693
FXUS63 KBIS 051147
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
647 AM CDT Sun May 5 2024

.KEY MESSAGES...

- Strong southeast winds across western and south central North
  Dakota late this morning through tonight, with gusts as high
  as 50 mph in southwest parts of the state.

- Widespread showers and isolated thunderstorms are expected
  late tonight through Tuesday, especially in far western North
  Dakota.

- There is a conditional risk of strong thunderstorms Monday
  afternoon, mainly across southern parts of the state.

- Windy conditions continue on Monday (central and eastern North
  Dakota) and Tuesday (southwest North Dakota).

&&

.UPDATE...
Issued at 647 AM CDT Sun May 5 2024

Patchy fog from south central into southeast North Dakota
appears to have mostly dissipated as of 645 AM CDT. However,
denser patchy fog has now settled over parts of the Devils Lake
Basin and Turtle Mountains area. This newer fog should burn off
by mid morning. Otherwise, the forecast for today remains on
track.

&&

.DISCUSSION...
Issued at 412 AM CDT Sun May 5 2024

Patchy fog has developed across south central parts of the state
this morning, but does not appear to be dense as of 4 AM CDT. The
fog should lift by mid morning.

A strong upper low over the western CONUS will push upper level
ridging over the Northern Plains today, setting up a strong
southerly return flow with a surface pressure gradient tightening
between low pressure over the Northern Rockies and high pressure
over the Upper Mississippi River Valley. Southeast winds are
forecast to increase to 25 to 35 mph across western and south
central North Dakota this afternoon, with gusts as high as 50 mph.
The Wind Advisory has been expanded east of the Missouri River to
include McLean, Burleigh, Kidder, Emmons, Logan, and McIntosh
Counties, though the strongest winds are still expected in southwest
parts of the state. Rising heights will allow for a warming of the
boundary layer, with forecast highs returning to slightly above
normal readings in the mid 60s to lower 70s. However, the southeast
flow, which is notorious for poor BL mixing, along with a mid level
capping inversion will likely prevent high temperatures from
reaching the warmer 50th percentile of NBM guidance (closer to mid
70s across the western half of the state).

A nocturnal low level jet will intensify over the Northern Plains
later this evening through tonight. There is some uncertainty on how
much of this will be reflected at the surface, especially given the
well-document NBM high bias for winds in nocturnal southerly flow
regimes. It is possible that sustained speeds could stay near, if
not above advisory criteria through the night as the core of the LLJ
translates through. For this reason, the Wind Advisory for all
counties has been extended in time until 4 AM CDT Monday morning.
The stronger overnight winds combined with increasing clouds will
keep low temperatures much warmer than recently, with forecast
values in the upper 40s to mid 50s.

The aforementioned western CONUS low is expected to pivot into a
negatively-tilted trough while maintaining a closed circulation over
the Northern Plains on Monday, initiating strong surface
cyclogenesis over western North Dakota. Chances for showers, along
with a few elevated thunderstorms given HREF mean MUCAPE approaching
500 J/kg, will begin increasing over western North Dakota later
tonight. High-resolution guidance then favors shower and isolated
thunderstorm activity becoming more widespread across western and
central North Dakota through the morning hours on Monday.

There is a conditional risk for a few thunderstorms to become
strong, possibly even severe, Monday afternoon, but it would require
several ingredients to line up properly, the probability of which
remains low at this time. Forcing and shear should be more than
ample for stronger convection, with effective bulk shear likely
exceeding 40 kts. The main question is whether enough low level
moisture will be advected into the warm sector to create sufficient
buoyancy for surface-based deep moist convection. Both the HREF and
NBM are showing lower to mid 50s surface dewpoints with dewpoint
depressions less than 10 degrees Fahrenheit. The resultant buoyancy
forecast carries more uncertainty though, ranging from effectively
none to as high as 1000 J/kg. Because this is expected to be such a
strong dynamical system, it does seem reasonable that the low level
moisture advection could be underestimated by model guidance. Should
all these ingredients line up both spatially and temporally, low-
topped supercells could develop along the northward advancing
occluded front. Impressive low level kinematics and thermodynamics,
including 0-500m SRH over 100 m2/s2, very low LCL heights, and long-
curved hodographs with strong streamwise vorticity available for
right-movers to ingest, would support a tornado threat. The
background wind profile alone could support a convective damaging
wind threat. Large hail would be the least expected hazard given the
likelihood of low CAPE, a low equilibrium level, and stronger low
level shear. It is important to note that should this threat
materialize, which is a low-probability but reasonable worst-case
scenario, it would likely be very limited in both space and time,
most likely migrating through portions of southwest and/or south
central North Dakota during the mid to late afternoon hours. On the
other end of the forecast spectrum, a reasonable best-case outcome
is for the shear and buoyancy to not line up, resulting in no strong
convection at all.

The exact evolution of the surface low Monday evening through
Tuesday carries some uncertainty, with some guidance suggesting a
double-barrel low system from southern Saskatchewan to northwest
South Dakota. All guidance does seem to eventually congeal on a sub-
985 mb MSLP low on Tuesday, but with location uncertainty still
ranging from southern Saskatchewan into western North Dakota.
Cluster analysis of global ensemble systems remains evenly split on
a northwest vs. southeast outcome for the placement of the surface
low on Tuesday. In spite of this, there has actually been a
consolidation of mean QPF among these clusters. Far western North
Dakota maintains medium to high chances for exceeding 1 inch of
rainfall through Tuesday, as most ensemble solutions now bring the
wrap-around deformation band back into the state. Elsewhere, we are
still expecting a dry slot to significantly limit rainfall amounts
for at least portions of central North Dakota over the Monday night
through Tuesday time frame, but there is still some uncertainty in
its exact placement and timing.

Near to below normal temperatures are favored through at least the
middle of the week, with highs mostly in the 50s and 60s, coolest in
the southwest on Tuesday and Wednesday where higher chances for rain
and greater forecast cloud cover exist. Ensemble guidance now seems
to favor a quicker deamplification of the low on Wednesday, but
chances for lighter rain showers will remain in the forecast into
Thursday, decreasing over time. A chaotic synoptic pattern is then
hinted at by some deterministic guidance later in the week, but with
general troughing to the east and ridging to the west. A slow
transition to warmer weather in time for next weekend is favored by
ensemble 850 mb temperature anomalies and NBM surface temperature
distribution trends.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 647 AM CDT Sun May 5 2024

Patchy fog will remain possible at KJMS early this morning,
reducing visibility as low as IFR levels at times. Otherwise,
VFR conditions are expected through the forecast period.
Southeast winds will become strong across the western half of
the state this afternoon through tonight, with sustained speeds
around 20-30 kts and gusts as high as 30-40 kts. If overnight
wind speeds end up lower than what is currently forecast, low
level wind shear could become a concern. Later in the night,
chances for showers and possibly a few thunderstorms will begin
to increase in western North Dakota.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 10 AM CDT /9 AM MDT/ this morning to 4 AM
CDT /3 AM MDT/ Monday for NDZ001-009-010-017>020-031>034-
040>045.
Wind Advisory from 2 PM this afternoon to 4 AM CDT Monday for
NDZ021-035-036-046-047-050.

&&

$$

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