Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 142035

Area Forecast Discussion
National Weather Service Aberdeen SD
335 PM CDT Mon Aug 14 2017

.SHORT TERM...(This Evening through Tuesday Afternoon)
Issued at 324 PM CDT Mon Aug 14 2017

One area of low pressure over south central MN is producing mostly
cloudy skies over the CWA. As this low pushes eastward, skies should
begin to clean, especially by this evening with the lose of heating.
With the low level moisture, along with light winds, patchy fog will
be possible for locations along and east of the James River. Western
MN will have the best potential of seeing fog tonight/Tuesday

An area of low pressure over eastern MT will slide eastward this
evening, bring convection into the western Dakotas. One area of
convection should track east-se along an instability gradient, which
runs along our far southwest portion of the CWA. Timing of
convection looks best between 7-10Z. Another area of pcpn should
progress mainly across ND, or with the upper level low pressure.

The evolution of the convective potential on Tuesday is a little
more unclear. An upper level shortwave will cross the central
Dakotas on Tuesday with showers and thunderstorms possible in
north central SD. Limited instability, but slow storm motion
should set the stage for locally heavy rainfall.

.LONG TERM...(Tuesday Evening through Monday)
Issued at 324 PM CDT Mon Aug 14 2017

Main focus for the long term portion of the forecast will be the
thunderstorms and heavy rain threat for Tuesday night into
Wednesday. This potential is tied to a shortwave currently moving
through Nevada ahead of an upper trough near the western coast of
the U.S. Portions of this shortwave will move towards the region
tonight into Tuesday, while the main trough and shortwave move
through Nebraska/Kansas Tuesday night and then northeast into
northern Iowa or southern Minnesota on Wednesday, as the PV
anomaly rotates northeast. Thus, expect a surface low to become
centered in the lee of the Rockies in Colorado tonight and then
continue east-northeast through Wednesday on the eastern edge of
that PV anomaly. While the main low looks to stay south of the
area, an inverted trough will stretch NNE into the area on Tuesday
into Tuesday night. That combined with the shortwave and
broad/weak 850-500mb warm air advection, expect
showers/thunderstorms to be ongoing at the start of the long term
period over much of the area and pushing east-northeast. While the
mid-level warm air advection is generally weak, models showing
decent low level WAA over the southeast half of the CWA (on the
east side of the inverted trough) starting around 21Z tomorrow and
continuing through at least 06Z Wednesday. Expect that to be the
focus of the best precipitation chances into Tuesday night.

Believe the severe threat will be limited Tuesday night. MUCAPE
values are generally between 500-1000 J/kg, although they could
peak a little higher over the far southeast part of the CWA (best
instability is south of the CWA). While there is some effective
shear (20-30kts), the elevated CAPE is pretty skinny (NCAPE sub
0.1). That being said, heavy rain is definitely a concern.
Southwesterly flow aloft developing behind tonight`s departing
upper level ridge will lead to PWAT values increasing through
Tuesday and peaking in the 1.5 to 2 inch range over eastern SD and
western MN late Tuesday afternoon and into Tuesday night. Based
off NAEFS mean PWAT values over eastern SD (between 1.5-1.75"),
that puts the area around the 97.5 percentile for mid August
climatology. Several 12Z model runs have started to focus the best
heavy rain threat over the southeast half of the CWA (especially
towards the Watertown area and southeast). Deterministic and
ensemble members are showing a variety of specific amounts in that
area, but they generally range between 1-3 inches. Thus, have
added heavy rain wording to the grids for late Tuesday night for
areas east of the James River. Overall, think the heavy rain
threat will be largely dependent on where the surface low and PV
anomaly track and the forecast will be fine tuned over the next

The upper trough will shift east on Wednesday, gradually bringing
dry northwesterly flow and subsidence. Thus, expect decreasing
clouds from west to east across the region with the surface high
moving in. This surface high will remain into Thursday for much of
the region, but would expect some diurnal shower/thunderstorm
development over western SD ahead of the next surface trough and
shortwave moving through on northwesterly flow aloft. Models vary
on the intensity of the shortwave and in turn surface features,
so will just follow a general blend of the models for
timing/location of precip chances Thursday night into Friday.

Behind that wave, an upper ridge over the western CONUS will
shift east and over the area for much of the weekend. This will
lead to temperatures trending up to normal or even above normal
for this weekend. At this time, much of the weekend should be dry,
but there are some hints of a weak shortwave rounding the ridge
late Sat afternoon into Sunday morning that could spark an
isolated storm. Finally, a strong upper trough moving through
southern Canada will likely drag a cold front (and associated
storms) across the area around the Sunday night into Monday
morning time-frame.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday Afternoon)
Issued at 1225 PM CDT Mon Aug 14 2017

Low CIGS will continue impacting all terminals except KPIR through
this afternoon. CIGS are slowly lifting with MVFR conditions
improving to low end VFR. A storm system will cross the region
later tonight with showers and thunderstorms possible. With timing
and placement issues among models, will leave pcpn out of the




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