Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Des Moines Iowa
356 PM CDT Friday Jul 31 2015

Short term /tonight/...
issued at 317 PM CDT Friday Jul 31 2015

Weak cold front has settled across far southern Iowa...a bit further
north than models had indicated...and will stall this evening just
over the Missouri border back into Omaha. There is roughly 3500
j/kg of cape that will shift into southwest Iowa from east central
Nebraska by 00z and shift across southern Iowa this evening. Add to
this weak forcing along the boundary...fairly steep low level lapse
rates according to forecast soundings and diminishing cinh and we
should see at least isolated to widely scattered thunderstorms from
southwest into far southern Iowa this evening. Deep shear is absent
however Storm Prediction Center mesoanalysis does show an area of effective bulk shear
across northwest Iowa that will sink into southern Iowa this
evening. There is also a shortwave and weak trough pushing across
the southwest this evening. This should be enough for a storm or
two to produce damaging wind and/or large hail. At 230 PM area
radars was showing storms developing over east central Nebraska but
the storms are not moving very fast at all and unless they rapidly will be 00z or a little after before they affect the
forecast area. Any threat of severe would diminish some after
sunset as we lose a some instability but we could still see storms
with moderate rainfall into the late evening.

Long term /Saturday through Friday/...
issued at 317 PM CDT Friday Jul 31 2015

Forecast will be active for beginning of period with
precipitation trends Saturday through Sunday...and chances through
period. Saturday approaching boundary and wave to push east
through the day...bringing chances for thunderstorms. Current
thinking is for initiation further west into NE/South Dakota area...pushing
east along with band of Theta-E advection. Some advection begins
to push into northern Iowa in the early afternoon...but expect
best chances for precipitation to come later with boundary pushing
in. Models are not in good agreement with the placement of best
forcing...but generally have trended towards the north as most
models have a northern push. Severe weather will be possible with
good instability expected to build in during the daytime heating.
Cape values will be near 2000-3000 j/kg. Shear will be sufficient
to maintain storms which may develop...and favorable lapse rates
will keep main threats hail and wind...with low level shear
somewhat lacking for a higher tornadic potential. Heavy rain also
cannot be ruled out...but precipitable water values are somewhat low near 1.5

Behind system northwesterly flow will persist across area through
beginning of week...with boundary stalled out across Iowa. This
will help to keep temperatures near going with highs generally in
the upper 80s and lows in the 50s and 60s. Though will see
continued chances for precipitation at times with waves pushing
through flow. Wave pushing south Monday night will bring chances
for rain...though models are again not in good agreement with
placement of this system. GFS continues to be further
south...though European model (ecmwf) is north...but have trended towards GFS for
now. Stronger system pushes east beginning to break down ridge
across rockies and push it eastward late Tuesday and
Wednesday...though confidence continues to decrease through the
period with precipitation trends as models diverge on solutions.
GFS continues to be more aggressive...bringing precipitation
through faster and further north and have trended this way for
now...along with cooler temperature trends.


issued at 114 PM CDT Friday Jul 31 2015

Cold front dropping across taf locations and will be in southern
Iowa by 01/00z. An unstable airmass across SW Iowa in vicinity of frontal
boundary and an upper level disturbance moving across SW Iowa may
trigger thunderstorms and rain but should miss taf locations. Thunderstorms and rain coverage may
expand 00z through 04z until wave passes. A stronger shortwave will
approach Iowa near the end of the taf forecast period increasing high clouds.
Surface flow will be variable with winds nearly at 10-15kts becmg west to SW
after 03z then nearly after 01/15z.


Dmx watches/warnings/advisories...


Short term...fab
long term...awb

National Weather Service Glossary of Abbreviations