Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
1200 am CDT Thursday may 7 2015
Short term /tonight/...
issued at 326 PM CDT Wednesday may 6 2015
The area saw southerly flow pick up significantly today...with many
areas seeing sustained winds of 20mph and gusts as high as 40 to 45
miles per hour as a low pressure system worked its way towards and into western
Kansas this afternoon. With cloud cover around much of the
day...little instability was able to develop with MUCAPE values
barely reaching 1000j/kg. Showers and storms though fired up along a
weak warm frontal boundary draped across central Iowa...oriented
roughly northwest to southeast from Estherville to Waterloo then
almost straight south from there. Individually they will be short
lived and pulse along the front...quickly working their way out of
the County Warning Area. Severe weather potential will be near none absent of any
good support from deep shear and cape.
The area will see some short lived respite from thick cloud cover
and precipitation this evening before another round of precipitation moves in
towards the end of the period...06-12z. It will be driven by the
exiting low pressure center trekking northeast from western
Kansas into central Nebraska and eastern South Dakota as it shoots
a short wave into the area and a wave of vorticity and Theta-E
advection enters the region. Resulting Omega values approaching
-35 to -40 microbars/S gives weight to the idea that thunder will
be certainly be present. With the absence of added instability and
deep shear...which also becomes more unidirectional by
morning...severe potential will be low once again...but enough
lift will certainly be present for some thunder. Strong southerly
flow and dew points in the upper 50s to low 60s will also keep
overnight lows up and relatively warm in the low 60s.
Long term /Thursday through Wednesday/...
issued at 325 PM CDT Wednesday may 6 2015
Little has changed in overall forecast thinking since yesterday.
The overall weather pattern will remain wet and active through the
weekend then turn cooler and quiet early next week. On Thursday
and Thursday night our forecast area will remain within a zone of
moisture advection on southerly surface flow...ahead of a weak
cool front slowly approaching from the northwest. Depending on the
persistence of debris from early morning convection...as discussed
in the short term section...there should be some destabilization
by peak heating time allowing storms to develop by late Thursday
afternoon or evening in a broken band of thunderstorms ahead of
the surface front. These storms will be dominated by thermodynamic
processes as there appears to be little deep forcing support and
shear fields are generally weak. There is some modest deep layer
speed shear but low level shear is modest at best and surface
convergence along the frontal boundary is also unimpressive.
Depending on the degree of instability this will most likely
result in clusters or lewp segments with some marginal hail/wind
threat as outlined by Storm Prediction Center. Have maintained likely probability of precipitation from the
afternoon into Thursday night but at this time anticipate that any
severe weather will be pretty isolated. Heavy rain may be more of
a concern given the orientation of the boundary parallel to the
steering flow and pretty decent moisture transport and precipitable water fields
depicted by short range forecast models.
The weak surface front will sag southeast across our area...likely
aided by the Thursday night convection...and stall around
southeast Iowa/northeast Missouri from Friday through Saturday.
One or two subtle shortwaves moving through the southwesterly flow
aloft should be sufficient to generate periodic showers and storms
around and northwest of this decayed surface boundary...likely
affecting our southeastern counties at times...thus probability of precipitation continue
to be largely confined to those areas. There is a continued
marginal threat of some heavy rain impacts but the severe weather
threat should be low during this time.
In terms of hazardous weather impacts in the long term forecast
period...all eyes are on the potent storm system approaching Iowa
later this weekend. The GFS...ECMWF...and Gem are all in general
agreement in swinging a deep and developing cyclone from the
southern High Plains up to Kansas and possibly western Iowa
between Saturday evening and Sunday evening. While there are
discrepancies in the track and timing of smaller scale
features...it is likely that this will place our area within the
warm sector on the eastern flank of the cyclone during the day on
Sunday...with a warm front probably arcing somewhere across the
County Warning Area. This is a prime synoptic set up for severe
thunderstorms from Sunday afternoon into Sunday night. Certainly
there is still some wiggle room for the details such as degree of
instability or capping...precise location of the front at
convective initiation time...et cetera to change in the next few
days...but it must be noted that there is some potential for
significant severe weather and even tornadoes on mothers day.
Clearly we will be keeping a very close eye on this potential
over the next few days. As the large cyclone passes near or just
northwest of Iowa on Monday it will push a cold front across the
state...followed by high pressure building in and leading to cool
and dry weather from late Monday through Wednesday of next week.
issued at 1157 PM CDT Wednesday may 6 2015
Area of showers and storms will progress into western Iowa
overnight and eventually across central Iowa through Thursday
morning. While conditions will be primarily VFR...locations
experiencing showers and storms will see MVFR to localized IFR
conditions with restrictions to ceilings and visibilities. Will
see the VFR conditions in most locations by midday into the
afternoon on Wednesday before more convection develops in the area
on Wednesday late afternoon and evening.