Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 151744
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1142 AM CST Thu Feb 15 2018

.SHORT TERM.../Today through Tonight/
Issued at 350 AM CST Thu Feb 15 2018

Iowa resides within a region of very weak surface flow early this
morning, and near-surface humidity is very high due to the snow
melting that occurred yesterday. This is causing widespread
development of fog and low stratus clouds across the region.
However, satellite imagery depicts thick cirrus clouds streaming
overhead and this has so far limited radiative cooling, with
temperatures remaining in the 30s over most of our forecast area.
Visibilities have fallen below 6 miles pretty much everywhere, but
visibilities under a mile have been very isolated and infrequent
thus far. The cirrus clouds are expected to hold through sunrise, so
dense fog potential should continue to be mitigated and an advisory
is not envisioned at this time.

After sunrise visibility will gradually improve as temperatures warm
and humidity decreases, but some light mist may linger through much
of the day as snow continues to melt. Meanwhile a cool front will
approach from the northwest, reaching Estherville by around noon or
shortly thereafter and surging across the forecast area this
afternoon and evening. The front will be accompanied by a swath of
thicker clouds and there is some potential for light precipitation
to be generated within the region of cold air advection just behind
the frontal boundary. However, mid-level forcing associated with the
system is fairly modest and largely shunted off to our north and
northeast. In addition, forecast soundings indicate layers of dry
air limiting precipitation potential. If precipitation does occur,
it would likely be in the form of light snow across our northern
areas, and possibly flurries further south. Have maintained low POPs
to account for this, but impacts should be negligible. It should be
noted that, given the anticipated layers of dry air in the column,
it is possible we could see brief periods of freezing drizzle if ice
crystal introduction shuts down at times, but confidence in this is
too low to include in the outgoing forecast at this time. It is also
possible that roads that become wet due to snow melt may freeze as
temperatures plummet behind the front this evening and tonight. This
will bear watching through the day.

Cool high pressure will move quickly in from the west and northwest
overnight, blowing out any light precipitation and sending
temperatures falling quickly. Depending on the relative timing of
decreasing winds and falling temperatures, we could see wind chills
approaching 20 below zero in our northwest by sunrise Friday, but
given the uncertainty in this scenario and the marginal nature of
forecast values, no advisory is planned at this time.

.LONG TERM.../Friday through Wednesday/
Issued at 350 AM CST Thu Feb 15 2018

Colder temperatures will be in place on Friday in the wake of the
cold front that will pass through the state tonight. The day will
begin breezy though the wind will gradual diminish through the day
as high pressure arrives. Very dry air will also be in place as
evidenced by PWAT values around 0.05 inches, therefore, an abundance
of sunshine is expected. Return flow will arrive Friday night as the
area of high pressure moves off to the east with warmer conditions
arriving for the weekend. There will be a chance for light snow
Saturday morning as the theta-e advection surge moves across the
area, followed by afternoon temperatures in the 30s and 40s.

By Sunday, a large sub-tropical high pressure system will be moving
west towards the far southeast CONUS while a upper trough carves out
across the northern Rockies. This will amplify the southwest flow
across Iowa late Sunday into Monday and should lead to a boundary
stalled across the state. Dew points in the 40s to even low 50s are
projected to reach south central and southeast Iowa by late Sunday
night and into Monday. Have increased Sunday night lows but likely
still may need to increase several degrees. Much of the
precipitation type will be in the form of rain or drizzle on the
warm side of the boundary. The cold side of the boundary could pose
problems. The setup is looking more favorable for freezing rain with
cold air undercutting the warmer air aloft which could result in
significant ice accumulations if this sets up just right. Will need
to continue to monitor this situation. Monday could have
temperatures near 60 with a few thunderstorms southeast and readings
in the 20s to low 30s with freezing rain or snow northwest. The
boundary placement will be critical. Currently do not have freezing
rain in the forecast but may need to be ramping up potential soon.

The boundary should continue to settle southeast Monday night and
Tuesday. Both the ECMWF and GFS suggest icing potential with highs
in the teens and 20s Tuesday and only slightly warmer Wednesday.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1143 AM CST Thu Feb 15 2018

Patchy fog or haze will cause localized reduction in vsbys to MVFR
conditions but overall VFR conditions expected through 21Z.  There
will be a period of low cigs as a surface low scoots east with
colder air pushing in behind it.  Stronger winds will be associated
with the push of cold air as speeds increase to 17G26kts.  Clearing
expected after 09Z along with diminishing wind.

&&

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

&&

$$

SHORT TERM...Lee LONG TERM...Donavon AVIATION...FAB



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