SPC AC 221247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0747 AM CDT Wed Jul 22 2020

   Valid 221300Z - 231200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE
   MIDDLE/UPPER OHIO VALLEY ACROSS THE MID-ATLANTIC TO PORTIONS OF
   WESTERN NEW ENGLAND...

   ...SUMMARY...
   The greatest potential concentration of strong to severe
   thunderstorms today -- with damaging wind and isolated severe hail
   possible -- should be from the middle/upper Ohio Valley across the
   Mid-Atlantic to portions of western New England.

   ...Synopsis...
   The upper-air pattern this period will be characterized by troughing
   over the West and Northeast.  In between, a progressive, amplifying
   synoptic ridge will move from the central/northern Rockies across
   the central/northern Great Plains.  A closed cyclone -- evident in
   moisture-channel imagery with a center offshore from SFO -- is
   forecast to move slowly eastward, reaching the Central Valley of CA
   by 12Z tomorrow.  As this occurs, a positively tilted shortwave
   trough -- initially crossing 50N140W in the northeastern Pacific --
   will dig southeastward and cross the coastal Pacific Northwest near
   the end of the period.

   Farther east, a negatively tilted shortwave trough from northern NV
   to central AZ will eject northeastward to the central Rockies,
   contributing to general thunderstorm potential in its path.  A
   well-defined shortwave trough -- located over Lakes Superior and
   Michigan initially -- will deamplify and accelerate eastward toward
   southern QC.  Embedded in the westerlies to its south, a series of
   small, convectively aided/generated vorticity maxima will cross the
   lower Great Lakes, Ohio Valley and Mid-Atlantic regions.

   The 11Z surface analysis showed a low near GRB with cold front
   southwestward to northwestern MO, becoming quasistationary west-
   northwestward to an area of convection over southern NE.  This
   boundary should weaken through the day and shift somewhat northward
   over the central Plains.  A warm front, extending from the low
   across north-central Lower MI, central Lake Erie, and north-central
   PA, should shift northward today over southern ON and portions of
   western/northern NY.

   ...Ohio Valley to Northeastern CONUS...
   Multiple episodes of scattered thunderstorms are expected to cross 
   this broad outlook area through early evening, offering damaging to
   severe gusts and isolated large hail.

   The threat over much of the NY/western New England portion and from
   about central PA/northeastern WV eastward should be related to the
   leading perturbation -- whose UVV field is contributing to a band of
   convection now crossing parts of western PA and northern WV. 
   Satellite imagery shows considerable clearing ahead of this activity
   on both sides of the warm front.  Several hours of sustained
   diabatic heating, amidst surface dew points commonly from the mid
   60s to low 70s F, will offset weak midlevel lapse rates enough to
   yield 500-1500 J/kg MLCAPE.  A well-mixed boundary layer, with
   inverted-V thermodynamic profile in forecast soundings, will support
   strong/isolated severe convective gusts capable of damage.  Isolated
   supercells are possible in northeastern parts of the outlook area,
   under somewhat stronger midlevel winds contributing to 35-45-kt
   effective-shear magnitudes.

   Some uncertainty exists in the northward extent of airmass recovery
   across the mid/upper Ohio Valley behind the first wave, and ahead of
   somewhat weaker, trailing perturbations over parts of IL/IN.  Still,
   with weak MLCINH and locally favorable heating/buoyancy forecast,
   clusters of thunderstorms with wind-damage potential are expected. 
   Vertical shear and convective organization should lessen with
   southward extent toward the southern Appalachians and Tennessee
   Valley region.

   ...Central/northern Plains...
   Thunderstorms may develop this afternoon into early evening near
   remnant outflow/differential-heating boundaries left by prior
   overnight convection/precip, some of which lingers across portions 
   of central/southern NE at this time around MCVs located near ODX and
   LXN.  The MCVs should move eastward through the day over
   lower-theta-e air and weaken.

   This is a low-confidence outlook area.  The conditional potential is
   strong that a storm can become severe -- and supercellular -- in the
   parameter space expected near the boundaries this afternoon/evening,
   with a threat for large hail and severe gusts.  However, there is
   considerable uncertainty on the location/strength of the boundaries
   and whether such convection will develop at all, given:
   1.  Rising heights aloft behind the dual MCVs,
   2.  The presence of some overhead EML air for capping, and
   3.  Lack of other means of forcing other than the remnant
   boundaries.

   Outside cloudy areas, forecast soundings suggest that rich low-level
   moisture, with surface dew points in the 60s to low 70s F, and steep
   low/middle-level lapse rats, will foster peak MLCAPE of 2000-3000
   J/kg.  Strong veering of winds with height will impart considerable
   curvature to low-level hodographs that will enlarge around 00Z as
   the low-level jet strengthens, with 200-300 J/kg effective SRH
   possible (especially along favorably oriented boundaries). 
   Elsewhere, ongoing convection rooted on an elevated front over
   northern SD may produce isolated hail near severe limits, but is
   expected to weaken through the remainder of the morning.

   ...Northwestern Great Basin/northeasternmost CA...
   Isolated to widely scattered thunderstorms are expected to develop
   this afternoon across this region, as strong diabatic heating strips
   away MLCINH and orographic forcing aids in convection initiation. 
   Strong difluence but weak absolute speeds are forecast for the
   mid/upper flow field over the western part of this area, with weak
   deep shear and small storm-motion vectors.  Somewhat greater
   cellular translation is likely from north-central NV eastward.  A
   very deep and well-mixed boundary layer is expected, amidst steep
   low/middle-level lapse rates exceeding 8 deg C/km, and sufficient
   low-level moisture to support high-based thunderstorms.  Downdraft
   types will range from dry beneath virga, to marginally/briefly wet
   cores at ground level, with both offering isolated severe gusts. 
   This area overlaps with isolated to scattered dry-thunderstorm
   hazards for fire concerns; please see the day-1 SPC fire-weather
   outlook for more discussion.

   ..Edwards/Dean.. 07/22/2020

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