Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 191828
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
128 PM CDT Sat May 19 2018

.SHORT TERM.../Today through Tonight/
Issued at 411 AM CDT Sat May 19 2018

The western upper level short wave is finally making progress east
this morning as the blocking system to the east is beginning to
depart towards the central Great Lakes. In response, a broad region
of showers and thunderstorms are currently ongoing across a large
portion of Nebraska and Kansas early this morning. This activity is
beginning to move into southwest Iowa as the low level jet is
shifting east and theta-e advection is spreading north into the
state. Still have some uncertainty on potential for a few showers
and thunderstorms to move into central Iowa today as it moves into a
more stable environment. A few of the CAM solutions suggest possibly
some potential along a sagging cold front into central Iowa this
afternoon but do not expect anything widespread.

Much more confidence in thunderstorm potential late this afternoon
and into this evening over central and southern Iowa as the short
wave energy arrives. SPC still has a slight risk over the southern
two tiers of counties and an enhanced risk just touching the far
southwest. Any surface based activity will be in the vicinity of the
primary warm front which continues to look to remain just south of
the state. That said, another area of instability may develop near
the sagging cold front into central Iowa though will need some sun
today to aid destabilization. Any tornado threat would be near the
Missouri border or south while the damaging wind and large hail
threat would extend farther north. The potential for locally heavy
rainfall remains into this evening however the convective complex
should be progressive as the steering flow increases to 30 to 40
kts. Drier mid-level air will arrive behind the complex and should
bring and end to the precipitation for a while overnight before
chances increase again towards morning.

Temperatures today will be cooler with the cold front entering from
the north and potential for clouds to linger through much of the
day. Highs will be in the 60s north to 70s south.

.LONG TERM.../Sunday through Friday/
Issued at 411 AM CDT Sat May 19 2018

The weather will be fairly quiet from Sunday through Tuesday with
weak, generally zonal steering flow aloft. On Sunday and Sunday
night the lingering effects of a departing trough will help clouds
and precipitation persist then slowly dissipate, resulting in
cool and showery weather. In fact high temperatures on Sunday
will only be in the upper 50s to mid 60s across most of Iowa.
Sufficient instability for thunderstorms will be found across
about the southeast half of the forecast area, mainly during the
day Sunday, but will be limited enough and with weak enough
forcing that severe weather is not anticipated. On Monday and
Monday night a modest surface ridge will cross the region,
bringing a brief dry spell and a nice, comfortable day with highs
in the upper 60s and lower 70s on Monday afternoon. The trough
will then move off to the east, but its influence should keep
things generally dry on Tuesday with perhaps just a few showers or
benign storms in western Iowa in the morning.

By Tuesday the 500 mb pattern will be characterized by a broad
trough of low pressure over the western U.S., pinching off ahead
of it a modest ridge extending from the High Plains up into the
northern Rockies. Subsequently the broad, relatively weak trough
will move northeastward up over the northern Rockies, with the
ridge in advance of it, then later move southeastward over the
Midwest and near Iowa at the end of the week. Initially, from
Tuesday night into Thursday, the modest 500 mb ridge will move
over our region but multiple, disorganized waves of vorticity
ejecting from the broad western trough will move through it,
causing atmospheric destabilization over Iowa. This results in
several days of POPs being carried in the forecast, as there is no
single feature to be discerned as bringing a greater precipitation
threat, but the pattern and broad instability generally support
multiple rounds of thunderstorm chances through this period.
Climatologically, this pattern would support the highest storm
chances coming in the late afternoon/evening each day coincident
with heating, and overnight each night in conjunction with the
nocturnal low-level jet. Severe thunderstorm chances are somewhat
nebulous during this time, there is a non-zero threat but it does
not appear anything high-end or particularly organized is likely.

Between Thursday night and Saturday, as alluded to earlier, the
approaching 500 mb trough will move southeastward across the
Dakotas, then over Wisconsin and Iowa and into the Great lakes by
the latter half of next weekend. Between Thursday night and Friday
evening, southwesterly 500 mb flow ahead of the approaching trough
will support continued destabilization and one or more additional
rounds of thunderstorms. It appears the trough of the axis, with
an attendant surface cool front, will cross our forecast area
around Friday night or Saturday morning and finally push the
unstable airmass out of our area for a while, bringing cooler and
drier weather from late Saturday into the following Sunday.
Whereas daily high temperatures from Tuesday through Friday will
generally range in the upper 70s to mid 80s, next weekend behind
the front it appears we will be in store for highs more in the
range of mid to upper 70s, though at this range there is still
considerable uncertainty.

To summarize: Cool and showery Sunday into Monday, then a brief
dry and quiet period into Tuesday, then warm, humid and
intermittently stormy for several days before becoming cooler and
quiet again next weekend.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 128 PM CDT Sat May 19 2018

Small cluster of storms will affect KDSM early this afternoon and
likely affect other sites to the north in some form later in the
afternoon. Will try to highlight precip and thunder trends in the
short term, but default to VCSH wording later as confidence
decreases. A frontal boundary is also bisceting Iowa SW-NE at 18z.
As this front sags south and east, confidence does increase that
MVFR to IFR ceilings will develop into Sunday morning.

&&

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

$$

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



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