May 13, 2018 1300 UTC Day 1 Convective Outlook
Updated: Sun May 13 12:55:36 UTC 2018 (20180513 1300Z Day 1 shapefile | 20180513 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20180513 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 59,603 10,887,631 Chicago, IL...Aurora, IL...Rockford, IL...Naperville, IL...Joliet, IL...
MARGINAL 245,502 32,097,083 Indianapolis, IN...Columbus, OH...Denver, CO...Washington, DC...Omaha, NE...
Probabilistic Tornado Graphic
20180513 1300 UTC Day 1 Tornado Probabilities Graphic
Probability of a tornado within 25 miles of a point.
Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.
Day 1 Tornado Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
2 % 10,938 990,321 Aurora, CO...Thornton, CO...Greeley, CO...Cheyenne, WY...Broomfield, CO...
Probabilistic Damaging Wind Graphic
20180513 1300 UTC Day 1 Damaging Wind Probabilities Graphic
Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point.
Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Day 1 Wind Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
15 % 59,624 10,858,588 Chicago, IL...Aurora, IL...Rockford, IL...Naperville, IL...Joliet, IL...
5 % 245,010 32,223,961 Indianapolis, IN...Columbus, OH...Denver, CO...Washington, DC...Omaha, NE...
Probabilistic Large Hail Graphic
20180513 1300 UTC Day 1 Large Hail Probabilities Graphic
Probability of hail 1" or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of hail 2" or larger within 25 miles of a point.
Day 1 Hail Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
15 % 36,703 1,763,011 Rockford, IL...Cedar Rapids, IA...Davenport, IA...Iowa City, IA...Dubuque, IA...
5 % 151,464 14,409,889 Denver, CO...Omaha, NE...Wichita, KS...Aurora, CO...Lincoln, NE...
   SPC AC 131255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Sun May 13 2018

   Valid 131300Z - 141200Z


   A few strong to severe thunderstorms are possible today from parts
   of the Midwest to the Mid Atlantic, the southern/central Plains, and
   a portion of the Colorado Front Range into the Laramie Mountains
   vicinity of Wyoming.  Damaging wind and hail will be the primary

   A blocky, split-flow regime over western North America will continue
   to dominate the upper-air pattern over the country today.  A cut-off
   cyclone centered over the Great Basin will continues to fill
   gradually while drifting erratically near its present position.  To
   the east, ridging will prevail from the Ozarks to the Dakotas. 
   Farther downstream, a series of weak, substantially convectively
   induced or enhanced perturbations will traverse a belt of west-
   northwesterly flow from the Upper Midwest to the Mid-Atlantic and
   New England.  Meanwhile, another broad/cut-off cyclone, weaker than
   the western low and presently situated over the eastern Gulf -- will
   move slowly and erratically, with a probable net northward shift
   over the northeastern Gulf through the period.  This feature will
   contribute to general thunderstorm potential over FL, as well as a
   growing potential for heavy rain (per WPC precip-forecast products).

   At the surface, 11Z analyses showed a wavy/quasistationary frontal
   zone extending from a low over northeastern NM east-northeastward
   over southern KS, central MO, central IN and southwestern PA,
   intersecting a strong outflow boundary that extended southeastward
   across northern VA to the southern Delmarva Peninsula near WAL.  The
   front may shift northward today over the lower Missouri Valley and
   parts of IA, but elsewhere should remain near its present position,
   with only minor/mesoscale movements.  A dryline now over the
   northern TX Panhandle and southeastern NM will mix eastward again
   today, reaching the eastern Panhandle, southwestern KS, and the
   Permian Basin region of west TX by mid/late afternoon.

   ...Southern/central Plains...
   Again today, afternoon thunderstorms are expected to initiate on and
   very near the dryline, as related convergence and intense diabatic
   heating overcome CINH in the basal EML capping layer.  With greater
   moisture and similarly steep midlevel lapse rates in place compared
   to previous days, buoyancy will be much larger.  MLCAPE is likely to
   rise above 3000 J/kg within 50-75 nm east of the dryline, and some
   values near 4000 J/kg are possible.  Though strong directional shear
   will exist in support of transient supercell characteristics,
   vertical shear will be modest -- with effective-shear magnitudes
   generally 25-35 kt and small 0-1-km hodographs.  Multicells will be
   the dominant storm mode for most of this event.

