Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 181729

Area Forecast Discussion
National Weather Service Bismarck ND
1229 PM CDT Mon Sep 18 2017

Issued at 1222 PM CDT Mon Sep 18 2017

Current forecast looks on track. Made some minor adjustments to
temperatures, bumping highs up a bit northwest. Otherwise no
significant changes to the forecast.

UPDATE Issued at 854 AM CDT Mon Sep 18 2017

Showers have exited the southern James River Valley. Have removed
precipitation here for the rest of the morning. Patchy dense fog
continues over portions of the southwest/south central, covered by
current forecast. No significant updates.

Issued at 652 AM CDT Mon Sep 18 2017

Forecast remains on track.  No major updates with this issuance.


.SHORT TERM...(Today and tonight)
Issued at 339 AM CDT Mon Sep 18 2017

Highlight is early morning scattered rainshowers and isolated
thunderstorms prior to sunrise far south central and into the
James River Valley.

Latest GOES16 water vapor imagery shows a mid level shortwave over
far north central South Dakota, pushing toward our southern border
and producing an area of showers and thunderstorms around Mobridge.
High resolution models, both in simulated composite reflectivity
and precipitation, continue to trend with showers reaching the
James River Valley through 12z, and have followed the above guidance.

For today, mostly sunny conditions as a weak transitory ridge
shifts through, allowing the flow aloft transition to southwesterly.
High temperatures will mostly be in the 70s. The exception will be
in the Turtle Mountains, where highs will be in the mid 60s.

Water vapor also shows an upper low and associated trough now
along the British Columbia coast will slide south and east today,
with a strong 700mb-500mb shortwave trough to arrive in southern
Alberta into central Montana tonight. In response, a developing
surface low pressure and potent cold front evolve across eastern
Montana tonight. Strong low level moisture advection occurs across
western and central ND with 850mb winds of between 40kt and 50kt.
Large scale ascent and low level frontogenetical forcing begin to
increase tonight in the west. Expect isolated to scattered rainshowers
west and north central after midnight. Lows will be in the 50s.

.LONG TERM...(Tuesday through Sunday)
Issued at 339 AM CDT Mon Sep 18 2017

Severe thunderstorms Tuesday afternoon and evening highlights the
long term period.

Confidence growing for severe thunderstorms capable of producing
damaging winds, large hail, and a few tornadoes along and east
of Highway 83 Tuesday afternoon and evening. SPC Day 2/Tuesday
portrays a quick transition for an increasing risk of severe
weather - from a Marginal Risk along Highway 83 (Minot to
Bismarck), to an Enhanced Risk across the James River Valley.
Have continued to follow the slower NAM/ECMWF which is in line
with the severe weather parameters/threat indicated by SPC. The
GFS continues to be faster with the cold frontal progression, and
hence, has a narrower area/timeframe for severe weather, which
suggests the James River Valley.

Mixed Layer Cape/0-1km and 0-6km Bulk Shear/0-1km Storm Relative
Helicity/ and Mixed Layer LCL Heights all within acceptable
values for the development of tornadic supercells, especially
over the James River Valley Tuesday late afternoon and early
evening. Per NAM, Strong synoptic scale ascent (500mb-300mb)
overlapping/aligning with lower to middle level (850mb/700mb)
frontogenesis, is maximized in the mid to late afternoon over
the James River Valley. Severe weather will wane in the mid evening
as the mid/upper level trough and forcing shift farther east.
It will be windy Tuesday ahead of an advancing cold front. Highs
will range from the lower 60s west to upper 70s southeast.

For Wednesday, breezy, dry and cooler behind the departing system.
The dry weather will be short lived, as a longwave trough in the
west sends more shortwaves into western and central ND with
continued chances for showers Thursday through Sunday. A cooling
trend will occur late in the long term period. Expect high
temperatures in the upper 60s and lower 70s Thursday, then upper
50s for the weekend.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1222 PM CDT Mon Sep 18 2017

A cold front will move into western North Dakota late tonight and
into central ND by around 18 UTC Tuesday. VFR conditions today
and this evening. Low stratus may spread from southeast ND into
central portions of the state after midnight. Have introduced some
IFR-MVFR ceilings at KJMS and just a mention of an MVFR layer at
KBIS and KMOT. Will need to monitor. Brought VCSH into KDIK, KISN
and KMOT after 12 UTC. Winds begin to shift at these sites as well
12-18 UTC. More significant thunderstorm activity possible, most
likely at KJMS after the 18Z TAF period.




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