18/06/2015
THERE IS A SLGT RISK OF SVR TSTMS FROM MT SOUTHEAST ACROSS THE
NORTHERN AND CENTRAL HIGH PLAINS...
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE MIDWEST/OH VALLEY TO
THE MID ATLANTIC...
...THERE IS A MRGL RISK OF SVR TSTMS OVER THE HIGH
PLAINS...ARKLATEX...AND MUCH OF THE EASTERN U.S....
...SUMMARY...
SCATTERED SEVERE STORMS WITH HAIL AND WIND ARE POSSIBLE ACROSS THE
NORTHERN AND CENTRAL HIGH PLAINS THIS AFTERNOON AND EVENING. OTHER
STORMS...MAINLY POSING A THREAT FOR LOCALLY STRONG TO DAMAGING
WINDS...WILL BE POSSIBLE DURING THE HEAT OF THE DAY FROM THE MIDWEST
AND OHIO VALLEY...EAST TO THE MID-ATLANTIC...AND ACROSS THE
SOUTHEAST.
...OVERVIEW...
A BELT OF MODEST QUASI-ZONAL MID-LEVEL FLOW WILL PERSIST OVER THE
NORTHERN ONE-THIRD OF THE CONUS THIS PERIOD. FROM WEST TO
EAST...LOW-AMPLITUDE SHORT WAVE PERTURBATIONS WITHIN THIS FLOW WERE
IDENTIFIED IN WATER V***R IMAGERY AND SHORT-TERM MODEL GUIDANCE
ACROSS THE PACIFIC NORTHWEST...CENTRAL PLAINS...GREAT LAKES AND MID
ATLANTIC EARLY TODAY. AT THE SURFACE...A WAVY FRONTAL ZONE RESIDES
ON THE SOUTHERN FRINGE OF THE STRONGER FLOW ALOFT. THE FRONT IS
ILL-DEFINED OVER THE MID ATLANTIC BUT BETTER FOCUSED WEST OF THE
APPALACHIANS WHERE IT EXTENDS WEST-NORTHWESTWARD TO A WEAK LOW
APPROACHING LAKE MI. THE FRONT CONTINUES WEST FROM THE LAKE MI
SURFACE LOW TO WHERE IT INTERSECTS A MATURE MCS OVER NEB...AND THEN
LIES WEST-NORTHWESTWARD ACROSS THE HIGHS PLAINS TO SOUTHERN MT. A
VERY MOIST MARITIME TROPICAL AIRMASS LIES COINCIDENT WITH AND SOUTH
OF THE FRONT...ALONG MUCH OF ITS EXTENT. IN CONCERT WITH STRONGER
FLOW AND FORCING ACCOMPANYING THE SHORT WAVE TROUGHS ALOFT...AND
HEATING OF THE DAY...THE MOST ORGANIZED THUNDERSTORMS WILL LIKELY
OCCUR ALONG/NEAR THE FRONTAL ZONE THROUGH TODAY...WITH MORE
SPORADIC/RANDOM CONVECTION OCCURRING ACROSS THE WARM SECTOR SOUTH OF
THE BOUNDARY.
WHAT REMAINS OF TROPICAL CYCLONE BILL IS MANIFEST AS A SLOW-MOVING
DEEP-LAYER CIRCULATION OVER SOUTHEAST OK THIS MORNING. THIS
DISTURBANCE...PRESENTLY REMOVED FROM THE STRONGER STEERING CURRENTS
OF THE WESTERLIES AND TRAPPED BETWEEN LARGE SCALE RIDGING TO ITS
WEST AND EAST...WILL DRIFT TOWARD THE OZARKS THIS PERIOD.
...MT SOUTHEAST ACROSS THE NORTHERN/CENTRAL HIGH PLAINS...
SHORT WAVE RIDGING ALOFT AND GENERALLY CLEAR TO PARTLY CLOUDY SKIES
SHOULD PROMOTE DESTABILIZATION DURING THE DIURNAL HEATING CYCLE
AHEAD OF THE PACIFIC NORTHWEST TROUGH EXPECTED TO BRING WEAK HEIGHT
FALLS TO THE NORTHERN ROCKIES LATER TODAY. ADEQUATE LOW LEVEL
MOISTURE FOCUSED ALONG RESIDUAL FRONTAL BOUNDARY...AND UPSLOPE
FLOW...WILL CONTRIBUTE TO HIGH ELEVATION CONVECTIVE DEVELOPMENT
DURING THE AFTERNOON. STORMS WILL BE SUSTAINED OFF THE HIGHER
TERRAIN BY STRENGTHENING MID-LEVEL WESTERLY FLOW...INCREASING LARGE
SCALE ASCENT...AND DOWNSTREAM CORRIDOR OF GREATER DESTABILIZATION.
EAST-SOUTHEASTERLY LOW LEVEL FLOW WILL VEER AND STRENGTHEN WITH
HEIGHT RESULTING IN FAVORABLE KINEMATIC PROFILES FOR SUPERCELLS.
