SPC AC 032323

   Day 1 Convective Outlook AMEND 1
   NWS Storm Prediction Center Norman OK
   0623 PM CDT Mon Sep 03 2018

   Valid 032320Z - 041200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING
   ACROSS PARTS OF WESTERN NORTH DAKOTA...

   AMENDED FOR ADDITION OF TORNADO PROBABILITIES IN IOWA

   ...SUMMARY...
   Severe storms capable of large hail and damaging wind are expected
   this afternoon and evening over parts of western/central North
   Dakota, with other strong to locally severe thunderstorms possible
   across the Midwest, south Florida, and northern New England.

   ...Amendment Update...
   Tornado probabilities have been added in Iowa to account for a local
   threat of a couple tornadoes associated with a quasi-warm front /
   baroclinic zone extending from central Iowa eastward. Higher
   effective helicity near the boundary, combined with a moist
   low-level environment, may support another tornado or two this
   evening. See MCD 1420 for more details.

   ...20Z Outlook Update...
   Some changes to categorical and severe probabilistic lines have been
   made, mostly to account for the latest trends concerning
   destabilization and ongoing convective development.

   ...East of the lower Great Lakes...
   In most areas still not impacted by thunderstorm activity, surface
   temperatures have warmed to near 90F, in the presence of dew points
   near or above 70F.  This appears to be contributing to moderately
   large CAPE in excess of 1500 J/kg, which may support a continued
   gradual increase in thunderstorm intensity through late afternoon. 
   Aided by 25-30+ kt westerly flow in the 700-500 mb layer, as far
   south as the central tier counties of New York state (near the
   northern periphery of the subtropical ridge), some of this activity
   may be accompanied by localized potentially damaging wind gusts
   before diminishing this evening.

   ...Central High Plains...
   There are no changes to severe probabilities (less than 5 percent)
   across this area.  Modest deep layer shear associated with a 30-35
   kt mid-level speed maximum may be supporting the sustained ongoing
   cluster of storms across eastern Colorado.  Activity is occurring in
   the presence of a relatively moist environment, which appears to be
   contributing to CAPE in excess of 1000 J/kg.  However, lower/mid
   tropospheric lapse rates across the region are generally modest to
   weak, and the boundary layer is not expected to become particularly
   warm or deeply mixed, which still seems likely to minimize any risk
   for severe wind gusts and hail.

   ..Picca/Kerr.. 09/03/2018

   .PREV DISCUSSION... /ISSUED 1131 AM CDT Mon Sep 03 2018/

   ...Northern High Plains...
   In response to CINH-reducing heating and strengthening
   convergence/lift along a prefrontal trough, widely scattered to
   scattered thunderstorms should form late this afternoon over western
   ND/perhaps extreme eastern MT in a south-southwest/north-northeast
   band. Coverage should diminish with southward extent into
   northwestern South Dakota due to stronger capping and weaker lift.
   Severe gusts and large hail will be possible from a blend of
   multicellular and supercellular storm modes.

   As the warm front moves northward, associated warm and moist
   advection will act in tandem with diabatic heating to destabilize
   the boundary layer, while boosting surface dew points into the upper
   50s to low 60s F range. Resultant MLCAPE of 1500-2500 J/kg,  along
   with 35-45-kt effective-shear magnitude and substantial veering of
   wind with height, will support several hours of severe risk as mode
   evolves from discrete to quasi-linear. Late this evening, as the
   boundary layer diabatically stabilizes and the mid/upper-level
   shortwave trough deamplifies, convection should diminish in
   intensity and coverage with time.

   ...Florida...
   Latest NHC guidance suggests that Tropical Storm Gordon will
   continue to gradually strengthen and organize as it continues slowly
   west-northwestward over South Florida. While the overall initial
   modest strength/organization of the system and its weak mid-level
   winds will tend to limit the overall tornado threat, strong
   east/southeasterly winds within the lowest few km AGL will support
   some transient supercells. A brief tornado or two could occur aside
   from a nearshore waterspout risk.

   ...Eastern IA/southern WI/northern IL...
   Have introduced severe probabilities for expectations of at least
   some severe-weather potential across the region this afternoon
   through early evening. Similar to prior days, it appears that
   thunderstorm development will be focused along a zone of decayed
   outflow and differential heating. Zone of moderate instability will
   support stout updrafts, with moderately strong mid-level winds
   supportive of as much as 35 kt effective shear, which could yield
   some transient supercells aside from more common
   east/northeastward-moving clusters. Localized wind damage would be
   the most probable severe hazard.

   ...Northern New England...
   Scattered thunderstorms are expected to develop over portions of
   southern Quebec, and perhaps portions of Vermont/New
   Hampshire/northwestern Maine close to the Canadian border. This
   activity should move eastward across northern New England, offering
   the potential for isolated severe hail and gusts.

   A plume of large-scale lift ahead of an Ontario shortwave trough
   will coincide with a diabatically heated boundary layer and an
   eastward-shifting moist axis, characterized by surface dew points
   generally in the mid/upper 60s F. Forecast soundings feature MLCAPE
   in the 500-1500 J/kg range, locally higher -- but weakening with
   northward extent over Maine where antecedent clouds will persist
   longer. Deep-layer shear will strengthen as the shortwave trough
   approaches, with 30-40-kt effective-shear vectors developing in
   support of some convective organization. Nearly unidirectional flow
   with substantial westerly component will limit convergence and
   storm-relative flow in low levels. Convection should weaken with
   eastward extent and time over southern/eastern Maine.

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