878
FXUS63 KBIS 180252
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
952 PM CDT Tue Sep 17 2024

.KEY MESSAGES...

- Isolated to scattered severe thunderstorms are possible across
  western into central North Dakota this evening and tonight.

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

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

&&

.UPDATE...
Issued at 911 PM CDT Tue Sep 17 2024

Thunderstorms are currently skirting the far northwest corner
of Bowman county. Numerous thunderstorms this evening have been
lifting north across eastern Montana this evening but have
remained west of the forecast area. Going forward, it appears
the potential for s severe thunderstorm watch for western ND is
diminishing. The convection presently skirting far southwest ND
began just south of Harding county, and a 63 mph gust was noted
as the thunderstorm developed. Analyzing the SPC mesoanalysis
page and surface analysis could provide some clues as to why
convection is currently not as abundant. The 850-300 mb mean
wind is pretty much due north, but there is a surface trough
that extends N-S across far western SD, but as you get into
northwest ND, it turns more northwest towards GGW. This may have
provided enough of a low level focusing mechanism for the
activity to cross over into Montana before it reached ND. Also,
the effective and 0-6km shear both indicate small but definite
min in shear right on the MT/ND/SD border. The 00Z HRRR was
about the only CAM that depicted that SD convection and it
definitely pushed it west into MT before reaching ND. The 01Z
HRRR is already too far east with stronger convection over
Harding County, and the next wave of convection will be with the
main shortwave impulse as it lifts into the Northern Plains.
This convection is currently just north of I-90 in western SD
and has quite a bit of dense cirrus to the north. So the
question now becomes, will this convection be severe as it lifts
into ND. There will definitely be strong effective shear as it
lifts into ND. Bufkit HRRR soundings indicate the instability
aloft diminishes quickly between 9 and 11 PM CDT. There is a
temporary bump in southwest environmental winds over the next
few hours over southwest ND and it`s possible that convection
could help force down some stronger gusts, but in general, we
think the threat for organized severe weather continues to
diminish (I hate these strong shortwaves lifting around upper
lows though, as it seems like there`s always some strong wind
gusts somewhere). By the time the next round of convection
pushes in, we may just see some heavier rain showers and
embedded thunder. 00Z HRRR forecast soundings over southwest ND
don`t support severe storms as the next wave moves in after
midnight.

So with all that, we have cut back on the eastern extent of pops
tonight. We used a blend of the latest HRRR and the time lagged
HRRR for the finer details in pops through 12 UTC. We kept the
likely or higher pops confined to the west through around 09Z,
before, extending them eastward late tonight along the warm
front on the International Border. Updated text products will be
transmitted shortly.

UPDATE
Issued at 611 PM CDT Tue Sep 17 2024

No significant changes for the early evening update. Convection
has developed over eastern Montana, near Glendive. Convection is
also moving through Northeast Wyoming at this time. MLCIN has
dissipated over southeast Montana and into far southwest ND but
MLCIN remains just to the east of here. DCAPE of 1000-1200 J/KG
over the MONDAK area so damaging winds would be the greatest
threat over southwest ND. The main question is how far east the
wind threat would extend as activity lifts into southwest ND
this evening.

&&

.DISCUSSION...
Issued at 225 PM CDT Tue Sep 17 2024

At the moment, surface low pressure sits across eastern Montana,
while a deep upper level trough spans much of the western CONUS,
slowly moving northeast throughout the day. Low stratus across the
north continues to slowly dissipate, while winds continue to
increase in speed as daytime heating continues. Temperatures
this afternoon mostly range from the lower to upper 80s, with a
few areas in the west and south central potentially exceeding
90. Some portions of the central and James River Valley regions
could see some rather humid conditions, with minimum relative
humidity values in these areas ranging from 60-70%.

