Area Forecast Discussion
Issued by NWS Sioux Falls, SD

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FXUS63 KFSD 302341

Area Forecast Discussion
National Weather Service Sioux Falls SD
641 PM CDT Sun Apr 30 2017

.SHORT TERM...(This evening through Monday)
Issued at 407 PM CDT Sun Apr 30 2017

Upper level low centered along the NE/KS border this afternoon, with
precipitation lifting northward through Nebraska out ahead of the
feature. This band of precipitation will continue to lift into the
southern portions of our CWA through the late afternoon and evening,
pivoting to the north across much of the remainder of our area
through the overnight hours as the low moves into IA/MO by late
tonight. There will be a fairly tight gradient along the western
edge of the precipitation shield, and the Chamberlain to Huron
corridor may see little precipitation overnight. Still a very tricky
forecast for tonight, with precipitation type dependent on near
surface temperatures with warmest air residing in the near surface
layer. Models are depicting robust QPF amounts overnight, with the
heaviest axis of precipitation, of around 1 inch, occurring
southwest to northeast from the central MO River Valley to southwest
MN. Currently seeing rain over the entire CWA with temperatures in
the upper 30s to lower 40s in areas that precipitation is occurring.
Back to the southwest, snow is being reported through north central
NE where readings are in the mid 30s. From obs and reports through
the day, it appears that the rain mixes with snow once the
temperature reaches around 36 or 37 degrees, while all snow is
reported once it drops to 34 or 35 degrees. It is these values that
were kept in mind when formulating the forecast for snow tonight.
Based on that, as temperatures begin to drop into the mid 30s over
our area by later this evening, expect rain to mix with or change to
snow over a good portion of the CWA. Current thinking is that lows
overnight will remain above freezing, through possibly touching
freezing over the higher elevations of southwestern MN. As touched
on in previous discussion, snowfall accumulations tonight are
dependent on many factors - actual surface temperatures, ground
temperatures, wetness of the ground, and precipitation rates. With
the high QPF amounts forecasted, do expect rates to become high
enough to drop temperatures and allow for snow to accumulate in the
axis of heaviest precipitation as talked about above.

To the specifics, current thinking is that amounts may accumulate 2
to 3 inches from the central MO River corridor northeastward to near
I-90, then where overnight lows will be even colder, looking at 3 to
5 inches possible through east central SD into southwestern MN. That
thinking was in agreement with previous headlines, though had to
expand the Winter Weather Advisory for 4 more counties in
southeastern SD and southwestern MN.

On Monday the upper level low pulls off into the western Great Lakes
region, with rain and snow collocated with the trowal ending from
southwest to northeast through the afternoon. It will be another
windy day, and with clearing in our west highs will climb to around
60. To the east where clouds and precipitation will remain, looking
at highs in the lower to mid 40s.

.LONG TERM...(Monday night through Sunday)
Issued at 407 PM CDT Sun Apr 30 2017

For the remainder of the week, continued to follow the GFS and ECMWF
as they are very close with the details. Strong low pressure in
northeast Wisconsin will continue to lift northeastward and fill in
southeast Canada. Any lingering precip will be confined Monday
evening to our extreme eastern sections of southwest MN early in the
evening before exiting. To the west, a stripe of light accumulating
rainfall may develop along and west of the James River during the
evening hours, along a vort filament which is associated with a weak
short wave moving down the backside of the departing upper low.
After midnight this wave weakens, and thus the deeper layer moisture
exits which will end the measurable precip threat.

Toward the middle of the week, we are dealing with a northwest flow
aloft as an upper ridge builds in the western United States, with
the upper low which is impacting us now still filling over southeast
Canada. A broad deformation impacts our area bringing skittish off
and on chances for rainfall. Tuesday afternoon, the highest pops,
which are only chance pops, are warranted in our northwest zones
along with a slight chance for isolated TSRA as cold air aloft
produces some weak instability in that area. More significantly, a
decent short wave dives southeast Wednesday afternoon and evening
bringing with it a stronger chance for showers and storms. Elevated
instability off of both the nam and gfs show 100 to 200 J/kg and
directional wind shear is pretty strong east of I 29 in the late
afternoon. However speed shear is pretty marginal at this time as
well as lapse rates. After this, the forecast becomes dry as the
aforementioned upper ridge moves closer to the plains.

Temperatures will just see a general warming pattern all week, and
are based on mixing down 850mb temperatures. Both the gfs and ecmwf
show an upper low diving down northwest flow aloft through the
western Great Lakes area Friday night and early Saturday. This wave
does not appear to be bringing in any backdoor cooling into our area
for Saturday if the 925-850mb temperatures pan out on both models.
So this weekend will are still looking at a lot of lower to mid 70s.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 621 PM CDT Sun Apr 30 2017

Widespread precipitation will continue across the region bringing
MVFR to IFR conditions through Monday afternoon. Rain will
transition to snow late this evening, dropping visibilities to
less than three miles. Visibility may drop from 1 to 1/2 mile at
times with moderate snow. Therefore, LIFR conditions are possible
with snow across KFSD. A gradual improvement in ceilings is
expected by late Monday morning, with VFR conditions expected
after 21Z. VFR conditions will prevail across KHON site through
the TAF period. Gusty northwesterly winds to 28 knots are also
expected to continue at all sites.


SD...Winter Weather Advisory from 10 PM this evening to 9 AM CDT
     Monday for SDZ060>062-065>070.

     Winter Weather Advisory from 10 PM this evening to noon CDT
     Monday for SDZ039-040-054>056.

MN...Winter Weather Advisory from 10 PM this evening to noon CDT
     Monday for MNZ071-072-080-089-097-098.



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