Area forecast discussion
National Weather Service Des Moines Iowa
1151 PM CDT Tuesday may 21 2013
Short term.../tonight/
issued at 346 PM CDT Tuesday may 21 2013
Steeping low-level lapse rates due to both cold air advection in the 700 to 850 mb
layer and diurnal late may heating led to the quick development of a
stratocu deck over much of Iowa this morning. The clouds were thickest
in the northwest...which limited daytime highs to the low to middle
60s. Drier air filtering in from the west will allow low temperature
readings to fall off into the low 50s...though these could be
complicated by the increasing clouds overnight.
Thinner cumulus over southern and eastern Iowa should lessen in
coverage by sunset...but clouds will build back in as the low shifts
to the east across southern Minnesota. Chances for precipitation will
increase after 06z across northern Iowa as the surface low tracks
across the region. Weak isentropic lift along the 300 to 310 k
surfaces on the backside of the low will be the main driving force
for precipitation. This process was reflected well in the high res east
WRF-arw model and used this as a base for pop chances for the
period. Middle to upper level vorticity maximum may also spark a few stray
showers over far northern Iowa close to the low center during the
earlier overnight hours...but was not confident enough to mention in
the forecast wording.
Long term.../Wednesday through Tuesday/
issued at 346 PM CDT Tuesday may 21 2013
Pattern looks active right into early next week outside of break
Thursday into Friday. Period should begin essentially with
extrapolation of what is currently ongoing over Minnesota as it drifts
southward into Wednesday. Deeper forcing mainly remains from eastern Minnesota
into WI...but moisture is fairly deep with soundings showing
column saturating. This should amount to scattered showers and
possibly a rumble of thunder or two during the day and lingering
into tomorrow night before exiting by daybreak Friday. Temperatures should
be fairly cool...held down by plenty of clouds...precipitation and cooler
temperatures behind departing cyclonic flow.
Upper level ridge will then enter the MO valley with fair weather
into at least early Friday. Theta-E advection then quickly returns Friday
night which will likely result in west to east mesoscale convective system development.
Ridge continues to weaken into early next week which will result in
prolonged southwest upper flow. GFS is a little more bullish than
the European model (ecmwf) with European model (ecmwf) showing a stronger Great Lakes high presence
and warm advection shifted a bit farther west. Resultant precipitation
from this pattern would be a string of elevated convective
complexes. Surface based potential does not look too great with
Lee side trough staying to our west and only subtle warm frontal
influences in prolonged generally southeast surface flow. However
mcss could be stronger by the weekend when elevated instability
increases and even the European model (ecmwf) nudges this MUCAPE eastward into west
and south Iowa. Certainly no specifics at this point but Hydro
situation will need to be watched as many river sites in the
northeast half of the state are already at the 90th flow
percentile or higher and any appreciable recovery period is not foreseen.
&&
Aviation...22/06z
issued at 1151 PM CDT Tuesday may 21 2013
Main concern will be the showers and ceilings lowering with the
upper level low moving east in the next 24 hours. Current upper
level system remains to the west but is slowly progressing east
overnight. This will result in an area of showers over northeast
Nebraska and southeast South Dakota to move southeast into the
forecast area. The main area of MVFR/IFR ceilings is west of a cold
front which will swing southeast through 00z Thursday. The timing
of the showers has been slowed by 1-2hrs from previous package but
otherwise trends remain the same. Still a hint at isolated
convection during the afternoon Wednesday...but confidence remains
low regarding categorical coverage so have added thunderstorms in the vicinity to southern
and eastern taf sites where more significant cape is expected.
Winds will increase after 18z most areas as stronger gradient
crosses the region.
&&
Dmx watches/warnings/advisories...
none.
&&
$$
Short term...skow
long term...small
aviation...rev
