Area Forecast Discussion
Issued by NWS Des Moines, IA

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711
FXUS63 KDMX 301142
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
642 AM CDT Sun Apr 30 2017

.SHORT TERM.../Today through Tonight/
Issued at 322 AM CDT Sun Apr 30 2017

Cool and wet weather continues with more rounds of rainfall expected
today. Currently drizzle/light rain is expanding across about the
northwest half of Iowa, while the southeast is dry. However, more
showers and thunderstorms remain to our south across Kansas and
Missouri. The surface low, currently over the southern
Kansas/Missouri border, will gradually occlude and become stacked
with the mid/upper level low today before crossing Iowa tonight.
During the day we will see another south/southwest to
north/northeast moving swath of rain cross our forecast area,
similar to the last two days, followed by a relative dry slot late
today and this evening, again very similar to the last two days. One
difference today is that there will be some elevated instability,
especially across our south and east, and have maintained isolated
thunderstorms in the forecast to reflect this. Short-term
precipitation trends will be tricky and we will continue to update
the near-term forecast throughout the day. The high temperature
forecast in our south and east is also problematic, as it remains to
be seen to what extent warmer air within the so-called dry slot may
work its way into those areas and to what extent it will stay south
of the border. In any event, as the stacked low moves across Iowa
late tonight into Monday morning it will bring cold rain, very low
clouds, and pull even cooler air into northwest Iowa. This will
likely result in some snow mixing with the rain right around sunrise
Monday in our northwestern counties. However, little to no
accumulation is anticipated.

.LONG TERM.../Monday through Saturday/
Issued at 322 AM CDT Sun Apr 30 2017

Monday through Tuesday...Confidence: Medium to High

Main concerns over the next several days will be temps...precip type
and winds.  Extraordinary system to our south at this time...not so
much due to extent of severe weather but for extent of rainfall and
flooding. 00z H850 subjective analysis tonight shows subtropical
High off the Southeast Atlantic Coast with another blocking ridge
extending from Lake Superior back west to Northern California.  Only
a modest teleconnection with a trough exiting New Foundland suggests
that the system will eventually have a slow progression north Sunday
night into Monday.  Meanwhile...H850 moisture is more typical of
June/July with 15 to 20C dewpoints streaming into the Arklatex and
also across Missouri.  Tremendous lift is resulting in extreme
rainfall there. For us...a baggy area of low pressure will consolidate
after occlusion by gradually lifting north tonight into north
Central Iowa to West Central Wisconsin. This will keep the west in
a developing deformation axis Monday with H850 temperatures of 0C
to -4C at 12z with the cold air advection spreading east southeast
with time. By mid to late afternoon H850 temperatures from 0C
northeast to -4C northwest. Early in the morning from 12 to 16z
there may be a period of -RASN over the west and north...though as
we move through the morning hours the boundary layer may warm
enough to alleviate snow chances as 1000|500mb thickness remains
above 540dm for the balance of the morning/afternoon hours.
Despite that...there will be sufficient mixing and rather strong
wind gusts/surface winds Monday from the northwest. It is possible
that there may be a few flakes of snow from time to time mixed
down through the column with gusts of wind though chances should
remain slim. The winds will remain brisk...ceilings low...and
showery precipitation will keep highs in the lower 40s over the
far north to the upper 40s/near 50 in the south. Typical weather
for Kodiak Island in the Gulf of Alaska but not so much here.
Monday highs will be nearly 20 degrees below normal. Winds will
reach 15 to 25 mph with gusts near 40 mph at times. By late day
the precipitation will be pulling out of the southwest with
lingering light rain northeast...and there may be a little sun
over the west areas toward dusk. This will set the stage for a
cold night with lows in the mid 30s north and upper 30s in the
south. However...winds will remain brisk in the evening then
steady overnight keeping frost out of the picture Tuesday morning.
Tuesday will be more pleasant with some clouds northeast...
sunshine elsewhere and highs back in the 50s to mid 60s.

Tuesday Night through Saturday...Confidence:  Medium

By Tuesday night into Wednesday a digging H500 trough will swing
southeast into the Central Plains.  With associated height falls and
increased forcing through the column clouds and light rain will be
possible.  Another wave will pass over the area Thursday.  Soundings
suggest some mid level instability...with showers likely by afternoon.
Models also push out some limited CAPE due to increasing lapse
rates with cold air aloft. This might promote some iso thunder but
given some uncertainty on coverage and timing...will leave any
mention out of products for now. Temperatures Wednesday and Thursday
will be on the mend with low to mid 60s for highs each day. The
best is yet to come...with a developing ridge at H500 for Friday
and into the weekend. Though Friday may be still be transition
day...the Euro is now slowing the warming aloft as yet another
weak wave drops south into the western Great Lakes...the GFS is
advancing the warmer air east at a faster clip. By Saturday there
is greater consensus for much warmer temperatures and this trend
holds through the weekend. Both Saturday and Sunday especially...
should be well into the 70s.

&&

.AVIATION.../For the 12Z TAFS through 12Z Monday morning/
Issued at 642 AM CDT Sun Apr 30 2017

IFR or lower conditions will prevail for much of the next 24
hours and beyond, with some only spotty breaks of MVFR conditions.
Rain showers will overspread the area again late this morning and
afternoon, with thunderstorms possible at OTM/ALO late in the day
but at too low a probability to include in the TAFs at this time.
Tonight a more uniform light rain will settle in, with ceilings
and visibilities dropping even further and LIFR conditions
becoming more likely after dark. No appreciable improvement is
then foreseen until at least Monday afternoon and perhaps beyond.

&&

.HYDROLOGY...
Issued at 322 AM CDT Sun Apr 30 2017

Though many streams and rivers will see some rise over the next 1
to 3 days due to the additional rainfall of yesterday and today...
forecasts now suggest that most if not all will remain below
action stage in our area. We will continue to monitor rainfall in
the next 24 hours...and update with any changes. The northern
portion of the forecast area may still see rainfall in excess of
1.5 inches through 18z Monday which may result in a reevaluation
of some main stem rivers and smaller tribs at that time. For
now...few if any impacts are anticipated.

&&

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

&&

$$

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



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