Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
329 PM CDT Friday Aug 22 2014
Short term /tonight/...
issued at 327 PM CDT Friday Aug 22 2014
Through early evening...storms have fired in two areas across
central Iowa. The northern most line of storms is associated with a
frontal boundary that is ever so slowly meandering south. Flow is
weak and there is little convergence and no shear to speak of so the
storms will become outflow dominant and should fall apart by 00-01z.
Further south storms have fired along a moisture axis. This axis
was evident on radar for much of the day and storms fired after
sufficient heating destabilized the atmosphere. These storms will
be a bit more robust and produce locally heavy rainfall...some
stronger wind gusts and possibly some small hail. Again...these
storms should move east southeast and diminish shortly after 00z.
The rest of early to middle evening should be relatively quiet though
things will fester through the evening so isolated storms will still
For the rest of tonight...upper ridge over Iowa will get pushed to
the east with the axis by 12z Sat into western Illinois by a deepening
trough over The Rockies. Several weaker shortwaves will eject out
of the base of the trough pushing across northern/western Iowa. In
addition...the frontal boundary from a surface low in western Kansas
will lift northward as the surface low shift to the northeast. To
the west of Iowa there is pretty impressive shear and strong Theta-E
advection that will work into the west and north and gradually shift
more north and east. One again the 4km WRF had a decent handle on
all of the convection of this afternoon and tonight so I did put
some weight on its solution for tonight. Storms should fire out
west of Iowa this afternoon and evening and will probably merge into
a cluster of storms that will then shift into the southwest third of
so of Iowa during the overnight. There may be isolated storms
throughout the evening but the main precipitation event will occur over the
southwest between 08z-12z before lifting north. Some storms will
produce heavy rainfall but there may also be a couple severe storms
as well...mainly over the far western portion of the County Warning Area. It will
be a muggy night with lows in the upper 60s over far northern Iowa
and 70s elsewhere. Dewpoints will stay in the upper 60s and low to
middle 70s so fog will once again develop...especially across the
Long term /Saturday through Friday/...
issued at 327 PM CDT Friday Aug 22 2014
Active weather pattern expected at least through the middle of next
week...with hot and humid summertime weather giving way to Fall-like
Middle/upper-level wind speed maximum plunging southward along the Pacific
coast...on the west side of the western U.S. Trough...will help
faciliate its intensification before it lifts out. In response...the
eastern U.S. Ridge will amplify into this weekend. A vorticity maximum over
the SW U.S. Will continue lifting northeastward. By the beginning of
the long term forecast period...lingering convection from overnight will
continue lifting northward ahead of a northward-advancing frontal
boundary. The boundary is expected to be in the vicinity of the Iowa-
Minnesota border...likely just north of it...by late afternoon Saturday.
South of the boundary...temperatures will warm more than what was
seen today. Previous forecast discussion acknowledged mixing issues over
the past few days and on Saturday...and agree with that thinking
however expect it to be more than was seen today as evidenced by
stronger surface winds than was seen today. Thus have nudged temperatures
up a degree or two from the going forecast for Saturday afternoon.
Additional convection is possible Saturday night as another
middle/upper-level disturbance moves through the central/northern
plains. Best forcing will remain west and north of the County Warning Area...and
have painted probability of precipitation as such.
Sunday is expected to be the warmest day of the forecast period with Iowa
solidly within the warm sector. NAM has continued trend from
previous forecast of being the warmest with forecast highs well into the
90s. NAM is suggesting more mixing on Sunday than on Saturday as
evidenced by lower dewpoints and stronger surface winds. For now
have not changed Sunday/S maximum temperatures much from previous forecast. Heat
index values will be as high as around 100f.
Higher chances of rain showers/thunderstorms and rain occur Sunday night as upper-level trough
to our west ejects northeastward and pushes a boundary into Iowa.
Models suggest the boundary will hang up in the state...likely
making it as far southeast as central Iowa. Instability and forcing
will be plentiful enough to result in at least scattered rain showers/thunderstorms and rain
during the night. As the upper level trough ejects...additional
upstream energy will help another trough to develop in its place over
the northern U.S. Rockies.
With boundary remaining in the region...have kept slight chance probability of precipitation
going into Monday. After this point...the model solutions diverge
leading to increasing forecast uncertainty. GFS brings the second
trough through faster than the Euro. Expect the boundary to move
through the County Warning Area by Wednesday. Have increased probability of precipitation beginning Tuesday
and kept them in place through late in the day on Wednesday.
After the frontal passage...a few days of Fall-like weather will
prevail with high temperatures across the northern County Warning Area possibly not leaving
the lower 70s on Wednesday.
issued at 1242 PM CDT Friday Aug 22 2014
Patchy stratus will plague northern taf locations for a couple more
hours but the fog has finally lifted/dissipated. In general there
will be VFR ceilings but brief periods where stratus may drift over taf
locations will yield IFR ceilings. Some of the hi-res models suggest
storms to develop along/south of I-80 around 21z. Sat already
showing cumulus bubbling and with the boundary approaching I could not
leave kdsm and kotm dry 21z-00z. Storms will be isolated at best
though so just a mention of thunderstorms in the vicinity in those tafs. Late tonight a
better chance for storms SW of a line from kfod to kotm with fog
and MVFR/local IFR conditions once again north of that line.