SPC AC 011300

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0800 AM CDT Sat Jul 01 2017

   Valid 011300Z - 021200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   MID ATLANTIC AND NEW ENGLAND...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
   SOUTHERN APPALACHIANS TO MISSISSIPPI...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISKS BETWEEN EAST TEXAS AND NEW ENGLAND...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   NEW MEXICO TO THE SOUTHERN HIGH PLAINS...

   ...SUMMARY...
   Isolated to scattered severe thunderstorms, primarily capable of
   wind damage, are forecast across the Northeast southward into the
   Mid-Atlantic states.  Strong to locally severe thunderstorms are
   also possible across the southern Appalachians and into Mississippi.
   Other strong-severe storms are possible this afternoon and tonight
   over parts of New Mexico across the southern High Plains.

   ...Synopsis...
   In mid/upper levels, a cyclonic flow field will shift eastward
   across parts of the Upper Midwest, Great Lakes and Northeast.  This
   will occur as a series of vorticity maxima and shortwaves rotate
   through the southern semicircle of a loosely organized, broad
   cyclone centered over northern ON and eventually the James Bay
   region.  A weak, leading, positively tilted perturbation -- now
   located from Lake Erie to eastern KY and supporting precip over
   portions of PA/NY -- will eject northeastward across the Mid
   Atlantic and New England today.  Upstream, a somewhat stronger
   shortwave trough was evident in moisture-channel imagery from Lake
   Michigan to eastern KS.  This feature should weaken gradually as it
   ejects northeastward to southern ON, Lake Ontario and OH by 00Z,
   then over southern/central QC by the end of the period.  A stronger
   shortwave trough -- occasionally manifest as a closed 500-mb low --
   will move southeastward from the Lake Winnipeg region to the Lake
   Superior region by 12Z.

   At the surface, an occluded low was analyzed at 11Z over eastern
   Lake Superior with a triple point over northwestern Lake Huron.  The
   warm front extended from there east-southeastward across the
   Adirondacks and VT/NH, and should move northward today, with some
   diffusion amidst clouds/precip shifting eastward out of the Lake
   Ontario region and adjoining parts of NY.  The cold front was drawn
   from Lower MI to the northern Ozarks in MO, through a weak low near
   TUL, southwestward across the Permian Basin of west TX.  Outflow
   boundaries preceded the front from southwest TX to western portions
   of TN and KY.  A new low should develop on the triple point by
   mid/late afternoon over southern QC, with the 00Z cold-frontal
   position from central parts of NY/PA southwestward across AR.  The
   front will become quasistationary and ill-defined from there to
   southwest TX.  

   ...Northeast/Mid Atlantic...
   Scattered thunderstorms are forecast to develop this afternoon in
   multiple/broken corridors along and ahead of the front, moving
   northeastward while posing a risk for damaging gusts.  Isolated
   large hail is possible.  A tornado cannot be ruled out portions of
   the Adirondack/northern New England region where low-level shear
   will be greatest nearer to the warm front.  Overall convective
   coverage/intensity should wane after 00Z.

   Large-scale ascent will be greatest but sustained diabatic surface
   heating weakest over the northern areas -- NY/New England -- given
   the ongoing areas of precip across western/central NY and western
   PA.  Still, the heating that does occur, in tandem with weak MLCINH
   and lift along the surface front, prefrontal confluence zones/
   troughs and differential-heating/outflow boundaries, will be more
   than enough to support convection by afternoon.  Forecast soundings
   suggest that 30-40 kt of midlevel (i.e., 700-500-mb) winds and the
   surface southerly component will support 30-40-kt effective-shear
   magnitudes.  Areas of locally backed surface winds and/or
   vorticity-enhanced boundaries further will support some supercell
   potential in addition to multicells and small bows. 

   The severe potential is expected to transition from a multimodal
   threat over the north -- where shear is stronger and buoyancy weaker
   but still adequate -- to multicellular wind and marginal hail in the
   higher-CAPE/weaker-shear MD/VA region.  Forecast soundings suggest
   surface temps heating into the 90s F, with dew points upper 60s to
   mid 70s and 1.5-2-inch PW -- offset modest mid/upper-level lapse
   rates to yield 1500-2000 J/kg MLCAPE, locally higher.  Shallow but
   well-mixed boundary layers will support the potential for damaging
   gusts, a few perhaps exceeding severe limits.  

   ...Southern States...
   Scattered thunderstorms in clusters are expected to form this
   afternoon along and ahead of the aggregate outflow boundary from
   prior convection, offering the potential for damaging downburst
   winds, isolated gusts near severe limits, and isolated large hail. 
   This should be mainly a diurnal threat, but with isolated severe
   possible into the mid/late evening.  Forecast soundings suggest
   well-mixed subcloud layers, but still with rich boundary-layer
   moisture, contributing to MLCAPE ranges from 2000-3000 J/kg in MS to
   500-1000 J/kg across eastern TN.  The area will reside beneath, or
   just off, the southern fringes of the cyclonic-flow field aloft,
   somewhat stronger over TN than farther southwest, with 20-30 kt
   500-mb flow and nearly unidirectional vertical wind profiles
   overall.  Organized multicells, outflow-dominant clusters and
   perhaps a few bows are possible, and transient supercell evolutions
   may occur relatively early in the convective cycle and/or near any
   vorticity-rich boundaries. 

   ...Southern Rockies/High Plains...
   Isolated to widely scattered thunderstorms are expected to form in 
   higher terrain of central/northern NM and move generally
   southeastward, with isolated large hail and damaging gusts possible.
   Surface diabatic heating will remove CINH effectively, while
   residual surface moisture and a weak upslope-flow regime contribute
   to buoyancy and lift.  

   Surface dew points generally in the 50s and low 60s F, overlain by
   steep midlevel lapse rates, will support MLCAPE 1500-2500 J/kg,
   amidst well-mixed subcloud layers with high DCAPE.  Even though
   low/middle-level flow will remain modest, strong directional shear
   and favorable storm-relative boundary-layer winds will enable
   convection to persist for several hours, at least into this evening,
   across the southern High Plains.  Some of this activity may
   aggregate into forward-propagating/cold-pool-driven bands or
   clusters this evening over the High Plains, leading to a somewhat
   more-concentrated damaging-wind threat than the broadbrushed 5% area
   now depicts.  Given the storm-scale to mesoscale dependency of such
   processes, in the absence of any substantial large-scale/upper-level
   support, predictability is too low at this time to narrow down a
   specific swath of greater wind probabilities.  However, an upgrade
   may be needed for part of this area as mesoscale trends warrant
   today.

   ..Edwards/Peters.. 07/01/2017

   CLICK TO GET WUUS01 PTSDY1 PRODUCT

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z