107
FXUS63 KBIS 041327
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
827 AM CDT Sat May 4 2024

.KEY MESSAGES...

- Low to medium chances (20 to 40 percent) for rain showers will
  diminish this morning across the far north central. More
  isolated showers (~20 percent chance) are possible this
  afternoon across the north central and northeast.

- Expect warmer highs today through Monday before we cool back
  into normal to below normal categories for most of the work
  week.

- Gusty southeast winds are likely Sunday through Monday.
  Sustained winds up to 35 mph will be possible with gusts to 50
  mph.

- Chances for rain return late Sunday evening and continue
  through the work week. The wettest period will be Monday
  through Tuesday when most of western and central North Dakota
  will see high chances (70 to 90 percent) of rain.

&&

.UPDATE...
Issued at 825 AM CDT Sat May 4 2024

For mid-morning update main change was to address the
persistant low clouds that continue to drop south through the
area, which are now starting to nudge into south central North
Dakota. On the west side, advancement has slowed down some,
while towards the east clouds continue to move right along.
Therefore, have adjusted cloud grids to address this
advancement.

UPDATE
Issued at 613 AM CDT Sat May 4 2024

For the most part, the weather is fairly quiet across western
and central North Dakota this Saturday morning. We have seen a
little bit of patchy fog develop across the far north, so we did
go ahead and add mention of this here until 15z. Otherwise, no
major changes were needed for this update.

&&

.DISCUSSION...
Issued at 347 AM CDT Sat May 4 2024

At the mid to upper levels, a trough axis currently sits over
central North Dakota and will continue to move east through the
morning as a western ridge starts to approach. Given the upper
level forcing of the trough, a few rain showers (20 to 40
percent chance) will remain possible across the far north
central, mainly north of the Highway 2 corridor.

Surface high pressure will move in today as the western ridge
nudges closer. Clouds should diminish from southwest to
northeast through the day and into the evening. We may see
enough diurnal heating and synoptic forcing for a few more
isolated showers across the north central and northeast this
afternoon (~20 percent chance). This activity should diminish
rather quickly around and after sunset given the loss of
heating. Highs will be a bit warmer today than in previous days,
ranging from the lower 50s to lower 60s. Areas that clear out
earlier today may end up a touch warmer.

The ridge axis then crosses the state on Sunday, which will
lead to even warmer temperatures and a mostly sunny sky. Highs
will range from the lower 60s around the Turtle Mountains, to
the lower 70s southwest. That being said, the surface pressure
gradient will tighten as a longwave trough approaches and a
surface low deepens significantly over northern Wyoming. This
gradient coupled with strong pressure falls and steep lapse
rates will lead to gusty winds out of the southeast. The latest
NAM/NAM Nest soundings have come in a bit stronger with winds at
the top of the mixed layer Sunday and Sunday night. For now it
appears that the strongest winds will be across the southwest
and south central where forecast soundings suggest the potential
for up to 50 knots at the top of the mixed layer. Elsewhere
across the west, we could see 40 to 45 knots at the top of the
mixed layer. All that being said, it certainly appears that we
will see higher winds than we usually see in these southeasterly
flow setups with some sustained values up to 35 mph and gusts
up to 50 mph. Wind headlines will likely be needed in future
shifts.

As the longwave trough approaches Sunday night, the
aforementioned surface low will move out of northern Wyoming
and into eastern Montana. An initial shortwave impulse will
overspread the low and we will see increasing chances of rain
across the west by the late evening hours. Several waves will
then move around the base of the potent trough and eject into
the northern Plains through the day on Monday as it takes on a
negative tilt. Another surface low moves into western South
Dakota by mid to late Monday morning and then deepens (sub 985
mb?) as it lifts into western or central North Dakota by the
late afternoon or early evening. By then, the system is forecast
to become stacked and nearly stationary or even retrograde back
into eastern Montana Tuesday into Wednesday. Cluster analysis
suggests that two distinct scenarios have started to show
themselves and at the moment it appears that each scenario has
about a 50/50 chance of happening. One scenario would have the
main low ending up in northeast Montana and the other would have
the low ending up somewhere across southwest North Dakota.
Scenario one would lead to the heaviest precipitation being
mainly over eastern Montana with a more pronounced dry slot over
central North Dakota. The second scenario would lead to some
heavier rain across the west (especially the southwest) and
maybe a less pronounced dry slot. No matter the scenario, it
does appear that we will at least see some dry slotting
somewhere across the central with the James River Valley and
vicinity being the most likely area.

Regarding thunderstorm potential on Monday, CSU Machine-
Learning guidance still suggests low chances for severe weather
in the afternoon and evening, but the signal continues to
weaken given meager instability in the forecast. Shear will be
strong and we could see some pockets of MUCAPE up to 500 J/kg or
so. Thus, a few strong storms may not be out of the question
but the severe threat continues to appear low.

The upper low remains stalled out over the northern Plains
through at least midweek before it washes out or moves out
towards the end of the week (models are having a tough time
deciding how this low predictability pattern evolves mid to late
week). This will mean more highs in the normal to below normal
categories (mainly in the lower 50s to lower 60s) Tuesday
through the rest of the week and continued chances for rain. The
period for heaviest rainfall will be Monday and Tuesday but the
highest NBM probabilities for an inch or greater have shifted
to the west, now maximized over portions of central and eastern
Montana. That being said, 72-hour NBM probabilities (ending 12z
Thursday) for an inch or greater still range from 40 to 60
percent across much of the west and central.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 613 AM CDT Sat May 4 2024

A band of MVFR to IFR stratus will continue to spread across
northern North Dakota this morning. KMOT and KXWA will likely
see MVFR ceilings until late this morning or early afternoon
while KJMS may see a period of MVFR ceilings this afternoon.
KXWA may also see brief ceiling reductions into IFR categories
over the next few hours. KDIK and KBIS should remain in VFR
conditions through the period. A few isolated showers may once
again develop across the north central and northeast Saturday
afternoon.

&&

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

&&

$$

UPDATE...JJS
DISCUSSION...ZH
AVIATION...ZH