Special Weather Statement, NWS Bismarck, ND

Area Forecast Discussion
National Weather Service Bismarck ND
104 PM CDT Thu Jul 20 2017

.UPDATE...
Issued at 1230 PM CDT Thu Jul 20 2017

Increasing concern for severe thunderstorm potential this
afternoon is the focus of this update with large hail and damaging
wind gusts being the main threats. A mid-level shortwave was
moving over central Montana early this afternoon, deepening a
surface low centered over southeast Montana. Two zones of
initiation will be the focus of storms this afternoon and
evening, one along a north-south oriented surface boundary over
the Montana/North Dakota border and another along a warm front
pushing north through western North Dakota towards the Canadian
border. Storms will initiate in an area of strong cross-boundary
effective wind shear and ML CAPE values at 1500-2000
J/kg...signaling large hail and damaging winds as the likely
threats. High-resolution models have been consistent run to run
with initiation between 20Z-22Z near the western North Dakota
border and moving on east. Uncertainty lies in the strength of
convection as they move further east later this evening, greatest
chance for severe weather will be early in initiation to the west.

UPDATE Issued at 913 AM CDT Thu Jul 20 2017

Convection over north central North Dakota has pushed off to the
east, while some light convection is starting to move into
northwest North Dakota. Still expect the main show to come later
this afternoon as latest run of high res models are advertising
strong convection developing around mid to late afternoon in area
of enhanced instability.

UPDATE Issued at 632 AM CDT Thu Jul 20 2017

Little change with this update other than for observed trends
through 11 UTC. Expect elevated convection to continue in the
Turtle Mountains area for the next one to two hours on the nose
of an 500 mb jet streak.

&&

.SHORT TERM...(Today and tonight)
Issued at 347 AM CDT Thu Jul 20 2017

Thunderstorm potential today and tonight and near-critical fire
weather conditions in the southwest this afternoon are the most
noteworthy topics in the short term.

Early this morning, elevated convection is occurring over north
central ND and is centered on McHenry and Towner Counties as of
08 UTC. While Convection-Allowing Models (CAMs) barely register
this activity in their simulations, perusal of plan view fields
suggests that this convection is likely occurring in association
with the warm, ascending branch of a frontogenetical circulation
centered in the 850 to 700 mb layer. RAP iterations through its
07 UTC cycle forecast that area of ascent to spread northeastward
with time, and perhaps re-establish itself further west by 12 to
14 UTC. Given weak MUCAPE for elevated parcels and recent trends
in lightning detection network data, a slight chance of storms is
prudent in the forecast over parts of north central ND through
about 15 UTC.

By this afternoon, the consensus of 00 UTC global models and more
recent CAMs is for a vorticity maximum currently observed across
west central MT to glide eastward into western ND amid seasonably
strong westerly flow aloft. A surface low will concurrently deepen
in eastern MT with increasing southerly flow across western ND and
distinct warming aloft with 850 mb temperatures forecast to reach
the 25-30 C range, supporting highs well into the 90s F. This will
set the stage for surface-based convective initiation, perhaps as
early as 19 or 20 UTC per some CAM simulations, and most favored
along a surface wind shift that should be close to the ND-MT state
line. Steep mid-level lapse rates near 8 C/km in the 700 to 500 mb
layer will be present, as will a very supercell-favorable wind
field characterized by strong directional shear with veering winds
aloft and 0-6-km and effective bulk wind differences around 50 kt.
However, the degree of available instability is rather uncertain,
with a wide range of afternoon dewpoint forecasts in guidance.
Most models hold MLCAPE in the 500 J/kg range thanks in part to
surface dewpoints dropping to the lower to mid 50s F in western
ND with afternoon mixing. That also yields considerable amounts of
MLCIN in forecast soundings from guidance such as the 00 UTC NAM.
We believe such a scenario would tend to marginalize the severe
threat, though certainly not eliminate it given the otherwise very
favorable scenario for large hail and damaging winds. Updraft
helicity forecasts from the 00 UTC NCEP WRF-ARW and 3 km NAMNest
reflect this scenario with peak values near 60 m2/s2. In contrast,
the RAP suggests much higher surface dewpoints this afternoon in
western ND, in the lower 60s F, in support of MLCAPE around 2000
J/kg. RAP-fed HRRR and ESRL-HRRR run-maximum updraft helicity
values are accordingly much higher. This would seem like the
lower-probability scenario, but afternoon mesoscale trends may
ultimately dictate an increase in severe probabilities if surface
moisture does indeed end up being higher than most models suggest.

