Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 151139
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
539 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 12Z TAFS through 12Z Friday morning/
Issued at 539 AM CST Thu Feb 15 2018

A cold front will approach from the northwest today, but ahead of
it winds will remain light and low-level moisture will remain high
due to continued snow melt. Areas of fog early this morning will
slowly improve after sunrise but persist through much of the day,
especially at DSM/OTM. For the most part visibility will be MVFR,
but IFR or lower conditions are likely at times at DSM/OTM in the
next several hours and have handled this with TEMPO groups. When
the cold front comes through this evening/tonight increasing north
northwest winds will scour out any remaining fog, but may also
bring MVFR ceilings into tonight. This will be reassessed
throughout the day for subsequent TAF issuances.

&&

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

&&

$$

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



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