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692
FXUS63 KBIS 181445
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
945 AM CDT Wed Sep 18 2024

.KEY MESSAGES...

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- Strong southerly winds are forecast today, with sustained
  winds up to 30 mph.

- Showers and thunderstorms are possible (30-50% chance) across
  western and northern North Dakota this afternoon through
  tonight.

- An active pattern continues through the weekend, with low
  chances for showers and thunderstorms most days.

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

.UPDATE...
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Issued at 945 AM CDT Wed Sep 18 2024

Showers have almost entirely moved out of the area to the
northeast, with clearing across much of the southwest and
central areas of the state. With the heightened winds and
slightly reduced relative humidity values, near critical fire
weather conditions will be possible across the south central.
However, the current live herbaceous fuel moisture values in
that area exceed 140%, which should help reduce the risk of
critical fire weather conditions. Otherwise, the forecast
generally remains on track.

.UPDATE...
Issued at 647 AM CDT Wed Sep 18 2024

Main surface low is now located in northern Montana, with the
center analyzed just west of Glasgow. A line of storms that
developed along a frontal boundary has continued to lift north
and is almost entirely out of the forecast area, although
scattered convection has developed behind this line in central
North Dakota. A modest swath of showers is ongoing on the west
side of the surface low, which is expected to eventually move
into western North Dakota later today as the low progresses
northeast. Freshened up PoPs with this update, which did bring
precipitation chances into the west a few hours earlier, with
potential for showers and thunderstorms to start in the early
afternoon.
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&&

.DISCUSSION...
Issued at 332 AM CDT Wed Sep 18 2024

This morning, there were two surface lows analyzed across the
region, one in southwest North Dakota and one in northeast Montana,
with a stationary front connecting them. Aloft, a deep closed low
was placed over northern Wyoming, with sharp southerly flow over the
Dakotas on the east side of the low. Widespread showers and
thunderstorms developed just ahead of the upper low as midlevel
heights were falling, and on the nose of a low-level jet that was in
southern North Dakota. As of 830 UTC a relatively consistent
line of storms extends from Williston to Jamestown, with
scattered convection on either side of the main line in central
North Dakota. SPC mesoanalysis indicates the environment is
characterized by 30-40 knots of bulk shear and 1000-1500 J/kg of
MLCAPE, although MLCIN is pretty high and thus keeping storms
subsevere.

Storms will continue to lift north through the morning as the
eastern surface low and attendant front move north as well.
Precipitation chances return fairly quickly this afternoon, however,
as the western surface low becomes stacked with the upper low and
convection wraps around the southern side. Showers and thunderstorms
will likely be limited to western and northern North Dakota, with
blended PoPs mainly in the 30 to 50% range.

The main other weather concern for today is strong winds from the
influence of the stacked low. Winds will be southerly through the
day today, sustained up to 30 mph, before shifting southwest to
westerly tonight as the stacked low lifts northeast. At this point
we are expecting winds to stay below advisory criteria, although it
will likely be close in our southern counties along the South Dakota
border. Highs today will be a bit cooler, in the lower 70s to lower
80s. Lows tonight will mainly be in the 50s.

On Thursday, the upper low is progged to be moving into the southern
Canadian Prairies, keeping 20-40% chances for showers and
thunderstorms in northern North Dakota. A tight surface pressure
gradient will lead to another day of breezy winds sustained at 20 to
25 mph, and cooler air behind the system will lead to highs in the
upper 60s to mid 70s.

Temperatures will rebound just slightly on Friday as a shallow,
progressive trough moves through the region, followed closely by a
shortwave trough that ensemble guidance is showing moving east
through the southern Canadian Prairies to end the week. Model
consensus is advertising that this system will bring a cold front
and precipitation chances starting Friday night, leading to cooler
temperatures and on and off chances for showers through the weekend.
There is fairly high confidence in highs generally staying in the
60s Saturday through Monday, with little spread in NBM temperature
percentiles.

Towards the end of the extended period, ensemble guidance diverges
in its solutions, although there is some suggestion in cluster
analysis of broadly zonal flow before potentially upper ridging
building in to our west. CPC outlooks tilt towards above normal
temperatures through the end of September, with some signs of a
drier pattern emerging for the end of the month.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 647 AM CDT Wed Sep 18 2024

Showers and thunderstorms continue across northern and eastern
North Dakota, with some additional weak showers moving north
from South Dakota. IFR to LIFR stratus is still in place across
northern North Dakota, although fog has diminished so
visibilities are generally VFR outside of precipitation.
Winds are turning southerly behind the storms, with sustained
winds of 20 to 30 mph through the day today. Additional showers
and thunderstorms are possible in western and northern North
Dakota this afternoon through the night, with winds turning more
west/southwesterly late in the period. Confidence in
precipitation is highest at KDIK, with chances at KXWA/KMOT as
well. Conditions should return to VFR by 18Z before pockets of
low VFR to MVFR ceilings move in with the precipitation tonight.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Besson
DISCUSSION...Jones
AVIATION...Jones