Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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218
FXUS63 KFSD 272318
AFDFSD

Area Forecast Discussion
National Weather Service Sioux Falls SD
618 PM CDT Thu Apr 27 2017

.SHORT TERM...(This evening through Friday)
Issued at 339 PM CDT Thu Apr 27 2017

Cold temperatures again the main concern for tonight. Clouds are
clearing from southwest to northeast this afternoon, and will
continue to do so through this evening. Clearing skies and light
winds through most of the night should allow temperatures to drop
into the mid to upper 20s tonight, mainly north of a line from Huron
to Spirit Lake. will not be issuing any frost/freeze headlines
tonight due to likely patchy nature of significant freezing and the
fact that impacts in the northern areas are still marginal at worst.
Will cover the cold and frost in social media posts. For late
tonight and Friday, could see a few scattered rain/snow showers
spreading into southern areas, with no widespread snow accumulation
expected. These will change back to all rain by mid morning. Highs
on Friday should be pretty similar to today except for warmer in the
east where they`ve been in the clouds all day today and will be the
last to cloud up tomorrow.

.LONG TERM...(Friday night through Thursday)
Issued at 339 PM CDT Thu Apr 27 2017

The focus in the long term is the significant spring storm
expected to begin late Saturday night and continue into Monday
morning.

Friday night will be another cool night across the region. With
winds staying above 5 mph and clouds slowly increasing,
temperatures will not be as cold as tonight but still expected to
be either side of freezing with the coldest temperatures around
Huron and Brookings which will be clear the longest. On Saturday,
a strong upper level wave will begin to dig into the southwest
United States as a strong 300 mb jet streak sets up along the US-
Canadian border. Strong southerly flow at 850 mb will begin to
advect moisture and a mid level front toward northern Iowa and
northern Nebraska during the afternoon. There will be quite a bit
of dry air in place during the day so it will take a while to
saturate the low levels of the atmosphere and we kept PoPs
between 20 and 40 percent south of a Wayne NE to Storm Lake IA
line.

For Saturday night through Monday morning, there are still
significant model differences. The NAM is the fastest and farthest
to the east with the system keeping most of the heavy
precipitation east of I-29. The ECMWF is the slowest and farthest
west with heavy precipitation back into the James Valley and
continuing into Monday afternoon. In terms of progression, the
Canadian is similar to the ECMWF as are a minority of GEFS
members. So have generally followed a slower solution in timing.
In terms of location, there is still a significant amount of
uncertainty. Precipitation will focus along the 700 mb front and
the interaction between the 700 mb front and the upper level PV
anomaly later Sunday afternoon and Sunday night favors the
development of moderate to heavy stratiform precipitation. The
question is where that will occur and when the heavier
precipitation will begin. The biggest concern is the impact of
convection over the central US. If widespread convection does
develop, this could act to both delay the development of
precipitation over the area and shift the area of precipitation
farther to the east - from far northeast Nebraska into
southwestern Minnesota rather than back into southeastern South
Dakota. It could also result in lower amounts of precipitation as
moisture will be forced to go around the convection and this will
limit available moisture for the expected frontal band. With the
storm still 3 days out, it is difficult to determine the exact
impact of convection so have broadened out the probability of
precipitation to 30-50 PoPs back to Huron and Chamberlain but
still keeping the highest probability east of a YKN to BKX line.
We did also increase the PoPs on Monday morning with the
expectation that a slower system is more likely at this time given
that upper lows tend to be slower to move out of the desert
southwest in spring as cyclogenesis commences.

In terms of QPF, the expectation is that there will be a band of
precipitation with this system developing sometime on Sunday and
continuing through much of Sunday night and perhaps into Monday
morning. It is very likely there will be a sharp gradient in the
precipitation. However, with so much uncertainty in the location
of this band, we opted to broaden out this gradient. This means
that precipitation goes from 0.10 from near HON to 9V9 to 1" from
Brookings to Tyndall and then a large area around an inch east of
that line. In all likelihood, given the strong frontogenesis and
ample moisture, the band will be more narrow than our QPF shows
and maximum amounts within the band could be higher than 1.25".
Also precipitation will likely decrease much more rapidly to the
northwest of the band.

The other concern is precipitation type. Confidence is increasing
that many locations will see some snow mixed with rain if not all
snow. Snow is most likely on Sunday night but there could be a
mix as early as Sunday afternoon. Accumulating snow would occur
where precipitation rates are the highest within the band. Also
accumulating snow will be terrain dependent so that persons along
the Buffalo Ridge in southwestern Minnesota are more likely to see
accumulating snow than areas near I-29 or around Marshall. Given
that snowfall amounts are sensitive to both the precipitation
rate, and thus location of the band, and the temperature in the
lowest 1000 ft of the atmosphere, it is still too early to
estimate amounts since an error in temperature of even 1 or 2
degrees could be the difference between no snow and a few inches
of snow. For right now, persons planning travel any time from late
Sunday afternoon into early Monday morning across far eastern
South Dakota, northeastern Nebraska, northwestern Iowa, and
southwestern Minnesota, should prepare for the *possibility* of
accumulating snow impacting travel.

In terms of temperatures, after lows in the mid to upper 30s on
Saturday night, temperatures will rise very little on Sunday
except in south central South Dakota where precipitation is not
as likely. As rainfall increases, strengthening northerly winds
will continue to advect drier air into the area and evaporative
cooling will prevent temperature from rising more than 3 to 5
degrees with highs expected to only be in the upper 30s to lower
40s. Sunday night will see temperatures fall to around freezing
along the Buffalo Ridge and mid 30s elsewhere. On Monday, the
combination of clouds, precipitation and maybe snow cover, at
least early in the morning, will keep highs from 40 to 45 along
and east of I-29. Much warmer air will move west of I29 and skies
should become at least partly sunny by afternoon so that highs
will warm into the upper 50s.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 558 PM CDT Thu Apr 27 2017

VFR conditions will prevail through the TAF period for KHON and
KFSD sites. MVFR ceilings return by late Friday morning for our
southern areas, including KSUX site where scattered rain/snow
showers are possible.

&&

.FSD WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
IA...None.
NE...None.
&&

$$

SHORT TERM...MG
LONG TERM...Schumacher
AVIATION...JH/05



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