Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area Forecast Discussion
National Weather Service Des Moines IA
346 AM CDT Thu Jul 28 2016

.SHORT TERM.../Today/
Issued at 341 AM CDT Thu Jul 28 2016

The Midwest remains in somewhat active WNW flow today with water
vapor imagery showing upstream short waves back into Montana. This
will continue to develop a slightly higher amplitude Upper
Mississippi Valley/Great Lakes wave eventually placing Iowa more on
the back side of cyclonic lower tropospheric flow. Much of the
larger scale forcing and response with this wave appears to stay
just north and east of Iowa however with surface based convection
seeming to be our only mechanism for precip today. Low level lapse
rates do steepen to varied degrees with the cooler GFS less
aggressive than the warmer Nam/ECMWF solutions. Although not
widespread, high resolution models do show widely scattered weak
convection developing in the cyclonic flow during peak heating,
mainly over the northeast third of the forecast area which
eventually will be enhanced by weak inverted trough formation.

The aforementioned model temp spread will lead to diminished
confidence in temps and convective potential. The cooler end of the
spectrum would result in little temp rebound and convection not
capable of a repeat landspout/weak tornado situation. Examination of
the warmer Nam soundings however show more pronounced 0-3km Capes,
exceeding 200 j/kg at times, which are a noted percentage of the
total MLCapes. This could enhance the landspout environment near the
inverted trough. The normally more mixed and aggressive RAP/HRRR
soundings do not support the warmer solutions however so have played
more of a blend for temps which is not too far from the previous
forecast and also makes sense considering the high res models
depiction of stratus advancing southward today. Confidence will not
be there to mention landspouts in the Hazardous Weather Outlook, but
it will be something to be monitored if warmer temperatures
materialize.

.LONG TERM.../Tonight through Wednesday/
Issued at 341 AM CDT Thu Jul 28 2016

The long term begins as a rather unsettled period but with no real
focus for thunderstorms.  We will be in a general zonal flow with
numerous shortwaves passing through the flow.  Any of these
shortwaves could result in isolated to widely scattered storms but
just where is the problem and confidence is too low to keep pops in
each period so I generally went dry. However...similar to today,
soundings indicate decent 0-3KM shear and instability across the
Northeast and LCL's are low in the late afternoon hours so I kept
pops going in this area with the thought that there is some
possibility of landspouts again. However this is a little too far
out to mention this in any products and confidence is too low to
consider this because of model uncertainties. It does warrant
watching in the coming model runs. The next best chance for
thunderstorms comes late Sunday into Monday as a warm front lifts
towards Southern/Southwest Iowa. One thing that is pretty certain
is that temps will be warming next week and we will be back in the
lower 90s across at least Southern Iowa. As for pops...went with
persistence and tried to tie better pops with shortwaves and some
kind of forcing or theta-e advection but this will likely change
with each model run and forecast issuance.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1142 PM CDT Wed Jul 27 2016

With showers and storms this evening having generally faded out,
main concern in TAF period becomes timing stratus deck expected
to expand out of MN/WI. Currently CIGs are primarily MVFR with
pockets of IFR there, and have dropped KMCW, KFOD, and KALO into
MVFR for a period overnight. Kept KDSM and KOTM out of MVFR with
some uncertainty on southern spatial coverage and timing. Toward
end of period, scattered storms will be possible, especially
around KMCW, KFOD, KOTM, but again uncertainty in timing and
coverage is low at the time and have kept mentions out.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SHORT TERM...Small
LONG TERM...FAB
AVIATION...Curtis

National Weather Service Glossary of Abbreviations