FXUS63 KDMX 250811

Area Forecast Discussion
National Weather Service Des Moines IA
311 AM CDT Sun Aug 25 2019

.DISCUSSION.../Today through Saturday/
Issued at 311 AM CDT Sun Aug 25 2019

Cloudy skies, increasing humidity, and scattered sprinkles will
be the norm today and tonight. Thunderstorms will move through on
Monday, with some threat of severe weather across about the
southeast half of Iowa, followed by a few days of dry and quiet
weather through Thursday. Rain chances may return again at the end
of the week.

Forecast discussion...
Currently, a large surface high pressure
area remains anchored over the Great Lakes, with east/southeast
flow over Iowa promoting dry air entrainment in the lower levels.
Meanwhile a broad 500 mb trough is approaching from the west,
promoting modest but widespread forcing for ascent and warm air
advection aloft. This is resulting in scattered light echoes
moving across the area early this morning, but mostly not reaching
the surface or only doing so as sprinkles fighting through the
dry air. Little change is expected in this scenario for the next
24 hours or so as the 500 mb trough moves overhead, though we may
see a slightly higher incidence of measurable rain, barely, over
our western and northern counties at times today/tonight where the
low-level dry air intrusion will be a bit less pronounced. In any
event, have maintained only very low POPs in the forecast through
tonight, with little to no QPF and only light showers and
sprinkles advertised.

A second, more vigorous 500 mb trough will approach rapidly from
the west/northwest late tonight, pushing a surface cool front and
promoting more widespread and robust convective development. This
should manifest as a swath of showers and thunderstorms along and
just ahead of the front, reaching far northwest Iowa by sunrise
Monday. As the front moves across our forecast area during the
day, and perhaps lingering into the evening in the east/southeast,
convective intensity should gradually increase with diurnal
destabilization and the maturing of the 500 mb trough overhead.
The strongest storms will be likely ahead of wherever the front is
located by around mid-afternoon, generally across about the
southeast half of the service area. SPC outlooks have outlined a
marginal risk of severe thunderstorms in this area, which is
reasonable given the degree of shear present. However, if the
stratus clouds that are anticipated throughout the day break up
more than expected, allowing for more substantial
warming/destabilization, the severe weather threat may increase
accordingly. This will bear close watching from today into Monday.
There is also some threat of locally heavy rainfall from the
storms on Monday given the ambient moisture in place by that time,
however, given the antecedent conditions we should be able to take
a good amount of rain so impacts will be minimal outside of
perhaps a very isolated/urban-centric flooding threat.

Once the system and its storms blow out to the east/southeast on
Monday evening and early Monday night, several relatively cool and
dry days are in store. A northwesterly, cyclonic steering pattern
aloft and northwest breezes at the surface will keep temperatures
in the 50s for lows and 70s for highs. A few light showers may be
possible during the afternoons Tuesday and Wednesday, but
probability/coverage are too low to advertise in the forecast and
they would have no impact even if a few do occur. By Thursday the
mid-level flow will flatten off a bit allowing for some low-level
return flow and warm air advection, pushing temperatures up into
the lower 80s across portions of Iowa. However, this will be
short-lived as another 500 mb trough approaches and matures over
the region at the end of the week. Long-range models have
displayed considerable variability in their solutions for the
evolution of this late-week system, so while rain is likely across
at least parts of Iowa, the location, timing, and magnitude of any
such precipitation are very uncertain at this time. Will maintain
low 20-30 POPs around Friday and Saturday for now and wait for a
clearer signal before making any real changes to the outer
forecast periods.


.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1138 PM CDT Sat Aug 24 2019

VFR conditions expected to prevail through at least Sunday
afternoon. Model guidance continues to trend toward lower cigs
after 21z Sunday, mainly across western and southern Iowa. Have
included mention of MVFR cigs at KFOD, KDSM, and KOTM.





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