SPC AC 201258

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0758 AM CDT Tue Jun 20 2017

   Valid 201300Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS MAINLY OVER
   WESTERN KANSAS...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR
   TORNADOES...FROM SOUTHEASTERN LOUISIANA ALONG PARTS OF THE
   MISSISSIPPI/ALABAMA COAST...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   CENTRAL PLAINS SLIGHT RISK...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   GULF COAST SLIGHT RISK...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   NORTHERN WYOMING AND SOUTHERN MONTANA TO SOUTH DAKOTA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   ILLINOIS...

   ...SUMMARY...
   Severe thunderstorms are expected this afternoon and evening over
   parts of the central High Plains.  A tornado risk will develop this
   afternoon and remain through tonight over the central Gulf Coast. 
   Isolated severe storms also are possible over parts of Illinois and
   the mid Mississippi Valley, as well as from the northern Rockies and
   High Plains into South Dakota.

   ...Synopsis...
   The upper-air pattern will be characterized by three primary/
   synoptic features through the period:
   1.  Broadly cyclonic flow from the Upper Midwest across the Great
   Lakes and Ohio Valley to New England.  A series of low-amplitude
   shortwaves will traverse that flow field and help to maintain mean
   troughing through the period, before heights rise on day 2.  One of
   these perturbations -- currently located over western Lake Superior
   and northern MN -- will dig southeastward to lower MI and northern/
   western IL by 00Z. 
   2.  An intense anticyclone anchored over AZ, with ridging eastward
   across the Red River region, through a col over MS/AL, connecting to
   the Atlantic subtropical ridge.
   3.  A fetch of fast, nearly zonal, northern-stream flow across the
   Pacific Northwest, southwestern Canada and northern Rockies,
   containing shortwaves ejecting from the base of a cyclone over the
   Gulf of Alaska.  One of these perturbations -- now evident in
   moisture-channel imagery approaching the west-central/southwest
   coast of BC -- is forecast to reach the Canadian Rockies along the
   BC/AB border around 00Z, then eject east-northeastward to SK
   overnight. 

   At the surface, 11Z analysis showed a cold front from central QC
   southward across southeastern NY and eastern VA, becoming
   quasistationary southwestward over the southern Appalachians to
   northern LA and north-central TX, then a warm front to a weak low
   over southeastern CO.  Frontolysis is expected along most of this
   boundary throughout the period.  Meanwhile, another cold front was
   analyzed from eastern Lake Superior across western WI, southern MN,
   and west-central SD, into southeastern MT.  The SD/MT segment of
   this front will decelerate/stall today, while the remainder proceeds
   southeastward across portions of the upper Great lakes and mid/upper
   Mississippi Valley.  A weak front/trough will move slowly
   southeastward across parts of southern NE and northern KS,
   intersecting a lee trough in eastern CO. 

   ...Central High Plains...
   Scattered to widely scattered thunderstorms should develop this
   afternoon near the weak front/trough over northern/western KS, with
   isolated, shorter-lived convection possible southwestward to
   northeastern NM moving off the Raton Mesa region.  The greatest
   concentration should be over the KS part of the outlook area, with
   initial multicellular and perhaps messy supercellular modes growing
   upscale into a southward- to southeastward-moving, cold-pool-driven
   complex offering damaging wind.  

   Activity will develop in an environment of seasonally hot/well-mixed
   boundary layers, increasing low-level moisture accompanying the
   retreating/weakening warm front, and diabatically minimized MLCINH,
   beneath steep midlevel lapse rates.  Forecast soundings depict
   preconvective MLCAPE 2000-4000 J/kg, amidst 40-50-kt effective-shear
   magnitudes.  While low-level flow will remain modest, considerable
   veering of flow with height will contribute to the favorable deep
   shear.  It is uncertain how far south and southeast any resulting
   MCS will propagate before weakening to severe levels; however, the
   outlook area has been expanded somewhat eastward and southward to
   account for possible evening/early-overnight penetration of damaging
   gusts through a shallow near-surface stable layer. 

