Oct 2, 2017 1300 UTC Day 1 Convective Outlook
Updated: Mon Oct 2 12:59:09 UTC 2017 (20171002 1300Z Day 1 shapefile | 20171002 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20171002 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 14,747 140,078 Garden City, KS...Dodge City, KS...
MARGINAL 121,791 5,405,654 Minneapolis, MN...St. Paul, MN...Sioux Falls, SD...Sioux City, IA...Bloomington, MN...
Probabilistic Tornado Graphic
20171002 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 % 10,688 50,501 No Major Population Center in Risk Area
Probabilistic Damaging Wind Graphic
20171002 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
5 % 83,224 4,570,385 Minneapolis, MN...St. Paul, MN...Sioux City, IA...Bloomington, MN...Plymouth, MN...
Probabilistic Large Hail Graphic
20171002 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 % 14,679 139,871 Garden City, KS...Dodge City, KS...
5 % 100,729 3,494,782 Minneapolis, MN...Sioux Falls, SD...Sioux City, IA...Bloomington, MN...Plymouth, MN...
   SPC AC 021259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Mon Oct 02 2017

   Valid 021300Z - 031200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   WESTERN KANSAS AND SOUTH-CENTRAL NEBRASKA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
   THE NORTHERN TEXAS PANHANDLE TO PARTS OF NORTHEASTERN COLORADO AND
   THE UPPER MISSISSIPPI VALLEY...

   ...SUMMARY...
   Several episodes of strong to severe thunderstorms are expected from
   afternoon at least into late evening, in a corridor from the central
   Great Plains to the upper Mississippi Valley.  At this time, the
   greatest potential for organized severe weather appears to be over
   parts of western Kansas into south-central Nebraska.

   ...Synopsis...
   A high-amplitude large-scale pattern, characterized by eastern
   ridging and western troughing, will dominate the continental U.S.
   this period.  Associated cyclonic flow will continue to cover much
   of the West; however, the trough will reorient toward a positive
   tilt, in response to the behavior of several shortwaves embedded in
   that flow field.  A leading/closed 500-mb low will meander over
   eastern MT the whole period.  A trailing shortwave trough -- now
   evident in moisture-channel imagery from southwestern UT to near the
   southern tip of NV, will eject northeastward.  By 00Z, this
   perturbation should reach the UT/CO border region.  The shortwave
   trough and its vorticity field then should accelerate northeastward,
   weaken and become deformed amidst confluent flow rimming the MT
   cyclone.  Meanwhile, another shortwave trough -- now extending from
   the inland Northwest across Vancouver Island, should pivot southward
   and strengthen into a closed cyclone over northern CA by 12Z.  

   At the surface, 11Z analysis showed an occluded front from southern
   MB southward across western MN.  The occlusion triple point over the
   Siouxland region was ill-defined amidst stabilized air from precip,
   but the associated quasistationary front became better-defined
   southwestward across eastern NE, northern/western KS and northern
   NM.  The High Plains portion of the front may retreat northward for
   several hours this afternoon into this evening, beneath neutral to
   weakly positive height-change fields aloft.  A dryline, drawn
   initially from northeastern NM southwestward between ELP-LRU, should
   mix northeastward across the western Panhandles and into
   southwestern KS today south of the front. 

   ...Central Plains to Upper Mississippi Valley...
   Multiple episodes or loci of strong to severe convective development
   are possi ble within this corridor.  Some spatial overlap and/or
   mergers between them comprise the outlook areas as a whole.  The
   most probable initial regimes are:

   *  Dryline to vicinity of cold-frontal intersection:
   Isolated to widely scattered, surface-based thunderstorm development
   now appears increasingly probable mid/late afternoon, near the
   dryline, along and south of its intersection with the cold front.  A
   growing consensus of convection-allowing guidance support this
   scenario, which appears reasonable given the heating expected just
   south of the front along the dryline, along with related removal of
   MLCINH in forecast soundings, in conjunction with convergence along
   the front itself.  Planar progs and forecast soundings suggest the
   parameter space into which this activity would move will favor
   supercell structures with large hail and isolated damaging gusts
   possible.  Surface dew points low-mid 60s F and diabatic heating
   along and just ahead of the dryline should yield MLCAPE exceeding
   2000 J/kg in a narrow corridor, juxtaposed with 45-55 kt
   effective-shear magnitudes and 200-300 J/kg effective SRH.  

   Hail-model application to forecast soundings suggests that at least
   isolated hail near 2 inches in diameter may occur, especially with
   any sustained/discrete supercell.  A sig-hail probability line may
   be introduced in a later outlook if confidence increases in
   coverage/duration of supportive storm mode.  A tornado cannot be
   ruled out, especially with any discrete or semi-discrete storm that
   remains surface based while interacting with the baroclinic zone. 
   Northern members of this convective plume eventually may merge with
   the southern part of the post-frontal band described below. 

   *  Post-frontal, High Plains initiation:
   As large-scale lift aloft related to the Great Basin shortwave
   trough approaches late this afternoon, widely scattered to scattered
   thunderstorms should develop near the Foothills and eastward across
   the High Plains of northeastern CO.  This process will be supported
   further by a substantial easterly/upslope component to the low-level
   flow. Initially dependent mainly on elevated inflow parcels, this
   activity should encounter an increasingly unstable layer above the
   surface, resulting from low-level theta-e advection, and may produce
   isolated large hail.  Potential exists for upscale growth into a
   wind-favoring bowing segment or cluster, whose downward momentum
   transfer may become strong enough to penetrate the
   near-surface/post-frontal stable layer with near-severe gusts in a
   few locales.  This convection may last long enough to merge with the
   southwestern parts of the next regime. 

   *  Post-frontal band, central Plains northeastward:
   Convective coverage should increase from late afternoon through
   evening behind the surface front, from the central Plains into parts
   of the upper Mississippi Valley region, offering the risk of at
   least isolated large hail.  The main limiting factor for hail
   coverage/intensity may be the presence of too many storms and
   related modal interference, though this also may support greater
   than currently depicted potential hail coverage early in the cycle
   of convective development, prior to excessive mergers/interference. 
   Elevated MUCAPE 1000-2000 J/kg and favorable deep shear are expected
   over this region.

   ..Edwards/Dial.. 10/02/2017

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