Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
1117 PM CDT Sat Aug 1 2015
..updated for 02/06z aviation forecast discussion...
Short term /tonight/...
issued at 313 PM CDT Sat Aug 1 2015
A wind shift boundary behaving like a warm front has lifted
northeastward across the area today and now stretches from
northeastern South Dakota down into far northeastern Iowa.
Instability is maximized around and just ahead of this
boundary...however so far there has been very little convective
development across Minnesota indicating that daytime heating has not
yet overcome a capping inversion...particularly with more robust
lift ahead of a 500 mb shortwave approaching from the northwest not
having arrived yet. It appears that as this lift progresses later
this afternoon and evening stronger storms will initiate somewhere
near the ND/SD/MN triple point and move east
southeastward...possibly affecting our northern/northeastern
counties around sunset or later. However...the storms may pass
further north or weaken before they reach our area.
Further southwest...weak but persistent convection across central
and eastern Nebraska has produced a high cloud shield that is
limiting heating and destabilization across much of central Iowa.
This should preclude any organized storm development through this
evening...although as the low level jet kicks in overnight it may be
sufficient to produce some elevated storms that are not expected to
be severe. Given the overall scenario heretofore described...have
lowered inherited probability of precipitation across much of the area but maintained chance
category especially north/northeast and then central after midnight.
Confidence is relatively low however. Other than storms and their
related effects...the forecast for tonight is straightforward with
modest south southwest breezes...dewpoints well into the
60s...and high clouds keeping temperatures from falling too far.
Long term /Sunday through Saturday/...
issued at 313 PM CDT Sat Aug 1 2015
Models have come into better agreement for beginning of
extended...and have become more progressive with boundary pushing
through state. Previous runs hung boundary across Iowa with
persistent chances for precipitation through beginning of week.
Current trends are for boundary to push south of County Warning Area by late
Sunday. Therefore...have kept trend of precipitation in the west
early in the period with potential lingering overnight
convection...and generally trended drier through late Monday. One
exception is with pop chances in the southwest late Sunday
afternoon. Models indicate sufficient instability
though...soundings indicate fair cap and may be enough to limit
development...but with boundary pushing south have gone slight
chances mention in at this time.
Behind system will see cooler temperatures for beginning of week.
Next best chances for precipitation come late Monday as a more
active weather pattern returns. Heavy rain will be possible Monday
night with good moisture pushing north...precipitable water values around 1.6
inches or higher...and soundings indicating deep moisture. Weak
ridge in west will keep generally northwesterly flow across Iowa
with impulses pushing through flow. Models are in general
agreement with best chances for precipitation across Iowa. Timing
of waves will be more difficult...though best signal is for
around Thursday...and have trended towards the GFS for now.
Temperatures will generally be held down by precipitation trends
and cooler northwesterly flow aloft...and have gone below normal
for early August. However...may be too cool...depending on
placement of precipitation and clouds throughout the extended.
issued at 1117 PM CDT Sat Aug 1 2015
Removed mention of rain showers/thunderstorms and rain overnight from the terminals. Latest
indications are that support will not be existent for precipitation
overnight. Thus...VFR ceilings and visibilities are expected. Winds will be
higher on Sunday than today. Slight chance of rain showers/thunderstorms and rain late afternoon
and evening across eastern Iowa...but have not added mention to the
Kalo or kotm terminals at this time.