Severe Storm Outlook Narrative (AC)
Issued by NWS

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000
ACUS01 KWNS 310601
SWODY1
SPC AC 310600

DAY 1 CONVECTIVE OUTLOOK CORR 1
NWS STORM PREDICTION CENTER NORMAN OK
0100 AM CDT SUN AUG 31 2014

VALID 311200Z - 011200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE CENTRAL PLAINS TO THE
UPPER MS VALLEY...

...THERE IS A SLGT RISK OF SVR TSTMS FROM PART OF THE MID-ATLANTIC
STATES TO SRN NEW ENGLAND...

CORRECTED TO INCLUDE THE EASTERN SLIGHT RISK AREA IN THE SUMMARY.

...SUMMARY...
SEVERE THUNDERSTORMS ARE EXPECTED TO DEVELOP ALONG A COLD FRONT
SUNDAY AFTERNOON AND EVENING FROM MINNESOTA SOUTHWESTWARD ACROSS
WESTERN IOWA...AND CENTRAL AND EASTERN NEBRASKA INTO KANSAS.  LARGE
HAIL...DAMAGING WINDS...AND A FEW TORNADOES ARE EXPECTED WITH THIS
ACTIVITY.  A FEW SEVERE STORMS WILL ALSO BE POSSIBLE THIS AFTERNOON
AND EARLY EVENING ACROSS NORTHERN VIRGINIA AND MARYLAND TO SOUTHERN
NEW ENGLAND.

...SYNOPSIS...
A BROAD NWRN STATES UPPER TROUGH WILL AMPLIFY TODAY ACROSS THE NRN
AND CENTRAL PLAINS.  A PROGRESSIVE SHORTWAVE TROUGH...CURRENTLY
MOVING EWD THROUGH THE NRN INTERMOUNTAIN WEST PER WATER VAPOR
IMAGERY...WILL TRANSLATE THROUGH THE PARENT LARGE-SCALE
TROUGH...REACHING THE CENTRAL DAKOTAS/NEB BY 01/00Z.  THIS MIDLEVEL
FEATURE WILL THEN MOVE THROUGH THE MID MO VALLEY...REACHING THE
UPPER MS VALLEY BY 12Z MONDAY.  A COLD FRONT WILL ADVANCE EWD ACROSS
THE UPPER MIDWEST...WHILE THE TRAILING PORTION MOVES SEWD ACROSS
NEB/KS.

AN EXTENSIVE CORRIDOR OF RICH TROPICAL MOISTURE /PW EXCEEDING 2
INCHES/ WILL SPREAD THROUGH THE OH VALLEY AND MID ATLANTIC STATES TO
SRN NEW ENGLAND TODAY.  A SHORTWAVE TROUGH...EMBEDDED WITHIN THIS
MOISTURE PLUME...WILL TRACK FROM KY/TN TO CENTRAL PA/SRN NY BY LATE
THIS AFTERNOON/EARLY EVENING...AND THEN UNDERGO SUBSTANTIAL
WEAKENING AS IT MOVES INTO SRN NEW ENGLAND LATE SUNDAY NIGHT.  WEAK
HEIGHT FALLS WILL PRECEDE THE KY/TN SHORTWAVE TROUGH ACROSS THE
CENTRAL APPALACHIANS TO SRN NEW ENGLAND...AS A COMPACT SHORTWAVE
TROUGH MOVES EWD FROM LOWER MI TOWARD NRN NEW ENGLAND THIS FORECAST
PERIOD.

...UPPER MS VALLEY TO CENTRAL PLAINS...
EARLY IN THE PERIOD...ELEVATED THUNDERSTORMS WILL LIKELY BE ONGOING
ALONG THE NOSE OF LLJ ACROSS PORTIONS ERN ND AND NWRN MN.  THIS
ACTIVITY SHOULD BE ROOTED AT OR ABOVE 850MB WITH THE GREATEST THREAT
WITH THESE STORMS BEING MARGINALLY SEVERE HAIL BEFORE THEY PROPAGATE
INTO SRN MANITOBA AND ADJACENT NRN ONTARIO ATTENDANT TO A
PROGRESSIVE LEAD SHORTWAVE TROUGH AND SURFACE LOW.

