May 18, 2017 1300 UTC Day 1 Convective Outlook
Updated: Thu May 18 12:43:31 UTC 2017 (20170518 1300Z Day 1 shapefile | 20170518 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Public Severe Weather Outlook

The SPC is forecasting ...Outbreak of tornadoes and severe thunderstorms expected over parts of the southern and central Plains this afternoon....   Please read the latest public statement about this event.  View What is a Watch? clip.

Categorical Graphic
20170518 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
HIGH 24,613 927,395 Wichita, KS...Enid, OK...Hutchinson, KS...Dodge City, KS...Derby, KS...
MODERATE 29,364 624,445 Salina, KS...Stillwater, OK...Emporia, KS...Ponca City, OK...Yukon, OK...
ENHANCED 43,807 4,681,691 Oklahoma City, OK...Kansas City, MO...Tulsa, OK...Overland Park, KS...Kansas City, KS...
SLIGHT 237,824 23,765,563 Dallas, TX...Fort Worth, TX...Cleveland, OH...Arlington, TX...Toledo, OH...
MARGINAL 238,629 33,375,858 San Antonio, TX...Detroit, MI...Indianapolis, IN...Columbus, OH...Austin, TX...
Probabilistic Tornado Graphic
20170518 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
SIG SEVERE 60,354 1,991,726 Wichita, KS...Lawton, OK...Enid, OK...Salina, KS...Stillwater, OK...
30 % 24,387 927,558 Wichita, KS...Enid, OK...Hutchinson, KS...Dodge City, KS...Derby, KS...
15 % 29,560 622,068 Salina, KS...Stillwater, OK...Emporia, KS...Ponca City, OK...Yukon, OK...
10 % 20,356 1,178,020 Oklahoma City, OK...Wichita Falls, TX...Lawton, OK...Edmond, OK...Bartlesville, OK...
5 % 67,117 4,463,411 Tulsa, OK...Overland Park, KS...Kansas City, KS...Topeka, KS...Abilene, TX...
2 % 157,197 18,750,130 Dallas, TX...Fort Worth, TX...Kansas City, MO...Cleveland, OH...Arlington, TX...
Probabilistic Damaging Wind Graphic
20170518 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
30 % 74,524 5,793,107 Oklahoma City, OK...Tulsa, OK...Wichita, KS...Overland Park, KS...Kansas City, KS...
15 % 250,911 23,960,030 Dallas, TX...Fort Worth, TX...Kansas City, MO...Cleveland, OH...Arlington, TX...
5 % 232,708 32,332,891 San Antonio, TX...Detroit, MI...Indianapolis, IN...Columbus, OH...Austin, TX...
Probabilistic Large Hail Graphic
20170518 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
SIG SEVERE 76,682 3,134,717 Oklahoma City, OK...Wichita, KS...Abilene, TX...Norman, OK...Wichita Falls, TX...
45 % 24,528 904,050 Wichita, KS...Enid, OK...Hutchinson, KS...Dodge City, KS...Derby, KS...
30 % 49,119 2,172,783 Oklahoma City, OK...Tulsa, OK...Edmond, OK...Midwest City, OK...Moore, OK...
15 % 251,175 25,869,541 Dallas, TX...Fort Worth, TX...Kansas City, MO...Cleveland, OH...Arlington, TX...
5 % 243,538 33,362,549 San Antonio, TX...Detroit, MI...Indianapolis, IN...Columbus, OH...Austin, TX...
   SPC AC 181243

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0743 AM CDT Thu May 18 2017

   Valid 181300Z - 191200Z

   ...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
   CENTRAL AND SOUTHERN KANSAS TO NORTHWESTERN OKLAHOMA...

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   HIGH RISK FROM WESTERN OKLAHOMA TO CENTRAL KANSAS...

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   MODERATE RISK FROM PARTS OF NORTHWEST TEXAS TO PARTS OF SOUTHEASTERN
   COLORADO...NORTHERN KANSAS AND EXTREME WESTERN MISSOURI...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK AND EXTENDING INTO EASTERN COLORADO...WESTERN
   MISSOURI...EXTREME SOUTHERN NEBRASKA...AND THE TEXAS RIO GRANDE
   VALLEY NEAR DEL RIO...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHEASTERN
   INDIANA TO WESTERN NEW ENGLAND...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING BOTH
   SLIGHT RISKS AND BETWEEN THEM OVER THE MID MISSISSIPPI VALLEY...

   ...SUMMARY...
   An outbreak of severe thunderstorms with tornadoes, very large hail
   and wind damage is expected to develop across parts of the southern
   and central Plains from this afternoon into this evening and
   overnight.  Some strong, long-lived tornadoes are expected, along
   with hail to near 4 inches in diameter, over parts of Kansas and
   Oklahoma.  Large hail, tornadoes and wind damage also will be
   possible outside of these areas across parts of the southern and
   central Plains.  Damaging gusts and sporadic hail also are expected
   across parts of the lower Great Lakes region to western New England
   this afternoon and early evening.

