Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
650 am CDT Tuesday may 26 2015
Short term /today/...
issued at 348 am CDT Tuesday may 26 2015
Active pattern to continue through much of the period...with the
main western US trough to lift northeastward through Tuesday. This
upper wave will approach the region tonight and move through the County Warning Area
Tuesday before pushing into the Great Lakes region Tuesday night.
Southerly flow ahead of it will continue to push moisture northward
into the area...with an elongated area of low pressure and
associated warm front stretched northeastward across the County Warning Area tonight
into Tuesday. An upper vorticity maximum currently over central/northwest Texas will
lift northward toward the region as well...and with energy from the
main upper trough and this secondary shortwave trough lifting
through the region late tonight...expect an area of thunderstorms to
develop across the County Warning Area mainly during the evening/late night hours.
Deep layer shear stronger in the evening then weakens into the
overnight hours...with instability waning through the overnight
hours as well. Given the amount of shear/instability some severe
storms possible with the latest Storm Prediction Center outlook showing a slight risk
across much of the County Warning Area for tonight. Large hail and damaging winds
would be the main threats...with an isolated tornado threat as well
however it looks slightly better further south and west of the County Warning Area.
Thunderstorm chances continue into Tuesday as the upper wave moves
through the County Warning Area...and the surface trough axis to push east by late
Tuesday/Tuesday night. Weak cold frontal boundary to sag south of
the state for Wednesday/Wednesday night before lifting back
northward Thursday. High pressure to briefly build into the state as
well Wednesday so expect mainly dry conditions Wednesday/Wednesday
night. NAM tries to hang the boundary across the south...with some
additional thunderstorm development but the majority of the models
push it south of the County Warning Area. Thus have gone with the consensus and
stuck with dry conditions for Wednesday night. Another upper trough
to build into the western US for Thursday into next weekend...with
southwesterly flow setting up again and chances for thunderstorms
return for late week. A stronger cold front to drop through the area
next weekend with a large area of high pressure expected to build
into the area for Sunday into Monday. Temperatures to remain near
average for the first few days of the period with a turn toward
cooler than average toward the weekend and into early next week.
Long term /tonight through Monday/...
issued at 348 am CDT Tuesday may 26 2015
Precipitation trends will remain the primary concern through the forecast
period. Current potent short wave along the eastern OK/Kansas border will
push a sharp lobe of thermodynamic and kinematic forcing through Iowa
today. Much of this synoptic scale forcing will be through the
Great Lakes by 00z...however lingering weak low level convergence
may be sufficient for a few isolated showers or storms during the
evening central and southeast. Any precipitation threat then looks to fade
with loss of heating leading to a break through at least Wednesday night
with short wave ridging and little in the way of lift or moisture.
The next precipitation window will be associated with subtle remnants of
current California/Nevada trough. Its approach into Thursday will rebuild deeper
moisture and instability starting another prolonged period of probability of precipitation.
Forcing will initially be driven by weak/moderate isent lift and
moisture transport from Thursday into Thursday night. Model differences
then arise into Friday however with 00z GFS spinning up a much
stronger middle level wave than the 00z European model (ecmwf). This results in a
deeper surface reflection and noted qg forcing lingering over
Iowa...especially northern sections...from Friday night through Sat night.
Although the 00z deterministic European model (ecmwf) suggests a weaker
solution...its 12z ensemble run is more GFS-like with the CMC/gdps
hinting toward a stronger solution as well. Precipitable waters and warm cloud
depths are not extreme during this period...but still well above
normal and capable of producing moderate to heavy efficient rains
considering the slow system movement. GFS quantitative precipitation forecast through Sat is four
to six inches over northern Iowa and while it is likely overdone due to
being the stronger outlier....it is indicative of higher end
potential. This will certainly need to be monitored considering
the narrow swath of two to three inch rains that occurred since
Monday across portions of Crawford...Carroll and Calhoun counties.
Regardless of how the magnitude of this system plays out...all
solutions suggest somewhat drier weather and continued cooler
temperatures to start the next week. Probability of precipitation will be on the decrease
with highs staying no better than the 60s.
issued at 650 am CDT Tuesday may 26 2015
Showers with a few thunderstorms to continue to impact the taf
sites through the morning hours...with redevelopment expected
throughout the afternoon into the early evening hours.
Precipitation should come to an end during the evening...with
dry conditions into Wednesday. Ceilings will remain the in the lower
VFR to MVFR range today. Dips into LIFR are possible this morning
in the precipitation. Mainly low VFR to higher MVFR ceilings possible
for tonight...with winds shifting to the west/northwest this
afternoon into tonight.