Severe Storm Outlook Narrative (AC)
Issued by NWS

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000
ACUS01 KWNS 231340
SWODY1
SPC AC 231339

DAY 1 CONVECTIVE OUTLOOK RESENT 1
NWS STORM PREDICTION CENTER NORMAN OK
0839 AM CDT SAT JUL 23 2016

VALID 231300Z - 241200Z

...THERE IS AN ENH RISK OF SVR TSTMS PORTIONS ERN DAKOTAS AND MN...

...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING ENH RISK FROM WI TO
ERN ND...

...THERE IS A MRGL RISK OF SVR TSTMS SURROUNDING SLGT RISK AND
EXTENDING TO CENTRAL HIGH PLAINS...

...THERE IS A MRGL RISK OF SVR TSTMS PORTIONS ERN NY AND NEW
ENGLAND...

...SUMMARY...
SCATTERED SEVERE STORMS WILL BE POSSIBLE ACROSS THE UPPER MIDWEST
TODAY INTO THIS EVENING.  DAMAGING WIND AND LARGE HAIL WILL BE THE
MAIN THREATS.  OTHER STRONG STORMS WITH A THREAT FOR MAINLY
DOWNBURST WINDS ARE EXPECTED TODAY FROM EASTERN NEW YORK INTO NEW
ENGLAND.

...SYNOPSIS...
IN MID/UPPER LEVELS...MAIN BELT OF WLYS WILL PERSIST GENERALLY
WITHIN A COUPLE HUNDRED MI EITHER SIDE OF CANADIAN BORDER...BUT WILL
BECOME MORE AMPLIFIED THROUGH PERIOD.  HEIGHT FALLS ARE UNDERWAY
ACROSS MUCH OF ROCKIES AND GREAT PLAINS REGIONS AS STG SHORTWAVE
TROUGH PROCEEDS EWD ACROSS PORTIONS WY/MT.  LATTER FEATURE IS FCST
TO MOVE ENEWD ACROSS NRN PLAINS...REACHING MB/ONT AND ND/MN BORDERS
BY 12Z.  THAT TROUGH LIKELY WILL BE PRECEDED BY CONVECTIVELY
ENHANCED/INDUCED MINOR PERTURBATIONS ACROSS NRN PLAINS AND UPPER
MIDWEST REGIONS...INCLUDING MCV NOW EVIDENT IN COMPOSITE-
REFLECTIVITY ANIMATIONS OVER ERN NEB.  OPERATIONAL RAP MODEL HAS
BEEN REASONABLY WELL-TUNED TO THIS MCV FOR PAST FEW HOURS...AND THIS
FEATURE IS PROGGED TO BE OVER WRN WI BY 00Z...EXTRAPOLATING TO NRN
LM OVERNIGHT.  MEANWHILE...DOWNSTREAM TROUGH CURRENTLY LOCATED FROM
LH NWD OVER ONT IS EXPECTED TO STRENGTHEN AND MOVE QUICKLY SEWD
ACROSS MID ATLC TODAY...REACHING AXIS FROM SRN NEW ENGLAND TO
DELMARVA BY 00Z...THEN MOVING OFFSHORE.

AT SFC...FRONTAL ZONE WAS DRAWN AT 11Z FROM SRN NEW ENGLAND ACROSS
CENTRAL/NRN PA THEN WNWWD OVER SRN WI...SRN MN AND NERN/N-CENTRAL
SD...TO LOW OVER NWRN SD.  COLD FRONT EXTENDED FROM THAT LOW SWWD
ACROSS CENTRAL/WRN WY.  WARM FRONT SHOULD MOVE NWD DIFFUSELY TODAY
OVER MN AND ERN ND...ITS BAROCLINICITY MODULATED BY CONVECTION...
PRECIP AND CLOUD COVER OVER PORTIONS MN/DAKOTAS.  COLD FRONT IS
EXPECTED TO MOVE EWD ACROSS NRN PLAINS...REACHING ERN DAKOTAS...
CENTRAL NEB AND NERN CO BY 00Z.  COLD FRONT SHOULD REACH MN
ARROWHEAD...WRN IA...SERN NEB AND NERN KS BY 12Z.  MEANWHILE...WEAK
COLD FRONT AND SFC TROUGH PRECEDING MID/UPPER-LEVEL SHORTWAVE TROUGH
SHOULD CROSS NERN STATES THROUGH PERIOD...OVERTAKING PRE-EXISTING
BAROCLINIC ZONE AND SFC TROUGH.

