Area Forecast Discussion
Issued by NWS Sioux Falls, SD
FXUS63 KFSD 250351
Area Forecast Discussion
National Weather Service Sioux Falls SD
1051 PM CDT Mon Apr 24 2017
.SHORT TERM...(This evening through Tuesday)
Issued at 255 PM CDT Mon Apr 24 2017
The primary forecast concern for this period is the potential for
moderate rainfall on Tuesday.
A few high-based showers are currently moving through southwestern
Minnesota this afternoon. While not a lot of rain, these showers are
temporarily preventing temperatures from rising into the 60s. Once
the showers exit temperatures should again rise to late afternoon or
early evening highs in the mid to upper 60s. Otherwise, a strong
late April front will continue to slowly move to the east. The
front just moved through Huron and Mitchell with temperatures
falling 10 degrees in less than an hour. Behind the front,
temperatures have fluctuated between the upper 40s and lower 50s and
this colder air will continue to move east reaching I-29 around
sunset and into much of northwest Iowa and southwest Minnesota after
midnight. By 12Z Tuesday the front will be just southeast of Storm
Lake. Temperatures over most of the area will be in the upper 30s to
lower 40s with only areas southeast of a Sioux City to Spencer
around 50 degrees.
A very strong upper level wave will approach eastern Nebraska on
late Monday night and Tuesday morning. This will essentially cause
the front to become stationary from east central Nebraska into
north central Iowa. At the same time, a strong 850 mb front will
set up from near Vermillion to Sioux Falls to Marshall Minnesota.
Expect a large area of light rain to develop across Nebraska
after late tonight and spread into the Missouri Valley on Tuesday
morning. This area of rain will spread across most of southeastern
South Dakota, southwestern Minnesota and northwestern Iowa by early
afternoon. Within this area of light rain, a band of moderate
rainfall associated with the frontal circulation is expected to
develop from Vermilion to Worthington and Windom Minnesota. Sioux
City will be on the southern edge of the heaviest rainfall while
Sioux Falls will be on the northern edge. Within this band over a
half inch of rain is possible during the afternoon. Otherwise, most
places east of the James River should receive over a quarter inch
of rain. As with any frontal band, the exact location is still
uncertain and could be as far north and west as Marshall and Yankton.
The other significant weather is the temperatures. After mild
temperatures for the last couple of weeks, much colder air will
accompany the rain. Most areas outside of northwestern Iowa will
not even see 50 degrees. In fact with lows around 40 on Tuesday
morning, if the precipitation moves in fast enough, temperatures may
only rise a couple of degrees during the day. Regardless, high
temperatures will be in the low to mid 40s before the rain begins
for most locations with temperature steady or even falling slightly
during the afternoon. One cannot completely rule out some snow
mixing in around Huron, De Smet or even Mitchell if the NAM 925
temperatures below 0C are correct. At this time, expect that there
will be just enough near surface warm air to keep precipitation all
rain during the day.
.LONG TERM...(Tuesday night through Monday)
Issued at 255 PM CDT Mon Apr 24 2017
Tuesday night will see rainfall gradually diminish from southwest to
northeast as the deeper lift and moisture move northeast. The dry
slot aloft will move into the area in the evening from about 500mb
and above but with the freezing level and colder air below this
precipitation chances will continue after about 3z the dry air will
begin to extend below the ice bearing layer and precipitation
chances will decrease, especially with no threat for convection.
While a few locations may see some wintry precipitation, the better
chance should be north and west of a Chamberlain to De Smet line
where either snow or sleet will be possible. At this time no concern
for any measurable amounts.
Wednesday morning will see a small chance for mixed precipitation
then clouds will very slowly decrease from west to east. A stout
north wind and cloudy conditions will only make highs in the 40s
feel that much cooler. In fact wind chill values could fall to 20 to
25 degrees on Wednesday morning.
Wednesday night into Thursday will see a threat for frost or
freezing as a ridge of high pressure builds south, especially west
of Interstate 29. Lows should be in the mid to upper 20s in central
SD with 30 to 35 in northwest IA.
After a cold start on Thursday morning temperatures will not rebound
too much. While there is a good chance we will scour out the low
level moisture mid and upper level moisture will likely increase so
any sunshine on Thursday could be short lived.
Friday through Monday looking either cool or cool and wet each day.
Generally highs will be 45 to 55 depending upon timing of any
rainfall while lows will be from the lower 30s to lower 40s. The
models are in decent agreement with forcing and moisture through the
period but enough differences pinning down timing for rainfall still
difficult. Overall with a digging trough over the Rockies during
this time there will be multiple weak to moderate strength waves
rotating northeast into the area. Looking like Saturday night into
Sunday is when the main trough swings onto the Plains. The main
message is a good chance for rain at some point in time from Friday
into Sunday night. If we had to wager a best guess for precipitation
it would be Friday south of Interstate 90 then again Saturday night
into Sunday most locations. Below normal temperatures unfortunately
will likely continue through the next week as well.
.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1048 PM CDT Mon Apr 24 2017
MVFR has already slipped down into KHON from the north as chilly
air pours into this area. With the cold air will be lowered
ceilings, with a strong disturbance impacting our weather on
Tuesday. North to northeast winds will become quite brisk on
Tuesday as the day progresses, eventually reaching frequent gusts
of 25 knots by afternoon and evening. In addition with the
rainfall, widespread lower end MVFR to IFR conditions are expected
late tonight and Tuesday.