Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 260842
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
342 AM CDT Wed Sep 26 2018

.DISCUSSION.../Today through Tuesday/
Issued at 341 AM CDT Wed Sep 26 2018

Benign weather on tap for the next day or so.  Aloft we will be in
northwest flow as the upper trough will still influence the region
but high pressure at the surface and dry forecast soundings will
keep clear skies and seasonally cool temps in place through tonight.

For Thursday, another shortwave will drop down the flow and deepen
the trough over the Upper Midwest while at the surface, a low will
traverse the Northern Plains.  As the low moves into the Northern
Great Lakes and Canada, a secondary cold front will drop across the
state during the day Thursday.  Soundings show some meager mid level
moisture with this system but there is enough convergence along the
boundary that a few sprinkles/light showers could develop along the
boundary mainly late Thursday afternoon or evening.

For Friday through Saturday, models are fairly consistent with a
high pushing down limiting any precip Friday to southern Iowa but
soundings are very dry so I think Friday...especially as the day
wears on...will be mainly dry.  There are some slight chance PoPs in
the forecast grids which I left, mainly because this is the first
run where models have converged on a solution so confidence is not
high yet but a lowering of PoPs from chance to mainly slight chance
was warranted and the best way to go.  With the high overhead for
Friday, temps will be quite cool and may struggle to reach the lower
60s.

Friday night we may see our first frost event mainly across the
northern third of the state as lows bottom out in the mid 30s. The
caveat that I see for this is the potential for dry air to work into
areas mainly between highway 30 and highway 20 which would limit the
frost potential but we will have to see how this plays out.
Somewhere across the north frost appears likely.

Over the weekend we will have our next precip chance. The upper flow
will become more zonal.  A shortwave is progged to move across the
Central Plains with a surface low developing in response to this. A
warm front will set up stretching across the state. This frontal
boundary will stall over us into the first half of next week. The
boundary will be the focus for shower and thunderstorm development
going forward.  Instability doesn`t really work it`s way back into
the region before Sunday afternoon so light showers is all that is
expected through then.

Monday night through Tuesday look interesting as models indicate an
intense low moving into the west coast off the Pacific Ocean while
over the CONUS a trough digs into the west coast.  There is some
indication that moisture from this low will get tapped by the
digging trough and may get transported into the region.  If this
would occur, we could see some bigger storms return to the region.
We would already be seeing highs well into the 70s and maybe lower
80s and that moisture would push dewpoints over IA well into the 60s
with a frontal boundary around.  Models vary quite a bit on details
so for now we are just carrying at least chance Pops in each period
with a mention of thunderstorms but the evolution of this will bear
watching for the possibility of stronger storms in the
Tuesday/Wednesday timeframe.

&&

.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1150 PM CDT Tue Sep 25 2018

High confidence in VFR conditions throughout this TAF period.
Winds will likely be too strong for fog formation this morning.
Winds will gust above 10 kts Thursday afternoon and then should
die down before sunset.

&&

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

$$

DISCUSSION...FAB
AVIATION...Kotenberg


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