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Day 1 Outlook Convective Tornado
Hail Wind
Categorical Day1 1300Z Outlook
  
Images courtesy of the NWS Storm Prediction Center
 Forecast Discussion

  
000
ACUS01 KWNS 011254
SWODY1
SPC AC 011252

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0752 AM CDT Mon Jun 01 2020

Valid 011300Z - 021200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTHWESTERN/CENTRAL MONTANA...

...SUMMARY...
Strong/isolated severe storms may affect parts of southwestern to
central Montana this afternoon.

...Synopsis...
In mid/upper levels, the large-scale pattern will feature troughing
near the U.S. West Coast and from eastern Canada southward, offshore
from most of the Atlantic Coast.  The southern part of the western
trough will become both closed and cut off over the next couple
days, evolving into a split-flow pattern as a cyclone forms today
west of northern Baja.  In the northern stream, the associated
shortwave-trough segment now over western portions of WA/OR will
eject northeastward to northern ID and northwestern MT by 00Z, with
a weaker vorticity banner now over northern CA pacing it into
western MT. 

A longwave ridge will remain over the Great Plains States, albeit
with these embedded shortwave troughs: 
1.  A strong northern-stream perturbation -- associated with severe
weather in the Northwest yesterday--will move eastward from central/
southern SK to northern ON through the period.  
2.  A weaker perturbation -- initially somewhat in phase with the SK
trough to the northwest -- is evident in moisture-channel imagery
over the Red River of the North (near the western MN border).  This
feature will pivot southeastward across Lake Michigan to western
Lower MI around 00Z, weakening as it moves toward the central
Appalachians overnight.
3.  A broad, weak, cut-off cyclone, that has been over northern MX
for a few days, is centered over southwest TX.  This feature will
contribute to general thunderstorm potential as it drifts northward
across west-central TX through the period, orbited by several
slow-moving vorticity maxima.

At the surface, 11Z analysis showed a cold front from central MB
southwestward across northwestern SD and southeastern MT, to a weak
low near COD, then diffusely southwestward toward northern UT.  This
front should move to Lake Superior, southern MN, southern SD, and
east-central/southeastern WY by the end of the period.  A warm front
-- initially analyzed across northeastern SD and western IA to near
MKC -- should move northeastward to eastern Lower MI and OH by 12Z
tomorrow.  A quasistationary frontal zone from southern GA to
southern LA and east TX became a warm front over north-central/
northwest TX.  This boundary should move northward over the southern
Plains today while becoming more diffuse.

...Northern High Plains...
Isolated to widely scattered thunderstorms may develop over the
higher terrain of southwestern MT and move northeastward this
afternoon/evening, offering strong to isolated severe gusts. 
Favorable diurnal heating of higher terrain and adjoining High
Plains should yield well-mixed, inverted-v-shaped thermodynamic
profiles on a sounding diagram, with enough midlevel lapse rates and
lingering low-level moisture to yield pockets of 200-500 J/kg
MLCAPE, atop DCAPE values potentially doubling that.  Strong
mid/upper winds will contribute to effective-shear magnitudes
potentially exceeding 50 kt, as well as fast storm motions with
fairly efficient momentum translation into downdrafts. 

The air mass downstream over central MT should become relatively
shallow and buoyancy-starved, given the combination of modest
midlevel lapse rates (despite any modest large-scale lift from the
approaching shortwave trough) and lack of more-robust low-level
theta-e.  However, any forced ascent of marginal low-level moisture,
from cold-pool driven processes related to the earlier convection,
may permit a marginal severe threat to linger into central MT before
diminishing.

Farther east, the elevated moisture plume may advect northward/
northwestward late in the period across the western Dakotas and
eastern MT; yet, by that time, surface-based buoyancy and low-level
lapse rates will be near nocturnal minima.  This, and a lack of
consistent guidance suggesting more than weak elevated development
at night, indicate too much uncertainty to maintain an unconditional
severe outlook for that area.  Farther south, isolated, short-lived
thunderstorms are possible this afternoon from the Black Hills
southward to eastern CO.  Strong gusts cannot be ruled out on a very
localized basis, given the deeply mixed subcloud layers.  However,
lack of substantial forcing other than boundary-layer heating,
combined with weakness of deep-layer flow/shear, suggest severe
potential is too minimal and unorganized for an outlook.

..Edwards/Gleason.. 06/01/2020

$$