Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 210203

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

Issued at 900 PM CDT Thu Jul 20 2017

Storms have been on a weakening trend over the past few hours, so
have trimmed off the back side of the watch. If storms continue on
current trend, anticipate additional areas can be trimmed off in
the not so distant future, particularly over the north. Latest
high-res models continue to show the southern convection
meandering to the east, but the last few runs have kept the
strongest convection to the south of the area.

UPDATE Issued at 609 PM CDT Thu Jul 20 2017

Intense boundary-layer heating across eastern Montana has aided
thunderstorm development/intensification across western North
Dakota. This activity is evolving just downstream of a pronounced
surface low primarily south of a well-defined wind shift where
low-level lapse rates are steep and dew points are holding in the
50s. Multiple reports of large hail have come in with this
activity and storms will likely linger well into the evening hours
as low-level jet is expected to increase across the western
Dakotas over the next several hours, partly in response to
upstream short-wave trough that is expected to flatten the
northern Plains ridge during the overnight hours.


.SHORT TERM...(This evening through Friday)
Issued at 214 PM CDT Thu Jul 20 2017

Water vapor imagery shows a mid-level impulse sliding eastward
through Montana over the axis of a southern plains high. At the
surface a deepening low was found over southeastern Montana with
strong southerly flow centered over the Montana/North Dakota
border. Steep mid-level lapse rates, surface dew points reaching
the low 60s, and cross-boundary effective bulk shear in the 40-50
knot range has built an environment ample for supercells across
western North Dakota. Currently convection has initiated in an
area of 850 mb warm air advection over the northwestern corner of
the state. Convection-allowing models have consistently depicted
further development along a surface convergence zone over far
western North Dakota where a cumulus field has started to
develop. With the strong deep layer shear profile and inverted V
signatures on the western forecast soundings, large hail and
damaging winds will be the main threats for any storm that forms
in this area, especially early in its life cycle. A non-zero
tornado risk does exist, highlighted by the SPC Day One update.
Convective activity will shift eastward into the evening hours
with likelihood for severe modes decreasing as storms move into
central North Dakota. Showers and thunderstorms will be possible
through the overnight hours, especially north central on the nose
of a strengthening low level jet.

Friday...a closed upper level low will meander east over the
southern Canadian Prairie, shifting a surface boundary eastward
through the state. A seasonably moist warm sector and strong bulk
shear values will allow for another possible round of severe
weather, especially for central and eastern North Dakota. The 850
mb thermal ridge axis stays centered over the upper high plains
until later in the evening, keeping high temperatures in the 80s
and 90s across the state.

.LONG TERM...(Saturday through Thursday)
Issued at 214 PM CDT Thu Jul 20 2017

As the previously mentioned Canadian low exits the area early
Saturday, flow shifts to the northwest with a surface high
moving in, leading to a dry post-frontal environment and gusty
surface winds. 12Z global model runs mostly agree on northwesterly
flow shifting out by the beginning of the week, showing signals
of a somewhat active pattern and above average temperatures.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 609 PM CDT Thu Jul 20 2017

Thunderstorms pushing across the area this evening will bring
areas of IFR visibility, along with the threat of damaging wind
gusts and large hail. The greatest chances for severe weather are
generally along and east of Highway 83. Thunderstorm chances
remain going into Friday, with the threat of severe pushing over
eastern into portions of central North Dakota.




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