Special Weather Statement, NWS Bismarck, ND

Area Forecast Discussion
National Weather Service Bismarck ND
239 PM CDT Tue Jul 4 2017

.SHORT TERM...(This evening through Wednesday)
Issued at 239 PM CDT Tue Jul 4 2017

Hot weather will continue through Wednesday, along with a chance
of thunderstorms.

Scattered, surface-based thunderstorms are expected to develop in
the 20 to 23 UTC time frame from Hettinger northeastward toward
the Bismarck/Mandan area. That area is favored for initiation on
the nose of the low-level thermal ridge, immediately ahead a pre-
frontal wind shift, and in advance of the weak mid-level impulse
implied by the area of enhanced cloudiness moving from southeast
MT into southwest ND as of 19 UTC. Pre-convective MLCAPE ranges
from 1000 J/kg at Hettinger, to 2000 J/kg at Bismarck, and 3000
J/kg at Jamestown. The degree of bouyancy, driven in part by steep
low- and mid-level lapse rates, coupled with weak, but veering
winds aloft will be sufficient for a few marginally-severe storms
with hail and gusty winds. However, overall, the risk will be kept
in check by weak deep-layer wind shear, with effective shear
magnitudes less than 25 kt. CAMs through the 17 UTC HRRR support
this scenario with maximum updraft helicity values generally less
than 50 m2/s2 as convection moving eastward, past the Missouri
River by 00 UTC, and exiting the James River valley by 06 UTC.

The weak cold front moving southward this afternoon is expected to
stall along the ND/SD border tonight, then return northeastward as
a warm front Wednesday afternoon. A strong thermal ridge will once
again flex its muscles eastward into western ND behind that front
with 850 mb temperatures of 26 to 28 C from Williston to Bismarck
and values around 30 C in southwest ND. Forecast soundings show an
impressively-deep well-mixed layer to almost 600 mb, all of which
supports highs around 100 F in western and south central ND. Note
that record highs are closer to 110 for July 5th, and thus should
be safe given currently-forecast thermal fields. The 12 UTC NAM
appears reasonable with its low-level dewpoint forecasts east of
the north-south oriented warm front given observed trends both
ahead of and behind that boundary today. Thus, we expect surface
dewpoints in the upper 50s to lower 60s F in central ND Wednesday
afternoon and evening in support of weakly-capped MLCAPE upwards
of 3000 J/kg. Models unanimously agree on significantly-stronger
winds aloft by then with 0-6-km wind shear around 45 kt along with
notable anvil-level flow. Forecast soundings from the 12 UTC NAM
are therefore strong matches to environments which have produced
significant hailstones over 2 inches in diameter. That being said,
this threat is conditional on convective initiation, which is not
a sure bet given weak mid-level height rises at 500 mb during peak
heating. We do, however, believe that high-based, initially-weak
convection could emanate from the intensely-heated air mass in
western ND by late afternoon, then intensify as it crosses the
warm frontal zone in central ND. Moreover, glancing influence from
a weak impulse crossing southern Canada could also aide convective
initiation in north central ND by Wednesday evening.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 239 PM CDT Tue Jul 4 2017

Above-normal temperatures and below-normal precipitation will be
the theme through the long term forecast period. Amplification of
the strong 500 mb ridge centered over the western U.S. late this
week will allow somewhat cooler northwest trajectories to develop
over our region, resulting in highs mainly in the 80s F Thursday
and Friday. However, global ensemble guidance continues to show
the ridge shifting back eastward beginning this weekend with yet
another period of potential 100 degree weather in the offing by
the early to middle part of next week, especially if the 00 and 12
UTC ECMWF runs have anything to say about it. It's worth noting
that the vast majority of recent deterministic GFS cycles have
been on the far cold edge of 850 mb temperatures from its ensemble
and so its numbers may be negatively-influencing blended forecast
guidance. Finally, CIPS analog guidance also suggests a generally
dry period, as expected with only weak shortwave troughs rounding
the 500 mb ridge.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 239 PM CDT Tue Jul 4 2017

A cold front will continue to advance towards KBIS by 00z Weds, and
then into KJMS by 06z Weds. A line of showers and thunderstorms will
develop ahead of the front, thus a tempo tsra has been added for
KBIS between 21z and 00z, possibly earlier and will continue to
monitor and amend as necessary. The thunderstorms look to move
into KJMS between 01z and 04z. Gusty winds to 45kt and small hail
possible with these storms. Elsewhere, vfr conditions with
northwest to northerly winds behind the front.

&&

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

SHORT TERM...CJS
LONG TERM...CJS
AVIATION...KS

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Special Weather Statement
National Weather Service Bismarck ND
149 PM MDT TUE JUL 4 2017

NDZ044-045-042030-
Sioux ND-Adams ND-
149 PM MDT TUE JUL 4 2017

...A STRONG THUNDERSTORM WILL AFFECT SOUTHEASTERN ADAMS COUNTY UNTIL
230 PM MDT...

At 149 PM MDT, Doppler radar was tracking a strong thunderstorm over
White Butte, or 9 miles west of Lemmon, moving east at 20 mph.

Nickel size hail and wind gusts to 50 mph will be possible with this
storm.

This storm will remain over mainly rural areas of southeastern Adams
County.

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

&&

LAT...LON 4595 10245 4605 10246 4610 10200 4605 10200
      4605 10196 4606 10194 4595 10193
TIME...MOT...LOC 1949Z 268DEG 15KT 4597 10235

SIMOSKO