Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 211737
AFDBIS

Area Forecast Discussion
National Weather Service Bismarck ND
1237 PM CDT Fri Jul 21 2017

.UPDATE...
Issued at 1236 PM CDT Fri Jul 21 2017

Pops were increased north central ND early this afternoon in
response to a convective line that developed - likely associated
with an upper level impulse moving east across the northern part of
the state today. Latest CAM models still favoring south central ND
later this afternoon for severe storm development...so increased
pops south central this afternoon/evening when the mid-level cap
may break as temps increase. Activity expected to linger in the
James Valley through the evening hours.

UPDATE Issued at 1013 AM CDT Fri Jul 21 2017

PoPs were increased across northwest and north central ND through
the morning with this update in respect to the elevated convection
crossing that area in association with a weak impulse translating
eastward. MUCAPE up to 1000 J/kg and effective-layer shear values
of 40 to 50 kt suggest some potential for severe weather with this
elevated convection. Moreover, there is a non-zero chance that the
convection will become more surface-based and thus intensify in
the next few hours, but recent RAP iterations suggest MLCIN won`t
weaken sufficiently for that until 19 or 20 UTC. The environment
across central and eastern ND this afternoon is still expected to
be characterized by MLCAPE of 2500-3000 J/kg and effective shear
around 50 kt. Anvil-level flow will also be significant in support
of potential supercells with very large hail initially, and a low,
but non-zero tornado potential given a discrete supercell moving
through this CAPE setting. Upscale growth could occur eventually
as cold pools aggregate by late afternoon or evening. CAMs through
the 13 UTC HRRR and 12 km NAMNest remain somewhat dispersive with
regard to the exact convective evolution, but may be displaying a
bit more clustering in advertising initiation occurring close to
Bismarck/Mandan in the 21-23 UTC time frame. Again, there`s still
some uncertainty with how the morning convection will play out in
north central ND, but if trends in recent CAMs continue with the
next few hourly-update cycles, our confidence in the specifics of
timing and evolution may increase by midday.

UPDATE Issued at 635 AM CDT Fri Jul 21 2017

Not much change with this forecast update for today, as the severe
potential remains unchanged, as well as the uncertainty in the
CAMs through the 10 UTC HRRR. Did add PoPs for thunderstorms
across northwest North Dakota this morning. This could be a signal
for an upper level shortwave across eastern Montana that could
yield a round of earlier initiation north central around midday.
However, this remains somewhat uncertain.

&&

.SHORT TERM...(Today and tonight)
Issued at 401 AM CDT Fri Jul 21 2017

Severe potential this afternoon and evening central, and near
critical fire weather conditions southwest highlight the short
term forecast.

See the fire weather discussion below for details regarding near
critical conditions southwest this afternoon and early evening.

Regarding severe convective potential, uncertainty still remains
in how the event may evolve as highlighted by run to run
inconsistencies across the CAMs through the 07 UTC HRRR. However,
the potential for large, destructive hail to 2 inches in diameter
or greater, damaging wind gusts to 75 mph, and a tornado or two
are possible across most of central North Dakota this afternoon
and evening.

A cold front approaching the US Highway 83 corridor around 18 UTC
with a likely pre-frontal trough, with a weak stationary front
across the US Highway 2 corridor will be the surface focal points
for initiation. Aloft, a few weak impulses across southeast
Montana this morning will propagate into south central North
Dakota by midday. A second stronger wave across southern Canada
arrives late this afternoon and early evening along the
international border. A moist boundary layer characterized by
surface dewpoints in the mid to upper 60s, possibly in the 70s
across the James River Valley, beneath steep mid level lapse rates
yields strong MLCAPE values of 2000-3000 j/kg. Deep layer shear
of 30-40kts, locally higher, perpendicular to the cold front/pre-
frontal trough favors discrete supercells in a highly unstable
environment. Thus, all threats with severe convection are
possible as mentioned above. The one limiting factor regarding the
tornado potential is weak low level flow and resulting SRH and
shear. However, given the strength of instability, relatively low
LCLs and likely supercells, a tornado or two is possible. Timing
of initiation is uncertain, as a couple rounds of initiation are
possible with the various forcing features. However, initiation as
early as 18-20 UTC is possible along the pre-frontal trough with
the earlier impulses, with a later round of initiation possible
with the stronger Canadian wave. A special 18Z sounding will be
launched today at Bismarck.

.LONG TERM...(Saturday through Thursday)
Issued at 401 AM CDT Fri Jul 21 2017

Critical fire weather conditions Saturday highlight the extended
forecast. See the fire weather discussion below for details
regarding the possibility for critical conditions southwest and
south central Saturday.

Will continue to monitor the potential for severe convection on
Monday as a cold front is favored by the 00 UTC global suites to
propagate across North Dakota.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1236 PM CDT Fri Jul 21 2017

An upper level impulse moving east across northern ND late this
morning is expected to continue moving east this afternoon. Latest
convective cells have organized into a line near Bottineau/KMOT/New
Town. This elevated activity should continue moving east.

Otherwise, at noon CDT, a surface low had entered northwestern ND
and was located near KISN/Tioga/Stanley area. This low is forecast
to reach central ND near KBIS/KMOT by 21z-00z, and near KJMS by
around 03z. Dry air and westerly winds west of the front with
east/southeast winds and humid air ahead of it is forecast.
Thunderstorm initiation along this boundary is expected this
afternoon, with best chances near KBIS by around 21z-00z according
to the latest high res models.

Thunderstorms should increase in coverage late this afternoon into
the evening near KMOT/KBIS and into KJMS around 00z-04z. Severe
thunderstorms are expected with large hail and damaging winds.
Uncertainty regarding exactly where/when convection will initiate,
will continue to mention VCTS at KMOT/KBIS/KJMS through 03z, and
through 06z at KJMS.

&&

.FIRE WEATHER...
Issued at 401 AM CDT Fri Jul 21 2017

Near critical fire weather conditions are possible behind a cold
front this afternoon across southwest North Dakota. Relative
humidity is expected to fall to 15 to 20 percent with high
temperatures in the 90s. The source of uncertainty is forecast
wind behind the front at 15 to 20 mph sustained from the west,
which at this time may fall short of critical levels and occur
during a narrow window. This will closely be monitored.

For Saturday, afternoon humidity between 15 and 25 percent,
northwest winds sustained at 20 to 25 mph with higher gusts, and
temperatures in the 80s are forecast across southwest and south
central North Dakota. When combined with dry fuels, critical fire
weather conditions are possible. A Fire Weather Watch is in
effect.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Saturday afternoon through Saturday
evening for NDZ020-031>035-040>046.

&&

$$

UPDATE...JV
SHORT TERM...PA
LONG TERM...PA
AVIATION...JV
FIRE WEATHER...PA



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