SPC AC 071256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0756 AM CDT Sun Aug 07 2022

   Valid 071300Z - 081200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   ARIZONA...THE UPPER MIDWEST AND CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Isolated severe thunderstorm gusts are possible across parts of
   Arizona, the Upper Midwest and central High Plains this afternoon
   and evening.

   ...Synopsis...
   In mid/upper levels, a zonally elongated anticyclone -- initially
   from parts of the central Plains to northern AZ -- will be the
   dominant feature for this period.  The western end of the associated
   ridging is expected to shift somewhat northward over southern CO and
   northern AZ, with weakly increasing heights.  Meanwhile, farther
   north, a strong shortwave trough was apparent in moisture-channel
   imagery over portions of southern SK to central MT and the WY/ID
   border region.  This perturbation is forecast to move eastward to
   the Dakotas by 00Z, then become aligned from Lake Superior to
   northern IA by 12Z tomorrow.

   At the surface, 11Z analysis showed a cold front from southwestern
   QC across northern Lower MI, central WI, southeastern MN (behind a
   great deal of convective outflow there), to a weak low near HLC, and
   across parts of east-central/north-central CO.  By 00Z, the front
   should extend from a weak low over the Upper Mississippi Valley
   southwestward across central IA (possibly where another weak low may
   be located), then over southeastern NE, north-central/southwestern
   KS, to north-central NM.  By 12Z, the front should extend from a low
   over eastern WI to near a DBQ-MKC-ICT-TCC line.

   ...AZ...
   Widely scattered to scattered thunderstorms are forecast to develop
   from midday through afternoon, initially over the Mogollon Rim and
   other higher terrain of central/southeastern AZ.  As the
   aforementioned northward shift of mid/upper ridging occurs, flow in
   those layers will become more easterly than in previous days, even
   northeasterly near anvil level.  This will encourage convection to
   move southwestward over lower elevations, where a strongly heated,
   favorably moist, yet well-mixed subcloud layer will encourage both:
   1) Strong-severe gusts from early convection moving off the higher
   terrain, and
   2) Discrete propagation/redevelopment of activity along successive
   outflow surges -- and especially, outflow-boundary collisions where
   lift will be maximized on mesobeta and smaller scales.

   The preconvective environment over the lower deserts should be
   characterized by 500-1500 J/kg MLCAPE (locally/briefly higher), atop
   steep low-level lapse rates supporting around 1000-1500 J/kg DCAPE
   at peak surface temperature.  Deep shear will increase with
   southward extent away from the ridging aloft, into associated
   easterly/northeasterly gradient winds, airing with organization and
   faster cell motion.  That, along with higher moisture content in the
   southern AZ storm-inflow region, also may support marginally severe
   hail.  Convection may persist into the evening over southern/western
   areas, especially if any clustering/small-MCS evolution can occur. 
   Some portion of this area may be considered for an upgrade, if
   mesoscale trends and shorter-fused convective guidance lessen the
   evolutionary uncertainty.

   ...Upper Midwest...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon, most probably along the remnant outflow/differential
   heating boundaries now evident south of the front across portions of
   IA.  Multicellular modes are expected, with gusts being the main
   concern.

   Potential northward into southern MN is more uncertain and
   conditional, given the persistent stabilizing effects of precip
   between the boundaries and the front, the southward movement of the
   front, and the uncertainties on boundary retreat and heating into
   what is now relatively stable air.  Near and south of the outflow
   boundaries, surface dewpoints will remain commonly in the 70s F,
   acting in concert with heating to offset modest mid/upper-level
   lapse rates, and boost peak MLCAPE into the 1500-3000 J/kg range. 
   Water-loaded downdrafts, into enough of a well-mixed subcloud layer
   to maintain or accelerate them, may offer damaging to isolated
   severe gusts.

   ...Central High Plains...
   Widely scattered thunderstorms should develop this afternoon over
   portions of central and southeastern CO, focused mainly near the
   southeastward-moving front and over a post-frontal upslope-lift
   regime near the Palmer Divide.  Strong/isolated severe gusts are the
   main concern.

   Diurnal heating of the elevated terrain, and around the frontal
   convergence maxima, should combine with favorable moisture for that
   altitude (surface dewpoints in the 50s to low 60s F) to render a
   favorable buoyant environment.  Despite modest midlevel lapse rates
   near the ridging aloft, MLCAPE values commonly 1000-1500 J/kg,
   locally near 2000 J/kg, are possible, atop a well-mixed boundary
   layer.  Though midlevel winds will be around 10-20 kt, post-frontal
   low-level northeasterlies will contribute to about 25-35-kt
   effective-shear magnitudes in support of multicellular organization.
   Nonsupercellular "landspout" tornado potential also may exist,
   especially near the front and any mass-convergence lines farther
   northwest, where pre-existing low-level vorticity can be stretched
   in early updraft stages.

   ..Edwards/Marsh.. 08/07/2022

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