Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 220446

Area Forecast Discussion
National Weather Service Bismarck ND
1146 PM CDT Fri Jul 21 2017

Issued at 1146 PM CDT Fri Jul 21 2017

We extended the Severe Thunderstorm Watch until 06 UTC downstream
of ongoing strong and severe convection. Extrapolation of current
activity suggests it will likely exit the James River valley in a
few hours. In the interim, some severe threat continues both with
ongoing convection and new cells maturing over Sheridan and Wells
Counties ahead of the surface wind shift as of 0430 UTC. However,
inflow parcels should continue to slowly become more elevated in
time and that should slowly decrease the severe risk. Of course,
well-established mesocyclones in this setting will take time to
weaken even as parcels are forced through stronger capping as the
boundary layer slowly cools.

UPDATE Issued at 958 PM CDT Fri Jul 21 2017

Three distinct clusters of severe thunderstorms continue across
central ND as of 0245 UTC. We earlier expanded the Severe
Thunderstorm Watch and will likely have to consider extending it
in time for some counties before its scheduled 04 UTC expiration.
The environment remains characterized by MLCAPE of 2000-3000 J/kg
and effective shear of 40-50 kt. Despite increasing CIN for
surface-based parcels, non-hydrostatic pressure perturbations
associated with already-established mesocyclones may persist for
some time, even if updrafts become more elevated with time.

Otherwise, also added patchy smoke to the forecast overnight in
southwest and south central ND given the abundance of smoke seen
on GOES-16 imagery moving east of Montana.

UPDATE Issued at 603 PM CDT Fri Jul 21 2017

We refreshed hourly PoPs through mid evening with this update
based on observational trends. Attempts at convective initiation
have occurred along a density gradient (dewpoint discontinuity)
centered on Dunn County since 2145 UTC, but based on GOES-16
imagery it appears anvils have orphaned with that activity and
it`s not clear deep initiation will indeed occur in that area.
Mesoscale analysis data suggests mean LCL-LFC relative humidity
values are only near 30 percent, which may be indicative of dry
air entrainment to the detriment of deep convection. Nonetheless,
we will continue to monitor that area in case we need to expand
the Severe Thunderstorm Watch further northwest. Otherwise, in
south central ND other cumulus continues to be agitated and per
the mesoscale analysis, MLCAPE of 2000-3000 J/kg is now largely
uncapped. Additional convective initiation may thus occur in the
next 90 minutes with an attendant severe risk, including the
potential for supercells initially. The mean LCL-LFC relative
humidity values in the RAP-based SPC mesoscale analysis are also
higher south of Bismarck, implying less potential for dry air
entrainment. However, the degree of remaining MLCIN is still
somewhat uncertain given satellite trends to this point.


.SHORT TERM...(This evening through Saturday)
Issued at 301 PM CDT Fri Jul 21 2017

Severe potential this afternoon and evening central ND, and
critical fire weather conditions southwest and south
central on Saturday highlight the short term forecast.

See the fire weather discussion below for details regarding
critical conditions southwest and south central Saturday
afternoon and evening.

Currently, ongoing thunderstorm activity that developed across
northwest and north central ND continued from the Devils Lake Basin
westward into Mercer and McLean counties. This is believed to be
associated with an upper level impulse moving east across the state
today. At the surface, low pressure was near Watford City/Stanley
area of western Lake Sakakawea...with drier northwest winds to its
west and humid southerly to easterly winds to its east. A surface
trough extending from the low into central SD is forecast to move
eastward slowly tonight. Thunderstorms are expected to develop along
this boundary, with the high res CAMs indicating south central ND
and into the James Valley as the most likely area for increasing
thunderstorm coverage as the CAP erodes with temperatures rising
into the 90s.

MU Cape values of around 3000 J/kg in current mesoscale analysis
with generally 40 knots of 0-6km bulk shear.

A a severe thunderstorm watch will be in effect through much of this
evening south central to the James Valley.

.LONG TERM...(Saturday night through Friday)
Issued at 301 PM CDT Fri Jul 21 2017

A progressive pattern dominates the long term with occasional upper
level shortwaves in the westerlies moving across North Dakota. One
such wave on Monday afternoon/night will bring a chance of
thunderstorms to our area. Another wave will bring a chance
Wednesday/Thursday as well.

Highs in the 85 to 95 range Monday, and in the 80s for the rest of
next week is expected.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1146 PM CDT Fri Jul 21 2017

Scattered thunderstorms over east central ND and the James River
valley could produce local IFR conditions before they move out of
the area between 06 and 08 UTC. Otherwise, VFR conditions will
prevail elsewhere and area-wide on Saturday, though some smoke
will impact parts of southwest and south central ND, primarily
aloft. Northwest wind gusts to 30 kt are expected on Saturday.


Issued at 301 PM CDT Fri Jul 21 2017

For Saturday, afternoon humidity between 15 and 20 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 Saturday afternoon. When combined with dry
fuels, critical fire weather conditions are possible. A Fire Weather
Watch remains in effect for Saturday afternoon and early evening.


Fire Weather Watch from Saturday afternoon through Saturday
evening for NDZ020-031>035-040>046.



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