Area Forecast Discussion
Issued by NWS Des Moines, IA

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104
FXUS63 KDMX 211751
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1251 PM CDT Mon May 21 2018

.SHORT TERM.../Today through Tonight/
Issued at 356 AM CDT Mon May 21 2018

Upper level PV anomaly is moving across Iowa early this morning
and will be followed by a second lobe later this afternoon which
will move across far northern Iowa. A broad area of precipitation
is moving across the state and as of 3 am, was largely confined to
east of Interstate 35. Precipitation amounts up to one and one
half inches of rain have been recorded since last evening, mainly
over the southern portion of the state and should have a
widespread one quarter to one half inch over much of the area
before the precipitation diminishes today. Much of the area should
be mostly dry by mid to late morning. The exception will be to
the far north where additional showers may develop as the next
segment of energy arrives.

A few areas of fog over the west is still possibly this morning
and still may have a few areas with visibilities dropping to one
quarter mile. The fog then should diminish by mid morning.
Temperatures today will be limited by widespread stratus. The
stratus will begin to erode over the south during the afternoon
and this will help temperatures rise into the low to mid 70s over
that portion of the state. Temperatures over the north will be in
the low to mid 60s. Possible a few locations remain stuck in the
upper 50s with a dense cloud shield in place. No significant
weather is expected overnight though theta-e advection will be
arriving after 09z and could result in isolated showers and
thunderstorms prior to 12z Tuesday over the west.

.LONG TERM.../Tuesday through Sunday/
Issued at 356 AM CDT Mon May 21 2018

Not much change in overall thinking for the long term forecast
period tonight. From Tuesday through Thursday a weak 500 mb ridge
will remain parked overhead, while at the surface light easterly
flow will slowly turn through southeast to south by Thursday. The
result will be a slow warming trend and an increase in dewpoints
to levels not yet seen this season. By Wednesday and Thursday
afternoons high temperatures will range in the mid 80s with
dewpoints in the mid to upper 60s, giving us our first taste of
summer-like weather. Meanwhile, subtle shortwave impulses
overtopping the 500 mb ridge will provide opportunities for
convection at times, mainly during the overnight/morning hours
coincident with the influence of the nocturnal low-level jet. The
first of these events will come across northern Iowa late Tuesday
night into Wednesday morning, with a consistent signal for
precipitation for the past several days in a favored synoptic
pattern. Have increased POPs to categorical in our northern
counties late Tuesday night, and increased well into the likely
range on Wednesday morning, to account for this. There is also
some potential for heavy rain late Tuesday night up in our
northwest corner, as we will have high PWATs for the season and
weak steering flow. However, in that area it would take a lot to
get flash flooding so will not message that hazard at this time. A
second round on Wednesday night/Thursday morning may end up more
across Minnesota/Wisconsin, so will carry more modest chance POPs
during that time.

From Thursday night through Friday night we will see some change
in the pattern as a fairly weak trough approaches from the
northwest. Ahead of this trough broader, though still modest,
forcing will overspread the region and several model runs over the
past couple days have appeared to depict a nocturnal MCS-type
system somewhere over the upper Midwest late Thursday night/Friday
morning. While shear/flow fields are somewhat unimpressive, the
degree of instability expected supports some low severe weather
threat, however, there remains much uncertainty not only in the
overall evolution of this scenario but also in the location of
such an MCS even if one were to develop. The trough from the
northwest should finally pass through our forecast area on Friday
night or early Saturday morning, largely sweeping things out and
bringing an end to POPs for quite some time.

There may be a few lingering showers/storms in our east on
Saturday, but otherwise the period from this weekend through the
early part of next week appears dry and quiet. Temperatures will
moderate just a bit but highs will still range in the upper 70s to
mid 80s during that time. Dewpoints will be a bit lower and
overall very pleasant weather conditions are expected during
these outer periods. Enjoy it while it lasts!

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1251 PM CDT Mon May 21 2018

Widespread IFR ceilings are in place across central IA with some
MVFR to LIFR conditions as well. Patchy drizzle also lingered
north and east. These conditions will gradually improve to MVFR
or VFR by the late afternoon or evening hours but revert back to
IFR or less later tonight as radiation fog develops in light and
variable wind. Diurnal improvement to VFR is anticipated into mid
morning.

&&

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

&&

$$

SHORT TERM...Donavon
LONG TERM...Lee
AVIATION...Small



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