Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 191743
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1243 PM CDT Tue Sep 19 2017

.UPDATE...
Issued at 641 AM CDT Tue Sep 19 2017

Fog continues to migrate north/northwest and has spread
through far northeast area of our forecast zones. Added counties
up through KALO and along US20 for the next several hours as well.
/rev

UPDATE Issued at 511 AM CDT Tue Sep 19 2017

Expanded fog advisory north along I35. Current trends confirm fog
expanding north a bit yet this morning. Some spots more patchy but
overall enough to warrant increased coverage. Will likely see
some fluctuation in fog across the more southern edge of the
advisory and if trends show significant improvement between 6 and
730 am...will trim back on overall coverage. For now standing pat
in the south. /rev

&&

.SHORT TERM.../Today through Tonight/
Issued at 333 AM CDT Tue Sep 19 2017

Confidence: Medium to High

Main concerns will be fog early and then thunder potential for
mainly this evening. As expected...old boundary stretching across
Iowa from northwest to southeast at 06z continues to pool
moisture... low clouds and dense fog.  Have expanded the fog
advisory as of around 06z and see little reason that fog will not
hold on to at least 1 to 2 hours after sunrise...now at ~7:00 am
local time in Des Moines. Since the boundary is slowly expected to
lift somewhat toward morning...the area of thicker fog may migrate
slightly north with the boundary.  If so...will make some minor
modifications to the outline with time...through the morning rush
hour as needed.  The bigger picture today features a robust upper
level trough which has several embedded short waves tracking east
through the flow in the next 12 to 24 hours. This sets the stage
for more thunderstorm development as the system approaches later
this evening. Some of the high resolution models and to a lesser
extent...synoptic models suggest that there may be some isolated
shower/thunder activity as the front retreats north this afternoon
and instability increases. It is possible...but chances right now
remain rather low and will exclude mention in the forecast for
now. As the stronger warm air advection drives north this
afternoon with the retreating warm front...south southeast winds
will mix this afternoon and highs will move into the lower to mid
80s over the south with upper 70s to lower 80s over the north.
Winds will be gusty this afternoon from 15 to 25 mph at
times...especially along the leading edge of the advancing front.
With the strong short wave approaching from the northwest this
evening...strong to severe thunderstorms across the
Dakotas/Minnesota will build southwest along the boundary.
However...they will encounter an increasingly capped environment
into Iowa where H700 temperatures are expected to be 10 to 12C by
06z. This will limit much of the thunderstorm activity to along
and just behind the boundary where H700 temperatures will cool
enough between 06-12z to allow for some convection over the
northwest at that time. At this point in time the timing would
diminish the chances for severe storms over our forecast area
later this evening/overnight. Though there is also a signal for
some elevated convection over northeast areas prior to 12z along
the leading edge of the capped environment near Waterloo...chances
there are less than over the northwest tonight. Strong warm air
advection will continue overnight with lows only falling to the
lower to mid 60s in the north and in the upper 60s to lower 70s in
the south.

.LONG TERM.../Wednesday through Monday/
Issued at 333 AM CDT Tue Sep 19 2017

On the synoptic scale by Wednesday morning, an upper level cutoff
low will be centered over the Alberta province of Canada with a
broad trough of low pressure stretching from the northern Plains to
off the Northwest Coast. Meanwhile, ridging will be forming over the
eastern US while Jose will be off the Northeast Coast as a tropical
storm per latest NHC forecast. A 130 knot, 250mb jet streak will
help to dig the trough over the western US through the end of the
week while the ridge builds over the eastern US. This pattern will
largely persist through the weekend and into early next week.

For Iowa on Wednesday, a cold front will be moving through the
state. Dewpoints ahead of the front will be well into the 60s with
MUCAPEs of 3000 to 3500 J/kg at peak heating over the southeast part
of the forecast area. Lapse rates are modest in the mid-levels
between 6.5C to 7.0C per NAM/GFS forecast soundings at KOTM. Deep
layer shear is marginal peaking near 30 knots so severe threat would
be isolated to the southeast part of the forecast area late in the
day into the evening. There is also plenty of dry air above 750mb
with DCAPE values in excess of 1000 J/kg. So main concerns with the
strongest storms would be strong wind gusts and hail. Behind the
cold front, high pressure will be building in with drier air
featuring dewpoints in the 50s. However, the upper level support for
the cold front will be lifting towards Canada allowing the front to
stall. The front will lift back northward as a warm front later
Wednesday night into Thursday morning with isolated to scattered
showers/storms possible.

Any lingering showers or storms should end by midday Thursday
with Iowa between the western US trough and the eastern US ridge
with southwest flow prevailing from Thursday into early next week.
With 850mb temperatures near 20C by late Thursday and good mixing
taking place, highs will be well into the 80s over much of
central Iowa with a few places near 90 degrees. Winds were also
increased from Superblend incorporating some CONSMOS to account
for expected stronger winds. The warm weather and stronger winds
look to continue into Friday as a cold front slowly moves toward
the area from the Dakotas and Nebraska. With similar 850mb
temperatures and the southwest flow continuing, high temperatures
were increased with a 50/50 blend of the 00z ECMWF and 00z NBM.
This raised highs 1 to 3 degrees in most places above initial
Superblend guidance. Current record highs look to stand as
forecast highs are 4 to 7 degrees below long standing station`s
record highs (Des Moines, Lamoni, Waterloo, Mason City). While
record highs look to be safe, record high minimums may be in
jeopardy Friday morning with lows a few degrees either side of 70.
As was the case on Thursday, winds were increased through much of
the daytime period.

As the trough of low pressure over the western US slowly moves
eastward, so will the front with shower and storm chances very
gradually entering the western part of the state on Saturday. Most
of central Iowa will still have high temperatures 10 degrees
above normal. The showers and storms will slowly spread into
central Iowa by Sunday and push into eastern Iowa by Sunday
night. Rain and storm chances will linger into Monday with
temperatures closer to normal due the clouds and rainfall.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1233 PM CDT Tue Sep 19 2017

Widespread fog/low stratus from this morning has dissipated,
giving way to mainly VFR conditions over central Iowa early this
afternoon. Some terminals may still experience temporary MVFR cigs
through 20z, but otherwise VFR conditions will prevail through the
remainder of the day.

Low level wind shear will become an issue tonight due to strong
southerly winds developing between 1000-2000 feet AGL. In
addition, MVFR cigs and scattered -TSRA are possible in north
central Iowa Wednesday morning. A return to VFR conditions area
wide is expected by mid-morning as a cold front pushes east
through the state.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...REV
SHORT TERM...REV
LONG TERM...Ansorge
AVIATION...Martin


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