Severe Storm Outlook Narrative (AC)
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
ACUS01 KWNS 211237
SWODY1
SPC AC 211235

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0735 AM CDT THU AUG 21 2014

VALID 211300Z - 221200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE UPPER MS VALLEY ACROSS
THE UPPER MIDWEST TO THE OH VALLEY/CENTRAL APPALACHIANS...

...THERE IS A SLGT RISK OF SVR TSTMS OVER THE LOWER CO RIVER
VALLEY...

...SUMMARY...
SCATTERED SEVERE THUNDERSTORMS ARE EXPECTED FROM PORTIONS OF THE
UPPER MISSISSIPPI VALLEY INTO THE OHIO VALLEY AND CENTRAL
APPALACHIANS. DAMAGING WINDS AND LARGE HAIL ARE THE PRIMARY THREATS
WITH THIS ACTIVITY. STORMS WITH LARGE HAIL AND STRONG WINDS WILL
ALSO BE POSSIBLE ACROSS THE LOWER COLORADO RIVER VALLEY.

...SYNOPSIS...
AN ACTIVE SUMMERTIME SEVERE WEATHER PATTERN HAS BECOME ESTABLISHED
FROM THE UPPER MS VALLEY TO THE MIDWEST WHERE A BELT OF MODEST
MID-LEVEL FLOW WILL RESIDE ON THE NORTHERN PERIPHERY OF AN EXTENSIVE
SUB-TROPICAL RIDGE ACROSS THE PLAINS AND SOUTH. DISTURBANCES
EMANATING FROM A BROAD TROUGH ACROSS THE NORTHWEST AND NORTHERN
ROCKIES...AS WELL AS CONVECTIVELY INDUCED PERTURBATIONS...WILL ACT
TO DAMPEN THE UPPER RIDGE FROM THE MS VALLEY TO THE MIDWEST/OH
VALLEY THIS PERIOD. ASCENT AND ENHANCED FLOW WITH THESE DISTURBANCES
WILL SUPPORT/SUSTAIN BOUTS OF STRONG TO SEVERE STORMS WITH SOME OF
THIS ACTIVITY POSSIBLY GROWING UPSCALE INTO ONE OR MORE
FORWARD-PROPAGATING CONVECTIVE SYSTEMS.

...MN/IA/WI TO IL/IND/OH...
RELATIVELY WIDESPREAD CONVECTION WAS ONGOING FROM MN/IA TO SOUTHERN
WI...NORTHERN IL...AND PARTS OF IND/OH. OVER THE PAST SEVERAL
HOURS...ACTIVITY OCCURRING ACROSS MN/IA HAS BECOME SLIGHTLY BETTER
ORGANIZED AHEAD OF A SURFACE LOW AND WARM FRONT SITUATED ACROSS THE
MID MO RIVER VALLEY. OTHER SCATTERED STORMS TO THE SOUTH AND EAST OF
THE MN/IA MCS...FROM WI/IL TO IND/OH...ARE WITHIN A
NORTHWEST-SOUTHEAST CORRIDOR OF STRONGER ASCENT WHERE MID/UPPER JET
COUPLING HAS HELPED TO STRENGTHEN MASS AND MOISTURE FLUXES ON THE
NOSE OF A BROAD SOUTHWESTERLY LOW LEVEL JET. AIR PARCELS ORIGINATING
IN THE CAPPED AND VERY WARM/MOIST AIRMASS OVER THE GREAT PLAINS ARE
FUELING THE ONGOING CONVECTION IN THE PRESENCE OF RELATIVELY STRONG
30-50KT EFFECTIVE SHEAR...ESPECIALLY FROM MN/IA TO NRN IL.

