FXUS63 KDMX 161100

Area Forecast Discussion
National Weather Service Des Moines IA
600 AM CDT Fri Apr 16 2021

...Updated for 12z Aviation Discussion...

.DISCUSSION.../Today through Thursday/
Issued at 308 AM CDT Fri Apr 16 2021

Forecast Highlights:
-- Outside of a somewhat seasonable Sunday, below normal
   temperatures will persist through at least the next 7 days
-- Light rain over southern Iowa this afternoon into early Saturday
-- Turning cooler, breezy, with precipitation on Monday

Details: GOES-East upper level water vapor imagery shows the swirl
of clouds over Wyoming associated with the upper low, which is
advancing towards the region today. Higher level clouds associated
with this system have already spread over much of Iowa per
Nighttime Microphysics while low level clouds remain beneath in
some parts of the state. These clouds will limit frost formation
this morning as temperatures probably will not be able to fall to
the dewpoint. Ahead of the upper lows arrival, there will be
modest QG convergence that will spread over southern Iowa later
today into early Saturday that will help to foster light rain.
However, as has been mentioned the past few forecast cycles, the
rain`s arrival will be slowed down by dry air in the mid-levels.
This showed up in tonight`s 00z KOAX sounding between 850mb and
650mb. Winds in this layer are forecast to become from the east or
northeast and will advect drier air in through the day. High
resolution soundings show saturation even in our southwest
forecast area not being achieved until later this afternoon or
early this evening. With high confidence in delayed rainfall, have
pushed PoPs back to after 18z across the southwest and toward 6z
across our southeast. Over the past 24 hours, there may be a
slight trend northward and have PoPs to around the Highway 30
corridor. The rain will exit much of the area Saturday morning
with breezy winds from the north-northwest prevailing. A shortwave
will dive in behind this upper low on Sunday and may foster some
showers, but at this point they look to stay to the south or east
of our area. Otherwise, Sunday is looking to be a seasonable day
with highs near or a few degrees shy of 60.

Attention then turns to the north and an approaching trough and
surface cold front that will bring breezy winds, precipitation and
cooler conditions, at least compared to Sunday. The wind shift will
precede the precipitation as the front slopes back to north with
height. Cross sections show this quite well with cooler air being
funneled in at the low levels, mainly below 750mb. An above freezing
layer will be at most 1000 to perhaps 2000 feet deep with the rest
of the column falling below freezing as cold air advection ensues.
The low levels will initially be dry, but as top down saturation
occurs, precipitation will begin. Initially, precipitation type
looks to be rain given the above freezing temperatures in the lowest
levels and some time to cool the air aloft. The dry air at the
surface will allow for wet bulbing, which looks to have the biggest
impact on precipitation type over northern Iowa Monday morning. With
moderate lift in the dendritic growth zone, a changeover to a short
period of snow is possible. Overall, the precipitation window will
not be long at a given location and generally 6 or so hours as
drier air following closely behind that will dry the column.
Initial PoPs from National Blend of Models has improved from its
dry solution the past few nights, but still upped values given
the forcing expected with the boundary. With soil temperatures
well above freezing in the middle to upper 40s based on ISU soil
moisture network observations, accumulations, if any, will be
short-lived. Brisk winds from the northwest will gust between 25
and 30 mph as well.

Northwesterly flow will continue over the region as the trough drops
to the southeast through midweek. Temperatures were trended cooler
on Tuesday by a few degrees from initial guidance. GFS soundings
continue to indicated steep lapse rates and with the cold air aloft,
except at least some cloud cover. The CMC and the GFS and about half
of its ensemble members continue to advertise scattered showers on
Tuesday, but the CMC ensembles and the deterministic and most of the
ECMWF ensembles are dry. As high pressure passes through the
region on Thursday, return flow and a shortwave trough may bring a
return of showers toward late next week.


.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 600 AM CDT Fri Apr 16 2021

The primary challenge in this forecast is ceilings, which will be
either side of MVFR restrictions throughout the period. Otherwise,
winds will be light from a northerly direction. There will be a
chance of light rain that is expected to reach DSM and OTM this
evening before ending Saturday morning.





NWS DMX Office Area Forecast Discussion