Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
646 PM CDT Monday may 4 2015
Short term /tonight/...
issued at 342 PM CDT Monday may 4 2015
The frontal boundary currently resides along the Iowa-Missouri
border and is all but stationary at this point and should slowly
reverse course later today. Showers associated with a short wave
moving into the County Warning Area will continue to develop and move
northeastward. Along with it...Theta-E advection will continue
to move in to the central portions of the County Warning Area and help keep
showers going this afternoon and into the evening. More vigorous
storms will develop later this afternoon in the southwest along
the instability axis to the west...where MUCAPE values are 1000+
j/kg. Early storms are likely to be elevated and forced by Theta-E
advection and positive vorticity advection associated with the
shortwave. Additionally...the high res models have storms
developing in south central Iowa...around Ottumwa...where SBCAPE
is around 1500-2000 j/kg and clear skies earlier eroded the cap.
The last few model runs have kept showers and storms further
south...not reaching areas of north central and northeastern Iowa
until well after 00z and slowed down the northward movement of
probability of precipitation along with the neighbors. The threat for severe weather is
limited with generally weaker shear values...but hail and winds
will be the main threats...especially if the convection GOES
linear later tonight.
Long term /Tuesday through Monday/...
issued at 342 PM CDT Monday may 4 2015
The key focus over the extended period was precipitation chances and
timing of precipitation during the middle of the week. Leaned toward the
nam12/arw-nmm WRF Tuesday...then a nam12/European model (ecmwf) blend for
Wednesday into Wednesday night. Late in the period...there was a
slight lean towards the European model (ecmwf) into the weekend.
Tuesday into Wednesday night...front looks to surge northward
during the morning Tuesday and storm activity will shift north
with the front through midday Tuesday. Theta-E advection and
frontogenetical forcing begins to diminish or phase out by the
early afternoon in northern Iowa and have low confidence in
storms/showers to persist much past 18z Tuesday. The arw-WRF
/nmm-WRF have a decent handle on the radar trends through the
morning hours Tuesday. Lowered probability of precipitation considerably Tuesday
afternoon to reflect this trend. Temperatures will rebound to
around 80 over the southern half of the state and may need to go
slightly warmer if cloud cover clears out sooner than anticipated.
Dry conditions over much of the forecast area Tuesday night into
Wednesday even though the GFS has the boundary draped across Iowa
and storms through much of the night. This is the first mention
of this trend via GFS so something to monitor if other models
begin to show this solution for Tuesday night into Wednesday.
Broad weak area of qg forcing slowly pushes into western Iowa by
Wednesday morning and transitions eastward throughout the day.
Several middle to upper level shortwaves move north to northeast in
conjunction with a parent negatively tilted 500mb trough moving
into the Central Plains by Wednesday morning. Fairly strong
moisture transport into the state Wednesday afternoon and evening
and have highest probability of precipitation going across the forecast area during this
time. Severe chances seem limited with the lack of any significant
instability and shear present.
Thursday and Friday...surface low and aforementioned 500mb trough
continue to push through the Dakotas into Minnesota Thursday. This
places central Iowa in southwest flow aloft with 850mb temperatures
warming to +13c to +14c Thursday afternoon. However...cold front
enters the western edges of the state by 00z Friday and should be
enough forcing over the western portions of the forecast area to
keep high chance/likely probability of precipitation going for Thursday into Thursday
night. Trended drier for Friday with the cold front located in
southern Iowa by this time.
Saturday through Monday...unsettled weather pattern continues
throughout the weekend with a series of shortwaves out ahead of a
deep upper level trough that pushes through the region by Sunday
afternoon into early Monday. This looks to be the next potential
for any widespread severe weather.
issued at 644 PM CDT Monday may 4 2015
Showers and thunderstorms possible over the area tonight with
thunderstorms most prevalent over southern Iowa. Ceilings are expected
to drop to MVFR with local IFR possible overnight into Tuesday
morning. Focus for the activity is anticipated to shift into
northern Iowa during the day Tuesday with VFR developing south.