Area Forecast Discussion
Issued by NWS Bismarck, ND

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FXUS63 KBIS 052005

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

.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 18Z TAFS through 18Z Saturday afternoon)
Issued at 1243 PM CDT Fri Jun 5 2020

Generally VFR expected through the TAF period. Areas of showers
and possibly thunderstorms will develop tonight after 09Z in
west/central North Dakota, though less certain further east at
KJMS. VCSH was used for all sites but KXWA where VCTS was used to
cover the higher probability of thunderstorms. Stronger
thunderstorms may produce hail and erratic winds.
Showers/thunderstorms diminish Saturday morning before
thunderstorms arrive from the southwest Saturday evening.

Low to mid level winds are expected to increase from the southeast
tonight. LLWS was added to KXWA-KDIK after 08Z. Winds across the
region will be gusty on Saturday.




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