   ...Central Gulf Coast...
   Potential Tropical Cyclone Three (PTC3 for short) is forecast by NHC
   to move generally northwestward across the central and northwestern
   Gulf through the period.  That motion, combined with the decidedly
   asymmetric nature of PTC3 (per forecast NHC wind radii) should
   spread a broad area of strengthening low-level winds in the
   northeastern quadrant over the outlook area, just above the surface.
    This will result in steadily enlarging hodographs through the
   period, within which any embedded, sustained convective elements may
   have an increasing potential to rotate.  Some uncertainties exist
   regarding local storm modes within the typically favored sector of
   PTC3's envelope, given 
   1.  Impacts on destabilization from the expansive cloud/precip
   shield, and 
   2.  Possible subtropical (instead of tropical) evolution of the
   system as discussed by NHC, which would impact kinematic and
   convective character.
   Still, the most favorable parameter space for tornado potential
   appears to be over parts of southeastern LA and parts of the
   immediate MS/AL coastal areas from this afternoon through tonight. 
   Outer bands and perhaps discrete cells may move inland amidst weak
   buoyancy (MLCAPE 200-500 J/kg) and 0-1-km SRH strengthening above
   300 J/kg. 

   Refer to the latest NHC advisories for forecast track/intensity and
   watch/warning information related to PTC3.

   ...MT/WY to SD...
   Widely scattered thunderstorms may develop over western parts of the
   region this afternoon, moving rapidly eastward to east-southeastward
   across the outlook area through overnight hours.  Isolated damaging
   wind and large hail are possible during the afternoon and evening
   over the northern Rockies/northern High Plains part of the outlook
   area, transitioning to an isolated hail threat late overnight in SD.
   Large-scale lift preceding the northern-stream shortwave trough,
   combined with diabatic heating of higher terrain, will steepen
   deep-layer lapse rates across this region in the presence of at
   least marginal low-level moisture.  Resulting convection will move
   over well-mixed subcloud layers supporting the potential for
   momentum transfer to surface and strong/locally severe gusts. 
   Favorable deep shear for isolated large hail also is apparent in
   forecast soundings.  Lack of more robust low-level moisture will
   preclude a greater unconditional severe threat. 

   Overnight, a southerly, 30-45-kt LLJ over the central Plains is
   expected to increase theta-e above the surface, and potentially  in
   inflow parcels available to convection moving east-southeastward to
   eastward over SD.  This may be in the form of new elevated
   development rooted near 700 mb or extension/ rejuvenation of earlier
   storms moving out of MT/WY.  Hail and strong gusts each are
   possible, given the presence of dry air below the storm-inflow
   region, though the gust potential should be mitigated to a
   considerable extent by diabatically stabilized near-surface air.  

   ...IL, mid Mississippi Valley...
   Widely scattered to scattered thunderstorms are expected to
   intensify and move southeastward from midday through late afternoon
   across this area, offering sporadic hail/gusts near severe levels. 
   Activity is expected to develop near and ahead of the cold front,
   and in a zone of large-scale lift/cooling aloft preceding the
   shortwave trough, each impinging on a diabatically destabilized
   boundary layer and related increase in low-level buoyancy.  Despite
   a "CAPE robber" layer of relatively warm air around 500 mb, forecast
   soundings suggest favorable low-level parcel accelerations amidst
   steep low/middle-level lapse rates, and a well-mixed subcloud layer.
   MLCAPE potentially topping 1000 J/kg (commonly above 500 J/kg), and
   effective-shear magnitudes 50-60 kt, indicate the potential for
   organized storm modes including a few supercells with mainly hail
   risk early, perhaps transitioning upscale to quasi-linear segments
   and bows with time.  The severe threat should lessen through evening
   as the near-surface layer cools.  Lack of greater low-level moisture
   will preclude a more robust severe threat.

   ..Edwards/Picca.. 06/20/2017

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