Scientific Forecaster Discussion
fxus63 kbis 231958
Area forecast discussion
National Weather Service Bismarck ND
258 PM CDT Mon Sep 23 2019
Short term...(this evening through tuesday)
issued at 256 PM CDT Mon Sep 23 2019
This afternoon, North Dakota sits between an eastward progressing
upper level ridge and a mid level shortwave approaching from the
west. A surface low pressure is analyzed over central Canada, with a
southward extending cold front/surface trough just out ahead of the
mid level shortwave. These features will continue to move eastward
through the short term period. The initial surface front and mid
shortwave do not appear strong enough to produce precipitation.
However, mid level heights will continue to fall behind the
shortwave as an upper level jet streak moves from the Canadian
rockies into Montana, eventually placing western and central North
Dakota under the left exit region. These combined forcing mechanisms
along with increasing positive vorticity advection should be enough to produce scattered
showers across the area late tonight into Tuesday morning.
Late Tuesday morning through mid Tuesday afternoon is looking drier
as the local area sits between frontal boundaries. A secondary cold
front tied to a surface meso-low that is progged to form over the
northern Red River valley is forecast to plunge south into northern
North Dakota by late Tuesday afternoon. Global and hi-res models
indicate scattered shower activity along the front as it moves in.
Strong wind shear, steep lapse rates, and a few hundred j/kg cape
warrants a slight chance of thunder mention with this activity.
Winds will pick up on Tuesday as the surface pressure gradient
tightens. Sustained northwest winds will reach 20 to 30 mph during
the afternoon, gusting to 30 to 40 mph. The strongest winds will
be in the far southwest where Wind Advisory criteria may be
approached. It will be a bit cooler on Tuesday, but still expect
highs in the mid 60s to lower 70s.
Long term...(tuesday night through monday)
issued at 256 PM CDT Mon Sep 23 2019
A colder and more active pattern is expected for the long term
period. While the synoptic features are similar, their progression
has slowed down compared to previous model runs.
Chances for scattered showers and isolated thunderstorms will
continue along the cold front as it moves south through the area
early Tuesday night. On Wednesday, much of North Dakota will lie
to the north of an upper level jet, while upper level longwave
troughing deepens over Hudson Bay. This will lead to a cooler and
breezy day. Models have backed off on the potential for diurnally
driven scattered showers and isolated thunderstorms. Despite
cyclonic flow aloft and steep lapse rates, moisture looks rather
limited. Still left a slight chance of showers and storms
mentioned across portions of northwest and north central North
Dakota Wednesday afternoon, per nbm guidance.
As mean troughing continues to deepen over Canada, a shortwave is
forecast to eject off the southern Canadian rockies and move into
the northern plains late Wednesday into Thursday. Strong mid level
positive vorticity advection and low level warm air advection should result in rain developing across
northwest and north central North Dakota on Thursday, spreading
south and east into Thursday night.
Even colder air will begin to filter in behind the departing
shortwave on Friday. Some precipitation may linger into Friday as
well, as yet another shortwave quickly moves across southern
Manitoba. While 850 mb temperatures may fall below 0 c across
northern North Dakota on Friday, surface temperatures should
remain well above freezing, so neither snow nor frost should be a
A forecast change from this time yesterday is that surface high
pressure is now favored over the northern plains Friday night. If
this trend continues, the first widespread frost event of the
season may occur Friday night into Saturday morning.
This weekend, an upper level low is forecast to deepen over the
Pacific northwest. Models are in agreement that broad isentropic
ascent will lead to rain developing across the area on Saturday. The
boundary layer looks rather dry Saturday morning, so precipitation
onset may be delayed beyond what model guidance shows, as models
typically saturate the column too quickly in these scenarios.
Precipitation is forecast to become more widespread on Sunday
with pieces of energy ejecting from the Pacific northwest low and
a tightening mid level baroclinic zone.
Temperatures may become problematic over northwest North Dakota
early Sunday. Some models continue to hint at sub-freezing 850 mb
temperatures and near freezing surface temperatures, resulting in
chances for a rain/snow mix or even all snow. However, gefs
plumes continue to show considerable temperature spread, and both
deterministic and ensemble long range guidance has trended
slightly warmer over the past 12-24 hours. Still think that a
rain/snow mix cannot be ruled out over northwest North Dakota
Saturday night into Sunday morning, but accumulating snow appears
rather unlikely, especially given the warm ground temperatures.
Unfortunately, this system could bring at least 0.5" of quantitative precipitation forecast to
areas that simply don't need it. Signals for anomalously high
moisture are beginning to emerge in naefs. Current deterministic
models show 1-1.5 in precipitable water (gfs) and warm cloud
depths over 3500 M (ecmwf), values more typically seen in the
middle of Summer. Additionally, there are signals in both
deterministic and ensemble guidance for over 1000 j/kg cape over
southeast parts of our forecast area on Sunday. It's still a long
ways off and a lot can change, but it is beginning to look like
the potential for impactful weather will be increasing late this
week into next week.
Aviation...(for the 18z tafs through 18z Tuesday afternoon)
issued at 1248 PM CDT Mon Sep 23 2019
VFR conditions are expected through this forecast period. A chance
of scattered showers will move from west to east across ND late
tonight through Tuesday morning, accompanied by 5,000-10,000 ft
ceilings. Expect southwesterly winds around 10 kts this afternoon,
decreasing tonight. Tuesday looks to be quite breezy, with west-
northwest winds increasing to 15-25 kts by the afternoon.