THE EXTENT AND LOCATION OF ONGOING ACTIVITY INTRODUCES CONSIDERABLE
COMPLEXITY INTO THE FORECAST FOR TODAY. LOW-CONFIDENCE SCENARIOS
INCLUDE 1) A CONTINUATION OF ONGOING CONVECTION WITH A GRADUAL
INCREASE IN ORGANIZATION AND INTENSITY AS DIURNAL DESTABILIZATION
OCCURS COINCIDENT WITH THE EAST-SOUTHEASTWARD-MOVING
SHORTWAVES/MCVS. 2) A DECREASE IN EARLY ACTIVITY AS LOW LEVEL JET
WEAKENS BEFORE NEW INITIATION OCCURS LATER THIS AFTERNOON ON THE
EDGE OF THE CAP FROM THE UPPER MS VALLEY TO UPPER MIDWEST...OR 3) A
COMBINATION OF 1 AND 2. DISPARITY EXHIBITED IN STORM-SCALE ENSEMBLE
GUIDANCE...INCLUDING TIME-LAGGED NAM4 AND HRRR FORECASTS...SUPPORTS
A BROAD AND RELATIVELY LOW-PROBABILITY CORRIDOR OF SEVERE WIND AND
HAIL POTENTIAL FROM THE UPPER MS VALLEY...ACROSS THE UPPER
MIDWEST...TO THE OH VALLEY/CENTRAL APPALACHIANS.

...LOWER CO RIVER VALLEY...
PER SATELLITE IMAGERY AND GPS PW DATA...A MOISTURE PLUME ORIGINATING
FROM T.S. LOWELL HAS BEEN ENTRAINED INTO THE EASTERN FLANK OF A
CLOSED LOW MOVING SLOWLY EAST TOWARD THE LOWER CO RIVER VALLEY. LIFT
AND COLDER AIR ALOFT ACCOMPANYING THE MID/UPPER LOW...IN CONCERT
WITH DIURNAL HEATING...WILL LIKELY RESULT IN A CORRIDOR OF GREATER
DESTABILIZATION AND THUNDERSTORM DEVELOPMENT ACROSS THE LOWER CO
RIVER VALLEY LATER TODAY. IN ADDITION TO LOCALLY STRONG WIND GUSTS
GIVEN HIGH DCAPE...SPC HAIL MODEL APPLIED TO FORECAST SOUNDINGS FROM
THE NAM AND WRF-ARW INDICATE THE POSSIBILITY OF SEVERE HAIL TO AT
LEAST 1 INCH IN DIAMETER. WHILE THE AREAL EXTENT OF THE SEVERE STORM
THREAT MAY REMAIN CONFINED...ENVIRONMENT APPEARS SUFFICIENT ENOUGH
TO WARRANT SLGT RISK 15 PERCENT HAIL/WIND PROBABILITIES.

...MID-ATLANTIC/NORTHEAST...
BACKGROUND CYCLONIC FLOW AND WEAK DPVA AMIDST A PLUME OF RELATIVELY
HIGH PW SHOULD FOSTER SCATTERED LOOSELY-ORGANIZED THUNDERSTORMS OVER
NY/PA AND DELMARVA THIS AFTERNOON. A BELT OF SOMEWHAT STRONGER
MID-LEVEL FLOW FORECAST FROM SOUTHEAST NY ACROSS PA/NJ MAY
CONTRIBUTE TO SLIGHTLY MORE ORGANIZED/PERSISTENT STORMS IN THESE
AREAS. WHILE A COUPLE OF STRONGER CONVECTIVE WIND GUSTS WILL BE
POSSIBLE AROUND THE TIME OF PEAK HEATING...WEAK LAPSE
RATES/INSTABILITY SUGGEST THE SEVERE WIND GUST POTENTIAL SHOULD
REMAIN LIMITED.

...NORTHERN ROCKIES TO EASTERN MT...
IN ADDITION TO OROGRAPHIC/DIABATIC INFLUENCES AMIDST LOW STATIC
STABILITY ACROSS THESE AREAS...FORCING FOR ASCENT WILL INCREASE LATE
IN THE PERIOD AS UPPER TROUGH DEEPENS ACROSS THE NORTHWEST. EXPECT
NUMEROUS SHOWERS AND THUNDERSTORMS IN THIS REGIME WITH SOME POCKETS
OF GREATER INSTABILITY AND SHEAR CONTRIBUTING TO MARGINALLY SEVERE
STORMS WITH HAIL AND GUSTY WINDS.

..CARBIN/COHEN.. 08/21/2014




USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.