Area Forecast Discussion
Issued by NWS Aberdeen, SD

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FXUS63 KABR 171749

Area Forecast Discussion
National Weather Service Aberdeen SD
1249 PM CDT Thu May 17 2018

Issued at 1049 AM CDT Thu May 17 2018

Isolated thunderstorms have developed across the James Valley and
portions of north central SD this morning. Activity should
continue to be isolated in nature over the next few hours, working
off of weakening forcing and an outflow boundary as the frontal
boundary sits further north at this time and is expected to stay
north of the area through the day.


.SHORT TERM...(Today through Saturday Morning)
Issued at 401 AM CDT Thu May 17 2018

A couple rounds of convection are expected over the next 48 hours or
so. The upper flow pattern features a western CONUS trough with weak
ridging aloft, though it should be noted that there is a weak trough
traversing this ridge in a southwest flow regime. The ridge re-loads
today, with the main trough digging out west. The shortwave that
is lifting across the area today looks to be mainly focused across
western South Dakota into central North Dakota. The tail end of
this feature moves into the area overnight. A third weak wave
lifts across mainly the east Friday. Each of these will present us
with a round of convection with varying degrees of intensity.

At the surface we have southerly flow with high pressure to the
southeast. A washed out front is moving into the far northeast
CWA, but this feature is moving against the low level gradient
and will be pushed back northwards. The only issue it will present
is possibly to fire weather given the number of spots that have
been requested over the last couple of days and will need to be
addressed if any are sought for that area. Otherwise we are
looking at a developing lee low and inverted surface trough west
river. The gradient increases to about 10mb across the state, with
a south/southeast component and mixed winds around 25kts. Given
the less efficient mixing direction and warm air aloft will trim
this back but gusts around 25mph should be achievable once the
shallow inversion mixes out. Given similar profiles, should also
come close to highs from Wednesday with initially low humidity,
though a more humid airmass is moving into the region.

The severe weather threat appears to be centered west river
associated with the upper wave and lee low/inverted trough as the
focus. CAMS suggest little convection in our area through 22Z.
MLCAPE tops out around 2000-3000 j/kg with a drop off from our west
river counties to along the Missouri valley. 0-6km shear is also
only on the order of 20-30kts. CAMS generate a line, consistent
with bulk shear vectors to the east, and rapid weakening with loss
of daytime heating and as convection moves into less instability.
Still the risk for severe weather exists this afternoon for our
west river/Missouri valley counties, mainly for hail and strong
winds given the shear environment.

Convection is anticipated to weaken overnight, the focus shifting
south of the CWA with the wave that will move over the FSD CWA. A 30-
40kt low level jet will be over the area, but MUCAPE is only on the
order of a few hundred j/kg with a drop off to the east.

The focus for severe weather moves east Friday. Flow aloft shows a
weak 500mb wave in the east initially with neutral flow thereafter.
MLCAPE is mainly along and south of highway 212 in the NAM and a
little farther north in the GFS. 0-6km shear is still unimpressive
and thus the lessened threat for widespread severe weather.
Additionally there will probably be some convective debris around and
BUFKIT profiles are more moist adiabatic Friday.

.LONG TERM...(Saturday through Wednesday)
Issued at 401 AM CDT Thu May 17 2018

Compared to 24 hours ago, the general trends remain similar in that
precipitation (remnant deformation zone/TROWAL-forced) at the start
of the period should be on a general decreasing/diminishing trend,
with conditions drying out for the rest of the weekend into Monday
before the upper level steering flow actually begins to acquire
enough jet stream strength to start moving subtle mid/upper level
short-waves (and precipitation chances) eastward from the Rockies
out onto the plains states next week.

The 850hpa airmass progged to be over the forecast area Saturday and
Sunday could be one to two standard deviations below May climo
normal. Cool surface high pressure centered over the region Saturday
night and Sunday night implies the need for some additional scrutiny
of low temperatures during those two periods. Likely enough cloud-
cover remains over the region Saturday night on enough surface
pressure gradient cold air advection northerly winds to keep
temperatures from falling off too much. But, if skies are clear
Sunday evening/night and winds go light/variable, temperatures could
fall below 40 degrees in spots. After Sunday, daytime insolation and
some modest low level waa next week should nudge temperatures back
up to near to slightly above climo normal next week.


.AVIATION...(For the 18Z TAFS through 18Z Friday Afternoon)
Issued at 1234 PM CDT Thu May 17 2018

VFR conditions are expected for all terminals, except KMBG
towards the end of the TAF period, which may see some MVFR
ceilings. Potentially severe thunderstorms are expected to move
into KMBG and KPIR this evening. Low level windshear is expected
during the overnight and early morning hours at all TAF sites.
KMBG and KPIR appear to have greater LLWS around midnight, while
the more eastern sites of KABR and KATY hold onto their LLWS until
shortly after sunrise.




SHORT TERM...Connelly
AVIATION...Telken is the U.S. government's official web portal to all federal, state and local government web resources and services.