A PLUME OF STEEP MIDLEVEL LAPSE RATES WILL SPREAD E/NEWD FROM THE
CENTRAL PLAINS TO THE UPPER MS VALLEY.  MEANWHILE...SOUTHERLY
LOW-LEVEL WINDS AHEAD OF THE COLD FRONT WILL ADVECT MOISTURE NWD
WITH SURFACE DEW POINTS IN THE MID-UPPER 60S SPREADING INTO ERN
NEB/IA TO THE UPPER MS VALLEY.  THESE FACTORS COMBINED WITH SURFACE
HEATING WILL RESULT IN STRONG INSTABILITY /MLCAPE 2000-3000 J PER
KG/ ACROSS THE CENTRAL PLAINS TO CENTRAL MN.

TSTMS MAY DEVELOP BY EARLY-MID AFTERNOON ALONG CENTRAL NEB PORTION
OF THE COLD FRONT.  THIS WILL OCCUR AS FORCING FOR ASCENT ATTENDANT
TO WEAK HEIGHT FALLS WITH THE LARGE-SCALE TROUGH AND AHEAD OF THE
PROGRESSIVE NRN INTERMOUNTAIN WEST SHORTWAVE TROUGH REACH THIS AREA.
EFFECTIVE BULK SHEAR WILL INCREASE ACROSS NEB/NRN KS AS A 50-60 KT
WLY MIDLEVEL JET EMERGES FROM THE CENTRAL ROCKIES.  STRONGER SURFACE
HEATING FROM CENTRAL NEB INTO KS RESULTING IN A DEEPLY MIXED
BOUNDARY LAYER WILL TEND TO RESULT IN DAMAGING WINDS AND LARGE HAIL
BEING THE PRIMARY THREATS IN THE CENTRAL PLAINS.  GIVEN VERY STEEP
LAPSE RATES /EXCEEDING 8 C/KM IN THE 700-500-MB LAYER/...STRONG
INSTABILITY AND EFFECTIVE BULK SHEAR OF 40-50 KT SUGGESTS VERY LARGE
HAIL WILL BE POSSIBLE.

MEANWHILE...STORMS ARE EXPECTED TO DEVELOP ALONG THE REST OF THE
COLD FRONT BETWEEN 21-00Z FROM MN THROUGH ERN SD...ERN NEB...AND WRN
IA WITH BOTH QUASI-LINEAR AND SUPERCELL STORM STRUCTURES.  ALL
SEVERE HAZARDS CAN BE EXPECTED.  THE GREATEST SEVERE WEATHER THREAT
IS EXPECTED ACROSS ERN NEB INTO WRN/CENTRAL IA AND SRN MN FROM LATE
THIS AFTERNOON INTO THE EARLY EVENING.  THIS REGION WILL HAVE A
JUXTAPOSITION OF A STRENGTHENING SSWLY LLJ ENHANCING LOW-LEVEL
SHEAR...LOWER LCLS...AND EFFECTIVE BULK SHEAR VECTORS CROSSING THE
COLD FRONT SUPPORTING SOME DISCRETE SUPERCELLS AND/OR EMBEDDED
SUPERCELLS WITHIN THE DEVELOPING LINEAR STRUCTURE.  STORM MERGERS
INTO THE EVENING SHOULD RESULT IN AN ELONGATED MCS-TYPE COMPLEX
ADVANCING EWD ACROSS THE UPPER MS TO LOWER MO VALLEYS.

...NRN VA/MD/ERN PA/NRN NJ/SERN NY/SRN NEW ENGLAND...
A CORRIDOR OF MODERATE INSTABILITY /MLCAPE UP TO 1000-1500 J PER KG/
IS EXPECTED TO DEVELOP THIS AFTERNOON FROM NRN VA/MD TO SRN NEW
ENGLAND...DESPITE THE PRESENCE OF WEAK MIDLEVEL LAPSE RATES.  THIS
DESTABILIZATION WILL BE LOCATED ALONG THE ERN PERIPHERY OF EARLY DAY
CLOUDS/PRECIPITATION EXPECTED TO EXTEND FROM THE OH VALLEY INTO
CENTRAL PA/NY.  CONVERGENCE IN VICINITY OF A SW-NE ORIENTED
DIFFERENTIAL HEATING BOUNDARY AND WEAK HEIGHT FALLS SPREADING ACROSS
THIS REGION SHOULD PROVE SUFFICIENT FOR DEVELOPING TSTMS FROM
AFTERNOON INTO THE EARLY EVENING.  EFFECTIVE BULK SHEAR OF 30-40 KT
WILL SUPPORT ORGANIZED STORMS WITH DAMAGING WINDS BEING THE PRIMARY
THREAT AS ACTIVITY SPREADS FROM WEST TO EAST.

..PETERS/DEAN.. 08/31/2014



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