May 1, 2018 1300 UTC Day 1 Convective Outlook
Updated: Tue May 1 12:48:24 UTC 2018 (20180501 1300Z Day 1 shapefile | 20180501 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
20180501 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
ENHANCED 42,907 1,830,766 Omaha, NE...Lincoln, NE...Council Bluffs, IA...Bellevue, NE...Salina, KS...
SLIGHT 66,316 2,820,062 Des Moines, IA...St. Joseph, MO...Waterloo, IA...Ames, IA...West Des Moines, IA...
MARGINAL 121,092 8,019,652 St. Paul, MN...Madison, WI...Rockford, IL...Fort Collins, CO...Cedar Rapids, IA...
Probabilistic Tornado Graphic
20180501 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 17,113 266,896 Salina, KS...Junction City, KS...Great Bend, KS...
10 % 17,084 268,692 Salina, KS...Junction City, KS...Great Bend, KS...
5 % 33,319 1,703,572 Omaha, NE...Lincoln, NE...Council Bluffs, IA...Bellevue, NE...Manhattan, KS...
2 % 98,363 5,273,492 Madison, WI...Des Moines, IA...Cedar Rapids, IA...Topeka, KS...Greeley, CO...
Probabilistic Damaging Wind Graphic
20180501 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 % 28,992 1,488,729 Omaha, NE...Lincoln, NE...Council Bluffs, IA...Bellevue, NE...Salina, KS...
15 % 40,146 1,902,404 Des Moines, IA...St. Joseph, MO...Waterloo, IA...Ames, IA...West Des Moines, IA...
5 % 117,836 7,129,340 Madison, WI...Rockford, IL...Cedar Rapids, IA...Topeka, KS...Green Bay, WI...
Probabilistic Large Hail Graphic
20180501 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 42,630 1,583,098 Lincoln, NE...Bellevue, NE...Salina, KS...Manhattan, KS...Hutchinson, KS...
30 % 42,943 1,828,542 Omaha, NE...Lincoln, NE...Council Bluffs, IA...Bellevue, NE...Salina, KS...
15 % 66,302 2,785,282 Des Moines, IA...St. Joseph, MO...Waterloo, IA...Ames, IA...West Des Moines, IA...
5 % 123,091 8,294,965 St. Paul, MN...Madison, WI...Rockford, IL...Cedar Rapids, IA...Topeka, KS...
   SPC AC 011248

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0748 AM CDT Tue May 01 2018

   Valid 011300Z - 021200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   THROUGH EARLY TONIGHT FROM CENTRAL AND NORTHERN KS TO SOUTHWESTERN
   IA...

   ...SUMMARY...
   Severe storms are expected mainly across north-central Kansas into
   southeast Nebraska from late afternoon through early tonight.  A
   strong tornado or two, very large hail, and damaging winds will all
   be possible.

   ...Synopsis...
   A midlevel low will become established over southern CA and the
   lower CO River valley by tonight, and the downstream southwesterly
   flow aloft will aid in deepening a lee cyclone today across eastern
   CO/western KS.  A quasi-stationary front will extend northeastward
   from the lee cyclone across southeast NE to the upper MS Valley,
   while a dryline will mix eastward into southwest KS and to the
   eastern TX Panhandle to the south of the triple point.  This pattern
   will maintain northward moisture transport from the southern to the
   central Plains, and will support severe storm development near the
   triple point along both the stationary front and a short distance
   southward along the dryline.

   ...Central Plains to the upper MS Valley through tonight...
   Surface temperatures warming into the 80s into southwest KS and
   boundary-layer dewpoints increasing into the low-mid 60s east of the
   dryline (per overnight observations) will contribute to MLCAPE
   values in excess of 2500 J/kg and weakening convective inhibition
   near the triple point and dryline by mid afternoon.  Thunderstorm
   development is likely in the 21-23z time frame, with more
   uncertainty along the dryline with southward extent toward the KS/OK
   border, in an environment with weak background height rises.  The
   initial storms should form in the more deeply mixed boundary layer
   close to the dryline, with steep lapse rates and effective bulk
   shear near 50 kt favoring supercells capable of producing very large
   hail.  Storms may tend to cluster more along the front into NE, with
   more discrete supercells persisting farther south in KS.  The threat
   for tornadoes will increase during the evening as the storms
   encounter richer moisture with eastward extent (toward the I-135
   corridor in central/north central KS), and as low-level shear
   increases rapidly in the 23-02z time frame with a strengthening
   low-level jet.  

   The synoptic pattern is largely consistent with past Plains cases of
   the evening low-level jet strengthening and late evening significant
   tornadoes.  However, low-level moisture is 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 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 perhaps a tornado or two, from the triple
   point northeastward along the quasi-stationary front into the upper
   MS Valley.

   ...Northeast CO this afternoon/evening...
   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 drive weak
   surface-based buoyancy, while deep-layer vertical shear will be
   favorable for supercells.  A few storms will form this afternoon in
   conjunction with subtle speed maxima ejecting northeastward from the
   Four Corners, and the storms will move across northeast CO before
   weakening later this evening.  Isolated large hail and strong wind
   gusts will be possible.

   ..Thompson.. 05/01/2018

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