INITIALLY DISCRETE STORMS WILL BE CAPABLE OF PRODUCING ISOLATED VERY
LARGE HAIL...AND PERHAPS EVEN A TORNADO. CONSOLIDATION OF UPDRAFTS
SHOULD OCCUR DURING THE EVENING WITH THE POSSIBILITY FOR A
FORWARD-PROPAGATING MCS AND DAMAGING WIND THREAT CONTINUING EAST
FROM SOUTHERN MT TO WESTERN DAKOTAS LATE.
MORE ISOLATED STORM ACTIVITY IS INDICATED OVER THE HIGH PLAINS ALONG
A LEE/THERMAL TROUGH FROM WY SOUTH TO CO/KS. ONE POSSIBLE AREA OF
GREATER STORM INTENSITY AND/OR COVERAGE MAY EVOLVE OVER WESTERN KS
AND ADJACENT AREAS OF EASTERN CO. SURFACE HEATING/MIXING OVER THESE
AREAS WILL CONTRIBUTE TO STRONG DESTABILIZATION WITH AFTERNOON
MLCAPE FORECAST TO RANGE FROM 2000-3000 J PER KG. LEE TROUGH/MCS
OUTFLOW/FRONT INTERSECTION WILL RESIDE AMIDST THIS STRONGLY UNSTABLE
AIRMASS WHERE WEAK LOW LEVEL EASTERLIES WILL BE TOPPED BY
NORTHWESTERLY FLOW AROUND 20KT. MAGNITUDE OF INSTABILITY MAY
COMPENSATE FOR WEAK TO MODEST EFFECTIVE SHEAR AROUND 25-30 KT TO
SUPPORT A FEW ROBUST MULTICELL STORMS WITH LARGE HAIL AND DOWNBURST
WINDS. ACTIVITY SHOULD DECAY QUICKLY AFTER DARK GIVEN LACK OF
STRONGER LARGE SCALE FORCING AND SHEAR.
...MIDWEST/OH VALLEY...
AS STATED ABOVE...GREATEST CHANCE OF ORGANIZED/PERSISTENT STORMS
SHOULD EVOLVE NEAR THE FRONT...AS WELL AS SURFACE LOW MOVING INTO
LOWER MI THIS AFTERNOON...WITH MORE SCATTERED STORMS ACROSS WARM
SECTOR AIRMASS. MID-LEVEL LAPSE RATES ARE NOT PARTICULARLY STEEP BUT
MLCAPE WILL STILL REACH 1500-2500 J PER KG GIVEN RICH MOISTURE
ACROSS THE REGION /PW 1.75-2 INCHES IN SOME AREAS PER MORNING GPS
DATA/. THE BELT OF 40-50KT MID-LEVEL FLOW AND STRONGER ASCENT
ACCOMPANYING THE GREAT LAKES SHORT WAVE TROUGH WILL TEND TO OUTRUN
GREATER DESTABILIZATION OVER THE CORN BELT AND LEND SUPPORT FOR
CLUSTERS/LINES OF STORMS POSING PRIMARILY A DAMAGING WIND THREAT
FROM IND AND LOWER MI EAST ACROSS OH/PA/WESTERN NY DURING PERIOD OF
MAX HEATING.
...APPALACHIAN CREST TO TIDEWATER VA...
DIFFERENTIAL HEATING ON THE EAST SLOPES OF THE APPALACHIAN HIGHLANDS
WILL SPUR CONVECTIVE DEVELOPMENT BY AFTERNOON AS LOWER ELEVATIONS
BECOME STRONGLY UNSTABLE. BACKGROUND SYNOPTIC ASCENT SHOULD SUSTAIN
CONVECTION EASTWARD AMIDST DEEP-LAYER UNIDIRECTIONAL SHEAR ADEQUATE
FOR UPDRAFT PERSISTENCE/ORGANIZATION. IN ADDITION TO THE POTENTIAL
FOR CLUSTERS AND/OR BROKEN LINES OF STORMS CAPABLE OF SCATTERED
DAMAGING WINDS AND MARGINAL HAIL...CELLS TRACKING NEAR/ALONG
SOUTHEAST TO NORTHWEST ORIENTED BACK-DOOR QUASI-STATIONARY BOUNDARY
OVER THE DELMARVA REGION COULD SPAWN A BRIEF TORNADO OR TWO.
...ARKLATEX/BILL...
ANY BREAKS IN CLOUD COVER NEAR THE CYCLONIC CIRCULATION ASSOCIATED
WITH THE REMNANTS OF BILL COULD RESULT IN LOCALLY MODEST
DESTABILIZATION THIS AFTERNOON. IF THIS DESTABILIZATION OCCURS
WITHIN EAST-SOUTHEASTERN QUADRANT OF THE CIRCULATION WHERE STRONGER
CONFLUENT LOW LEVEL FLOW RESIDES AND IS FORECAST TO
PERSIST...DAMAGING WIND AND/OR ISOLATED TORNADO POTENTIAL MAY
INCREASE.
...SOUTHEAST...
SCATTERED/RANDOM PULSE STORMS WITH SHORT LIFETIMES AND LIMITED
INFLUENCE IN ANY PARTICULAR LOCATION WILL AGAIN POSE A CHANCE FOR
DOWNED TREES AND POWER LINES AS SOUTHEAST HEATS UP AND DESTABILIZES
BENEATH RIDGING AND WEAK FLOW ALOFT.
