Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 121128
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
628 AM CDT Sat Aug 12 2017

.SHORT TERM.../Today through Tonight/
Issued at 403 AM CDT Sat Aug 12 2017

Large area of high pressure in place through Iowa and into the
Northern Plains this morning. Patchy fog has been developing over
portions of north central Iowa. Localized visibilities below 1sm
miles will be possible through 8 am. High pressure will dominate
the weather for much of today. There is a low possibility that a
stray shower or two develops this afternoon. Confidence is low
enough that no precipitation is in the forecast through the
afternoon. High temperatures will be near to a few degrees warmer
than Friday.

The area of high pressure will move slowly southeast today and be
centered over eastern Iowa and western Illinois by this evening.
The pressure gradient and return flow will increase across
Nebraska and the Dakotas today as the high settles southeast and
low pressure presses east from Wyoming and Colorado. A vigorous
short wave/PV anomaly will drop southeast into the Dakotas today
and overnight and will spread kinematic forcing into Iowa
overnight. This short wave in combination with a developing low
level jet focused into Nebraska and South Dakota should lead to
thunderstorm development by this afternoon. These storms are
expected to move into northwest Iowa tonight. Still in question
how far these storms move into Iowa as they will encounter the
more stable and drier air mass to the east with the area of high
pressure still in the vicinity.

.LONG TERM.../Sunday through Friday/
Issued at 403 AM CDT Sat Aug 12 2017

The forecast concerns were focused on pops tomorrow night into
Monday, temperatures and winds Tuesday, and pops (or lack thereof)
Thursday into Friday. Models are in fair agreement with the
overall synoptic trends through the middle of the week with the
typical timing differences wrt to any system to impact the
forecast area late in the period.

Sunday through Monday...Large and dominant surface high pressure
finally shifts east enough to allow a developing surface low over
the Dakotas to push into the state by late Sunday morning. The
forecast area looks to be impacted by two successive upper level
shortwaves Sunday into Monday. The stronger system (coincident
with the surface low) is likely to push across the state by early
Monday during the 09-15z time frame and will quickly push
eastward by Monday afternoon. Both systems are moisture starved
but there is enough forcing with the parent system over northern
Iowa to possibly see up to a quarter inch by Monday morning.
Still not enough precipitation to help with the drought
conditions.

Tuesday...weak upper level ridging builds into the state and
provides some good southerly flow throughout the day Tuesday ahead
of a deepening trough over the Rockies. The GFS is slightly
stronger with this deep trough, but even with the slightly weaker
ECMWF, Tuesday is shaping up to be a warm and breezy day. Tweaked
up maximum temperatures and winds Tuesday to coincide with a fair
amount of deep mixing by the afternoon. Plus, with the drought
conditions leading to lesser ground moisture and thus slightly
lower surface dew points and allowing temperatures over the past
few days to easily go above guidance. Thus, confident to go warmer
a couple of degrees across the forecast area Tuesday and could
even tweak a bit higher.

Wednesday through Saturday...The aforementioned deep trough and
associated cold front track across the region on Wednesday into
Wednesday night providing the state`s best chance of any
widespread rain over the next week to 10 days. Have the highest
pops going in the morning Wednesday when the best theta-e
advection and decent frontogenetical forcing comes across the
state along and ahead of the cold front. System should quickly
push east by Thursday morning and went dry for the entire forecast
area after 12z Thursday into thursday night. Could even go dry by
Friday with the lack of any significant moisture as a weak
shortwave cuts across northern Iowa.

&&

.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 623 AM CDT Sat Aug 12 2017

VFR conditions are forecast through the period with the exception
of localized fog early this morning. Expect cumulus development
around 5 kft mid to late morning through the afternoon. The
cumulus may become broken, especially north. High pressure will
keep the wind light much of the period. Low chances for an
isolated shower or thunderstorms near KFOD late tonight.


&&

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

$$

SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Donavon



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