Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190831
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
331 AM CDT Thu Oct 19 2017

.SHORT TERM.../Today through Tonight/
Issued at 324 AM CDT Thu Oct 19 2017

A frontal boundary that entered northern Iowa Wednesday afternoon
and continued to settle slowly south overnight. The boundary was
fairly diffuse early this morning with the wind becoming quite
light but still was evident with dew points in the 30s north and
the mid to upper 40s south. The boundary will continue to settle a
bit farther south through mid morning before lifting north as the
upper flow transitions back to southwesterly.

The 850 mb temperatures will be warmer this afternoon than on
Wednesday though mixing depths are not expected to be as deep today.
Overall highs should be near to a few degrees warmer than Wednesday.
Dew points will remain quite dry far north and east while return
flow will help lift readings back into the low 50s central and
southwest. The arrival of the higher dew points will keep overnight
low temperatures warmer and in the upper 40s to mid 50s.

.LONG TERM.../Friday through Wednesday/
Issued at 324 AM CDT Thu Oct 19 2017

Friday...The upper ridge axis slips east while a trough digs into
the west.  The result for much of the Upper Midwest will be to have
increasing winds out of the southwest and warm temperatures...likely
the warmest yet for this week with highs in the upper 70s to lower
80s.  Dewpoints will likely have to be lowered if today`s forecast
soundings verify as there is some very dry air in place initially
and with a strong southwest wind, things will dry out even further.

Friday night we begin to transition to a cooler/wetter pattern.  The
upper trough crosses into the Rockies and deepens while the surface
low noses eastward.  Strong theta-e advection occurs Friday night
and while initially the dry air will win out, overnight there will
be deep enough moisture and instability for some elevated storms
mainly over the western quarter to third of the state.

Saturday morning the upper trough noses closer to the Upper Midwest
while the surface boundary begins to push across Iowa.  While
initially we may see some elevated storms from the increased warm
advection, forecast soundings show a very stout cap in place in the
morning which gradually breaks across the west by mid to late
morning, and across central Iowa around mid afternoon and east late
afternoon and evening and this should be the timing for storms as
the frontal boundary moves across the state.  Instability and shear
will be sufficient for some strong storms...and possibly severe
storms Saturday afternoon into the evening.  In addition, PWAT`s
approach an inch and a half so moderate to occasionally heavy
rainfall is also possible.

On Sunday the upper trough moves through pushing any remaining
precip into the Great Lakes area and bringing in cooler air. Models
begin ti diverge here though...mainly with the location of a closed
low that develops from the upper trough as a second upper trough
moves into the Northern Plains Tuesday into Wednesday.  We should
still be precip free...but temps will be affected.  At this point
the Canadian is really messing up model blends as it keeps precip
over the region for days, which doesn`t seem likely.  As a result,
low PoPs are in the grids in the out periods.  They have been
removed for Sunday night but beyond that there is low confidence in
any one solution and slight chance PoPs were left in.

The bigger story is that the Northern Plains trough will deepen and
allow some very cold air to enter the region.  Tuesday through
Thursday, highs will only get into the 50s with lows in the mid to
upper 30`s.  Some moderation in highs will occur next Thursday as
the deepening trough finally inches far enough east into the Great
Lakes to allow ridging to move into the west or southwest.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1144 PM CDT Wed Oct 18 2017

Surface ridge will slide across the state tonight with light and
variable winds along with widespread VFR conditions. As the ridge
passes to the east on Thursday, winds will increase from the
southeast. However, with the airmass remaining dry, widespread VFR
conditions will persist.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...FAB
AVIATION...Cogil


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