Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 210524 AAC

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
1224 AM CDT Sun May 21 2017

Issued at 1222 AM CDT Sun May 21 2017

See updated aviation discussion below.

UPDATE Issued at 630 PM CDT Sat May 20 2017

Used the MSAS analysis to update pops for the next few hours. A
dry slot is wrapping in to eastern SD and reducing rain to
scattered to isolated showers in spots. Will continue to monitor,
but for now bumped pops down to likely instead of categorical east
of the James.


.SHORT TERM...(This Evening through Sunday Afternoon)
Issued at 344 PM CDT Sat May 20 2017

Forecast challenges include evolution of ongoing precipitation event
inside the deformation zone and TROWAL airstreams, as well as frost
potential across the far (west river) western forecast zones late

Currently, the very northwestern corner of the forecast area is
seeing sunshine, while the rest of the forecast area is under cloudy
skies with areas of light rain ongoing. Temperatures where it`s
cloudy and rainy have been holding generally in the 40s today, while
portions of central and north central South Dakota where sunshine
has occurred some today have been able to warm into the 50s to
around 60 degrees.

Basically, look for good chances for precipitation (mainly rain in
the 12Z model soundings) to continue overnight tonight across the
forecast area in the deformation zone where warm conveyer belt
processes (and eventually TROWAL airstream) will continue to feed
low/mid level moisture in ascending fashion back around to the
northwest and west of the upper low circulation over this cwa. The
loan caveat with this scenario is the very noticeable dry slot
currently moving northward across far eastern SoDak and western MN.
It`s possible to see precipitation filling back in across the
northern/western edge of the dry slot, keeping the good chances for
measurable precipitation going overnight across the I-29 corridor.
But, seeing the current radar/satellite trends, a new/different
solution appears to be materializing whereby, the dry slot would
continue to push northward all the way up into far eastern NoDak and
central MN, effectively shutting off rainfall, and replacing it with
"periods of light to moderate drizzle" overnight across the I-29
corridor and points east. The potential for this will have to be
monitored closely. There could still be some areas tonight (mainly
over the Leola Hills and the Sisseton Hills) that see some flakes of
snow mix in with the rain mainly after 06Z tonight. Overall, there
remains the potential for a trace to upwards of a quarter inch of
precipitation by event`s end Sunday morning between the Missouri
River and the James River. East of the James River over into western
MN, there could be an additional quarter inch to half inch of
precipitation through Sunday morning.

The other scenario worth noting is the potential to see clearing
late tonight across the far western forecast zones as the
deformation clouds begin to gradually pull away towards the east.
Winds will be light and low level dry air still in place over that
area could lead to some coverage of frost for a few hours prior to
sunrise Sunday. Will need to monitor skycover/satellite conditions
closely over the next 6 to 12 hours.

Guidance did not really show much in the way of change to the
forecast lows (generally mid/upper 30s to around 4o) tonight or the
forecast highs on Sunday (50s east and 60s west).

.LONG TERM...(Sunday Evening through Saturday)
Issued at 344 PM CDT Sat May 20 2017

The period begins with a weak upper level shortwave crossing the
region within northwest flow. This system will have some instability
to work with on Monday afternoon with thunderstorms possible. Upper
level ridging and a surface high pressure will settle over the
region Monday night with mostly dry conditions expected through
Thursday. The ridge will breakdown by Thursday night as a broad
upper level trough develops over the Pac NW. The upper level trough
will slowly track eastward for the rest of the work week and into
the holiday weekend. While this trough is still a week out, the
phase of the MJO favors below normal temperatures with above normal
pcpn for Memorial Day weekend. While below normal temps are
possible, they will be nothing like today`s readings.


.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
Issued at 1222 AM CDT Sun May 21 2017

MVFR/IFR cigs will persist overnight at KATY/KABR and lift to VFR
Sunday afternoon. Low pressure will continue to draw rain over the
region for the next few hours. As the low moves east, winds will
increase out of the northwest to 15 to 35 kts.




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