...and even with the potential of faster
moving storms in the middle to late evening...rainfall rates and
amounts may still be on the moderate to perhaps heavy side due to
the persistence of elevated warm cloud depths and high precipitable waters . Any
additional rainfall will again push streams and rivers up and
perhaps cause some urban street or field flooding...but the 36
hour break from Saturday morning through 00z Sunday will help
mitigate the situation.

Beyond Sunday night...there remains slightly less certainty
regarding next weeks pattern...but two themes are beginning to
emerge. First...both the GFS/Euro suggest an additional fast moving
wave with scattered thunderstorms will cross the region the Monday night
Tuesday...prior to the southeast US h500 ridge building back west
into the Southern Plains and middle Mississippi River valley. As the
westerlies retreat farther north...h500 heights are expected to
build from around 580dm Monday to nearly 592dm by 00z Friday. Temperatures
will warm through the period with a general shift of the heavier
rain threat to northern Iowa and states north. Will continue to
monitor...and have more confidence in the warming temperatures than lack of do expect to see temperatures pushing toward 90
degrees over central and south toward the end of next week.
Otherwise...much of the upcoming week will continue in the upper 70s
to 80s from Monday through Wednesday with lows in the upper 50s to


issued at 1214 am CDT Sat Aug 30 2014

Ceilings and visibility trends will be the primary concern through Sat
morning as upper low and associated scattered convection passes
through Iowa. Conditions are quite variable across the state at 05z
ranging from LIFR to VFR. Although confidence in details is
low...generally kept scattered showers/isolated thunderstorms going
through 09z or so as system exits. Surface flow will be weak
behind surface trough passing through central Iowa at the moment and
coincident with plenty of lingering low level moisture so lowered
ceilings/visibilities before typical diurnal rebound into VFR conditions
later Sat. Category confidence low so started with IFR/MVFR
although either side of that may occur.


issued at 315 PM CDT Friday Aug 29 2014

Similar concerns as past few days. Most of the region continues
nearly saturated...with some locations not having much room for
additional rainfall before runoff may become a problem. Southeast
counties of our forecast area could take a bit more...fortunately
the current storms are working north across these locations.
Additional rainfall later this evening could lead to some isolated
problems over the west/southwest with field ponding/minor flooding
in rural and urban areas the main concerns. Will be monitoring for
any more organized threat this evening. By Sunday evening...more
widespread moderate rainfall expected. With nearly 36 hours of
quiet conditions expected from 12z Sat through about 00z Sunday...
soils should better be able to handle additional rainfall. Some
minor river flooding continues now near dsm and potentially some
additional rivers/streams rising to or slightly above fs over the
weekend or early next week across the area.


Dmx watches/warnings/advisories...none.



Short term...Lee
long term...rev

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