We do have some concern for near-critical fire weather conditions
in southwestern ND this afternoon as gusty south winds will exist
ahead of the surface trough. Even so, conditions appear a bit too
marginal for a Red Flag Warning at this time.

Otherwise, elevated convective potential and an attendant marginal
risk of severe storms will continue tonight from northwest through
central ND on the nose of a 30-40 kt low-level jet based near 850
mb.

.LONG TERM...(Friday through Wednesday)
Issued at 347 AM CDT Thu Jul 20 2017

Seasonably warm weather will continue through next week, with the
main weather highlights being thunderstorm potential Friday and
Friday evening, and gusty post-frontal northwest winds Saturday.

The 00 UTC GFS, NAM, and ECMWF remained steadfast in advertising
afternoon MLCAPE of 2000-3500 J/kg on Friday with a much richer
pool of low-level moisture marked by surface dewpoints deep into
the 60s F, and even near 70 F in the James River Valley. A weak
low-level frontal zone is expected to be located in central ND,
and deep-layer bulk wind differences are expected to be from 40 to
perhaps 50 kt in support of a conditional severe risk. However,
forecast soundings suggest residual capping will persist through
the afternoon in many locations at the base of the warm elevated
mixed layer near 700 mb. This could keep storm coverage somewhat
low, but all of the 00 UTC global and CAM guidance nonetheless
supports at least scattered intense storms, mainly in far south
central ND in close proximity to the low-level thermal ridge and
most intense diabatic heating in support of cap erosion, and in
north central ND along the Canadian border where mid-level height
falls will be focused as a strong shortwave trough scoots through
central Canada.

Saturday will feature gusty northwest winds on cold air advection
as the aforementioned Canadian shortwave trough drops into Ontario
and the low-level height and pressure gradient tightens. This may
yield near-critical fire weather conditions in the southwest again.

Beyond that, semi-active and quasi-zonal flow aloft is forecast to
be maintained across the northern Plains next week in support of
seasonably warm temperatures and an occasional storm chance.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1230 PM CDT Thu Jul 20 2017

VFR conditions will prevail across western and central ND this
afternoon. However, thunderstorms may develop across western
ND along a surface pressure trough this afternoon, and could
then move into central ND by this evening (although we are not
confident they will survive that far eastward). A few strong to
severe thunderstorms may be possible with large hail and damaging
winds.

&&

.FIRE WEATHER...
Issued at 347 AM CDT Thu Jul 20 2017

Near-critical fire weather conditions could occur in southwestern
ND each of the next three days. However, there is uncertainty with
how low humidities will get today, and we currently believe that
the most likely scenario is for minimum values near 20 percent. A
few of our model simulations suggest much lower humidity near 10
percent could impact Golden Valley and western Slope and Bowman
Counties in the late afternoon if a dryline moves through, but if
that is the case then it would be accompanied by west winds which
would likely be less than Red Flag Warning criteria. There's also
some uncertainty in the wind and humidity forecasts both Friday
and Saturday, but both days will be monitored closely.

&&

.BIS WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

UPDATE...AE
SHORT TERM...CJS
LONG TERM...CJS
AVIATION...JV
FIRE WEATHER...CJS

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Special Weather Statement
National Weather Service Bismarck ND
147 PM CDT THU JUL 20 2017

NDZ009-201930-
Williams-
147 PM CDT THU JUL 20 2017

...A STRONG THUNDERSTORM WILL AFFECT CENTRAL WILLIAMS COUNTY UNTIL
230 PM CDT...

At 147 PM CDT, a strong thunderstorm was located 12 miles west of
Williston, moving east at 40 mph.

Winds gusts to 40 mph and half inch hail are possible with this
storm.

Locations impacted include...
Williston, Ray, Epping, Bonetrail, Wheelock, White Tail Bay
Recreation Area, Lewis And Clark State Park, Spring Brook and
Blacktail Lake.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

LAT...LON 4843 10404 4852 10308 4813 10301 4814 10304
      4813 10309 4815 10310 4813 10321 4809 10325
      4809 10359 4811 10361 4812 10359 4813 10359
      4811 10364 4812 10369 4810 10374 4810 10404
TIME...MOT...LOC 1847Z 268DEG 37KT 4822 10387

HEINERT