SWODY2
SPC AC 220549
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1249 AM CDT WED APR 22 2009
VALID 231200Z - 241200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
WITHIN THE PREVAILING SPLIT UPPER FLOW...MODELS CONTINUE TO INDICATE
THAT BROAD UPPER TROUGHING WILL BEGIN TO EVOLVE ACROSS THE WESTERN
U.S. DURING THIS FORECAST PERIOD. THIS APPEARS LIKELY TO OCCUR AS
THE SOUTHERN PORTION OF A SPLITTING NORTHERN BRANCH TROUGH CONTINUES
TO SLOWLY DIG ACROSS THE PACIFIC NORTHWEST/NORTHERN INTERMOUNTAIN
REGION AND NORTHERN GREAT BASIN. AS THE NORTHERN PORTION LIFTS
ACROSS THE CENTRAL CANADIAN PROVINCES...TO THE SOUTH OF A VORTEX
NEAR NORTHERN HUDSON BAY...AND A SOUTHERN BRANCH TROUGH PROGRESSES
ACROSS THE SOUTHERN ROCKIES/PLAINS...UPPER RIDGING IS PROGGED TO
BUILD WITHIN THE TWO STREAMS...ACROSS MOST AREAS EAST OF THE
MISSISSIPPI VALLEY.
IN ASSOCIATION WITH THIS EVOLVING PATTERN...A SOUTHWESTERLY
LOWER/MID TROPOSPHERIC FLOW DEVELOPING OFF THE PLATEAU/ROCKIES IS
EXPECTED TO ADVECT INCREASINGLY WARM ELEVATED MIXED LAYER AIR ACROSS
MUCH OF THE PLAINS AND MISSISSIPPI VALLEY...PERHAPS AS FAR EAST AS
THE APPALACHIANS. THIS WILL LIKELY CAP AN INCREASINGLY MOIST GULF
RETURN FLOW ACROSS PARTS OF THE SOUTHERN PLAINS. THE MORE
SIGNIFICANT LOW-LEVEL MOISTENING MAY NOT REACH CENTRAL/ SOUTHEAST
TEXAS AND THE LOWER RIO GRANDE VALLEY UNTIL LATE THURSDAY
NIGHT/EARLY FRIDAY. AND...THIS STILL SEEMS LIKELY TO LIMIT
CONVECTIVE POTENTIAL ACROSS MOST AREAS EAST OF THE ROCKIES...WHILE
MOISTURE VALUES ALSO REMAIN RATHER LOW ACROSS MUCH OF THE
INTERMOUNTAIN WEST AND ROCKIES.
...UPPER MISSISSIPPI VALLEY...
DESPITE THE LACK OF GULF MOISTURE RETURN...MOISTENING IS CURRENTLY
EVIDENT IN GOES BLENDED TOTAL PRECIPITABLE WATER IMAGERY...ALONG A
LOWER/MID TROPOSPHERIC BAROCLINIC ZONE...IN THE WAKE OF THE LARGE
SCALE TROUGH NOW BEGINNING TO DEAMPLIFY AND LIFT OUT OF THE EASTERN
STATES. THIS PROVIDES SUPPORT FOR THE NCEP SREF...WHICH INDICATES
PRECIPITABLE WATER ON THE ORDER OF .5-1.0 INCH RETURNING TO THE
VICINITY OF A COLD FRONT ADVANCING SOUTHEASTWARD INTO THE REGION
DURING THE DAY THURSDAY.
