Area Forecast Discussion
Issued by NWS Aberdeen, SD

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000
FXUS63 KABR 190242 AAB
AFDABR

Area Forecast Discussion...UPDATED
National Weather Service Aberdeen SD
942 PM CDT Mon Sep 18 2017

.UPDATE...
Issued at 916 PM CDT Mon Sep 18 2017

The forecast is on track. Only adjustment was to knock low
temperatures down a couple of degrees. Already seeing stratus/fog
redeveloping across the far eastern forecast zones across and east
of the prairie coteau. Until coverage of fog/low stratus begins to
develop further north and west over the next few hours, just
seeing some scattered/broken high cirrus working west to east over
the region. Latest available CAM solutions continue to hammer away
at convective initiation (supercells developing) along or just
east of the Missouri River Tuesday afternoon around 20-21Z
somewhere between KPIR and KMBG. Low level CAPE/shear progs
(NAM12) are showing higher end values between 21Z and 03Z amid 45
to 55 knot deep layer shear, requiring some thoughtful
consideration for tornado potential over the next couple of
forecast shifts.

UPDATE Issued at 652 PM CDT Mon Sep 18 2017

See below for an aviation forecast discussion for the 00Z TAFs.

&&

.SHORT TERM...(This Evening through Wednesday Morning)
Issued at 351 PM CDT Mon Sep 18 2017

A low level jet of around 50kts will develop across western/central
South Dakota overnight. The environment aloft is stable however, so
this will just result in some low level wind shear for aviation. In
our east, models continue to show the redevelopment of low status
and possibly some upslope fog as a warm layer sets up around 1000-
1500ft AGL. Warm advection will continue through the morning, but in
areas not impacted by stratus, the strong low level jet will mix out
resulting in sustained winds around 30mph with higher gusts out of
the south. CAM solutions continue to show a high degree of
continuity on timing/location with storm development, between the
James and Missouri valleys after 21-22Z. Forecaster confidence is
therefore high as CAMS tend to do better with strongly forced
environments. The storm environment: 0-6km shear tops out around
+50kts with around 2000j/kg MLCAPE. CAMS show convection all along
the front, supporting SPC premise that a line of storms will be the
main convective mode, with embedded rotation due to the high shear
environment. Therefore the threat consists of hail/winds and QLCS
tornadoes - though that last one may be negatively impacted by the
duration of any stratus layer (limiting heating) across our eastern
CWA. As the evening progresses, a 60kt low level jet develops over
Minnesota so the other severe threats continue as the line moves
into the far eastern CWA.

On the backside of this front, we also have some cooler
temperatures, however we also maintain gusty northwest winds with
GFS BUFKIT mixed winds up to around 30kts, with mixed dewpoints to
around 40F. This would be late in the day which will limit the fire
weather threat.

.LONG TERM...(Wednesday through Monday)
Issued at 351 PM CDT Mon Sep 18 2017

Very good agreement in the long term with the upper level
flow pattern over the United States. Our region will be affected by
large scale upper level troughing west and northwest of our region.
At this time, it looks like it will be mainly dry from Wednesday
through Friday morning behind the exiting Tuesday night system. The
rest of the long term will have a surface front extending mainly
from southwest to northeast across the Upper Midwest with our CWA
being on the much cooler side. Several short wave troughs will lift
over our region from the southwest bringing good chances of showers
and thunderstorms to our CWA. The best chances look to be from
Pierre to Aberdeen and east at this time.

Temperatures will warm from Wednesday through Thursday from the 70s
to the 80s as southerly winds bring in warmer air. The cooler air
will then push in on Friday as the front sinks through with highs in
the lower 60s far northwest in the CWA to the lower 80s far
southeast. The rest of the long term will be below normal with highs
mostly in the upper 50s to the mid 60s across the CWA.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday Evening)
Issued at 652 PM CDT Mon Sep 18 2017

It still looks like KATY will be falling into IFR flight category
conditions (cigs and visbies) at or after midnight in good low
level warm air advection/moisture advection with upslope terrain
enhancement from the Prairie Coteau. Some guidance hints at low
stratus maybe sticking around through, at least, the first of the
day on Tuesday. There is also the distinct possibility of
convection rumbling through terminal airspace by early Tuesday
evening. The same goes for KABR tonight into Tuesday, although the
sub-vfr flight cat conditions at KABR may only materialize in the
form of stratus clouds.

Currently, it appears that KPIR/KMBG will avoid most of the sub-
VFR weather tonight into Tuesday. Will continue to monitor for the
potential for some early Tuesday morning ground fog/valley fog at
these two terminals. Otherwise, the main impacts are Low Level
Wind Shear that sets up by midnight and persists into Tuesday
morning before mixing out, and thunderstorms that are expected in
the afternoon along a cold front/wind shift. Timing and location
of initial thunderstorm cell development is yet to be determined,
but could be somewhere near the KPIR terminal by 21Z Tuesday.
Additional convection is possible between 21Z and 24Z Tuesday
at/near KMBG. Felt it was necessary to include some mention of TS
out at the end of the TAF valid period, since any convection that
does get going on Tuesday will have the potential to produce
severe weather, with large hail and damaging winds having the
highest probability of occurring.

&&

.ABR WATCHES/WARNINGS/ADVISORIES...
SD...None.
MN...None.
&&

$$

UPDATE...Dorn
SHORT TERM...Connelly
LONG TERM...Mohr
AVIATION...Dorn



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