FXUS63 KBIS 060249

Area Forecast Discussion
National Weather Service Bismarck ND
949 PM CDT Fri Jun 5 2020

Issued at 927 PM CDT Fri Jun 5 2020

We have a single convective cell northeast of Broadus Montana
that has been decreasing, but if it holds together should arrive
in southwest North Dakota around 945 PM MDT, most likely as a
shower or weak thunderstorm. But we are starting to see southeast
winds increasing at the Bowman airport and Sand Creek RAWS
station. This is in response to the increasing low level jet that
will play a role in storm formation late tonight. The storms late
tonight will primarily have a large hail threat.

Satellite imagery shows the mid level ridge axis now from
northwest Wyoming into eastern Alberta. As this axis moves east
late tonight we expect better mid level forcing for ascent, and
the low level jet will be maximized across western North Dakota
late tonight as well. Current forecast looks good in increasing
thunderstorms after midnight west.

UPDATE Issued at 638 PM CDT Fri Jun 5 2020

Current weak showers across east central North Dakota, which have
diminished this evening, and are likely associated with strong
mid level warm advection at h700 mb, are expected to end. They
will not be mentioned in the forecast.

Later tonight, a developing strong low level jet of 35 to 45kts
will increase low level convergence and an h500 shortwave trough
should bring low/mid level vertical motion and support scattered
thunderstorms. These are expected to be fairly high based and not
routed to the surface. Isolated severe thunderstorms with the
primary threat large hail are possible across western North
Dakota after midnight. The severe threat should diminish after
sunrise as the low level jet mixes down to the surface. Then we
expect increasing surface winds of 25 to near 40 mph across much
of west and central North Dakota. A wind advisory may be needed if
winds turn out to be stronger than that.

Current forecast looks ok for now.


.SHORT TERM...(This evening through Saturday night)
Issued at 222 PM CDT Fri Jun 5 2020

Chances for severe thunderstorms increase across western North
Dakota overnight with large hail being the primary threat. Then a
second wave of thunderstorms is forecast to increase from the
southwest late Saturday afternoon and become widespread across the
western and central Saturday evening. Large hail, damaging wind
gusts, and a non-zero tornado risk are the threats in far
southwest North Dakota through Saturday evening while large hail
will be the primary threat elsewhere as convection continues
through the overnight hours.

This afternoon, a mid-level ridge was building over the northern
high plains as two deep troughs were beginning to move over the
west coast. One, a closed low making its way over southern
California and the other over the Pacific Northwest coast. Theta-e
rich air at 700mb was advecting into western North Dakota this
afternoon as the synoptic warm sector continued to transition
eastward. On the northern periphery of this warm air mass, light
showers were ongoing from northeast Montana through south-central
North Dakota.

This showers should decrease in coverage by this evening as the
700mb frontogenetic forcing becomes more diffuse. The mid-level
ridge will quickly move over western North Dakota tonight with
flow becoming more southwesterly with embedded impulses emerging
from the cyclonic flow upstream. Very steep lapse rates accompany
the incoming cyclonic flow as well as strong mid-level winds. The
combination of steep lapse rates, effective bulk shear of 50-60
kts, and moistening at the elevated 850-700mb inflow layer should
provide an environment for strong convection, if not supercells,
to develop after 06Z tonight. While uncertainty in severe
magnitude lies in convective interaction with the broad area of
ascent...the potential for large hail exists given the steep lapse
rates and strong shear. Through Saturday, the overnight
convection moves off to the east, becoming less severe in the
morning as it encounters more stable air further east.

Late Saturday afternoon, a potent shortwave is forecast to lift
northeast into the western Dakotas. A corridor of surface based
instability should develop in far southwest North Dakota where
surface dew points will rise into the low 60s. Deepening low
pressure stretching from eastern Montana through western South
Dakota will provide the convergence axis for initial convective
initiation, which will then translate northward into southwest
North Dakota. Within this surface based convective environment,
effective deep layer shear of 50-60 kts as well as strong 0-1 km
shear of 15-20 kts will produce an environment capable of all
severe convective hazards. However storm mode will drive the
convective threat potential, as quick upscale growth looks likely
given the strong forcing and unidirectional wind shear forecast,
which somewhat dampens the tornado and large hail potential. This
will change as the event begins to play out and dominant storm
mode becomes more apparent.