   --- Technical Discussion ---

   ...Synopsis...
   In mid/upper levels, broadly cyclonic flow will cover much of the
   western and central U.S., anchored by a major cyclone now centered
   over UT.  The 500-mb low is expected to dig southeastward across the
   Four Corners through the period, reaching northwestern NM by the end
   of the period.  As this occurs, the leading lobe of an 85-100-kt
   250-mb jet will spread over the southern High Plains and western/
   central KS through tonight.  A leading shortwave trough -- currently
   apparent on moisture-channel imagery over WI, western Lake Superior
   and the MN Arrowhead -- will weaken and eject east-northeastward
   across southern QC to northern ME by 12Z.

   At the surface, 11Z analysis showed a cold front from southern QC to
   a low over the MI/WI border, southwestward across IL, becoming
   quasistationary over MO and the KS/OK border region, to a weak low
   over southeastern CO.  The low is expected to move erratically
   eastward along the KS frontal segment (becoming a slowly northward-
   moving warm front) through the period.  Today, the richer low-level
   moisture now over north TX and southern/central OK will advect
   northwestward over the outlook area.  The dryline will sharpen this
   afternoon from northern Coahuila northward across west-central TX to
   southwestern KS.  The cold front will move southeastward across the
   Great Lakes region, reaching parts of NY, New England and the Ohio
   Valley region overnight.

   ...Central/southern Plains...
   Scattered thunderstorms, including multiple supercells with
   significant-tornado, giant-hail and damaging-wind potential, still
   appear probable this afternoon anywhere over the moderate- and
   high-risk areas, and possible farther south where coverage should be
   more widely scattered to isolated.  The progged ingredient/parameter
   space is ideal for a multi-hazard severe-weather outbreak from the
   warm-frontal zone southward for 250-300 nm ahead of the dryline:
   *  Moderate to strong moisture and instability:  moist advection of
   a rich Gulf air mass with 60s to near 70 deg F surface dew points,
   diurnally heated along/ahead of the dryline to remove CINH, beneath
   roughly 8 deg C/km midlevel lapse rates, yielding 3000-4500 J/kg
   MLCAPE.  CAPE will diminish northward through the warm-frontal zone
   but the cut-off of surface-based inflow should be gradual, not
   abrupt.
   *  Favorable lift along the dryline and in the "bent-back" zone of
   deep forcing closer to the surface and upper lows,
   *  Favorable low-level and deep-layer shear, especially near the
   warm front and low, but also southward across the moist sector. 
   Low-level vorticity and hodograph size each will be maximized along
   and somewhat north of the warm front, with 300-400 J/kg effective
   SRH and 50-65 kt effective-shear magnitudes common.
   Any discrete/sustained supercells in this environment will be
   capable of cyclic, significant-tornado production as well as giant,
   destructive hail.

   A note of caution on this forecast:  Some uncertainties still exist,
   however, precluding any greater probabilities at this time -- mainly
   involving density and timing of convection.  Initiation of too many
   cells in early/middle afternoon (before warm-sector hodographs and
   CAPE each are biggest), and/or too close to each other at once, is
   possible in some parts of the current moderate/high risks.  This
   scenario, which some guidance suggests, would lend a greater wind
   threat and somewhat suppressed hail/significant-tornado risk with
   eastward extent, due to messier storm modes amidst strengthening
   deep-layer flow.  Evidence for such a quick transition still is
   inconsistent in the bulk of numerical guidance, and of course
   observationally.  While not compelling enough to modify the high or
   moderate risk-driving probabilities yet, this is a possibility to be
   monitored for potential adjustments to the outlook during the day.

   Regardless of timing, storm mergers and ultimate upscale growth of
   convective clusters appear likely, with embedded supercells, bows,
   and mesobeta- or smaller-scale damaging-wind swaths.  Therefore,
   wind probabilities have been offset eastward somewhat, relative to 
   the others.

   ...Northeast to mid Mississippi Valley...
   Widely scattered to scattered thunderstorms are expected to develop
   along and ahead of the front this afternoon as diabatic surface
   heating and low-level convergence zones (including the baroclinic
   zone) act against already weak MLCINH.  Well-mixed subcloud layers
   beneath 1000-1500 J/kg MLCAPE, and the presence of 30-40 kt
   effective-shear magnitudes will support enough multicellular
   organization, and perhaps transient supercell structures, for a
   hail/wind threat.  Nearly unidirectional prefrontal wind profiles
   are forecast, and some bowing clusters could develop that would
   enhance the wind-damage potential on smaller scales, especially over
   parts of NY or western New England, however, it is too soon to be
   specific enough with mesoscale details for a greater categorical
   corridor within the existing, somewhat eastward-expanded lines.  The
   severe wind/hail threat will be more isolated, marginal and
   transient this afternoon and early evening southwestward toward the
   middle Mississippi Valley where warm-sector surface winds and
   frontal convergence each should be weaker.

   ..Edwards/Dial.. 05/18/2017

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