Day 1 Outlook Day 2 Outlook Day 3 Outlook Days 4-8 Outlook

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

ACUS01 KWNS 071251
SPC AC 071249

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0749 AM CDT Tue Jul 07 2020

Valid 071300Z - 081200Z


Severe storms capable of intense damaging winds, large hail and a
few tornadoes are expected from central Montana eastward across the
northern Plains and into Minnesota, today into tonight.

The main upper-air feature for this forecast cycle is a synoptic
trough, located initially from southern BC southward across the
interior Pacific Northwest States to central CA. The northern part
of this trough should evolve into a closed 500-mb cyclone over the
Canadian Rockies by 00Z, which will move eastward over southern AB
through the remainder of this period.  As that occurs, a shortwave
perturbation -- now apparent in moisture-channel imagery over
central/eastern parts of WA/OR -- will eject east-northeastward over

By 00Z, this trough should reach southeastern AB and west-central/
southwestern MT.  The primary associated vorticity max is progged to
eject toward southwestern SK overnight -- albeit with some track/
timing uncertainty due to upscale-growing convective processes
discussed below that may result in a dominant MCV.  Elsewhere, a
broad, baggy weakness in the height field across the east-central/
southeastern CONUS will contain several slow-moving perturbations
and MCVs, supporting general-thunderstorm potential amidst weak
deep-layer flow and lack of vertical shear.

At the surface, 11Z analysis showed an outflow boundary from the
prior/overnight MCS, in an arc from southwestern NM across north-
central/southwestern IA, and southern to northwestern NE.  This
feature is expected to shift back northward across NE and portions
of SD through this evening, becoming more diffuse with time.  Cold
frontogenesis and related low-level cyclogenesis, over the northern
Rockies this morning into afternoon, will shift out across the
plains of MT/northern WY through the remainder of the afternoon into
evening.  By 00Z, the main surface low should be located near the
northern Black Hills, along a front arching from there northwestward
over eastern MT, and southwestward across south-central/southwestern
WY.  The low should pivot northeastward to east-central ND by around
12Z, with cold front southwestward across central SD, the NE
Panhandle and northern CO.

...Northern High Plains to MN...
Thunderstorms are expected to develop initially over the mountains
near the MT/ID border, closest to the large-scale DCVA/lift that
precedes the progressive Pacific Northwest trough.  Low/middle-level
lapse rates should steepen throughout the day over western MT from
that process, with weakening MLCINH and increasing boundary-layer
lapse rates related to diabatic heating.  With time and eastward
extent through the afternoon, this will support additional, widely
scattered to scattered thunderstorm development across other higher
terrain and foothills of southern MT and extreme northern WY,
eastward past the Bighorns.  Activity should encounter increasing
moisture, but also, well-mixed subcloud layers, as it moves
northeastward into a diurnally destabilizing inflow layer.

The severe threat will evolve largely according to storm mode.  A
time window of uncertain duration exists for initial convection to
evolve into supercells, offering large to very large hail and severe
gusts.  A few tornadoes may occur as well, particularly from
north-central/central MT into portions of the western Dakotas, as
moisture advection continues, LCLs lower some with eastward extent,
and a reservoir of residual low-level vorticity may linger near and
just north of the western extent of the old outflow boundary. 
Strengthening deep shear is expected across the northern High Plains
as the mid/upper trough approaches,

Meanwhile, areas of strong afternoon heating and sufficient moisture
(remaining behind previous days' convective activity) will support
peak preconvective MLCAPE ranging from around 1500-2000 J/kg in
central MT to nearly 4000 J/kg over north-central/northeastern SD,
along and north of the diffuse boundary.  A very conditional,
isolated but potentially significant severe threat in all forms may
occur in central/north-central SD near the retreating boundary, late
this afternoon into evening, where very large hodographs and rich
moisture are forecast. However, initiation remains in doubt during
daylight to early evening hours that far east, due to capping and
lack of stronger deep-layer lift.

One or two MCSs are expected to evolve from the earlier convection
and move across central/eastern MT, much of ND and perhaps northern
SD.  As this occurs, the main severe threat transitions to damaging
wind -- some of which locally may exceed hurricane force (65+ kt,
significant severe).  A broad, 50-60-kt LLJ should develop over the
Dakotas and northern NE overnight, with associated theta-e advection
and moisture transport above the surface supporting MCS maintenance
across the northern Plains, reaching parts of northwestern and
perhaps far northern MN late tonight.

..Edwards/Bryoles.. 07/07/2020