Later Saturday evening, an 850mb jet is forecast to strengthen
over western/central North Dakota, kicking off more widespread
convection across the state. A very unstable airmass and strong
deep layer shear will contribute to an environment conducive to
elevated supercells with large hail becoming the primary threat.

Breezy southeast winds will be widespread beginning Saturday
morning, becoming near advisory criteria through the day. At the
moment certainty was just low enough to hold back from issuing a
headline but this will need to be reevaluated.

.LONG TERM...(Sunday through Friday)
Issued at 222 PM CDT Fri Jun 5 2020

Chances for severe thunderstorms return Sunday afternoon. The
probability for all severe hazards including very large hail,
damaging wind gusts, and tornadoes is greatest in the southeast
portions of the state. Severe thunderstorms with the potential for
large hail will be possible in the western and central portions
of the state as well.

Sunday morning should offer a reprieve of strong convection with
positive mid-level height rises behind Saturday's shortwave and
ahead of the next shortwave set to emerge from the persistent
southwesterly flow.

By Sunday afternoon, a surface low will deepen over western South
Dakota in response to the ejecting mid-level shortwave. Poleward
advection of richer theta-e values will accompany the deepening
surface low along with the associated warm front forecast to lift
into southeastern North Dakota. Strong deep layer shear of 50-70 kts
and ample instability will produce an environment conducive to
severe weather as convection should form off the surface trough
convergence and warm front. Forecast hodographs in the warm sector
depict long, mostly unidirectional, hodographs supporting all modes
of convective hazards. Additionally, depending on how far north the
warm front ends up, a relative increased tornado risk should
accompany the enhanced low level helicity associated with the
frontal baroclinicity. However, the northward extent of the frontal
boundary and the associated magnitude of low level moisture return
will likely be the largest point of uncertainty as it will impact
severe weather coverage and threat potential in
southern/southeastern North Dakota. Based off the likely surface
track under the modeled upper level pattern as well as the location
of increased SREF calibrated severe probabilities, the most likely
area for surface-based severe weather should be the southern James
River Valley area, with the west and northern extent still
uncertain. Further west severe convection will still be possible,
though with the inflow level well off the surface severe hail should
be the more likely threat.

In other weather news on Sunday, temperatures should be the warmest
of the forecast period, especially central and east. The NBM and MOS
guidance agrees well with temperatures reaching the mid-upper 80s in
the central to around 90 in the southeast.

By Monday, the shortwave will lift northeast into Canada with an
associated cold front pushing eastward through the day. While the
deterministic 12Z GFS puts the frontal passage well east of our
area, NAM/ECMWF/CIPS all show the potential for the surface trough
to linger over the James River Valley which may deliver another
thunderstorm threat before the front pushes through Monday evening.

On Tuesday, the upper low is forecast to be mostly be closed off
over southern Canada with the Northern Plains under broad cyclonic
flow. This should lead to cooler temperatures with periodic chances
for precipitation through Wednesday. After that, while there is
increasing divergence in how guidance transitions the upper
low...the blended consensus keeps surface temperatures seasonably
low with generally low chances for precipitation.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 638 PM CDT Fri Jun 5 2020

The primary hazards to aviation include scattered thunderstorms,
low level wind shear, and gusty surface winds to 35 kts.

Scattered thunderstorms are forecast to develop after 06z across
western North Dakota, then move to central North Dakota after
sunrise. Increasing low level winds will create areas of wind
shear between 06-13z of 45 kts from 15 hundred feet to the surface

Then gusty surface winds of 25 to 35 kts are expected after 15z.




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