Area forecast discussion
National Weather Service Des Moines Iowa
1250 PM CDT Tuesday Jun 18 2013
Update...
issued at 945 am CDT Tuesday Jun 18 2013
Have sent out an update to refine probability of precipitation across the northeast and
southwest. Have increased probability of precipitation across the southwest to a sliver of
likelies given the coverage of lingering activity...and reduced
probability of precipitation across the northeast to slight chance given the small areal
coverage and likelihood that the majority of activity will skirt
by the County Warning Area just to the east. Soundings still indicate some
instability by the afternoon hours in the Waterloo/Mason City
area. Therefore have left a small area of slight chance
probability of precipitation...however main forcing will slide off to the southeast with
the upper level shortwave so do not expect widespread chances.
Also lowered maximum temperatures a few degrees across the far southwest in
vicinity of the thunderstorms/cloud cover...otherwise remainder of
forecast looks on track.
&&
Short term.../today/...
issued at 328 am CDT Tuesday Jun 18 2013
Combination of inverted trough extending from central Illinois into southwestern
Minnesota into the Red River valley...combined with rather strong shortwave
rotating southeast into southern Minnesota and speed maximum moving into northwestern Iowa has
resulted in vigorous convection in southwest Minnesota...albeit one main
cell. Though parameters are weak...being isolated it has been able
to maintain itself. Expect to see scattered convection develop in
the vicinity of the front as shortwave rotates through today. The best
chance is in the morning before the shortwave passes the area. Feel the
namdng5/European model (ecmwf) and to a decent amount the GFS has a pretty good
handle on the precipitation area. Initially discounted the nam12/hrrr
and some of the other hires models with the development over western
Iowa...however some development has occurred in response to the
passage of the speed maximum putting southwestern Iowa in the right inflow region.
Temperatures today will be a little cooler than yesterday with increase
clouds and and some cold air advection at 800 mb. Generally middle 70s to around 80...a
few degrees warmer in heat island areas.
Long term.../tonight through Monday/...
issued at 328 am CDT Tuesday Jun 18 2013
Weak ridge will remain in place across Central Plains for the
beginning of the period. Shortwave to push east across ridge will
bring precipitation chances for Wednesday. However timing is still
a bit uncertain...with models in poor agreement on timing of wave.
Have kept timing near going. Models do indicate a slightly further
south solution...with best precipitation chances across southern
Iowa/northern Missouri...and have begun to trend probability of precipitation that way for
late Wednesday and early Thursday. Do have some concerns for
severe weather late Wednesday afternoon...with cape values
1000-2000 j/kg indicated...and little in the way of cap. Weak cap
may be enough to hold off convection with little forcing. Hail/wind
would be the main threats from any severe storms. Temperatures
will warm steadily through the period under the ridge...with
primarily southerly flow at the surface.
Upper level low pushing east across Canada will drop a frontal
boundary across northern plains for Thursday/Friday. Models have
sped up timing a bit on this system...and have there fore cut back
on probability of precipitation across the south for late Friday. Large upper level low
pressure system expected to push onshore in Pacific northwest for end of week
will influence weather across Iowa for much of the remainder of
the period. As this is still off the shore and poorly sampled...do
not have high confidence in timing...though models are in decent
agreement. As low digs across The Rockies...ridge lifts across
Iowa...with shortwaves lifting through ridge.
However...timing/location again problematic. Latest model runs
keep tracks west and north of Iowa...and have trended towards this
solution for the weekend keeping probability of precipitation out of the south.
&&
Aviation...18/18z
issued at 1245 PM CDT Tuesday Jun 18 2013
Mainly VFR conditions expected through this evening...with
possibility for visibilities to drop to MVFR in some light fog late
tonight. Some instability around this afternoon suggesting
potential for an isolated thunderstorms and rain near kmcw...Kalo...and kotm.
However uncertainty is high as upper level disturbance is exiting
the area...this left out any mention of thunder in the tafs at
this time. Winds to shift around to the east/southeast through the
period...with moisture increasing aloft late tonight. Therefore
expect some low/middle VFR ceilings by early Wednesday morning across the
western taf sites.
&&
Dmx watches/warnings/advisories...none.
&&
$$
Update...beerends
short term...MS Jun 13
long term...awb
aviation...beerends
