Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 271145
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
645 AM CDT Sat May 27 2017

...Updated for 12z Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 340 AM CDT Sat May 27 2017

Main focus as well as questions lies with the possibility of severe
weather today, but also in the near term have had areas of dense fog
over eastern parts of the CWA. Areas around Waterloo have had
visibilities drop briefly to 1/4 mile at times early this morning.
Will monitor near term trends, but have issued an SPS for patchy
dense fog.

Rest of attention then is on later today and questions around
moisture return, the amount of sunshine, or perhaps lack thereof,
and how convection south of the state may play into our potential.
While winds are light at 8z, winds generally have had some sort of
northerly component early this morning. Winds should start to shift
and blow from the east after sunrise, but do not obtain a southerly
component until mid to late morning. This will limit the amount of
moisture return as convection and showers currently over Nebraska
and northern Kansas will be moving into Iowa. Secondly, the
aforementioned convection cluster out west is already spreading mid-
level clouds into western and southern portions of the state per
latest GOES-13/16 imagery. These clouds, which may allow for some
filtered sunshine initially, will limit our overall surface heating.
Still, there will be instability with MUCAPE values between 1500 and
2500 J/kg with bulk shear between 40 and 55 knots. Finally, a strong
line of thunderstorms is forecasted to move across areas south of
Iowa along a warm front later today. This may be too late in the day
to have a large impact on our storms, but something that will have
to be watched.

Bottom line, strongest storms that are able to develop will be
focused over areas along and south of Interstate 80, where a
damaging wind threat will be the main concern. Secondary concerns
will be brief tornadoes on the leading edge of the line of storms
along with perhaps isolated large hail.

.LONG TERM.../Sunday through Friday/
Issued at 340 AM CDT Sat May 27 2017

This period will be relatively inactive with little in the way of
sensible weather. The long wave trough axis and embedded short
waves currently moving through the Rockies should reach eastern
Iowa at onset, while the core of the upper low should move from
Manitoba into the northern Great Lakes by Monday. This pattern
will place Iowa in broad, deep cyclonic flow for an extended
period keeping us mainly dry and temperatures seasonal at best.
Some weak instability is evident north and east during peak
heating Monday, but any showers that develop will likely be sparse
and unorganized with little in the way of convergence. Thus PoPs
are no better than low chance category.

Our cyclonic flow should finally weaken into the middle of next
week as surface high pressure very slowly builds from the high
Plains into the MS Valley. This will gradually increase warm
advection and thus finally re-introduce low PoPs back into the
forecast south and west by Thursday and just beyond the valid
period to end the week.

&&

.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 645 AM CDT Sat May 27 2017

LIFR and IFR visibilities are impacting many eastern Iowa
terminals early this morning, but restrictions should improve into
VFR no later than mid-morning. A line of thunderstorms over
southeastern Nebraska and northeastern Kansas will move through
the state today. Impacts are expected to arrive to the western
central Iowa terminals late this morning and the eastern central
Iowa terminals this afternoon. Recent observations suggest if the
storms maintain a steady state, wind speeds will be 20 knots with
gusts to around 30 knots as they arrive. However, confidence in
the thunderstorms maintaining a steady state not high enough to
put that in at this time so have VCTS wording. After the storms
clear the area, should see a return to mainly VFR conditions.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 8 AM CDT this morning for IAZ027-028-
037>039-048>050-060>062-073>075-084>086.

&&

$$

SHORT TERM...Ansorge
LONG TERM...Small
AVIATION...Ansorge



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