Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181735

Area Forecast Discussion
National Weather Service Des Moines IA
1235 PM CDT WED MAY 18 2016

Issued at 1235 PM CDT Wed May 18 2016

Have updated the cloud cover with the CU field slightly
more extensive than previously thought. Have sprinkles in
for this afternoon as well with some weak instability present
on the soundings. Also touched up temps and dewpoints with
drier air mixing down across the north.


.SHORT TERM.../Today/
Issued at 306 AM CDT Wed May 18 2016

Main challenge will be early morning fog. Already have an SPS
this morning for the south. A couple of HIRES models leaning
toward more widespread dense fog development from
possible that an FG.Y may be needed closer to sunrise. For now
will hold off and monitor. Interesting mesoscale set-up early
today over the state. Very weak mesolow near Carroll and rather
weak boundary cuts across south central Iowa between US30 and I80.
South of the boundary dew points in the lower to mid 40s while
north lower to mid 30s. By 14- 15z the fog will break up...similar
to Tuesday with a generally nice recovery for temperatures during
the afternoon hours. Some minor concern regarding moisture over
the south and west being recycled as stratocumulus over the
west/south during the afternoon. Also a hint by the Euro and
ARW/NMM that there may be some additional afternoon showers west
of I35. Though confidence is not high...this would be near the
center of the weak low and developing return southeast flow aloft
where lift may be enhanced by the weak circulation and a
developing warm front across the Northern Plains. For now will add
14 PoP in area that may see the better opportunity for a few
showers between 18-23z this afternoon. This includes the metro and
areas west/northwest toward Sac County. With partly cloudy skies
by afternoon...highs today will be similar to a few
cases a degree or two warmer. Winds will once again by rather
light from the east southeast at under 10 mph.

.LONG TERM.../Tonight through Tuesday/
Issued at 306 AM CDT Wed May 18 2016

Main items of note during the long term period include
pcpn chances on Friday...pcpn potential on Saturday and then
active/severe weather potential next Monday through Wednesday.

Long term period will begin with relatively quiet wx this
evening. Surface pressure ridge...oriented from SW to NE...will
remain in place overnight allowing for another night of seasonably
cool temperatures...albeit warmer than those of this morning. 850
mb temps will be around +5C overnight...with coolest temperatures
toward the E.

Surface pressure ridge will retreat eastward during the day on
Thursday as WAA and height rises begin ahead of ejecting southern
stream CONUS shortwave. Pcpn chances with this feature are
relatively low on Friday as it slides across the CWA...but have
maintained slight chance pops with the best chances across the
southern third of the CWA. Although instability is just enough to
suggest potential for thunder...uncertainty of its potential along
with desire to maintain forecast cycle-to-cycle consistency has
led to keeping it out of the forecast at this time. Have sub-
slight chance pops into the northern CWA which is higher than the
previous forecast. Cannot totally discount pcpn chances across
much of the CWA given shortwave energy in the area combined with
elevated moisture.

Upper-level omega block then sets up across the CONUS...with the
CWA in the vicinity of the ridge...for the weekend as the
shortwave departs the region late Friday. Have kept the forecast
dry for the weekend...however guidance has increased pops slightly
from Friday night into Saturday. Makes sense given shortwave
energy progged to drop through the NW flow on the E side of the
upper-level ridge. GFS is most bullish with this feature and kicks
out around 1/4 inch QPF late Friday night into Saturday morning.
Moisture parameters seem high though given the setup. Have not
strayed far from the blend overall model consensus
keeps the CWA dry...and thus kept POPs below slight chance though.
At the very least...expect some increased cloud cover Friday night
into Saturday in conjunction with the shortwave.

Attention then turns to active weather potential for next week
beginning on CWA falls under the influence of an
upper-level SW flow regime...along with WAA. Western CONUS trof
kicks out a shortwave that rides NE along the NW side of the
upper-level ridge on Monday into Monday night. Surface reflection
will be a cold front that slides across Nebraska...reaching around
the Missouri River by Monday evening...placing most of the CWA in
the warm sector. Instability definitely supports thunder...and
0-6km bulk shear support marginal svr wx threat. Thunder/pcpn
chances continue Monday night with the front in the vicinity of
the CWA.

Relatively potent shortwave progged to ride NE through the SW
upper-level flow on Tuesday will bring an increased opportunity
for TSRA. It will help to kick the northern portion of the surface
front eastward...from IA northward...while the southern portion
remains quasi-stationary. Upper-level forcing then becomes more
nondescript for Wednesday...but models prog the quasi-
stationary/warm surface front to be in the vicinity of the CWA.
Potential will exist for additional strong to severe thunderstorms
on Wednesday given the amount of instability as well as shear


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1235 PM CDT Wed May 18 2016

CU field across the CWA this afternoon will diminish with
sunset, and should reform during the day Thursday especially
across the north. Lighter winds still, mainly out of the southeast
to south through the period. Might see a sprinkle/light shower
across the KFOD and KDSM sites this afternoon.


.DMX Watches/Warnings/Advisories...None.



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