151
FXUS63 KBIS 210944
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
444 AM CDT Tue May 21 2024

.KEY MESSAGES...

- Rainy and windy in the southern James River Valley today.

- Patchy frost could develop across western and parts of central
  North Dakota tonight into Wednesday morning.

- Medium to high chances (50 to 80 percent) for rain Thursday
  afternoon through Friday morning.

- Unseasonably cold Thursday night through Friday night, with
  lows in the mid and upper 30s and highs on Friday only in the
  mid 40s to mid 50s.

&&

.DISCUSSION...
Issued at 444 AM CDT Tue May 21 2024

A closed upper low over southwest Saskatchewan with a trough
extending south through Yellowstone National Park places southwest
flow aloft over the Northern Plains early this morning. At the
surface, a Colorado low is intensifying over northwest Kansas, with
an MCC to the north and east from Nebraska to Iowa. The low is
forecast to move into central Minnesota this afternoon. Showers in
the deformation northwest quadrant of the low are likely to reach
the southern James River Valley today, where we are carrying a 60 to
85 percent chance of rain. Deterministic QPF from the WPC contains a
very sharp gradient, ranging from around one tenth of an inch from
LaMoure to Ellendale to around half an inch in the southeast corner
of Dickey County. NBM probabilistic QPF fields also imply a sharp
gradient, but are actually shifted a little farther west with
chances for exceeding half an inch of rain as high as 40 percent
from LaMoure to Ellendale. The tight pressure gradient with the low
will also intrude the southern James River Valley today, with mean
boundary layer winds forecast around 30-40 kts. This could translate
to winds approaching advisory criteria of 30 mph sustained and 45
mph gusts across Dickey and eastern LaMoure Counties this afternoon.
Elsewhere for today, scattered showers could develop this afternoon
along a low level convergence zone across central North Dakota. A
thunderstorm or two cannot be ruled out with up to around 200 J/kg
CAPE, but a shallow LFC-EL layer minimizes this potential. The
southern James River Valley will be the coldest part of the forecast
area today under the rain and clouds, with highs in the mid to upper
50s. Expect 60s elsewhere for highs today.

A surface ridge is forecast to build into western North Dakota
tonight, with flow aloft beginning to transition from cyclonic to
anticyclonic. This should set the stage for strong radiational
cooling, although there are some questions on lingering cloud cover.
However, observations over the past few nights have shown a tendency
for temperatures plummeting into the mid and upper 30s in low lying
areas with light winds and little to no cloud cover. We are using
the 25th percentile of NBM guidance for low temperatures tonight,
which paints widespread values around 34 to 37 across the western
third of the state. If confidence in this forecast remains high, a
Frost Advisory will need to be strongly considered for tonight into
Wednesday morning.

Wednesday should be dry for most areas with surface to upper level
ridging over the Northern Plains. There are only low chances (10 to
20 percent) for showers from the Turtle Mountains to the Devils Lake
Basin in the remnant cyclonic flow from the departing low, and in
western North Dakota later this afternoon and evening as shortwave
energy ejecting from the base of a Pacific Northwest low begins to
approach the region. Highs on Wednesday are forecast to mainly be in
the 60s. A backdoor cold front is forecast to move into far northern
North Dakota Wednesday evening, which could allow temperatures there
later that night to fall into the mid and upper 30s.

The Pacific Northwest low is forecast to dig into the Northern
Rockies/upper Great Basin and eject into the Northern Plains
Thursday into Friday. Latest ensemble cluster analysis favors a
slower progression of the surface low/trough and its attendant
precipitation, which is focused over the Thursday night time frame
for western and central North Dakota (70 percent membership). But
there is still a lower probability outcome for a quicker passage
with much lower QPF (30 percent membership, mostly GEFS). All
ensembles seem to have shifted to the south with both the
upper/trough wave and in particular the surface low, for which there
are no longer any clusters that bring the low fully into North
Dakota. However, one of the clusters that contains over half of the
ECMWF members brings the major axis of the QPF shield from west
central to northeast North Dakota. The two dominant clusters each
have a 30 to 60 percent chance of exceeding half an inch of rain
with this system (the non-ECMWF majority cluster is shifted more
over southern North Dakota), but the two GEFS-driven minority
clusters only have low chances (10 to 30 percent) for exceeding just
a quarter of an inch, with zero chance in the northwest. The
southward shift of the forecast track of this system in all
ensembles has also lowered the chances for not only strong to severe
storms, but also thunderstorms in general.

The passage of this trough/low is forecast to drag an unseasonably
cool air mass into the Northern Plains. Prior to its arrival, there
is a larger spread in NBM guidance for highs on Thursday, likely
owing to arrival time uncertainty in both rain and clouds. But
forecast confidence is higher for mid to upper 50s across the north
than for lower to mid 60s across the south. There is much higher
confidence in lows in the mid to upper 30s Thursday night, even in
the presence of widespread precipitation. Wet bulb temperatures are
forecast to remain above freezing, which would preclude a mixed
hydrometeor phase. However, a handful of GEFS members and a majority
of ECMWF members are now showing potential for a geographically
small area of snow Thursday night into Friday morning. The high
temperature forecast for Friday has shifted notably colder from
previous iterations with the NBM now advertising only mid 40s to mid
50s, and think this could actually be too warm if clouds persist
through the day. Friday night carries the highest potential for
frost over a larger geographic area, with light winds, decreasing
clouds, and forecast lows currently in the mid 30s.

The NBM shows a quick warmup back into the 60s for highs through the
holiday weekend, but 850 mb temperature anomalies in 00 UTC global
ensembles do not seem to support that drastic of a warmup, so it
will be interesting to see how the forecast evolves. It follows that
there is growing ensemble support for another shortwave passage
Sunday into Memorial Day that would likely inhibit the warmup and
bring another round of rain chances through the region.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 444 AM CDT Tue May 21 2024

VFR conditions are expected through the forecast period. Light rain
will fall over the southern James River Valley later this morning
and afternoon. The rain could reach KJMS, but categorical
visibility restrictions are unlikely there. More widely
scattered showers could develop this afternoon from around KBIS
to east of KMOT. Northeast winds will increase to around 20 kts
at KJMS this afternoon, with gusts approaching 30 kts. 10-15 kt
winds are expected elsewhere today, with a northerly direction
in central North Dakota and northwesterly in western North
Dakota.

&&

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

&&

$$

DISCUSSION...Hollan
AVIATION...Hollan