Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 192107
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
307 PM CST Sun Nov 19 2017

.SHORT TERM.../Tonight through Monday/
Issued at 307 PM CST Sun Nov 19 2017

Ridge of high pressure has moved east into eastern Iowa and this has
led to return flow and warm advection moving into the area this
afternoon. The south/southwest winds will diminish overnight but
should still remain in the 5 to 10 mph range and enough to keep weak
surface warm advection continuing while much stronger warm advection
occurs from 900-850 mb. This disparity will result in a strong
inversion around 900 mb to start the day on Monday. In addition, the
warm advection will keep overnight lows much warmer than  This low
inversion will limit mixing potential on Monday, therefore, limited
highs in the upper 50s despite the thermal ridge and 850 mb
temperatures around 10C arriving. Southwest winds will become breezy
to gusty by late morning and into the afternoon, however, the
momentum transfer of mixed layer winds will be dampened by the
ongoing warm advection. A few gusts over 30 mph will be possible
though the wind speeds will remain below advisory criteria. Some
passing high level cirrus clouds at times tonight into Monday though
should not be dense enough to prohibit high temperatures.

.LONG TERM.../Monday night through Sunday/
Issued at 307 PM CST Sun Nov 19 2017

Temperatures will be below normal Tuesday and Wednesday with warmer
air arriving for Thanksgiving and Friday before colder air arrives
into next weekend. Chances for widespread dry weather across central
Iowa remains high through Friday morning.

A shortwave trough will pivot toward the Great Lakes around the
Hudson Bay low pressure Monday night. At the surface, a cold front
will move from the northwest to the southeast and be through much if
not all of central Iowa by 12z Tuesday. While mid and high level
clouds will accompany the front, low level moisture will be lacking
so looks to be dry despite a few models (e.g. GFS, WRF HiRes window)
indicated light precipitation. Behind this front, strong cold air
advection with gusty winds from the northwest will prevail as skies
become mostly sunny on Tuesday. 850mb temperatures will drop to
between -6 and -10C with a 24 hour change by 00z Wednesday of -15 to
-16C. Temperatures will also exhibit a non-diurnal trend over the
forecast area. Temperatures may fall through the morning before
briefly rising in the afternoon over northern Iowa, be fairly steady
over the central part of the state, and fall through the afternoon
over southern Iowa. Highs will range from around 30 degrees near the
Minnesota border to the low 40s over southern Iowa. BUFKIT NAM
soundings show winds at the top of the mixed layer around 40 knots
near midday with both the GFS and NAM showing winds at the top of
the mixed layer of 30 knots through around 5pm. Therefore, increased
winds from initial Superblend guidance by blending in CONSMOS from
12z to 22z Tuesday, which will help to accentuate the chill of the
day.

Throughout the day Tuesday into Tuesday night, a 1030mb high
pressure will be moving through the Dakotas into eastern Nebraska
becoming centered around Kansas City, MO by 12z Wednesday. With
cross sections and forecast soundings showing a fairly dry airmass
and winds dropping off in response to the high, especially across
western Iowa, did lower temperatures a degree or two over these
areas.

As the high pressure moves away to the east on Wednesday, a weak PV
anomaly will move from the southern Canadian prairies into the
western Great Lakes Wednesday afternoon into Thursday morning. The
precipitation looks to stay predominately north of the area with
GFS/ECMWF forecast soundings showing plenty of mid-level saturation,
but a lack of low level saturation. Therefore, will maintain a dry
forecast during this time. As the PV anomaly moves away, warm air
advection will begin on Thanksgiving Day as temperatures rebound to
near normal.

Temperatures are expected to continue to rise on Friday, though by
how much remains a bit in question. After the 18/12z ECMWF run
suddenly trended lower than the 18/00z run, the ECMWF 19/00z and
19/12z runs are back to much higher temperatures in the upper 50s to
perhaps upper 60s over the far southwest forecast area. The GFS over
its past several runs remains quite warm with 60s over at least the
southwest half of Iowa. Last three 00z runs of the North American
Ensemble Forecast System (NAEFS) at KDSM has been trending higher
with around 9C from the 17th run, 10C from the 18th run, and the
latest run being around 13C. While the interquartile range has been
decreasing over these runs, the range between the minimum and
maximum values remains around 20C weighted toward the minimum side.
Part of the large range in temperatures is uncertainty about what if
any clouds may impact the area later Friday as a shortwave trough
crosses southern Canada/the northern US states with a cold front
approaching the area. Forecast soundings from the GFS/ECMWF continue
to show varying increases of moisture in the middle and upper levels
of the atmosphere over northern Iowa Friday afternoon. Therefore,
confidence is highest in higher temperatures over southern parts of
the forecast area where concerns of any high level clouds are the
lowest. Highs are still weighted towards Superblend, which is
catching on to the GFS/ECMWF warmth to a small degree, with small
percentages of the GFS/ECMWF also incorporated. If it becomes more
certain that these clouds will be higher level and not persistent,
highs could easily be 5 degrees higher.

While the shortwave trough moves into the Great Lakes, surface low
pressure will move over southern Canada into the Great Lakes with
much of the precipitation remaining north of the area. However,
precipitation may brush the area as global models bring it close to
northern Iowa. As the cold front moves through the area Friday night
into early Saturday, colder air will arrive as cold air advection
and gusty northwest winds once again prevail across the state.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1126 AM CST Sun Nov 19 2017

Widespread VFR conditions through the forecast period. West to
southwest winds today will become more southerly overnight. The
wind will become breezy from the southwest by mid to late morning
on Monday.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...Ansorge
AVIATION...Donavon



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