963
FXUS63 KBIS 182358
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
658 PM CDT Wed Sep 18 2024

.KEY MESSAGES...

- Strong winds are forecast from through Thursday evening, with
  sustained winds up to 30 mph at times.

- Showers and thunderstorms are possible (40-60% chance) across
  western and northern North Dakota this evening through
  tonight.

- A cooling trend is forecast for the end of the week, with
  highs in the 60s by the weekend.

&&

.UPDATE...
Issued at 657 PM CDT Wed Sep 18 2024

The low pressure continues to spin and move north through
eastern Montana, bringing light bands of rain through North
Dakota. In the more unstable air in the northern James River
Valley and to the north, thunderstorms have formed. They should
stay non-severe with just localized heavy rain and maybe very
small hail. These should only last about an hour until sun down.
Winds should slowly diminish to around 20mph overnight.

UPDATE
Issued at 348 PM CDT Wed Sep 18 2024

This is just a brief update to include that we have issued a
Wind Advisory for our four southwest counties (Slope, Hettinger,
Bowman, Adams) from now until 4 PM CDT (3 PM MDT) Thursday.
Taking a closer look at model soundings, areas in the southwest
see some lowering of the mixing layer height to near the
surface, with winds at that height approaching 35 to 40 kts. The
Bowman airport AWOS recorded sustained winds of 38 mph about 30
minutes ago as well, which helps support the fact that these
winds are making it to the surface. While the advisory is just
the four counties, there is a potential for an expansion for the
daytime hours on Thursday, however that will be a decision to
be made a bit later as we see how this advisory evolves.

&&

.DISCUSSION...
Issued at 255 PM CDT Wed Sep 18 2024

The expansive, stacked low pressure system that brought us our
thunderstorms earlier this morning sits across eastern Montana
today, with a large band of precipitation wrapping around it. Across
western and central North Dakota, skies have cleared out, allowing
for some heating and drying to occur. Highs today range from the mid
70s to lower 80s.

Southerly winds continue to increase as the low slides eastward,
tightening the surface pressure gradient and allowing for sustained
wind speeds around 20-25 mph, gusting up to 30-35 mph, especially
across the southern tiers of counties. At the moment, winds are not
necessarily exceeding advisory criteria, but there is the potential
for some areas to see isolated speeds right around the lower
threshold of advisory criteria. Wrap-around precipitation may begin
to nose into the southwest in the afternoon hours, which may
potentially help limit the mixing at the surface, so there is no
Wind Advisory out at this time. We will continue to monitor winds
throughout the day, in case the mixing begins to overperform.
This precipitation should then continue to expand to the
northeast as the surface low tracks to the northeast into
southern Saskatchewan, bringing about 40-70% chances for showers
and an occasional thunderstorm to much of the west and north
through the evening and through the overnight hours. Portions of
the south central also see some near critical fire weather
conditions, given the increased wind speeds and minimum relative
humidity values around 20-23%, but our live herbaceous fuel
moisture values in that area exceed 130%, helping limit much of
the concern for critical fire weather.

As the aforementioned low continues moving northeast, wind speeds
will remain breezy overnight, with the wind direction becoming more
southwesterly, before becoming entirely westerly by Thursday
morning. Along the nose of the precipitation band moving into our
far southeast, winds are expected to increase in speed, with
portions of Bowman, Adams, and Slope counties potentially seeing
sustained wind speeds above 30 mph for a time. There is the
potential for these winds to remain elevated long enough for a Wind
Advisory, so we will have to keep a close eye on how the forecast
evolves in that area. Some forecast soundings indicate stronger
winds aloft (30-35 kts) mixing down to around 500-700 ft for a few
hours in that area, which could certainly support some stronger
winds there. The EFI for wind speeds does indicate parts of the
southwest as potential areas for strong winds, but the shift of
tails remains further west and out of our area. Some models,
including the HREF, also suggest wind speeds dying out overnight
before they really establish themselves in our area. All in all, it
is a somewhat strange setup that will need to be closely monitored
through the rest of the evening. Otherwise, overnight lows will
mostly remain within the 50s.

Through the day Thursday, the low pressure system will transit to
our north, resulting in continued breezy winds from the west.
Chances for showers and thunderstorms (mostly 25-50%) will slowly
cross the northern counties, before eventually lifting north into
Canada by Thursday evening. Wind speeds will once again remain close
to advisory criteria but staying just below, mostly across the far
south and far northwest. Given the strong influence from the low,
high temperatures will be a bit cooler tomorrow than they are today,
with highs mostly in the upper 60s to mid 70s.

Very brief upper level ridging will help bring highs back into the
upper 70s to lower 80s Friday. With the low finally moving entirely
out of our region, winds should decrease across the area. Friday
should remain mostly dry under the influence of the ridge, with very
low chances (roughly 15-20%) for showers in the evening.

Through the weekend, models suggest a large trough digging down
across the Canadian Prairies, with the trough axis positioned just
to the north of the International Border. As a result, upper level
flow becomes more westerly, bringing about cooler temperatures to
the state. High temperatures on Saturday and Sunday mostly remain in
the 60s, with overnight lows in the low 40s. Some areas in the west
may see lows in the upper 30s as well. Long range guidance has
shifted flow to be more westerly as opposed to southwesterly over
the past few runs, so chances for precipitation through the weekend
has decreased quite a bit, with only portions of the southeast
seeing 15% chances for rain. These dry conditions are forecast to
continue into next week, where cluster analysis suggests the
beginning of a warming and drying trend, with upper level ridging
building into the western CONUS.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 657 PM CDT Wed Sep 18 2024

Mainly VFR visibilities through the period, they could drop to
MVFR in heavier rain showers. VFR skies tonight before MVFR
ceilings move in west to east, Thursday late morning in the
west. Rain showers will continue overnight in the west and
north, ending in the afternoon Thursday. Winds overnight will
calm some, but remain breezy. Thursday morning winds will pick
back up to near 40kts from the west. Skies should clear at the
end of the period.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 3 PM MDT Thursday for NDZ040-041-043-044.

&&

$$

UPDATE...Smith
DISCUSSION...Besson
AVIATION...Smith