STRONGEST DESTABILIZATION DURING THE AFTERNOON HOURS STILL APPEARS
LIKELY TO BECOME FOCUSED ALONG A PRE-FRONTAL LOW-LEVEL THERMAL RIDGE
AXIS ACROSS NORTHWESTERN MINNESOTA AND PARTS OF SOUTHEAST NORTH
DAKOTA/NORTHEAST SOUTH DAKOTA...WHERE INHIBITION MAY BECOME WEAK
ENOUGH TO ALLOW THE INITIATION OF STORMS. A DEEPLY MIXED BOUNDARY
LAYER WITH CAPE OF 500-1000...IN THE PRESENCE OF A MODERATE TO
STRONGLY SHEARED...GENERALLY UNIDIRECTIONAL...SOUTHWESTERLY DEEP
LAYER FLOW...PROBABLY WILL BE CONDUCIVE FOR LOCALIZED DAMAGING WIND
GUSTS AND MARGINALLY SEVERE HAIL.
STORM DEVELOPMENT COULD EVENTUALLY BE ENHANCED BY FRONTAL FORCING BY
EARLY THURSDAY EVENING...BUT THE BETTER CHANCE FOR STORMS MAY EXIST
IN A STRENGTHENING WARM ADVECTION ZONE ON THE NOSE OF THE CAPPING
MID-LEVEL RIDGE...ACROSS PARTS OF NORTHEAST/EAST CENTRAL MINNESOTA
INTO NORTHWEST WISCONSIN AFTER DARK. THESE STORMS MAY CONTINUE TO
POSE THE RISK FOR AT LEAST MARGINALLY SEVERE HAIL.
...ERN GREAT BASIN INTO THE N CNTRL ROCKIES/NRN PLAINS...
DESPITE LIMITED MOISTURE...BOUNDARY LAYER HEATING...COUPLED WITH
OROGRAPHIC FORCING AND...AT LEAST WEAKLY DIVERGENT UPPER FLOW
DOWNSTREAM OF THE DIGGING UPPER TROUGH...LIKELY WILL CONTRIBUTE TO
AT LEAST SCATTERED STORM DEVELOPMENT THURSDAY AFTERNOON AND EVENING.
AFTER DARK...MODELS ARE SUGGESTIVE THAT THUNDERSTORM ACTIVITY MAY
BECOME CONCENTRATED IN A ZONE OF STRONGER FORCING ALONG A SLOWLY
SOUTHWARD ADVANCING COLD FRONT...ABOVE THE FRONTAL INVERSION....NEAR
OR JUST NORTH OF THE WYOMING/MONTANA BORDER EAST NORTHEASTWARD INTO
THE DAKOTAS.
...MID MS VALLEY INTO THE SOUTHEAST...
WIDELY SCATTERED STORMS MAY BE ONGOING EARLY THURSDAY IN A LOW/MID
LEVEL WARM ADVECTION REGIME ACROSS THE EAST CENTRAL PLAINS INTO THE
OZARK PLATEAU. THIS ACTIVITY IS EXPECTED TO DIMINISH EARLY...WITH
NEW STORM DEVELOPMENT POSSIBLY ACCOMPANYING A WEAK MID-LEVEL IMPULSE
LIFTING NORTHEAST OF THE MID MISSOURI VALLEY TOWARD THE LOWER GREAT
LAKES REGION. ADDITIONAL STORMS ARE POSSIBLE WITHIN A MOISTENING
CONFLUENT FLOW IN THE WAKE OF THE DEPARTING EASTERN TROUGH...AHEAD
OF A BUILDING SOUTHERN STREAM RIDGE...ACROSS PARTS OF THE TENNESSEE
VALLEY INTO THE EASTERN GULF STATES. ACTIVITY IS EXPECTED TO BE
ISOLATED TO WIDELY SCATTERED IN COVERAGE AT BEST...HOWEVER.
...SOUTHERN HIGH PLAINS...
AS CONVECTIVE TEMPERATURES ARE APPROACHED/EXCEEDED...ISOLATED TO
WIDELY SCATTERED STORMS ARE POSSIBLE LATE THURSDAY AFTERNOON NEAR A
DEVELOPING DRY LINE. THIS ACTIVITY MAY PERSIST INTO THE EARLY
EVENING...BEFORE DIMINISHING WITH THE LOSS OF DAYTIME HEATING.
..KERR.. 04/22/2009
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