Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 180430
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
1130 PM CDT Sun Sep 17 2017

.UPDATE...
Issued at 1129 PM CDT Sun Sep 17 2017

Going forecast remains in good shape. No changes needed.

UPDATE Issued at 925 PM CDT Sun Sep 17 2017

Have increased fog coverage a bit for later tonight over western
locations in line with latest guidance. Remainder of forecast
looks to be in good shape.

UPDATE Issued at 607 PM CDT Sun Sep 17 2017

Latest high-res models have backed off some on precipitation over
portions of south central North Dakota into the southern James
River Valley tonight, so have backed off a little bit on chances,
pulling most locations back to a slight chance. Have also added
the mention of fog over portions of the southwest and south
central as models are starting to suggest fog developing again
later tonight. Since it has not taken much for fog to form lately
with the moist ground, have gone ahead and added some in.

&&

.SHORT TERM...(This evening through Monday)
Issued at 321 PM CDT Sun Sep 17 2017

Forecast highlights in the short term period will be a slight chance
of showers tonight in the southwest and James Valley, along with
some patchy frost southwest. Otherwise a warming trend in the offing
for Monday.

Currently, satellite pics indicated the upper low associated with
our recent rains in North Dakota was over central Manitoba. Another
large low was pushing into the west coast of British Columbia,
leaving a zonal upper level flow aloft across the western US.

A weak upper level shortwave in the flow aloft was over southern
Montana/Wyoming this afternoon. This weak wave is forecast to move
east across southern North Dakota and South Dakota. Look for
increasing cloudiness this evening in southern North Dakota, with a
low chance of showers in the south central through the James Valley
tonight.

Weak upper level ridging aloft will occur on Monday over the Dakotas
ahead of the large British Columbia low that begins expanding
eastward into the Canadian and northern US Rockies on Monday. Look
for southerly winds and warm advection, resulting in afternoon highs
from the upper 60s in the Turtle Mountains to the 70s
elsewhere...upper 70s in the far southwest.

.LONG TERM...(Monday night through Sunday)
Issued at 321 PM CDT Sun Sep 17 2017

A large area of low pressure developing into the Pacific Northwest
and eventually the Northern Plains will highlight the long term
period.

Monday night through Tuesday night: An upper level low pressure
system over southern British Columbia Monday evening moves east into
southern Alberta Tuesday. As it does, a significant shortwave in the
base of the low ejects out of Idaho/Wyoming and moves east quickly
across the Dakotas Tuesday and Tuesday night. This will trigger
cyclogenesis in the lee of the Rockies of Montana, Wyoming, and
Saskatchewan, including a strong surface cold front. This cold
front moves east into the western Dakotas/Saskatchewan Tuesday
morning, sweeping east across the Dakotas/Manitoba Tuesday afternoon
and night. We can expect increased southerly winds ahead of the
front and westerly winds behind the front. The increased southerly
winds will also extend well into the mid levels of the atmosphere as
a low level jet that develops up the Plains. This will act to
transport plenty of moisture northward into the Northern Plains.
This scenario will favor good chances of showers and thunderstorms
across much of the Northern Plains. MU CAPE over 2500 J/Kg and 0-6Km
bulk shear of around 50 knots in the James Valley (NAM model)
will provide a favorable environment for strong to severe storms.
The GFS model again is slightly faster than the NAM/ECMWF, but
models are beginning to look a bit closer together regarding
movement/timing of this front. At this time it appears the best
chances for severe storms would be east of Highway 83, favoring
the Highway 281 corridor for Tuesday afternoon and perhaps into
the early evening hours.

In addition to the thunderstorm threat, strong winds associated with
the system will be of concern. Bufkit soundings are indicating
strong southerly winds aloft, but a surface inversion with perhaps
some stratus clouds in the near surface layers not allowing mixing
with 40 to 50 knots aloft will make for a mostly cloudy and cool day
with thunderstorms triggered by the front and upper level shortwaves
- mainly in the afternoon - and more likely in the James Valley.
However, forecast soundings are indicating the strong and gusty
westerly winds behind the front will translate to the surface. Thus
expecting a windy day for western and much of central North Dakota
with west winds of 20 to 30 mph with gusts nearing 40 mph at times.

Wednesday: The strong upper low remains over Alberta/Saskatchewan
and becomes more elongated as it deepens into a longwave trough
extending southwestward towards northern California. A strong
southwesterly wind component develops at upper, mid, and low levels.
This would result in gusty southwest winds for much of western and
central North Dakota.

Thursday through Sunday: The upper low/trough continues to deepen
and elongate, becoming a longwave trough extending from Hudson Bay
to its base in southern California by Friday. The trough begins to
make its way across the Rockies through the weekend. This will
result in an unsettled period with off and on chances of showers and
thunderstorms, with fairly seasonable temperatures with highs in the
60s to lower 70s and lows mainly in the 40s to lower 50s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1129 PM CDT Sun Sep 17 2017

Vfr cigs/vsbys this TAF period. A tempo -shra was included at KJMS
between 09z-12z Monday as an area of rainshowers will briefly clip
the terminal. Otherwise, dry elsewhere with southeast winds to 10kt
through the period.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...JJS
SHORT TERM...JV
LONG TERM...JV
AVIATION...JJS/KS



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