...UPPER MIDWEST...
SEVERAL ONGOING COMPLEXES OF TSTMS HAVE COMPLICATED THIS FCST AND
RENDERED OUTLOOK QUITE CONDITIONAL.  HOWEVER...TOO MANY MIXED
SIGNALS STILL EXIST...WITH INSUFFICIENT EVIDENCE AGAINST LATER
DEVELOPMENT OF SEVERE-WIND-PRODUCING MCS...TO JUSTIFY REMOVING ENH
AREA ATTM.  SEWD MOVEMENT OF RELATIVELY CONCENTRATED AREA OF TSTMS
WITH DAMAGING GUSTS AND ISOLATED LARGE HAIL STILL APPEARS POSSIBLE
IN AND NEAR ENH AREA...BUT SUBSTANTIAL ADJUSTMENT MAY BE REQUIRED IN
SUCCEEDING OUTLOOKS AS MESOSCALE CONVECTIVE/INSTABILITY TRENDS
WARRANT.

MAIN FACTOR ATTM IS ARC OF CONVECTION FROM ERN SD ACROSS WRN IA AND
SERN NEB...WHICH IS EXPANSIVE EXTENSION OF OVERNIGHT ACTIVITY
PROPAGATING ALONG ITS OWN OUTFLOW.  SFC ANALYSIS INDICATES SOME
THETAE/DEW-POINT DEFICIT WITH THAT OUTFLOW...BUT NOT ENOUGH YET TO
CATEGORICALLY RULE OUT CORRIDOR OF FAVORABLE MOISTURE BETWEEN IT AND
WARM FRONT TO SUPPORT SVR CONVECTION LATER TODAY.  TSTMS NOW OVER
CENTRAL ND MAY INTENSIFY THROUGH AFTN AS THEY ENCOUNTER
PROGRESSIVELY GREATER LOW-LEVEL MOISTURE WITH EWD EXTENT NEAR
NWD-MOVING WARM FRONT...WITH DEW POINTS IN PRECONVECTIVE WARM-SECTOR
AIR MASS GENERALLY IN UPPER 60S TO UPPER 70S F.  HOWEVER...
CONSIDERABLE CLOUD COVER IN THAT PROSPECTIVE MCS PATH WILL MUTE
DIABATIC/DIURNAL HEATING AND...WITH SUFFICIENTLY DENSE
CONCENTRATIONS BETWEEN NOW AND AFTN...MAY REDUCE MLCAPE
SUBSTANTIALLY FROM WHAT HAS BEEN PROGGED.  THIS CONDITIONALITY...IN
TURN...WILL INFLUENCE DOWNSHEAR DEVELOPMENT/PROPAGATION OF ANY
DIURNAL TO NOCTURNAL MCS -- BOTH IN TERMS OF PATH AXIS AND BREADTH.

...ERN NY/NEW ENGLAND...
WIDELY SCATTERED TSTMS SHOULD DEVELOP AND MOVE EWD TO SEWD ACROSS
OUTLOOK AREA MIDDAY THROUGH AFTN...OFFERING SPORADIC DAMAGING WIND
WITH ISOLATED SVR GUSTS POSSIBLE.  COMBINATION OF SFC DIABATIC
HEATING AND DCVA/COOLING ALOFT PRECEDING SHORTWAVE TROUGH SHOULD
DESTABILIZE TROPOSPHERIC COLUMN SUFFICIENTLY TO SUPPORT CONVECTION
FROM LATE MORNING THROUGH AFTN.  LOW-LEVEL CONVERGENCE WILL BE SMALL
GIVEN SUBSTANTIAL WLY COMPONENT OF NEAR-SFC WINDS...HOWEVER MLCINH
WILL BE WEAK AS WELL.  SFC DEW POINTS UPPER 50S TO MID 60S F ARE
FCST...DEPENDING BOTH ON LOCAL MIXING EFFECTS...ELEVATION AND GEN
INCREASE IN MOISTURE WITH SWD EXTENT.  THIS WILL SUPPORT PEAK
PRECONVECTIVE MLCAPE IN 500-1000 J/KG RANGE...ATOP REASONABLY
WELL-MIXED BOUNDARY LAYERS.  STG MID/UPPER WINDS ARE EXPECTED...
THOUGH DEEP SHEAR WILL BE LIMITED BY UNIDIRECTIONAL NATURE OF
DEEP-LAYER FLOW.  STRONGEST DOWNDRAFTS MAY TRANSPORT ENOUGH HIGHER
MOMENTUM FROM ALOFT TO SFC FOR DAMAGING GUSTS..ESPECIALLY WITH SOME
HELP FROM SUBCLOUD EVAPORATION.

..EDWARDS/MARSH.. 07/23/2016

$$


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