As the surface low slowly moves northeast, a warm front associated
with this low is expected to slowly press north, moving into
southern North Dakota later this evening. Decent height falls
extending into our west along with 0-6 km bulk shear values in the
30 to 40 kt range will help aid in the formation of some showers and
thunderstorms along this front. Instability remains marginal at this
time, with the main corridor of maximum MUCAPE (with values ranging
from 1500 to 2500 J/kg) displaced ahead of the front, across our
northern tiers of counties. The SPC has outlined a Slight Risk
(level 2 of 5) for severe thunderstorms across our far southwest,
with a Marginal Risk (level 1 of 5) extending a bit further east,
covering essentially the western third of the state. This threat is
rather conditional and dependent on two different things: the
evolution of the storms prior to entering the state, and the timing
of the front itself. Our current thinking is that these storms will
mostly be forming and strengthening in South Dakota, as indicated by
the most recent CAMs runs, so if these storms can get going prior to
entering the area, there`s a better chance they`ll remain severe as
they enter. However, the timing of the front also adds another layer
of complexity, as it seems like the most recent high resolution
models have slowed this front down a bit, with some models
suggesting an arrival time closer to midnight, perhaps around 10 or
11 PM. While prefrontal development of showers are certainly
possible, guidance is suggesting that this will be a mostly
nocturnal event, limiting the severe threat as the instability will
begin to decrease prior to the arrival of the storms. The nocturnal
low level jet is forecast to kick in later on in the night, but by
the time this jet is maximized, the line of showers and
thunderstorms will be well displaced from the shear maximum,
stretching across our northern tier of counties. Some late
re-intensification could occur, but it shouldn`t be too much of
a concern. Overall, the threat is certainly there, but remains
heavily conditional. Modest DCAPE along with relatively steep
low level lapse rates leads to the possibility for 60 mph wind
gusts and half dollar sized hail as the main threats. If severe
weather doesn`t occur, some showers and thunderstorms may still
roll through the area from the south (with chances ranging from
40-60%), moving north through the overnight hours.

Precipitation chances will linger across the west and central on
Wednesday, as the surface low continues to slowly move northeast.
The upper level trough will continue to move northeast as well,
helping filter in some cooler air as it does so. As a result, we can
expect to see the beginning of a cooling trend on Wednesday, where
high temperatures are forecast to mostly be in the 70s and lower
80s. With the approach of the surface low, the surface pressure
gradient is forecast to tighten as well, bringing about some breezy
to windy conditions across the entire area. Southerly winds are
forecast to be around 25 mph at times, with gusts up to 30 to 35
mph. Lows overnight into Thursday will stay mostly within the 50s.

Through the end of the work week and into the weekend, we can expect
to see continued cooling temperatures. As the initially mentioned
upper trough moves off to the northeast through the second half of
the week, troughing is forecast to quickly take its place to our
north, continuing to filter in cooler air to the area. At the same
time, another deep upper level low is forecast to drop south across
the west coast, before drifting slowly towards the east, across the
southern Rockies. These features will help keep much of the Northern
Plains under southwesterly flow aloft, which will result in a
somewhat active period of weather. Near daily chances (mostly 15-
30%) for showers and thunderstorms continue through this period,
though the chances for anything severe diminish quite a bit given
the lower temperatures and weaker forcing. For temperatures,
Thursday and Friday will continue to see highs mostly in the 70s,
with perhaps a very slight warmup on Friday as very subtle ridging
nudges its way between the two troughs. However, Saturday and Sunday
will see cooling once again, with temperatures decreasing into
mostly the 60s. Overnight lows may drop into the 40s during this
time as well.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 611 PM CDT Tue Sep 17 2024

VFR conditions to begin the 00Z TAF period. Low stratus remains
around the Turtle Mountains and may expand later this evening
and overnight, but at this time the consensus is that it will
remain north of KMOT. Otherwise mostly high cirrus to begin the
TAF period with scattered thunderstorms lifting south to north
later this evening and overnight. Included a tempo for Thunder
at KXWA, KDIK, KMOT and KBIS but higher probability looks to
remain over western ND. Did also include a VCSH at KJMS. Breezy
to windy conditions through the forecast period. Southeast winds
ahead of the convection shift more southerly behind the
convection. Windy south winds 15 to 30 knot late Wednesday morning
and through the afternoon.

&&

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

&&

$$

UPDATE...TWH
DISCUSSION...Besson
AVIATION...TWH