Aug 25, 2017 1300 UTC Day 1 Convective Outlook
Updated: Fri Aug 25 12:47:32 UTC 2017 (20170825 1300Z Day 1 shapefile | 20170825 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20170825 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SLIGHT 45,615 5,811,871 Houston, TX...Pasadena, TX...Sioux Falls, SD...Sugar Land, TX...Missouri City, TX...
MARGINAL 78,451 3,645,745 Corpus Christi, TX...Beaumont, TX...Sioux City, IA...College Station, TX...The Woodlands, TX...
Probabilistic Tornado Graphic
20170825 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
5 % 10,562 5,263,174 Houston, TX...Pasadena, TX...Sugar Land, TX...Missouri City, TX...Baytown, TX...
2 % 45,497 2,825,255 Corpus Christi, TX...Beaumont, TX...College Station, TX...The Woodlands, TX...Bryan, TX...
Probabilistic Damaging Wind Graphic
20170825 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 % 26,398 516,909 Sioux Falls, SD...Watertown, SD...Brookings, SD...Mitchell, SD...Yankton, SD...
5 % 97,897 8,989,906 Houston, TX...Corpus Christi, TX...Pasadena, TX...Beaumont, TX...Sioux City, IA...
Probabilistic Large Hail Graphic
20170825 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 % 27,046 192,403 Watertown, SD...Mitchell, SD...Huron, SD...Madison, SD...
5 % 54,809 1,092,205 Sioux Falls, SD...Sioux City, IA...Norfolk, NE...Aberdeen, SD...Willmar, MN...
   SPC AC 251247

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0747 AM CDT Fri Aug 25 2017

   Valid 251300Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   MIDDLE-UPPER TEXAS COASTAL PLAIN...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   EASTERN SOUTH DAKOTA...EXTREME NORTHERN NEBRASKA AND SOUTHWESTERN
   MINNESOTA...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   TEXAS COAST SLIGHT RISK AND EXTENDING DOWN THE LOWER COAST...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   NORTHERN SLIGHT RISK...

   ...SUMMARY...
   Tornadoes and locally damaging winds are possible today and tonight
   with stronger cells mainly north through northeast of the center of
   Hurricane Harvey.  Otherwise, a cluster of thunderstorms should
   develop across parts of South Dakota and northern Nebraska, offering
   locally damaging winds and/or hail during the afternoon and evening.

   ...Synopsis...
   In mid/upper levels, the pattern will include a broad area of
   ridging and weak flow aloft from the Southwest and Great Basin to
   Florida, within which Hurricane Harvey is becoming increasingly
   embedded as it moves northwestward toward the TX coast.  Per NHC
   discussions, this should result in a marked deceleration of the
   tropical cyclone near the coast and its very slow movement for a few
   days.  In the northern stream, a strong shortwave trough now over
   northern AB will eject northeastward toward Nunavut by 12Z.  To its
   southeast, a minor perturbation -- now evident in moisture-channel
   imagery over eastern MT -- is progged to move slowly
   east-northeastward across much of ND through the period.

   At the surface, a weak low was analyzed in extreme eastern WY near
   TOR, with a warm front arching eastward then northeastward across
   northern NE, southeastern SD and southwestern MN.  The low is
   expected to shift eastward across the northern Sandhills of NE
   today, while the warm front and collocated "inverted" trough moves
   slowly northwestward over eastern SD.  The low should move
   east-northeastward obliquely across the SD/NE border overnight,
   reaching the FSD/MHE area by 12Z.

   ...TX coastal plain...
   Hurricane Harvey is forecast by NHC to continue approaching the
   Texas coastal bend region, with landfall possible late in the day-1
   convective period.  Initial outer bands have outrun the most
   favorable low-level wind profiles and shear, and have exhibited
   outflow-dominant, messy modes.  However, more intense low-level
   winds just above the surface are expected to shift northward and
   ashore over the middle/upper TX coast today, in step with Harvey's
   motion, and in keeping with NHC's forecast pre-landfall
   intensification of its wind fields.  This combined process will
   spread increasingly favorable low-level hodographs across the
   outlook area through today and into tonight.

   With rich boundary-layer moisture a given, and weak CINH, pockets of
   relative clearing and related heating and warm advection (away from
   persistent heavy-rain areas) should boost MLCAPE into the 1000-1500
   J/kg range.  MLCAPE may locally exceed 2000 J/kg where any pockets
   of persistent insolation can develop.  These factors should ramp up
   the threat for a few tornadoes throughout the day, whether from
   supercells embedded in bands or discrete cells moving ashore from
   the Gulf.  Though instability should diminish over land after dark,
   low-level theta-e will remain rich enough to support sporadic
   mesocyclones and a tornado threat throughout the night as well.

   See the latest NHC advisory and graphics for forecast information on
   Harvey, including related tropical watches and warnings.

   ...North-central Plains...
   Widely scattered to scattered thunderstorms are expected to develop
   by late afternoon in the destabilizing area of relatively maximized
   low-level lift corresponding to the front/trough.  Large-scale
   ascent/destabilization aloft related to the mid/upper-level
   perturbation will support this activity as well, and strongly
   difluent flow aloft is expected near the shortwave trough, extending
   southward across the outlook area.  Diabatic heating of a moistening
   near-surface layer is expected east of the front/trough, combining
   with 60s F surface dew points and 6.5-7 deg C/km lapse rates above
   the boundary layer to produce MLCAPE ranging from 1500-3000 J/kg,
   locally higher.

   Any sustained/discrete storms will have the potential for at least
   brief/low-end supercell characteristics, given enlarging curvature
   and 0-1-km SRH in time series of forecast hodographs in the 22-01Z
   time frame.  However, a deep/mid-upper-level layer of weak flow will
   contribute to messy storm modes fairly quickly, with a band or arc
   of convection forecast to develop over the southeastern
   SD/northeastern NE region this evening.  Early hail-dominant
   discrete to semi-discrete configurations should transition to later
   wind-preferred bowing segments within the convective band.  Late
   evening into overnight hours, the activity will encounter
   progressively more stable near-surface air with a decreasing severe
   threat.

   ..Edwards/Smith.. 08/25/2017

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