   Regardless, the size, depth and "top-heavy" shape (e.g., 300-mb LI
   of -13 to -16 C) of the buoyant profile in forecast soundings
   suggest extremely intense updrafts, characterized by deep initial
   lofting of condensate and fast-growing hydrometeors from a layer of
   13-16 g/kg mean mixing ratios.  In turn, that should yield the
   potential for both severe downdrafts and large hail as cores
   collapse.  A few hours of upscale aggregation of multicellular
   clusters and generally eastward forward propagation are possible,
   extending the severe threat into parts of western OK, southwestern
   KS and northwest TX.  Activity should weaken with eastward extent
   from there, as a combination of outflows and nocturnal/diabatic
   cooling stabilize their inflow layers.  The severe threat becomes
   more isolated and conditional north and south of the "slight" risk
   area due to coverage concerns, though a combination of modified
   RAOBs, forecast soundings and a consensus of numerical guidance
   suggest an afternoon severe thunderstorm or two may fire off the
   dryline in the lower Pecos Valley region as well.

   A weak frontal-wave low may develop along the retreating boundary,
   with theta-e advection occurring across the region.  Boundary-layer
   destabilization will be strong diurnally, both from that factor and
   from diabatic surface heating, with MLCAPE and MUCAPE strengthening
   into the 2000-3000 J/kg range.  However, a stout elevated mixed
   layer and mid/upper-level ridging over the region will render late-
   afternoon severe potential very conditional.  If thunderstorms storm
   can sustain themselves in the relative convergence max near the low
   over western/central IA, upscale development with eastward/forward-
   propagating clustering and related severe-wind/hail potential may
   occur earlier and somewhat farther west than presently indicated.  

   Barring that conditionality, and resultant stabilizing effects, low-
   level warm advection, moisture transport and isentropic lift to LFC
   are expected to support evening/overnight thunderstorm formation
   mainly north of the boundary, offering a large-hail risk.  This may
   evolve to feature more of a wind threat with time, especially for
   activity close to the frontal zone.  Favorable mean winds, as well
   as 40-50-kt effective-shear vectors -- each aligned nearly parallel
   to the boundary --  may support a sustained severe threat into
   tonight for any organized thunderstorm clusters that can develop. 

   ...Central Rockies region...
   Widely scattered thunderstorms are expected to develop over portions
   of the Front and Laramie Ranges late this afternoon, offering
   isolated large hail and damaging gusts.  A tornado cannot be ruled
   out, with that threat strongly dependent upon meso-gamma to
   storm-scale processes.  A narrow corridor of favorable
   destabilization will occur over the mountains, foothills, and a
   small part of the adjacent High Plains, primarily within and north
   of a cyclonically curved convergence zone forming over the DEN area.
   Slightly rising heights aloft (with a lack of substantial upper
   perturbations) and the presence of a strong EML will hamper
   convection through most of the day; however, persistence of heating
   and upslope flow should be sufficient for storms to develop by late

   Forecast soundings show 800-1200 J/kg MLCAPE for a few hours around
   00Z in the northern lobe of the convergence zone, and in a narrow
   tongue northward toward the Laramie Range.  The easterly surface
   flow component north of the convergence zone will contribute upslope
   lift, favorable storm-relative boundary-layer winds, augmented deep
   shear with 40-50 kt effective-shear magnitudes, and locally
   elongated 0-3-km hodographs to support supercell potential. 
   However, such flow also will advect more stable, smaller-theta-e air
   from lower elevations, and keep the spatiotemporal window for
   optimally productive storm maturity rather narrow.  For now the
   threat remains too small-scale and conditional for an upgrade, but
   one may be needed should it become better-defined and more focused
   through the day.

   ...Central Appalachians to Middle Atlantic region...
   Scattered thunderstorms in episodic clusters are expected along and
   north of the front and outflow boundary through the period, offering
   isolated hail and possibly damaging gusts. Any activity accessing
   effectively surface-based inflow parcels along and south of the
   front would have a less-conditional damaging-wind threat, though
   coverage may be lower in the warm sector, with deep shear decreasing

   The outflow pool behind the boundary has been reinforced by
   overnight precip, as well as additional/ongoing clusters of showers
   and thunderstorms over southwestern/south-central PA, the eastern
   panhandle of WV, and northern VA.  This will hinder airmass recovery
   across northern VA into south-central PA today.  Progs are
   accordingly inconsistent on the progress of the warm sector back
   into those areas, with more confidence in destabilization over
   southwestern PA.  To the extent the boundary can retreat back under
   the southern rim of the stronger mid/upper level winds, and in
   combination with vorticity along the retreating boundary, a
   narrow/localized supercell threat may develop.  However, mesoscale
   conditionalities and uncertainties preclude greater than
   marginal-level unconditional outlook probabilities at this time.

   ..Edwards/Mosier.. 05/13/2018