May 1, 2018 2000 UTC Day 1 Convective Outlook
Updated: Tue May 1 19:55:16 UTC 2018 (20180501 2000Z Day 1 shapefile | 20180501 2000Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20180501 2000 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
MODERATE 11,229 156,273 Salina, KS...Great Bend, KS...
ENHANCED 31,241 1,641,888 Omaha, NE...Lincoln, NE...Council Bluffs, IA...Bellevue, NE...Manhattan, KS...
SLIGHT 71,036 2,831,142 Des Moines, IA...St. Joseph, MO...Waterloo, IA...Ames, IA...West Des Moines, IA...
MARGINAL 117,344 8,353,451 St. Paul, MN...Madison, WI...Rockford, IL...Fort Collins, CO...Cedar Rapids, IA...
Probabilistic Tornado Graphic
20180501 2000 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 19,733 304,470 Salina, KS...Junction City, KS...Great Bend, KS...
10 % 19,696 301,430 Salina, KS...Great Bend, KS...
5 % 36,652 1,719,900 Omaha, NE...Lincoln, NE...Council Bluffs, IA...Bellevue, NE...Manhattan, KS...
2 % 98,854 5,299,083 Madison, WI...Des Moines, IA...Cedar Rapids, IA...Topeka, KS...Greeley, CO...
Probabilistic Damaging Wind Graphic
20180501 2000 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 % 28,992 1,488,729 Omaha, NE...Lincoln, NE...Council Bluffs, IA...Bellevue, NE...Salina, KS...
15 % 42,647 1,942,344 Des Moines, IA...St. Joseph, MO...Waterloo, IA...Ames, IA...West Des Moines, IA...
5 % 132,365 7,435,408 Madison, WI...Rockford, IL...Cedar Rapids, IA...Topeka, KS...Green Bay, WI...
Probabilistic Large Hail Graphic
20180501 2000 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 48,465 1,634,879 Lincoln, NE...Bellevue, NE...Salina, KS...Manhattan, KS...Hutchinson, KS...
45 % 11,267 156,364 Salina, KS...Great Bend, KS...
30 % 31,025 1,640,867 Omaha, NE...Lincoln, NE...Council Bluffs, IA...Bellevue, NE...Manhattan, KS...
15 % 71,027 2,830,458 Des Moines, IA...St. Joseph, MO...Waterloo, IA...Ames, IA...West Des Moines, IA...
5 % 116,950 8,287,651 St. Paul, MN...Madison, WI...Rockford, IL...Cedar Rapids, IA...Topeka, KS...
   SPC AC 011955

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0255 PM CDT Tue May 01 2018

   Valid 012000Z - 021200Z


   Numerous severe storms are expected over the central Great Plains to
   the Mid-Missouri Valley. The most likely corridor is across parts of
   central Kansas between 5 to 9 PM CDT. Tornadoes, a couple of which
   could be strong, very large hail, and damaging winds will be

   The only appreciable change from the previous outlook involved the
   inclusion of parts of northwest OK into the categorical 2 (Slight
   Risk).  Recent runs of the HRRR suggest convective initiation near
   the OK/TX border and subsequent supercell-like storms evolving from
   this simulated convection.

   ..Smith.. 05/01/2018

   .PREV DISCUSSION... /ISSUED 1144 AM CDT Tue May 01 2018/

   ...Central Great Plains to the Upper MS Valley...
   12Z soundings across central OK and north-central TX sampled mean
   mixing ratios near 12 g/kg within a plume of 63-64 F surface dew
   points. Richer moisture with upper 60s and lower 70s dew points
   remains confined to south TX this morning. This suggests
   boundary-layer moisture return is probably one day too early from
   being optimal for a regional severe weather/tornado outbreak. Even
   so, confidence is relatively large in storm initiation and probable
   evolution in the late afternoon and evening, rendering increased
   large hail probabilities. 

   The dryline is expected to mix across the eastern TX Panhandle and
   western KS as surface temperatures warm into the 80s, creating a
   T-intersection with a quasi-stationary front arcing across
   west-central KS into eastern NE. A broad plume of MLCAPE from
   2000-2500 J/kg is anticipated ahead in the warm/moist sector.
   Guidance is consistent in ejecting a mid-level speed max ejects from
   the southern High Plains into KS by 00Z. This should aid in ascent
   for probable storm development in the 20-22Z time frame along the KS
   portion of the dryline. Decreasing certainty exists farther south
   along the dryline where weak mid-level height rises are anticipated
   to occur this evening as the broader trough over southern CA digs a
   bit farther south.

   50-kt effective shear, nearly perpendicular deep-layer winds to the
   dryline, and rather steep mid-level lapse rates will favor discrete
   supercells producing very large hail. A few of these supercells may
   become long-tracked across central KS and the threat for tornadoes
   will increase between 22-02Z as storms encounter richer
   boundary-layer moisture (toward the I-135 corridor) and as low-level
   shear increases rapidly in response to a strengthening low-level
   jet. The synoptic pattern is largely consistent with past Great
   Plains cases of the evening low-level jet strengthening and aiding
   in mid/late evening significant tornadoes. However, low-level
   moisture may remain on the lower margins of what is typical of the
   high-end events (with similar temperature profiles and hodograph
   structures). The slightly drier boundary layer will encourage a
   little quicker increase in convective inhibition, coincident with
   the strengthening low-level shear. Thus, it is not clear whether or
   not this scenario will evolve into a long-lived significant tornado
   threat near sunset, or if the inhibition increase will tend to limit
   the window of opportunity. Otherwise, clusters of storms (with
   embedded supercells) should persist well into the overnight hours
   with an attendant threat for large hail/damaging gusts and a couple
   tornadoes, from the triple point northeastward along the
   quasi-stationary front into the Upper MS Valley.

   ...Northeast CO...
   An upslope flow regime will help maintain boundary-layer dewpoints
   in the 40s across northeast CO through the afternoon, north of a
   stalled front. Surface heating in cloud breaks will yield weak
   surface-based buoyancy, while deep-layer vertical shear will be
   favorable for supercells. A couple supercells may form this
   afternoon in conjunction with the northern periphery of the
   mid-level speed max ejecting into KS. These storms should move east
   before weakening by mid-evening.  Isolated large hail and strong
   wind gusts will be possible.