Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 201733
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1233 PM CDT Sun May 20 2018

.SHORT TERM.../Today through Tonight/
Issued at 347 AM CDT Sun May 20 2018

The surface boundary currently extends from central Illinois into
central Missouri with northerly winds now in place across much of
Iowa. The boundary is quite shallow and only 3 kft deep or less
across much of the area. Southwest flow resides above the boundary
and with a vigorous shortwave lifting northeast through this flow
today, there will still be descent kinematic forcing across the
state and some modest theta-e advection. That said, the moisture
profiles across much of central and northern Iowa today will not be
overly conducive for precipitation and have begun to lower pops
across much of the area. Do expect scattered thunderstorms to
develop over southern Iowa this afternoon as the short wave forcing
arrives. While an isolated severe storms with marginal hail could be
possible, the overall outlook is low for severe weather potential.
Another area of better moisture profiles will be over northwest Iowa
and this area could be another focus area for precipitation/shower
potential.

Cold advection will continue to move into central Iowa today. Due to
the shallow nature of the boundary, mixing will be limited due to
the inversion aloft. The lack of mixing along with the potential for
stratus to linger much of the day will have an impact on high
temperatures. High temperatures central and north will be in the
upper 50s to mid 60s. Highs over southern Iowa should be warmer and
potentially reach the low 70s with the potential for more sunshine.
Overnight lows tonight will be near to above seasonal values with
clouds preventing a further drop.

.LONG TERM.../Monday through Saturday/
Issued at 347 AM CDT Sun May 20 2018

Monday through Monday night...
Lingering showers and a few thunderstorms, along with low-level
moisture and cloud cover, will slowly clear out to the east on
Monday with little additional impact. Late in the day and into
Monday night a weak surface ridge will move across Iowa with winds
dying off. There is some potential for fog development Monday
night into Tuesday morning, however, confidence is low due to
limited low-level moisture. The extent to which relatively
widespread rain falls between now and then will probably be a
primary driving factor. Otherwise, this period appears relatively
cool and quiet.

Tuesday through Thursday...
A weakening 500 mb trough will move northeastward from the western
U.S. up over the northern Rockies, preceded by persistent weak
ridging over Iowa and the Midwest. Subtle impulses and fragments
of vorticity ejecting out of the western trough will provide some
rather nebulous forcing for ascent at times and gradual
destabilization should occur in a broad warm air advection regime.
However, flow fields are very light and the instability, while
sufficient for thunderstorm development, appears to modest for any
organized severe weather threat in the face of unimpressive
forcing aloft. This will lead to several days of intermittent rain
and thunderstorm chances that are difficult to pin down at this
range, but will probably be most likely during the overnight
periods as the nocturnal low-level jet provides more impetus for
convection. While severe weather may be possible at times, the
overall threat appears low given the lack of any organized forcing
mechanism. Meanwhile temperatures will slowly increase over these
several days, peaking Thursday afternoon with highs in the low to
mid 80s across the state. There will also be a surge in dewpoints
to levels unseen so far this season, with mid/upper 60s likely
across central and southern Iowa by Wednesday and Thursday. This
will give us our first glimpse of summer heat and humidity that is
likely not far off as the end of May approaches.

Thursday night through Friday night...
The aforementioned western U.S. 500 mb trough, residing over the
northern/Canadian Rockies by late Thursday, will turn and drift
southeastward at the end of the week, reaching the Great lakes by
Saturday. A weak leading impulse ahead of the main trough will
move over northern Iowa on Thursday night and may provide focus
for somewhat more vigorous thunderstorms somewhere across the
region, though details remain unclear at this range. Additional
thunderstorms will then be likely from Friday afternoon into
Friday night as the broad forcing ahead of the large-scale trough
moves overhead. When the main trough axis moves over it will be
accompanied by a surface reflection that should cross our forecast
area somewhere around Friday night. Though any cold air advection
behind this initial boundary will be weak, it should signal an end
to scattered/widespread thunderstorm coverage for a while.

Next weekend and beyond...
The 500 mb trough will move overhead during the day on Saturday.
More showers and thunderstorms will likely develop just ahead of
the axis during the afternoon, but it appears this will occur
either just east of our forecast area or perhaps in our
eastern/southeastern counties before moving off to the east. For
much of the area Saturday will see clearing and drier weather, a
welcome respite after several days of humidity and scattered
storms. This trend will continue from Sunday into early next week
as a large surface high pressure ridge looks to dominate the
region for several days approaching the end of the month.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1233 PM CDT Sun May 20 2018

Widespread MVFR to IFR cigs are expected to continue across Iowa
through the forecast period.  Isolated showers will impact TAF
locations mainly tonight with scattered showers and thunderstorms
at/near KOTM.  Winds will continua to be northeast AOB 10kts.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...Lee
AVIATION...FAB



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