Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 191847

Area Forecast Discussion
National Weather Service Bismarck ND
147 PM CDT Mon Jun 19 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 147 PM CDT Mon Jun 19 2017

The risk of strong to severe thunderstorms from north central ND
into the James River Valley this afternoon will be replaced with
cool overnight temperatures and a day with light winds Tuesday.

As of 1845 UTC, low-topped convection is intensifying across far
southern Canada ahead of a strong mid-level vorticity maximum
that is forecast to reach northeast ND by early evening. Forcing
for ascent from resultant DCVA aloft and in association with a
southeast-moving surface cold frontal zone is still expected to
drive rapidly-southeastward-moving thunderstorms this afternoon.
Forecast and RAP-objectively-analyzed soundings still support a
weakly buoyant air mass with MLCAPE up to 500 J/kg, but intense
deep-layer wind fields are yielding 0-6-km bulk wind differences
of 60 to 70 kt in support of an organized, low CAPE, high shear
convective situation. CAMs through the 17 UTC HRRR and 15 UTC ESRL
HRRR remain in strong agreement with their support of potentially
severe convection with maximum updraft helicity tracks of 50 to
75 m2/s2 in the 20 to 00 UTC time range, especially in north
central ND where brief supercell structures may be favored before
upscale growth into a line perhaps occurs with expanding cold
pools. The high-based nature of the convection related to the
relatively dry boundary layer will favor evaporational cooling and
thus enhanced downdrafts, and the strong environmental winds will
favor strong momentum transfer, both in support of potentially
damaging wind gusts at the surface. The cold temperatures aloft
and strongly-tilted updrafts suggest potential for marginally-
severe hailstones with the most intense cores, as well, though
HAILCAST output from the 00 and 12 UTC NSSL WRF-ARW was very
unimpressive with peak model hail sizes less than an inch.

We expect convection to rapidly shift southeast of the area by
about 01 UTC. Surface high pressure will follow the cold front
into the area overnight and is expected to be in control of the
weather during the time of optimum radiational cooling prior to
sunrise. Given observed temperatures upstream in Canada beneath
that ridge axis and the tendency for guidance to underestimate
radiational cooling in these scenarios, we relied on the coldest
edge of 12 UTC model guidance for forecast lows tonight. Thus, we
are advertising widespread lows in the lower to middle 40s F.

The proximity of the surface ridge will ensure light winds for
once on Tuesday, while building heights aloft will yield a dry
day, with forecast highs from the 70s F in the James River Valley
to the mid 80s in southwest ND where southerly return flow and
warm advection will take hold by afternoon.

.LONG TERM...(Tuesday night through Monday)
Issued at 147 PM CDT Mon Jun 19 2017

Very cool, below-normal temperatures from late this week through
the weekend highlight the long term forecast period.

Based on the 00 and 12 UTC global models, our confidence remains
high that a mean 500 mb trough will develop over the region by
Friday in response to an upstream ridge building across British
Columbia. The transition from initially-zonal flow aloft will be
heralded by a dynamic shortwave trough and surface cold frontal
passage Wednesday afternoon. Mid-level warm air advection on the
nose of a 35-45 kt low-level jet in advance of that shortwave may
yield some elevated convection Tuesday night and Wednesday. Model-
derived soundings suggest enough MUCAPE in the face of steepening
mid-level lapse rates associated with a pulse of elevated mixed
layer air spreading across the northern Plains to support a few
thunderstorms in that batch of elevated convection. Any chance of
stronger, surface-based thunderstorms on Wednesday afternoon is a
topic of much lower confidence owing to uncertainty with both the
potential for early-day convection to slow destabilization, and a
more real possibility that the surface cold front will have moved
east of the local area by the time that wind shift is able to act
upon the destabilizing boundary layer. True to that point, the 12
UTC GFS has the front into the Red River Valley by the time that
occurs, while the 12 UTC NAM simulates a slow enough passage to
allow for afternoon storms in the James River Valley. We are more
certain that gusty northwest winds will likely follow that front,
in the west Wednesday, and area-wide on Thursday.

By Friday and Saturday, the 00 UTC NAEFS calls for temperatures
at 850 mb falling to the 4 to 8 C range underneath the trough,
which is near the 10th percentile of those temperatures for this
time of the year. The 00 UTC CIPS Analog guidance not only calls
for a 95 percent probability of below-normal temperatures for the
weekend, but its actual analog dates (most of which were from May
and early June) only had highs greater than 70 F in roughly 30
percent of the cases. The opportunity for precipitation in that
time is less certain, with the 00 UTC ECMWF slower and stronger
with a shortwave trough and thus wetter than both the 12 and 00
UTC GFS runs. The GFS ensemble plumes suggest wide spread in the
possibility of precipitation, as well, but it`s worth noting that
the 12 UTC GFS did indeed take a step toward the wetter side of
solutions, albeit with a faster wave timing than the ECMWF.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 147 PM CDT Mon Jun 19 2017

A rapidly-moving line of strong to possibly severe thunderstorms
is forecast to move southeast across north central and extreme
northwest ND into the James River Valley this afternoon, exiting
the area by 01 UTC. Localized MVFR to IFR conditions, damaging
surface winds and large hail are possible with the most intense
storms. Otherwise, VFR conditions will prevail for the 18 UTC TAF
cycle as high pressure builds into the region overnight. Gusty
northwest winds will also diminish this evening.




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