Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181726
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1226 PM CDT Fri Aug 18 2017

.SHORT TERM.../Today through Tonight/
Issued at 427 AM CDT Fri Aug 18 2017

Generally two items of note within this forecast time frame, showers
and storms today and then fog potential overnight.

First and foremost, showers/storms slated to move through today
associated with a couple of weak waves moving through the 500 mb
flow. The first of which currently located in the SD/IA/MN
intersection area with an associated weak surface boundary extending
SW back into eastern and central Nebraska. The second wave currently
situated in central ND. As the initial wave slides across northern
IA/southern MN, forcing across remainder of IA will be rather
lackluster with the weak surface boundary. However, do expect some
scattered convection to occur within an environment of modest CAPE
around 1500 J/kg by mid-afternoon. Effective shear to be sub-30 kts,
but with strong low-level lapse rates and increasing amounts of low
level dry air the further south you trek, isolated strong to severe
winds may be possible with evaporative cooling aided down drafts. As
the second wave approaches northern/northwest IA in the evening,
nothing more than a few elevated showers and storms expected before
dissipating overnight as surface high pressure takes over. POPs
reflect above thinking with prevailing chances wording across north
and primarily scattered further south.

Second, and less consequential, fog potential Fri night. Multiple
factors lining up, including increased moisture from any
showers/storms and clearing skies and light winds from incoming
surface high pressure to be situated over Iowa. Have put in patchy
fog wording for the time being across much of the CWA, especially
north, but would not be surprised if coverage ends up justifying
areas/widespread in spots.

.LONG TERM.../Saturday through Thursday/
Issued at 427 AM CDT Fri Aug 18 2017

Surface ridging will slide across the state on Saturday with
subsidence. Quiet weather is expected during the day into the
night with warm advection increasing overnight. Moisture plume
will move northward on Saturday night across the Plains with
convection expected in Nebraska. The moisture transport will
increase into Iowa on Sunday as the low level flow veers.
Instability will begin to increase with the theta-e advection and
there is the possibility of a few storms on Sunday in western and
northern Iowa. Otherwise, the warm sector of the system will be in
place across the state with highs reaching into the lower 90s in
central and southern Iowa, especially across the drought areas.

Convection will develop on Sunday night from southern South
Dakotas into southern Minnesota along a boundary. Low level
moisture transport will be focus on this area overnight with heavy
rains expected. In addition, initial development may produce some
severe weather given decent bulk shear parameters. This convection
is expected to edge into northern Iowa on Sunday night and linger
into midday Monday. Cloud cover for the eclipse on Monday will be
most problematic across northern Iowa where the convection is
expected. Farther south, the warm sector should still be in place
with a better chance of scattered cloudiness and ability to see
the eclipse occurring.

The front will settle into Iowa by late Monday with convection
redeveloping by late afternoon into the evening across Iowa.
Severe weather seems a possibility with this development along
with the threat of heavy rainfall given the available moisture.
Plenty can still change between now and then and will continue to
monitor but models becoming a bit more consistent with some decent
rainfall across the state. The weather appears a bit more quiet
into the middle of next week as Canadian high pressure settles
into the state with seasonably cool temperatures and limited
precipitation chances.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1220 PM CDT Fri Aug 18 2017

Primary forecast challenge of the latest terminal forecast
continues to be the potential for fog/visby restrictions Saturday
morning. Model guidance has come into sound agreement with the
lowest visby across north central Iowa between 09-14z, which may
drop sites into LIFR/VLIFR category at times. Fog is possible
further south toward KDSM/KOTM, although guidance suggests that
visby will likely remain at or above 2sm. This trend has been
reflected in the latest forecast. The fog will dissipate and give
way to VFR conditions by mid-morning.

Isolated to scattered TSRA activity is also possible this evening
and early overnight. The highest potential will be in north
central Iowa. A mention of TSRA has been included at KMCW after
02z. Confidence is not high at the moment enough to warrant
mention at other sites.

&&

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

$$

SHORT TERM...Curtis
LONG TERM...Cogil
AVIATION...Martin



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