Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Des Moines Iowa
1250 PM CDT Monday may 4 2015

..updated aviation discussion...

Short term /today/...
issued at 400 am CDT Monday may 4 2015

Some lingering light shower and isolated thunderstorm activity
ongoing across the County Warning Area mainly in vicinity of a frontal boundary
slowly dropping through the state. Additional activity further south
in an area of weak Theta-E advection in the low/middle levels. The
activity across the east has been diminishing over the past few
hours...and expect that trend to continue as upper level energy
continues to push east of the state. Another weak impulse moving
through eastern Nebraska looks to be firing some weak thunderstorms
across the northwest/wc portion of Iowa. This activity is expected to
continue over the next few hours as the upper wave pushes east
diminishing toward sunrise. With the area between stronger waves
aloft...expect a mainly dry period this morning across the County Warning Area
before activity redevelops toward middle/day and this afternoon as the
next stronger shortwave aloft approaches the state. Frontal boundary
expected to be stretching across the middle-section of the County Warning Area...with
strong push of Theta-E advection into the west central County Warning Area pushing
north/northeast through the afternoon hours. High-res models seem to
be handling things have kept probability of precipitation close to the NAM arw/nmm
solutions. Initial activity expected to be elevated...with storms
becoming more surface-based toward evening with the severe threat
increasing as deep layer shear increases and sufficient cape for
stronger updrafts.

Temperatures to be cooler today with cloud cover expected to be in place
throughout much of the day...and a relative cooler airmass in place
initially before warm air advection later today as the frontal boundary pushes back

Long term /tonight through Sunday/...
issued at 400 am CDT Monday may 4 2015

Convective trends will be the primary focus through the period
although details will be hard to come by and with the only
appreciable weakness late Tuesday into Tuesday night. Expect mesoscale convective system of some
form to be either in progress or developing at onset as short wave
currently crossing the Black Hills interacts with present 850 mb/700 mb
front and baroclinic zone. Cannot argue with Storm Prediction Center day 1 outlook.
Although southern Iowa should be north of the warm front by early
evening...models suggest convection could be rooted fairly low
with sufficient cinh reductions. This only slightly elevated
convection may then be able to realize veering and increased
effective shear near the low level frontal boundary. Expect
convection to gradually become more elevated into the night and
slowly lift northward as 850 mb/700 mb frontogenetical forcing and
Theta-E advection eventually reaches northern Iowa as MS valley short
wave ridge builds. Severe threat would appear to initially be wind
and hail...transitioning to more hail northward into the night.

This progression will continue into Tuesday before the front and
baroclinic zone pushes northward later in the day. This will cut off
precipitation for a time late in the day and into the night. Expect a
decent...and low confidence forecast...south to north temperature gradient
Tuesday with high differences potentially 10-15 degrees before warm
sector encompasses state overnight. Probability of precipitation will return Wednesday however
as remnants of current wave off the Southern California coast eventually
reaches the MO and then upper MS valleys. Kinematic forcing looks
to drive things initially and be followed by another somewhat non-
descript surge of Theta-E advection into the night. There may be a
brief lull Thursday afternoon...but peak heating convection is then
expected to develop to our west along a frontal boundary and
persist into the night in some form with steady SW flow aloft.
Increased middle level winds and resultant shear will likely make
this the next appreciable severe window to examine.

The surface boundary and associated deeper baroclinic zone will
then stay in the vicinity through the weekend before low currently
off the British Columbia coast makes a methodical progression through the base
of the western trough and reaches the plains sun. This surface
reflection will be much more organized than recent past with
triple point reaching northwestern Iowa during peak heating. This is quite a
ways out of course...but would seem to support an increased
tornado threat north and east of the low with squall line
potential along the front to the south. Certainly a severe window
to keep an eye on. Naefs specific humidity....precipitable water
and integrated water vapor transport are all often into the 90th
plus percentiles through the week so an mesoscale convective system of any significant
will be capable of producing locally heavy rains. GFS/European model (ecmwf) depict
two to four inch qpfs through sun over central and southern Iowa which
seems reasonable.


issued at 1223 PM CDT Monday may 4 2015

Frontal boundary will continue to slowly move south across the
state today. Ceilings should move out of MVFR and into VFR this
afternoon prior to storm and shower development. Storms will begin
to develop around dsm...fod...and otm sites around 22z to 00z and
work east and northeastward...likely dropping ceilings into MVFR at
those locations and eventually the other sites later this evening.
After storms and showers move through...all sites should hover
around VFR/MVFR ceilings through the morning. Gusty winds...from the
northeast to 20kts...behind the front will affect mcw and fod
this afternoon before calming down in the evening.


Dmx watches/warnings/advisories...


Short term...beerends
long term...small

National Weather Service Glossary of Abbreviations