Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 131442 AAB

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
942 AM CDT Wed Sep 13 2017

Issued at 940 AM CDT Wed Sep 13 2017

No significant updates to the forecast grids this morning. A weak
surface cold front is located over the northwest third of the CWA
(from northwest of Aberdeen and to the WSW to about half way
between MBG/PIR) and has been bringing in significantly drier
air. This front will slowly sag to the southeast, although models
vary on where it will stall. Thus, confidence is low on dewpoints
for areas along the Sisseton to Aberdeen, Redfield, and Pierre
area. The main adjustment was for the continuation of smoke over
N.C. SD behind the front into this afternoon. Visibilities are
still below 2mi at MBG and NWS Air Quality modeling of near
surface smoke is showing it lingering through the rest of the day
with generally light winds. Will be monitoring trends and may
have to expand into the tonight period.

UPDATE Issued at 652 AM CDT Wed Sep 13 2017

See below for an aviation forecast discussion for the 12Z TAFs.


.SHORT TERM...(Today through Friday Afternoon)
Issued at 429 AM CDT Wed Sep 13 2017

A few interesting things going on early this morning. First off,
weak shortwave is moving east over the area with a surface frontal
boundary also layed out across the CWA. Very weak low-level jet is
in place. There`s actually some elevated instability to work with as
well, with MUCAPE values around 1000 J/KG. Steep mid-level lapse
rates as well, although there really isn`t much shear to speak of.
All that said, we have seen last evening`s shower and storm activity
across central SD persist all the way east into the James valley. A
few storms produced some gusty winds over 40 mph. In fact, we`ve
seen some heat bursts over the past few hours within some of
these decaying thunderstorms. Many have been subtle, but the most
impressive one was noted at the Loyalton mesonet site, where the
temperature jumped to 91 degrees with a 41 mph wind gust. The
dewpoint tanked to 46 degrees as well.

We`ve also been noting the surface obs upstream across western ND
with lowered visibility due to smoke from western wildfires. Many
obs in the 3 to 6 mile range and some of this extends southwest into
northwest and north central SD. Have therefore included smoke in the
grids through the morning hours across north central SD. Now that we
are seeing surface obs reflect lower visibility, felt it was
necessary to include this in the grids.

Switching gears to later today, it appears we`ll see another hot day
as 850 mb temps are fairly similar to yesterday. Although, mixing
winds are not as good as they were yesterday. Still expected to see
highs ranging from the upper 80s to upper 90s across the CWA. The
frontal boundary will not move much today, and will basically bisect
the CWA. An area of low pressure will move northeast along this
boundary tonight and through the day Thursday. This will bring
scattered showers and thunderstorms to the area. MLCAPE does
increase to about 1000 to 1500 J/KG Thursday afternoon, although
bulk shear is only around 30 knots. Could see a few strong storms
Thursday afternoon and evening as well.

A stronger area of low pressure moves northeast into the area on
Friday. Models agree fairly well on the track, bringing it right
into central or eastern SD by 00Z Saturday. A nice temperature
gradient sets up over the CWA as suggested by the wide range of 850
mb temps across the area. Increased highs on Friday over the
southeast CWA where breezy southerly winds look to bring up some
rather warm temperatures ahead of the low pressure system. 80s will
be common in the Watertown area, while much cooler readings in the
low 60s will be all there is for highs over north central SD. A more
broad area of synoptic scale lift with this system should bring more
widespread precipitation to the area on Friday. Although, the slight
differences in the low track will determine just how far southeast
within the CWA the more widespread precipitation occurs. Right now
it appears at least central and north central SD would have the best
chance for rainfall, but even areas as far east as the James valley
show increasing chances as well.

.LONG TERM...(Friday Evening through Tuesday)
Issued at 429 AM CDT Wed Sep 13 2017

When the period opens, southwest flow aloft is in full swing with
shortwave energy embedded within it. As such, rain chances persist
from Thursday night through mainly Saturday. As long as the surface
frontal boundary is hanging on across the eastern forecast zones on
Friday/Friday night, still keeping an eye on convective potential
(storm mode?). Friday evening, models are all generating at least a
30-something knot low level jet into the boundary for
increased/stronger low level forcing. And plenty of deep layer shear
to support organized strong convection, too.

For a period of time from the second half of the weekend into early
next week, some shortwave ridging aloft will try and put a stop to
the precip chances over this cwa. However, all indications point to
the western CONUS longwave upper trof reloading and taking aim on
the northern plains, once more, by mid-week next week.

The trend in temperatures remains relatively unchanged. The 00Z
NAM12 solution out to 84 hours now reaches to 12Z Saturday and has
the coldest 925hpa thermal progs, so far, for Friday night into
Saturday. The 00Z ECMWF low level thermal progs have backed off just
slightly. Still thinking there will be more clouds around than sun
on Saturday, and with low level caa ongoing while a large surface
high pressure builds across the region, current forecast highs in
the upper 50s to mid 60s should continue to suffice. The cool
airmass sticks around on Sunday before airmass moderation and some
low level waa help nudge temperatures back up to near to above
normal during the first half of next week.


.AVIATION...(For the 12Z TAFS through 12Z Thursday Morning)
Issued at 652 AM CDT Wed Sep 13 2017

VFR conditions should prevail at all locations through Wednesday
night. That said, smoke will probably impact the visibility at
KMBG for a few more hours this morning before mixing out.
Isolated/scattered showers and thunderstorms could develop later
this afternoon across the western Dakotas. Will need to monitor
their progress east in case they make it as far as KMBG/KPIR
before dissipating.




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