Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Des Moines Iowa
1211 am CDT Wednesday may 6 2015

issued at 1211 am CDT Wednesday may 6 2015

Fog has developed across northern Iowa in area north of daytime
surface boundary. Short term hrrr and rap continue to show the fog
lingering through the early morning hours before dissipating
shortly after sunrise. Have also adjusted readings across the
north downward as warmer airmass never quite made it into northern


Short term /tonight/...
issued at 330 PM CDT Tuesday may 5 2015

The frontal boundary remains draped across central Iowa and has not
moved much at all today. It made a very slow progression northward
and currently sits just south of Highway 20...versus Interstate 80
this morning. Precipitation from earlier this morning faded as it
trekked northward and away from what little lift...mainly warm air advection
over the boundary...remained. Cloud cover that lingered well into
this afternoon across southern Iowa prevented SBCAPE values from
soaring...but have rebounded to 1500-2000 j/kg with none cin across
central and southern Iowa. Areas that saw sunshine will likely
reach their convective temperatures...generally in the upper 70s...and is
being realized now as some weak convection is currently firing up
in south central Iowa and west of Fort Dodge. Weak
shear...generally 20kts or less...and no cap will result in
numerous pulse storms that will limit severe potential as they
fight for energy. The hrrr appears to have the best handle on the
situation on current convection and has all of it gone by 03z with
the lack of any significant lifting mechanism over night.

Temperatures struggled to reach daytime highs with thick...and
generally low...clouds over much of northern Iowa. As a result many
locations still sit in the upper 50s. With that in mind...have
dropped lows only a couple of degrees since dewpoints will remain
fairly high and stop any crashing temperatures. Fog may be of concern
across the north as well...but may be limited with winds around
10kts. Sites that see winds drop a bit may see periods of fog.

Long term /Wednesday through Tuesday/...
issued at 330 PM CDT Tuesday may 5 2015

Generally southwesterly 500 mb flow across Iowa and the Midwest
will result in an active and fairly wet weather pattern through
the weekend. This pattern will be punctuated by two fairly robust
low pressure systems and an intermediary period of subtle
shortwaves moving through the aforementioned southwesterly flow.
The first big system is currently tracking across the southern
rockies in the form of a deep open wave...and will move up over
our region on Wednesday and Wednesday night. Meanwhile a large
closed 500 mb low currently moving ashore over Washington/British
Columbia will dive southward along the Pacific U.S.
Coast...reaching northern California by late Wednesday night. the first system departs quickly to our northeast
and the second system digs in and deepens over the southwest U.S.
It will maintain southwesterly flow over Iowa...through which
subtle shortwaves will move at the end of this week. Finally...the
large southwest U.S. Low will eject up across the Midwest sometime
around Sunday.

As the leading low pressure trough moves up into the region on
Wednesday it will push a large region of warm air and Theta-E
advection up across Iowa. Within this region widespread
thunderstorms should develop over Nebraska and Kansas
tonight...then slowly fade as they translate across our forecast
area during the day Wednesday. Will maintain likely probability of precipitation for most
of the day...however...confidence by Wednesday afternoon is not
high as it appears that in wake of the initial band of warm air
advection there may be little to force trailing convection and we
could see a lull similar to today. In any event...while there is
some marginal instability and decent deep layer shear...nearly
uniform low level winds and a lack of any kind of focus mechanism
lead to a low risk of severe weather which is well outlined by
Storm Prediction Center. On Wednesday night another expansion of thunderstorm coverage
will likely occur with the nocturnal low level jet on the eastern
flank of the upper low pressure system...but this activity will
mostly be to our west across Nebraska...South Dakota and
northwestern Iowa and once again with a low severe weather threat
in our area. Forecast probability of precipitation have been trended toward this direction
and timing.

Potentially the most active period for our area will come on
Thursday. It appears that one or two shortwaves moving overhead
will push a synoptic surface trough slowly across our area on
Thursday night. Ahead of this peak heating time late
Thursday afternoon and evening prognostic models unanimously
depict a swath of strong instability developing ahead of the
boundary...though this could be tempered by debris from early
morning convection. While upper level forcing is somewhat less
impressive during this time...deep shear in the 25 to 35 knots range
coupled with surface cape approaching 3000 j/kg would certainly
indicate a severe weather threat. This will bear watching as we
progress through the work week.

The forecast from late Thursday night through Saturday is
dependent on how far southeast the above discussed surface trough
can progress. It will likely stall somewhere over southeastern
Iowa or Missouri/Illinois...with subsequent shortwaves moving
overhead and kicking off one or more rounds of thunderstorms along
and northwest of the frontal zone. is unclear how far
southeast of our area the boundary will lie and thus how far into
our area these storms may penetrate. There is little to no model
consistency in this area...and for now have stuck to chance probability of precipitation
across our southeastern areas depicting a rough consensus
approach. In any event...the severe weather threat during this
time will be low although there is some potential for heavy rain
as storms may train along the frontal zone.

Finally...the second and stronger low pressure system discussed
earlier will make for an active period with several rounds of
thunderstorms between Saturday night and Sunday night. Given the
energetic nature of the system there will again be a potential for
severe weather...however at this range the critical details of
track and timing and thus the region of maximum severe weather
potential remain very unclear. In any wake of this
weekend system it appears that a brief respite of cool and quiet
weather may be in store for early next week.


issued at 1211 am CDT Wednesday may 6 2015

Widespread IFR to LIFR conditions are expected across northern
Iowa overnight affecting kfod...kmcw and Kalo with areas of dense
fog and very low ceilings. This fog will lift quickly on Wednesday
morning as daytime heating rapidly increases. VFR conditions can
be expected on Wednesday for most location from midday forward.
Convection is likely to move into the west by afternoon and
overspread the remainder of the area on Wednesday evening with an
increased threat of MVFR conditions at that time.


Dmx watches/warnings/advisories...
dense fog advisory until 9 am CDT this morning for Black Hawk-
Boone-Bremer-Butler-Calhoun-Cerro Gordo-Emmet-Franklin-Grundy-
Hamilton-Hancock-Hardin-Humboldt-Kossuth-Marshall-Palo Alto-



short term...Curtis
long term...Lee

National Weather Service Glossary of Abbreviations