Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Des Moines Iowa
629 PM CDT Tuesday may 26 2015

Short term /tonight/...
issued at 324 PM CDT Tuesday may 26 2015

Shortwave trough presently located over the upper Mississippi and middle-
Missouri River basins has been slow to depart today. Surface low
reflection presently over far SW Wisconsin has been equally slow to
depart. Elongated vorticity rotating around the base of the shortwave
trough is reflected at the surface as a surface low in SW Iowa...along
with an attendant trough.

The boundary layer has destabilized enough in west central...
southwest and south central Iowa today for convection to develop
this afternoon. Will have to watch convection there for landspout
activity at least until combination of low-level
cape...surface vorticity and low LCLs are maximized there. A few of
the updrafts there this afternoon have been rather potent as will also have to monitor for potential hail and
localized damaging wind threat.

Any convection that does develop...even non-storms...will be capable
of producing brief heavy rainfall. The shortwave trough has entrained
moisture-rich air...some of which has rotated down into Iowa. Flash
flooding is not expected however.

After sunset...precipitation chances will gradually diminish from SW to NE
across the County Warning the upper-level shortwave trough continues pulling
away...and negative vorticity advection along with associated downward motion sets in.

Lows are expected to drop further tonight than last
cooler air is pulled in on the back side of the departing system.
Made only minor tweaks to going forecast mins.

Long term /Wednesday through Tuesday/...
issued at 324 PM CDT Tuesday may 26 2015

High pressure at the surface will be pushing east through the day
on Wednesday while an upper level ridge moves across the region.
The result will be a warm and dry day...however it will be
relatively short lived.

By Wednesday evening we will be back into an increasing warm air
advection regime with a short wave expected to lift across Iowa
during the day on Thursday. With steepening lapse rates and
sufficient instability thunderstorms will be back in the forecast
for Thursday although shear is somewhat in question. While low level
shear looks sufficient for a few stronger updrafts...the shear aloft
is unidirectional and only around 20-25kts.

Friday looks to be the most active day in the extended as a
surface low lifts to the northeast and a cold front moves across
the forecast area. Qg forcing is fairly impressive across Iowa
Thursday night and Friday as well. Shear is a little better as
well so strong storms are most probable from late Thursday night
through Friday.

Precipitation over the weekend will depend on where the front sets up as it
is expected to stall. Latest models show the front hanging up
in Missouri then extending northeast across far eastern Iowa but
especially Illinois and Wisconsin. The bigger story will be a
large high pressure building into the northern plains and upper
Midwest. This will likely bring some well below normal highs to
the region for the weekend. It is quite likely we will be seeing
highs in the 60s to around 70 on Saturday with very cool overnight
lows. As cooler and drier air works into the state it also will
make it harder for storms to develop...despite frontal location.

The cooler and drier weather will persist into next week although
highs will moderate closer to seasonal readings by Monday.


issued at 626 PM CDT Tuesday may 26 2015

Low VFR and MVFR ceilings will persist through the beginning of
the period along with scattered thunderstorms and rain and rain showers as low pressure
system continues to lift northeast. Ceilings will lift to VFR
near 06z at sites. May see br/fog at sites near 12z though do not
have high confidence and have only trended this way for now.
Beyond 12z conditions will be VFR with winds becoming easterly.


Dmx watches/warnings/advisories...



Short term...zogg
long term...fab

National Weather Service Glossary of Abbreviations