Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180506

Area Forecast Discussion
National Weather Service Des Moines IA
1206 AM CDT Sun Mar 18 2018

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Sunday/
Issued at 309 PM CDT Sat Mar 17 2018

The primary concern through the period will be fog and stratus
trends into Sunday. The compact system that brought localized heavy
snow to central Iowa earlier today continues to shear and move away
leaving behind little mid level lift or moisture for precipitation.
However the low level flow is quite weak this afternoon and will
remain so into tomorrow as surface high pressure drifts through the
MO Valley into the Upper OH Valley by midday tomorrow.

Low level moisture and stratus remains trapped in the weak flow over
much of central and southern Iowa at 19z with little change
anticipated into the night. This will certainly suggest plenty of
stratus with fog possibly also developing, especially near areas of
deeper snow pack. Widespread moist soils due to recent rains will
also aid development. Radiational cooling will not contribute much
but with temp/dew point depressions already <5F it may not take much
for fog, which is also suggested by several high res models and the
SREF to some degree.

Fog should gradually dissipate later in the morning but only weak
return flow, and from a southeasterly direction, should keep plenty
of clouds and stratus around to end the weekend.

.LONG TERM.../Sunday night through Saturday/
Issued at 309 PM CDT Sat Mar 17 2018

From Sunday night into Wednesday the forecast is somewhat active
but relatively low impact. From Sunday night through Monday a deep
closed low pressure system will move eastward from the Oklahoma
panhandle, across the Ozarks, into the lower Ohio River valley. A
shield of precipitation will develop on the northern half of the
low, but with weak surface ridging and northeasterly flow across
Iowa, forecast soundings indicate a lot of dry air entrainment and
there should be a sharp cutoff to the northern edge of the
precipitation somewhere across central or northern Missouri. Have
removed POPs from our forecast area on Monday as a result and
taken them south of the border. The result will be dry, but cloudy
and somewhat cool weather on Monday with highs in the 40s to
perhaps lower 50s.

Between Monday night and Wednesday two weak 500 mb shortwaves
will move overhead, providing some broad forcing but without any
focusing features and still with a relative lack of moisture
source. The first shortwave will still likely generate some light
precipitation across about the northwest half of our area late
Monday night into Tuesday before fizzling out. Forecast soundings
and temperatures indicate this precipitation will come primarily
in the form of snow Monday night into Tuesday morning, changing to
rain during the day on Tuesday before ending. However, there is
some signal for a loss of ice introduction at times, and transient
pockets of freezing drizzle may be possible on Tuesday morning.
Confidence in this is low, and only rain/snow are included in the
forecast, with snow amounts around 1-1.5 inches roughly northwest
of a line from Audubon to Mason City. This should have minimal
impact as temperatures/roads warm above freezing after sunrise,
but may lead to slick conditions in some areas of northern and
western Iowa for the Tuesday morning commute. The second shortwave
moving over on Tuesday night/Wednesday morning is more nebulous
and has even less moisture to work with, precluding any
precipitation mention at this time.

From Wednesday afternoon through Thursday a thermal ridge will
move over the region, with the establishment of modest southeast
surface flow. This will allow for dry weather and only slow
moderation of temperatures during this time, with highs peaking
generally in the mid-40s to lower 50s on Thursday afternoon. By
Thursday night or Friday the large ridge will begin to break down,
as a large western U.S. trough ejects out several vorticity maxima
across the Midwest. Long range models show divergent solutions
with regards to the details of this evolution, resulting in
significant differences in timing of POPs, temperature
fluctuations, etc. The most likely time for precipitation in our
forecast area seems to be from Friday into Saturday, primarily in
the form of rain but perhaps with some wintry mix across northern
Iowa Friday night into Saturday morning. A generally active
steering pattern then looks to continue into the latter half of
the weekend and early the next week. We will be refining the
forecast for this period as the week progresses.


.AVIATION.../For the 06Z TAFS through 06Z Sunday night/
Issued at 1203 AM CDT Sun Mar 18 2018

Messy TAF forecast this morning. KOTM periods of LIFR expected to
generally stay in IFR. KALO tricky site as they are right on the
edge of MVFR versus VFR for this afternoon. KMCW also on edge of
IFR versus MVFR near sunrise. May need to adjust closer to

MVFR trending to linger longer into the overnight tonight, so have
mainly kept sites in MVFR to account for this. High confidence in
winds less than 10 kts throughout TAF period.




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