309
FXUS63 KBIS 121936
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
236 PM CDT Mon Aug 12 2024

.KEY MESSAGES...

- There are medium to high chances (40-80%) for showers and
  thunderstorms late Tuesday afternoon through Wednesday. A few
  storms could be strong to severe in western North Dakota
  Tuesday evening and into the night, and across much of
  western and central North Dakota on Wednesday.

- Very heavy rainfall and slow moving storms are also possible
  on Wednesday, leading to the potential for localized flooding.

- Temperatures will remain rather steady from Thursday into next
  week, with low chances for showers and thunderstorms each
  day.

&&

.DISCUSSION...
Issued at 228 PM CDT Mon Aug 12 2024

At the moment, surface high pressure is positioned across much of
North Dakota. Aloft, a weak ridge is sitting across much of the
central CONUS, with generally zonal flow across the Dakotas.
Throughout the daytime hours, cumulus has formed across essentially
the entire area, with the expectation that this cumulus will
dissipate heading into the evening hours. Troughing to the west has
allowed for very subtle moisture return, and given the ample daytime
heating under the clear sky this morning, the diurnal cumulus has
overperformed so far. We are not expecting any showers to form with
these, however a few prominent towers in the north central have
formed, with very weak radar returns present in one of them, near
Rugby. As the troughing begins nudging in from the west, the surface
pressure gradient has begun tightening up a bit, and we`ve seen a
slight increase in winds through the day, with a few areas seeing
some gusts up to 15 to 20 mph. High temperatures today are forecast
to be in the 70s, with a few areas seeing the low 80s in the far
northwest. Winds and cloud coverage should decrease overnight, with
overnight lows forecast to be in the mid to upper 50s.

As the trough to the west of us deepens and begins transiting the
High Plains, midlevel height falls ahead of the trough will help
provide enough forcing for the development of showers and
thunderstorms Tuesday night and into Wednesday. The SPC has outlined
Marginal Risks (level 1 of 5) for far western North Dakota on
Tuesday, and for much of central North Dakota on Wednesday. The
severe threat on Tuesday will remain mostly in the western parts of
the state, as the main axis of forcing will be situated across
Montana and Wyoming through most of the day. As that trough slowly
progresses eastward, a narrow corridor of instability is forecast to
develop across western North Dakota, helping aid in the development
of thunderstorms. The main limiting factor at this point appears to
be deep layer shear, with low values of 20 to 30 kts of 0-6km bulk
shear values forecasted across the area. Shower and thunderstorm
activity is likely, but the severe threat remains marginal at best.
Our current thinking for expected threats remains hail up to quarter
sized and winds up to 60 mph. Otherwise, Tuesday is forecast to
be the warmest day of the forecast period, as the low level
thermal ridge will be at its strongest ahead of the trough, with
highs in the lower to upper 80s. Given the continued approach
of the trough, breezy winds will also be possible across much of
the western and central portions of the state.

Wednesday will see the continuation of these showers and
thunderstorms, as the slow moving wave will continue making its way
across the Dakotas. This can be seen in the SPC`s Marginal Risk
shifting east, covering much of the central part of the state. Given
that its the same feature as before, the overall setup of this
severe risk is rather similar to Tuesday`s setup. Continued midlevel
height falls ahead of the trough will allow for the development of
showers and thunderstorms, however these showers and thunderstorms
will mostly form during the morning and afternoon hours. The chances
for severe weather decrease overnight as we cool off into the 50s,
but as daytime heating ramps back up again, the ample instability
will return. Once again, forecasted deep layer shear values appear
to be the limiting factor, lending to this being a rather marginal
threat. Another area of concern, however, is the forecasted
precipitable water (PWAT) values for much of our area, combined
with the slow moving nature of these storms, leading to concerns
for possible localized flooding. Current guidance is suggesting
the potential for PWAT values to approach 1.25 to 1.75 inches,
which is coming close to the climatological maximum for KBIS at
this time of year. Deterministic sounding forecasts also support
the potential for slow moving, training storms. As a result,
the WPC has outlined a Marginal Excessive Rainfall across much
of central North Dakota, in line with where chances for showers
and thunderstorms are maximized. Even though the severe threat
is marginal, we still do see rather high chances for rain during
the day Wednesday (ranging from 60 to 80%), which brings about
this risk for excessive rainfall. The main threats associated
with the severe weather on Wednesday will be hail up to quarter
size, winds up to 60 mph, and locally heavy rainfall. Otherwise,
highs on Wednesday will be in the mid 70s to mid 80s.

Heading into the weekend, current long range models suggest the
development of a ridge across the Rocky Mountains, slowly expanding
east heading into Friday. However, this expansion is expected to
halt, as the upper level flow is forecast to settle into an omega
blocking pattern, with troughing across the Great Lakes region and
Pacific Northwest, keeping this ridge across the Rockies in place.
As a result, from Friday onwards into the extended period, similar
conditions are being forecasted each day. Temperatures are forecast
to rise into the upper 70s to mid 80s, with the warmest temperatures
in the west, with overnight lows consistently in the 50s. While
we`ll be under the influence of a ridge aloft, the actual ridge axis
will be displaced to the west, allowing for a somewhat active
northwesterly flow aloft to be draped across the state. While
details are a bit fuzzy this far out, we can expect to see a few
shortwaves passing through this flow, allowing for low chances
(around 20 to 40%) for showers each day, the spatial extent of which
is uncertain at this time. The chances for severe weather remain
low, as this setup will not be very conducive for adequate moisture
return and midlevel forcing. The CSU machine learning guidance
currently keeps all severe chances out of the area from Friday
onwards.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1228 PM CDT Mon Aug 12 2024

VFR conditions are expected to prevail through the TAF period.
Fair weather cumulus has developed across much of the area,
resulting in a scattered deck around 4 kft, which is expected to
continue until the evening hours. Winds are initially out of the
southeast around 5 to 10 kts, beginning to increase in strength
around 16z Tuesday.

&&

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

&&

$$

DISCUSSION...Besson
AVIATION...Besson