869
FXUS63 KBIS 201435
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
935 AM CDT Mon May 20 2024

.KEY MESSAGES...

- Temperatures will trend near to below normal this week, with
  highs in the upper 50s to around 70 and lows in the mid 30s to
  upper 40s.

- Medium to high chances for rain in the James River Valley
  tonight through Tuesday, with lower chances elsewhere.

- Medium to high chances for showers and a few thunderstorms
  across all of western and central North Dakota Thursday
  through Friday.

&&

.UPDATE...
Issued at 934 AM CDT Mon May 20 2024

Fog has mostly lifted across the area but a few patches may
linger for another hour or so. Otherwise, the forecast remains
relatively quiet this morning. No major changes were needed for
this update other than to blend in the latest observations to
the forecast.

UPDATE
Issued at 654 AM CDT Mon May 20 2024

Patchy fog has developed across portions of the state early this
morning. The greatest impacts in our forecast area have been
across the Prairie Pothole region and southern James River
Valley, and southwest Williams County including the Williston
Airport, where visibility has fallen below one quarter mile. The
fog should lift between 8 and 9 AM CDT.

&&

.DISCUSSION...
Issued at 351 AM CDT Mon May 20 2024

Surface low pressure is located between Mobridge and Aberdeen, South
Dakota early this morning, with the parent mid/upper level wave back
to the northwest from southeast Saskatchewan into northwest North
Dakota. The wave continues to produce scattered showers across
northwest and north central North Dakota, which are forecast to
diminish and lift northeastward into southwest Manitoba through the
morning. Some parts of southwest and central North Dakota that have
remained cloud free for most of the night have seen temperatures
drop into the upper and even mid 30s. This includes Golden Valley,
western Slope, and western Bowman Counties, as well as areas from
around Beulah/Hazen to Garrison, Max, and Velva. Patchy frost could
develop in these areas early this morning.

Aside from the morning showers dissipating in the far north, a
mostly dry day is expected. Some models bring high based showers
into far south central and southeast North Dakota later this
afternoon and evening along a band of mid level frontogenesis. Given
forecast cloud bases around 10-15 kft (aside from diurnal cumulus),
this will likely manifest as virga. Elsewhere, cyclonic southwest
flow aloft could generate some light isolated to widely scattered
showers this afternoon and evening, with the highest chance of
occurrence (albeit still quite low) in the northwest. Highs this
afternoon are expected to be in the 60s.

A progressive shortwave over the western CONUS will force the
development of a Colorado low late tonight, with strong model
consensus on a northeast track into southern Minnesota on Tuesday.
Most of the rain from the wrap-around deformation band will miss our
forecast area, but the southern James River Valley could get clipped
with a few tenths of an inch. The synoptic pattern looks to remain
active thereafter, but stronger forcing is forecast to avoid the
Northern Plains through Wednesday. Nevertheless, some weaker
disturbances could generate a few showers at times Tuesday through
Wednesday. But most locations are likely to remain dry through the
middle of the week. The NBM keeps temperatures slightly below normal
over this time period, with highs in the 60s and lows in the upper
30s to mid 40s. However, should any overnight period end up with
light winds and a clear sky, temperatures could fall closer to
freezing. Tuesday night looks to the have the greatest frost
potential at this time, with surface ridging forecast over the
western half of the state.

A much stronger wave is forecast to dig into the Northern Plains
Thursday into Friday. The greatest ensemble spread is on the timing
of this system, with latitudinal displacement being a secondary
source of uncertainty. 00 UTC ensemble cluster analysis appears to
favor a slower and slightly more northward progression by a ratio of
3:2, and this scenario is heavily weighted by the ECMWF and Canadian
ensembles. The GEFS dominates the southern and quicker solution,
though its parent deterministic model now more closely aligns with
the former. Some parts of western and central North Dakota are
likely to receive at least a few to several tenths of an inch of
rain from this system, with the NBM showing a 40 to 60 percent
chance of exceeding half an inch across the entire state. Depending
on the timing and track of the system, there could be a strong to
severe storm risk. This appears more likely should the slower
progression come to fruition, though weak moisture return in the
warm sector could be a limiting factor. Daytime temperatures over
this two-day period are likely to be slightly cooler, especially
Friday when forecast highs are only in the mid 50s to lower 60s, but
overnight lows could be kept in the 40s by the increased cloud
cover. Ensemble guidance then hints at a slight warming trend for
the holiday weekend that ensemble mean height fields show could
feature another shortwave passage Sunday into Memorial Day.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 654 AM CDT Mon May 20 2024

Patchy fog has developed across parts of western and central
North Dakota early this morning. The fog has been dense at KXWA,
and visibility has been reduced as low as IFR levels at KMOT.
The fog should dissipate by 14Z. Low ceilings have also been
sporadically observed across the state this morning. Aside from
the LIFR ceiling associated with the fog at KXWA, MVFR has been
the lowest prevailing category, but a brief period of IFR
ceilings cannot be ruled out. By late morning, VFR conditions
are expected at all terminals and should continue through the
forecast period. Winds will become northwest around 10 kts
today, with some higher morning gusts from KBIS to KJMS.

&&

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

&&

$$

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