000
FXUS63 KDMX 190859
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
359 AM CDT Sat Aug 19 2017

.SHORT TERM.../Today through Tonight/
Issued at 353 AM CDT Sat Aug 19 2017

A thunderstorm complex dropped out of southwest Minnesota into Iowa
Friday evening and weakened as it approached the Highway 30
corridor. Closer to the Des Moines metro, what can only be described
as a cruel atmospheric tease, a storm looked poised to go over the
western part of the Des Moines. Instead, it veered to the east over
the far northern part of the metro before diving southward over the
eastern part, including the Iowa State Fair. And once again, drought
stricken southern Iowa missed out on rainfall. Overall across
central Iowa, locations that did receive rainfall generally saw less
than a quarter an inch, though a few places like Estherville picked
up over half an inch.

As the clock passes 3am, a few widely scattered showers and storms
are lingering southeast of the metro. These showers and storms will
continue to move to the southeast and should maintain or decrease in
intensity as 07z SPC mesoanalysis shows them in an environment
characterized by MLCIN of 75-100 J/kg with CAPE values less than
1000 J/kg. Regardless, they should move out of the forecast area by
sunrise as the shortwave trough pushes east of the state.

Meanwhile, GOES-16 Nighttime Microphysics RGB and 10.3-3.9 micron
channel difference as well as surface observations show patchy fog
with even some low stratus. Some of the patchy stratus over
southwest Minnesota is slowly moving southeastward and have adjusted
sky grid to include these clouds over the northern part of the
forecast area in the next few hours. Overall, fog trends from
guidance at 00z, 06z, and latest short range models have backed off
the intensity and to a much lesser extent the areal coverage. The
back off in intensity may be due to the rainfall not being overly
widespread and lingering clouds from the earlier convection over
eastern Iowa. CONSShort, HRRRs, WRF cores, and RAP along with the
SREF have the most likely area being over the northwest part of the
forecast area. Based off of observations and model guidance, will
continue patchy fog over much of central Iowa with a few areas of
denser fog across the northwest. This fog should dissipate within a
few hours after sunrise.

Behind the aforementioned departing shortwave trough, mid-level
ridging along with surface high pressure passing over the state will
provide for a dry day with ample sunshine. Temperatures will peak
within a few degrees of mid-August normals. After sunset this
evening, clouds will begin to increase from west to east over Iowa
as a subtle shortwave approaches from the west. Showers and a few
thunderstorms will reach central Iowa after midnight. There`s also a
chance for patchy fog over the northeast part of the forecast area
ahead of the clouds and precipitation.

.LONG TERM.../Sunday through Friday/
Issued at 353 AM CDT Sat Aug 19 2017

Aloft a strong low traverses Canada eventually dropping a decent
shortwave across the Northern Plains and Upper Midwest in the
Sunday/Monday time frame. At the surface a strong low develops on
the lee side of the Rockies. During the day Sunday, a couple of
weaker short waves will move across the state and we should be in
at least weaker warm advection with a weak boundary developing
across the state and sliding north near the MN border in the
afternoon. We will see scattered showers/storms from this
feature. There will likely be a break in precip between late
morning and mid afternoon but it is difficult to say where and for
how long. For this reason I kept general chance PoPs going
through the day. Then Sunday night, stronger warm advection will
bring in even warmer and more moist air, especially across the
southwest and west central portion of the state late Sunday night
through Monday. Given the amount of instability moving into the
west, and steep mid level lapse rates along with the boundary,
storms should re-develop near the boundary and move into at least
the western third by 12Z Mon. There would be some potential for
hail and strong wind with these storms some of which may approach
or touch severe limits.

During the day Monday and Monday night the surface low pushes
northeast dragging a cold front through the state. More storms will
develop along the boundary especially Monday afternoon and night,
some of which could be strong. Some caveats to the convective
potential and especially the severe potential on Monday, would be
the amount of cloud cover, the effect of the eclipse and how much
that would limit instability. Also the effect of the ongoing
precip on Monday and that impact on the environment. There appears
to be a window mainly Monday night with the frontal passage and a
stronger shortwave passing through that robust storms would
develop in the vicinity of the boundary.

Tuesday morning the front will continue to push to the southeast
thus ending precip across the southeast third to half during the
morning hours Tuesday.  Cooler air will surge in so while we will
see some clearing, we will also see some brisk wind in the
afternoon.

Ridging takes place Wednesday into the weekend for dry conditions
and cooler temps.  Friday night into the start of the next weekend
another shortwave pushes into the state from the west.  Models vary
on the location and intensity of the wave and while model blends
wanted to put likley pops in for this period, the discrepancy between
models and being that it`s day 7 I did knock PoPs back to the chance
category.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1209 AM CDT Sat Aug 19 2017

Primary concerns continue to be convection and fog development
tonight. Scattered thunderstorms are moving east and southeast
over central Iowa and will be close to KALO and may impact KDSM.
For now, just have VCTS for a few hours at the start of the TAF
period. Storms should continue to weaken as they move southeast so
do not have a mention at KOTM at this time.

Fog development continues to be advertised by the models, although
they have backed off a bit from how low the visibilities will
drop. GOES-16 Nighttime Microphysics RGB showing some low stratus
as well as fog over southwest Minnesota. For the northern
terminals, will continue to have IFR with some TEMPO LIFR
visibility restrictions. For the southern terminals, confidence is
not as high, but think KOTM has the better chance of MVFR
restrictions. With lower confidence at KDSM, just have TEMPO MVFR
conditions. After fog dissipates Saturday morning, high pressure
will move across the region with VFR conditions prevailing.

&&

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

$$

SHORT TERM...Ansorge
LONG TERM...FAB
AVIATION...Ansorge