Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 232322
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
622 PM CDT Sat Sep 23 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 340 PM CDT Sat Sep 23 2017

Bottom Line Up Front...
Main focus in the short-term continues to be record to near-
record temperatures. Sunday will bring another day of record to
near-record heat on Sunday. Precipitation chances trending
downward to only slight at best in our far west/NW CWA.

Temperatures...
20z analysis shows 1020mb sfc high remains anchored over the
Great Lakes region. Aloft, 500mb anti-cyclonic flow settles all
the way down to the Ozarks... with accompanying thermal ridge
crashing over the upper Midwest from MN through north of Lake
Superior, into southern Canada. 850mb temps over Iowa in the +18C
to +20C range, with the anticyclonic flow reinforcing WAA from
850mb via strong SW flow. For Sunday, 850mb temps may be a degree
or two cooler. Given similar sky cover, continued WAA, etc. very
difficult to go any more than a degree or two cooler than
persistence from today`s highs. Thus, have nudged guidance up
closer to persistence.

Precipitation...
A deep upper low remains more or less parked over the Four
Corners region of the CONUS... helping to dig a longwave
trough/height falls from Manitoba all the way into the Baja of
California. The past few days, models have been trying to resolve
an elongated sfc low slated to eject off the Rockies Sunday
morning and propagate northeastward, making it to central MN by
00z Mon. An attendant boundary is slated to push through Iowa as
this weak low punches into Canada by Wed. For short-term purposes,
this boundary passage is trending to be slower, weaker, and more
disorganized... perhaps clipping our far western/NW CWA by 00z
Mon... the end of the short term fcst period.

.LONG TERM.../Sunday night through Saturday/
Issued at 340 PM CDT Sat Sep 23 2017

Bottom Line Up Front...
Not only will the string of record heat will come to an end
Monday into Tuesday, but temperatures may actually be a few
degrees *below* normal by Tuesday afternoon. Rain chances return
for Monday into Tuesday.

Precipitation...
Building off of the above short-term discussion, the
aforementioned boundary looks to slowly migrate across Iowa during
the day on Monday into Tuesday. With the boundary trending
slower, have bumped up temps in the central to eastern portion of
our CWA to start blending into persistence from Sunday. Though
PWATs become a widespread 1.5 inches across the DMX CWA on Monday,
the best low-level saturation remains over western Iowa until
Tuesday morning. CAPE values on Monday look to be in the 500 to
900 J/KG range... of which, fcst soundings show the CAPE to be
thin with an elevated base. Thus, have started to knock down QPF
vs guidance to the one-tenth of an inch range across central
Iowa... which may still be too optimistic. Far western portions of
our CWA may be in the one-quarter to one-half inch range. As the
boundary pushes into central Iowa by Tuesday finally, low-level
frontogenetical forcing looks disjointed at best... a very far cry
from the Sat Sep 16 into Sun Sep 17 boundary passage. Therefore
have continued to knock down QPF into Tuesday.

By 00z Wed, the 12z GFS and ECMWF have Iowa into the strong
northerly flow with subsidence, dry air entrainment winning out as a
1025mb sfc high begins to head towards the Midwest. This high
dominates the wx pattern through Thursday as it parks over northern
Missouri. behind this sfc high, another sfc high looks to punch
southeastward from Alberta into the Dakotas...serving to keep Iowa
dry for at least the next 7 days.

Temperatures...
As discussed above, persistence a great place to be Sunday, and
ahead of the boundary Monday. Behind the boundary with round 1 of
strong CAA (thanks to our dominating high pressure), 850mb temps
look to bottom out around +5C to +9C Wednesday into Thursday
morning. The second shot of cold air from the second high looks to
bottom out 850mb temps near 0C by next weekend. So, we will go
from record temperatures in the low 90s this weekend, to
frost/freeze headlines strongly needing to be considered for next
weekend. Overall trends are for cooler, so I have nudged down a
few degrees already.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 620 PM CDT Sat Sep 23 2017

South to southeast surface flow through the forecast period. Model
sounding support sct cu Sunday afternoon with more cloudiness over
the northwest tafs. VFR throughout the forecast period.

&&

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

$$

SHORT TERM...Kotenberg
LONG TERM...Kotenberg
AVIATION...Johnson


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