Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 300451 AAC
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1151 PM CDT Sat Apr 29 2017

.UPDATE...
Issued at 908 PM CDT Sat Apr 29 2017

No significant changes to the going forecast. Only made minor
tweaks to the PoPs for late tonight to delay the start of the
precipitation into tomorrow morning based off existing dry air,
radar trends, and hires guidance.

&&

.SHORT TERM...(This Evening through Sunday Afternoon)
Issued at 349 PM CDT Sat Apr 29 2017

Very dry air has again settled across central SD. Dewpoints in the
low to mid teens and temperatures in the upper 50s have combined to
drop relative humidity values into the upper teens as many of the
high clouds have thinned out during the late morning and early
afternoon hours. Expect temperatures to fall into the low to mid 30s
across much of the area tonight. Could drop a couple degrees lower
than forecast over central SD where all the dry air has set up
today. Will need to monitor this area.

Sunday will bring increased moisture and increased northeasterly
winds, especially in the afternoon as the pressure gradient
increases between the sfc ridge retreating across western SD and
much of ND, and the nearing low sliding across eastern KS and into
far southeastern NE by the end of the day. Soundings indicate the
potential to mix down some gusts nearing 30kts over much of the
eastern half of the forecast area from 18Z on. The driest air will
again be across north central SD, where afternoon rh values may fall
to around 25 percent. On a positive note fire weather wise, this
will be the area of lightest winds, where northeasterly winds around
15kts combine with gusts around 20kts. Will still highlight this in
our fire weather products, as Sunday will be quite a contrast
northwest to southeast. Areas southeast of ATY could receive 0.25 to
0.50 inches of rain by the end of the day. Will continue to refine
the onset and timing as it gets closer. The 12Z NAM is on the
extreme slow end to bring rain/snow into to the area, and holds off
until after 18Z. Will continue more of a GFS/ECMWF blended solution,
with additional details from the CAM solutions through tonight. The
result was a slight eastern shift to the qpf Sunday afternoon.

.LONG TERM...(Sunday Evening through Saturday)
Issued at 349 PM CDT Sat Apr 29 2017

Main focus will be strong area of low pressure moving out of the
Central Plains into the Upper Midwest during the beginning of this
forecast period. Not much variation in terms of storm track among
the 12Z models as most take it out of NE Kansas into SW Iowa Sunday
night and early Monday morning before making it to SE Minnesota and
Western Wisconsin by Monday afternoon.

With not much change noted from previous model runs with regards to
this system...the western edge of the QPF remains across the James
River Valley. A fairly sharp gradient still exists in QPF from west
to east with the highest amounts still confined around the I-29
corridor, Prairie Coteau and west central Minnesota. Amounts in
excess of 1 inch of liquid are likely which will include several
inches of snow. Highest snowfall amounts still appear to be
concentrated in the Watertown/Clear Lake area and the higher
elevations of the Coteau. GEFS plumes for Watertown are centered
around the 5 to 6 inch mark. A few locations could get a little more
than this...possible 6-7 inches...especially along the Coteau ridge
as northeast winds run perpendicular to the ridge line and upslope
flow develops. With increasing confidence in this snowfall scenario
and after coordination with surrounding WFO`s, a Winter Storm Watch
has been issued for portions our northeastern and eastern counties.

The surface temperatures will be the driver of this snowfall event
and still remain a bit tricky to nail down. With that said, things
at this point look to play out like this...rain to initially fall
across the I-29 corridor and points east into MN and west into the
James valley during the evening hours Sunday. Eventually, a
rain/snow mix will become all snow then by midnight across eastern
areas and higher elevations. For points west of there in the James
valley, rain should begin to mix with snow after midnight. Except
for eastern fringes of the James valley, a changeover to all snow is
not expected in the rest of the valley and/or it will remain a
fairly short-lived snowfall event there. Snow and a mixture of
rain/snow will hold on the longest across the I-29 corridor through
the morning hours Monday before turning back over to rain. The
system should then depart by late Monday afternoon and evening with
the precip coming to an end.

The remainder of the forecast period will feature a return to
somewhat more tranquil conditions. A weaker disturbance moving
through toward the middle of the week could bring some lighter rain
showers to the forecast area Tuesday into Wednesday. After that, a
ridge begins to build into the region. Temperatures will gradually
moderate back closer to normal if not slightly above.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
Issued at 1149 PM CDT Sat Apr 29 2017

VFR conditions will continue for all sites except KATY through the
TAF period. At KATY, expect rain to move in during the early
evening hours and as temperatures cool through the evening, it
will change over to snow. Latest model trends delaying the start
of the precip and lower ceilings so have trended that directions
will this issuance and expect IFR conditions after 06Z.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Watch from Sunday evening through Monday evening
     for SDZ008-019>023.

MN...Winter Storm Watch from Sunday evening through Monday evening
     for MNZ039-046.

&&

$$

UPDATE...SRF
SHORT TERM...KF
LONG TERM...Vipond
AVIATION...SRF



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