Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 121720
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1120 AM CST Sun Nov 12 2017

.SHORT TERM.../Today through Tonight/
Issued at 340 AM CST Sun Nov 12 2017

Upper level wave approaching from the west will plow through the
state by 18Z with upper level ridging building into the state this
afternoon.  The best forcing with this wave moves across central
Iowa between the highway 20  and I80 corridors but quickly weakens
and dives southeast between 15Z and 18Z.  The better precip will be
tied to where the better lift is.  Current radar imagery shows two
areas of better reflectivities.  Over the southeast this is tied to
the inverted trough moving through the state and the precip along
the highway 20 corridor is tied to the forcing ahead of the
shortwave.  North of highway 20, drier air is already moving in as
noted by the clearing skies, colder temps and dewpoints in the 20s
vs 30s to around 40.  I think this drier air is negatively impacting
precip along the highway 20 corridor and while I knocked dow qpf a
little, it may not be enough...especially out west of I-35. As for
ptype...soundings show precip should mostly be rain but there may be
a brief period of a rain/snow or freezing rain mix along the back
edge of the precip where the colder air is pushing in.  For the rest
of the day, rapid drying will take place behind the upper wave and
with a surface high moving in, skies will be clearing from 18Z
through mid afternoon.  Temps will still be on the cool side with
upper 30s expected northeast to the upper 40s southwest.

Tonight will be a cool night but as the high slips east, a southerly
flow develops across the forecast area after 09Z.

.LONG TERM.../Monday through Saturday/
Issued at 340 AM CST Sun Nov 12 2017

High pressure will begin slide off to the east of Iowa Monday night.
This will allow for return flow and a low level moisture plume
originating from the Gulf to arrive by late Monday night into
Tuesday. This plume is already visible through central Texas and
will lift up with attendant low stratus.  Good moisture depths to
near 6 kft below a pronounced elevated mixed layer (EML) and
favorable conditions for collision coalescense with warm advection
and lift through the stratus layer, all suggest drizzle potential on
Tuesday. Reduced visibilities due to fog may also occur. The EML
remains stout through Tuesday night with very dry air positioned
from around the 850-600 mb layer. Some forcing with an approaching
short wave above this layer will arrive however, convective
instability will be due to the EML and have reduced thunder mention
and confined it to the very far southeast. High temperatures Tuesday
likely will be directly related to dew points and remain with in 2-4
degrees of the dew points as low level relative humidity remains
high.

High pressure will build into the Central and Northern Plains on
Wednesday followed by weak upper ridging for Thursday. Thursday will
be a bit cooler though with the low level flow originating from the
surface high pressure which will be over the western Great Lakes
region. The biggest forecast challenge and concern arrives toward
the end of the week as a strong PV anomaly/short wave arrives and
lee side cyclogenesis commences. The general trends remain the same
with the surface low rapidly deepening as it passes by Iowa and
lifts northeast into the central Great Lakes. Still a wide range of
potential impacts to Iowa from this system ranging from
thunderstorms to light snow to strong winds. This system is just now
moving into Alaska.  The system will then evolve into a closed upper
low over the Pacific just west of the Pacific Northwest region
before ejecting out towards the Plains. Details regarding this
system will continue to be fine tuned through the week.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1120 AM CST Sun Nov 12 2017

Low clouds still in place at KALO, KDSM and KOTM should clear out
in the next 2-4 hours as drier air moves in from the north.
Northerly winds expected to shift around to south into Monday. VFR
conditions expected tonight into Monday.

&&

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

&&

$$

SHORT TERM...FAB
LONG TERM...Donavon
AVIATION...Beerends



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