867
FXUS63 KBIS 171157
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
657 AM CDT Sat May 17 2025

.KEY MESSAGES...

- Near to sub-freezing temperatures across western and north
  central North Dakota this morning, and northern and south
  central North Dakota tonight into Sunday morning.

- Rain chances return Sunday through Tuesday, highest along the
  South Dakota border (near 100 percent) and lowest in the
  Turtle Mountains area (around 30 percent).

- Well below normal temperatures expected through Tuesday, then
  warming to near normal to end the work week.

&&

.UPDATE...
Issued at 657 AM CDT Sat May 17 2025

The mesoscale area of dense fog in southwest North Dakota has mostly
diminished. Confidence in the sky cover forecast for today is not
that high, with uncertainty in how long low clouds will persist in
the eastern half of the state and mid level clouds trying to push
across the Montana border. Otherwise, the forecast for today remains
on track. Numerous locations across western and far north central
North Dakota have seen temperatures fall to and just below
freezing. Gusty northerly winds across the eastern half of the
state will gradually weaken through the day. The Turtle
Mountains area continues to experience a local enhancement of
wind speeds likely due to orographic processes, but winds there
should begin to relax soon as well.


&&

.DISCUSSION...
Issued at 432 AM CDT Sat May 17 2025

The stacked low system responsible for the rain event of the past
few days is drifting into the western Great Lakes region early this
morning. In its wake, a low-amplitude upper level ridge with
positive tilt cuts through the region from near the Black Hills to
Lake Winnipeg, with surface ridging building into eastern Montana and
the western Dakotas. The surface to mid level cyclonic flow
associated with the downstream low still extends as far west as
central North Dakota, where cloud cover is prevailing. The clouds
have kept overnight temperatures slightly warmer than anticipated,
but they are still mostly in the mid to upper 30s. Meanwhile, a
clearer sky in western North Dakota has allowed some locations to
drop into the lower 30s. Becoming more pessimistic in the formation
of fog from radiational cooling processes. Nevertheless, a peculiar
looking narrow band of very low stratus and fog is migrating
westward across southwest parts of the state at the time of this
writing. For the most part, the ongoing Freeze Warning and Frost
Advisory are tracking well, and no changes are planned.

The surface ridge will drift eastward across the state today before
stalling over eastern North Dakota tonight. This should help clear
the ongoing low clouds, but the RAP interestingly keeps the eastern
half of the state socked in through the entire day. Think that may
be too aggressive given the solar angle this time of year, but
confidence in a sunnier day today has decreased, and there are also
now mid level clouds approaching from the west. These cloud
uncertainties translate to the high and low temperature forecasts for
today and tonight. While the high temperature forecast for today is
noteworthy for being well below normal, ranging from the mid 40s in
the Turtle Mountains to around 60 in the far southwest, of greater
importance is the low temperature forecast for tonight, for which
the NBM strongly favors lower 30s for most areas north and east of
the Missouri River. It is still likely that a headline for frost or
a freeze will be needed tonight into Sunday morning, but given the
aforementioned uncertainty as well as the ongoing headlines, will
delay making headline decisions for tonight until closer to midday,
which should hopefully give us a better handle on cloud trends.

A potent shortwave is forecast to dig into the Great Basin region
this evening, eject into the Central Plains by Monday, and then
possibly enter a Fujiwara effect with a trailing shortwave
originating from the Pacific Northwest. This setup will bring
another round of beneficial rain to southern North Dakota, with
confidence in rain decreasing to the north and east. Some lighter
showers could develop as early as late tonight in southwest North
Dakota, with CAMs then suggesting several waves/bands of showers
moving north through the western half of the state through the day
Sunday. The highest chances for rain, which are essentially 100
percent, arrive in southwest North Dakota late Sunday night and
spread eastward along and south of I-94 through Monday night, which
is the period of time that the lead shortwave will be closest to the
state while it closes off into an upper low circulation. The
prospects for rain in northwest North Dakota have increased due to
their forecast residence time under isentropic ascent Sunday night
into Monday. But north central parts of the state could be kept
mostly dry by a stout high pressure near Hudson Bay advecting drier
air in from the east. Forecast confidence in daily rainfall is
comparatively lowest on Tuesday when cluster analysis reveals a
slower departure of rain from non-GEFS based members. The latest NBM
shows event total probabilities of exceeding 0.25" from around 95
percent in southwest North Dakota to only 25 percent in the Turtle
Mountains. Furthermore, there are medium chances for exceeding an
inch of rain along and south of I-94 over the 3-day span of Sunday
through Tuesday. The increased cloud cover and precipitation will
keep daytime temperatures well below normal, with forecast highs
mostly in the mid 40s to mid 50s Sunday through Tuesday, coldest in
the southwest on Monday. Another item of note for this time period
is that sustained wind speeds could approach advisory criteria (30
mph) in parts of southwest North Dakota on Sunday and south central
North Dakota on Monday.

A transition to a warmer and drier pattern is favored by longer
range ensemble guidance for the second half of next week as an upper
ridge tries to build underneath a quasi-zonal northern stream. The
NBM shows high chances for daily maximum temperatures returning to
the 60s for Thursday and Friday. It also maintains daily low chances
for rain from Wednesday onward, but there are no strong signals for
another significant rainfall event at this time.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 657 AM CDT Sat May 17 2025

MVFR ceilings are expected across central and eastern North Dakota
this morning, with KBIS and KMOT near the edge of the low stratus.
The ceilings will likely raise and/or scatter from west to east this
afternoon, but forecast confidence in this occurring is only medium.
For western North Dakota, patchy dense fog remains possibly early
this morning, but has now cleared away from KDIK. VFR conditions are
otherwise expected, with mid level clouds pushing in from the west.
A few rain showers could develop in southwest North Dakota near the
end of the forecast period. Gusty northerly winds across the eastern
half of the state  will gradually weaken throughout the day and turn
northeasterly at around 5 kts tonight. For the western half of the
state, expect light winds turning easterly today and increasing to
10-15 kts late tonight.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Freeze Warning until 10 AM CDT /9 AM MDT/ this morning for
NDZ001>005-009>011-017>019-021-031>033-040-041-043-044.
Frost Advisory until 10 AM CDT /9 AM MDT/ this morning for
NDZ012-013-020-022-034-035-042.

&&

$$

UPDATE...Hollan
DISCUSSION...Hollan
AVIATION...Hollan