Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
648 am CDT Wednesday Jun 3 2015
Short term /today/...
issued at 324 am CDT Wednesday Jun 3 2015
Strong Theta-E advection ongoing across western Iowa early this
morning with MUCAPES increasing to 1000-2000 j/kg. The 40+ knots low
level jet focused across Kansas and Nebraska will shift east into
western Iowa over the next several hours. This should bring an
increase in thunderstorm development over the west through 12z. The
low level jet and moisture transport is expected to weaken rapidly
after 12z therefore any activity should weaken as well. The
exception may be the far southwest where greater instability will
remain. Overall...should have a mostly quiet period late morning
into the early afternoon. A couple areas to watch by middle to late
afternoon for redevelopment. One is the surface warm front that will be
across far southern Nebraska and near the Iowa/MO border. The focus
along this boundary should initially be to the west over south
central Nebraska. The other area is the boundary dropping southeast
through South Dakota and Minnesota. This may lead to redevelopment
over north central Iowa late in the period. The layer parcel static
stability by the afternoon is quite low so it will not take much
forcing to displace parcels for convection to trigger.
Precipitable water values will increase to near 1.5 inches today along with
relatively slow storm motions may lead to a few locations to top 1
inch of rain. Deep layer shear of 25 to 30 kts could lead to storms
to become organized for short periods of time leading to a large
hail threat along with potential for 60 miles per hour winds as dcape values
increase above 1000 j/kg. High temperatures today will be contingent
on cloud cover. With some sun...highs in the middle to upper 70s is
Long term /tonight through Tuesday/...
issued at 324 am CDT Wednesday Jun 3 2015
Active weather pattern expected for beginning of period with
thunderstorm chances throughout the end of the week as surface low
pressure system slowly pushes eastward under weak upper level
ridge across upper Midwest. Expect precipitation to be ongoing for
beginning of period with boundary dropping slowly across Iowa.
Models are very slow with progression of this boundary...generally
stalling it across central Iowa with continuing chances for
thunderstorms along it overnight...especially as low level jet begins to
increase and push into southwestern Iowa. Models remaining consistent
in initiating another mesoscale convective system overnight...and main concern will be
placement of this system. Models have begun to pull further south
with this...and given southern dive to currently ongoing mesoscale convective system
across NE...agree with this southern trend. European model (ecmwf) was first to
pull further south to near NE/Kansas boarder and NAM has followed
suit...with GFS and Gem much further north. Have therefore
trended towards European model (ecmwf) for much of forecast through end of the
week. Initiation is once again across central NE/Kansas and have
therefore cut back on probability of precipitation until later in overnight tonight.
As system pushes east into Iowa Thursday...will see
continued chances for thunderstorms...though expect to see
dissipation in the late morning hours before development again
near stalled boundary across central Iowa in the afternoon and
have trended forecast this way. Afternoon storms will have
potential to be severe Thursday...with cape values by late
afternoon near 1000-1500 j/kg with higher values across the south.
Shear values will be sufficient to allow for storms which
develop to persist...with best chances for severe storms in the
far southwestern portions of Iowa. However...weak cap noted which
will help to limit severe potential.
Most forecast uncertainty will be Thursday night with mesoscale convective system
potential once again. Concern will be where mesoscale convective system sets up...which
will be heavily dependent on preceding systems. However...with
favored southern trend to models have begun to shift chances
further south. European model (ecmwf) indicates that system may be south of Iowa
into Kansas/MO and may still be too aggressive with probability of precipitation Thursday
night. With system...will see lingering precipitation chances
into Friday before low finally pushes east of area.
Concern for heavy rain will persist with precipitation chances
through the end of the week. Precipitable water values remain high...generally
around 1.6 inches with strong moisture transport into Iowa as
well. Warm cloud depths will be high through the overnight
Wednesday nigh and again Thursday night. Main concern will be
with precipitation Thursday night as precipitable water values increase to near
1.8 inches and soundings indicate good saturation through column.
However...again lower confidence in heavy rain Thursday night
depending on placement of system which may be south of Iowa.
Saturday will see a brief break in precipitation chances. Flow
across Iowa will be mostly zonal through the period with large
upper level low digging into Southern California coast Friday and pushing
east helping to break down ridge somewhat. However...a deepening
upper level low drops south through Canada for late in the period.
This will result in continued active weather late Saturday and
Sunday before Canadian low pushes boundary south of Iowa.
Temperatures will be fairly steady through period with highs in
the 70s and 80s and lows in the 50s and 60s.
However...temperatures will be heavily dependent on convective
trends...and may be too cool on highs if skies clear and may be
too warm on lows with ongoing precipitation possible.
issued at 648 am CDT Wednesday Jun 3 2015
An area of thunderstorms will move across the central Iowa this
morning. A few patches of MVFR ceilings will be in the area as well.
Thunderstorms are expected to diminish late morning in the
afternoon. Some redevelopment is possible late afternoon and into
the overnight. MVFR ceilings may become more prevalent overnight as
well as moisture pools ahead of an approaching boundary.