Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 251149
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
649 AM CDT Thu May 25 2017

.SHORT TERM.../Today through Tonight/
Issued at 309 AM CDT Thu May 25 2017

Patchy fog is developing across parts of northeast Iowa early this
morning and will likely continue, or expand, through sunrise before
quickly dissipating. Otherwise the morning will be dry and mostly
sunny as a deep layer ridge passes overhead. Later today and tonight
a shortwave impulse ejecting out of a northwestern U.S. trough will
begin to erode and impinge upon the ridge, resulting in increasing
moisture and an opportunity for isolated convection. This will
likely begin in our northwest counties late today but be very
isolated, then gradually increase in coverage overnight, especially
after midnight, as a second impulse follows quickly on the heels of
the first one and has even more of an effect. Various forecast
models are having a difficult time resolving this scenario with
widely divergent depictions of timing and coverage of precipitation,
ranging from completely dry (NAM, SPC WRF) to isolated/widely
scattered showers (EC, GFS), to widespread showers and thunderstorms
(HRRR, RAP). However, given the nebulous forcing expected with the
impulses and the time needed to erode the preceding ridge, have
maintained only slight chance POPs, with virtually no mention of
thunderstorms.

.LONG TERM.../Friday through Wednesday/
Issued at 309 AM CDT Thu May 25 2017

Friday through Sunday Night: Confidence Low to Medium

The period of least confidence in the extended will the holiday
weekend. There continue to be evident differences between the GFS
and Euro model for Saturday...though Friday/Sunday are pretty
much in agreement with regard to precipitation chances. In the
near term Friday will see a weak front pass over the state with a
chance for some showers/isolated thunder as the trough swings
through the region. For Saturday the GFS is trending toward the
Euro while the Euro continues to downplay precipitation chances
overall...it still shows a chance of some light showers/isolated
thunder in the afternoon and early evening hours as the boundary
and upper level support in the form of a H500 wave track east of
the region in the evening hours. By Sunday there is some consensus
between the GFS/Euro in despite another trough approaching from
the northwest... there appears to be less chance for any
widespread shower coverage during the day. Temperatures Friday
will be mild and in the mid to upper 70s. By Saturday into Sunday
a trend toward only slightly cooler H850 temperatures will lower
highs only a couple of degrees at worst but still mild. Sunday
night will see a more pronounced push of cooler air beginning to
build south into Minnesota/Iowa. The GFS/Euro are still at odds
with regard to coverage of precipitation during the overnight
hours...with the Euro mainly focused on diurnal heating trends and
the GFS perhaps...overdoing the coverage after sunset.

Monday through Wednesday:  Confidence Medium

Monday the upper level low begins to swing south/southeast with
increasing upper level support over the area.  While the Euro swings
another upper level trough across the region...the GFS is slightly
delayed in the timing of the trough. The pattern continues through
Wednesday with several additional upper level waves dropping south
on the back side of the parent H500 upper level trough over Ontario.
This will likely keep some clouds and chances for light showers over
the east Tuesday...but confidence on coverage/timing is rather low.
Temperatures continue to lower with each upper level wave and a new
surge of Canadian air dropping south. This remains the bright spot
in the models...as both the GFS and Euro agree that temperatures
will most likely remain pleasant and slightly below normal for late
May into early June.

&&

.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 649 AM CDT Thu May 25 2017

Patchy fog will produce local IFR conditions early this morning,
but burn off quickly with VFR conditions after 13 or 14Z. Isolated
showers may affect the area late today through tonight, but
probability and expected coverage are too low for even VCSH at
this point. Low clouds will begin to move back in toward the end
of the TAF period, but it appears that any associated MVFR
ceilings will not occur until after 12Z Friday.

&&

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

&&

$$

SHORT TERM...Lee
LONG TERM...REV
AVIATION...Lee


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