Area Forecast Discussion
Issued by NWS Des Moines, IA

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291
FXUS63 KDMX 242033
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
333 PM CDT Sun Sep 24 2017

.SHORT TERM.../Tonight through Monday/
Issued at 327 PM CDT Sun Sep 24 2017

Confidence: Medium

Main challenges will remain the onset/coverage of showers and
thunderstorms through Monday afternoon along with high temperatures
Monday. Isolated showers and thunderstorms have developed across the
western areas of the forecast area today with convective heating.
The front remains slow to move east with the blocking ridge aloft
continuing to mute its progress.  The front will gradually advance
east Monday with increasing showers and isolated thunderstorms ahead
of the boundary.  Just behind the boundary instability weakens
considerably and have adjusted weather type behind the boundary
tomorrow to reflect the lack of instability.  Overnight lows tonight
will remain quite mild ahead of the boundary with southwest winds
continuing...dropping only to around 60 northwest to the upper 60s
to near 70 southeast. The bigger challenge remains tomorrow`s high
temperatures. The sharp boundary poses plenty of opportunity for
error over a small corridor.  The front looks most likely to reach a
line from near Clarion southwest to Ames to near Atlantic.  Mild
conditions will continue ahead of it with highs in the 80s with much
cooler afternoon temperatures behind the boundary only in the lower
to mid 60s. Other than highs tomorrow...there is also some question
about the rainfall amounts expected across the region.  The airmass
remains relatively moist with precipitable water values exceeding
1.5 inches over the entire area between 18z and 00z Tuesday.  This
combined with some low level instability ahead of the boundary could
improve some areas chances of seeing +1 rainfall. However...a closer
look at warm cloud depths shows that the deeper warm cloud depths
remain over northwest areas of the state where lift through that
column is less robust. That said... some locations might see heavier
rainfall totals with more spotty higher amounts out ahead of the
front tomorrow. WPC has outlooked our central/northwest with a
marginal risk for excessive rainfall tomorrow.  More realistically
...the better dynamics point to the northwest though most areas that
receive showers might see a brief heavier downpour if some of the
warm cloud depth/lift elements line up.  Have increased PoP over
most of the area tomorrow with the exception of the southeast where
the slowly advancing showers and iso thunder will reach last prior
to the end of the period.


.LONG TERM.../Monday night through Sunday/
Issued at 327 PM CDT Sun Sep 24 2017

Bottom Line Up Front...
Summer is over! Normal to even slightly below normal temperatures
in the 60s and 70s, with lows in the 40s and 50s, are expected to
grace Iowa during this long-term fcst period. The only
significant rain chance comes at the beginning of the forecast
period, associated with the boundary passage on Tuesday.


Precipitation...
Building off of the short-term discussion, the aforementioned
boundary looks to slowly migrate across Iowa during the day
Tuesday. While this boundary looks decently well-organized across
western Iowa on Monday, it begins to fall apart as it makes it
into central Iowa. On a larger scale, weak cyclonic flow aloft
over the Gulf of Mexico pinching off best moisture access ahead of
the front. The low-level frontogenetical forcing profile really
tells the story as the forcing becomes disjointed after 00z Tue
and never regains its composure while over the DMX CWA.
Instability is meager at best, with only a few hundred joules of
CAPE across Iowa. So have slight thunder chance in...certainly no
svr threat Tuesday.

Though PWATs are a widespread 1.5+ inches, sounding profiles across
central Iowa confirm lack of decent saturation in the low-levels
which will make for very inefficient rain production. Thus, have
continued where I began yesterday afternoon with knocking down
QPF vs guidance. Since this output was basically in place since
yesterday afternoon, confidence becoming high in that this will
not be an interesting boundary passage in central Iowa...with
sprinkles to light showers being far and away more likely than
moderate and/or heavy rainfall in central Iowa.

At this point in time, there are no reasonable significant
precipitation chances for the remainder of this long-term forecast
period.


Temperatures...
850mb temperatures this past weekend were generally in the +17C
to +20C range across Iowa. By Wednesday afternoon, temperatures at
850mb look to be in the +5C to +9C range, which is close to 0 std
dev... suggesting near-normal temperatures is a good base
starting point... which guidance happens to be near already.
Synoptically, leading the surge to knock temperatures back down
to normal will be a 1025 mb high slated to propagate down from
Manitoba, making it into western Iowa by 12z Wed, and into
southern Iowa by 12z Thu.

On the heels of this high is another, even more expansive 1030mb
high that should make it into central MN by Friday afternoon. This
second high will advect even cooler temperatures into the upper
Midwest. Though long-range models and their ensembles in good
agreement with the overall synoptic features of this second sfc
high, still some run-to-run discontinuity and disagreement with
temperatures. The 12z Sun ECMWF is several degrees cooler than the
12z GFS, so we will have to see how this plays out as it will
impact frost potential in Iowa.


&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1237 PM CDT Sun Sep 24 2017

Main challenge is timing of precipitation. Mixing this afternoon
will relax by 00-01z. Blocking pattern continues to delay the
progression of the cold front over eastern South Dakota/Nebraska.
The front is finally expected to reach western Iowa by 12z
Wednesday. With most of the rain/iso thunder behind the boundary...
will see cigs gradually lower to near MVFR by the end of the
period near KFOD/KMCW along with increase in shower chances to
VCSH/-SHRA at those two locations. Elsewhere VFR conditions will
remain with southwest winds. /rev


&&

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

$$

SHORT TERM...REV
LONG TERM...Kotenberg
AVIATION...REV



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