Scientific Forecaster Discussion
fxus63 kdmx 171140
Area forecast discussion
National Weather Service Des Moines Iowa
540 am CST Fri Nov 17 2017
Short term.../today through tonight/
issued at 350 am CST Fri Nov 17 2017
Will see surge of low level moisture this morning with strong
southerly flow across the state. This surge will be accompanied by
widespread low clouds and eventually light precipitation.
Soundings indicate saturation in a near surface layer deepening to
around 4- 6kft by midday. This will be sufficient for drizzle
production and even some light rain, especially in the east where
the moisture will be deeper. Given the push of higher dewpoints
across relatively cooler ground, may also see the development of
some fog although likely not dense at this time. Despite the
extensive cloud cover, temperatures will be much warmer today with
highs 10 to 15 degrees warmer than Thursday.
A surface front will pass through central Iowa this evening with
colder air spreading quickly into the state overnight. There may
be a brief period of a bit denser fog prior to frontal passage
this evening but any of this should quickly lift with passage of
the boundary. Strong northwest winds are then expected overnight
with strong cold advection. Meanwhile, mid level forcing will
increase substantially with the approach of the upper level
system. The increased lift will saturate the soundings with rain
becoming likely across the southern half or so of the forecast
area after midnight. The cooling temperatures on the northern edge
of the precipitation will likely be sufficient for a bit of
snowfall before the precipitation ends. However, the bulk of
precipitation will fall as rain with any snow accumulations
remaining very light.
Long term.../Saturday through Thursday/
issued at 350 am CST Fri Nov 17 2017
Rain changing to snow Saturday morning was the main focus for the
extended. Otherwise, temperatures and dry conditions are likely
throughout the Holiday week. Models are in great agreement with
timing of precip exiting the forecast area Saturday morning and
leaned toward a blend of the GFS/NAM/sref. Still some differences
in the temperature and moisture profiles Saturday morning, but the
overall trend is slightly cooler than previously anticipated.
Thus, a slightly better shot of snow further southeast.
Saturday...with the cold air advection blasting into the state tonight,
temperatures look to continue to fall during the morning hours
Saturday. The overlap of the decent forcing within the dendritic
layer and ice introduction is a small window and thus snow
accumulation will be limited. The GFS has shown a fairly dry layer
below 850mb to the surface, but the frontogenetical forcing
between 700-600 mb should be strong enough to allow precipitation
to make it to the surface. Surface temperatures range b/T 34-37f
from 12z to 17z from the southwest to northeast portions of the
forecast area. Have mention of a rain/snow mix mainly due to the
near freezing surface layer. Snow will be falling aloft, it's just
a matter of how much is melted prior to reaching the surface.
There is a low probability of a light dusting from Adair County
northeast through Dallas, portions of Polk to as far northeast as
Black Hawk and Bremer. Highest totals going in the location are a
only few tenths of an inch. With the strong cold air advection bringing in much
drier air, ice introduction is quickly lost before 18z. Winds atop
the mixed layer range from 35 to 40 knots throughout the day and
kept with the gusty winds until 00z Sunday.
Warm air advection looks to provide a pleasant day across the forecast area and
nudged up maximum temperatures a couple degrees. Another cool down
mid-week before temperatures look to rebound on Thanksgiving back
to around normal. Dry weather pattern throughout the Holiday week.
Aviation.../for the 12z tafs through 12z Saturday morning/
issued at 540 am CST Fri Nov 17 2017
Widespread IFR to MVFR conditions are expected for most of the
period. Ceilings will be below 1kft for much of the time with
strong and moist southerly flow across the state today. Areas of
drizzle and light rain will be common with some restrictions to
visibility as well. A cold front will pass through central Iowa
this evening with strong northwest winds expected late tonight.
Areas of rain will also be common after midnight, especially
across the southern half to two-thirds of the area.