SPC AC 200539
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1239 AM CDT FRI MAR 20 2009
VALID 211200Z - 221200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
AN INTENSIFYING LARGE-SCALE TROUGH WILL MOVE FROM THE ERN PACIFIC
ONTO THE WEST COAST DURING THE DAY TWO PERIOD...INDUCING PRONOUNCED
DOWNSTREAM RIDGING FROM THE CANADIAN PRAIRIES SWD INTO THE WRN GULF
OF MEXICO. LATEST MODEL GUIDANCE IS RELATIVELY CONSISTENT IN
SUGGESTING THAT A COUPLE OF SMALLER-SCALE IMPULSES WILL PRECEDE THE
PRIMARY TROUGH. ONE IS EXPECTED TO TRANSLATE ENEWD THROUGH THE NRN
ROCKIES...GREAT BASIN AND LOWER CO VALLEY INTO THE NRN HIGH PLAINS
SWD INTO THE CNTRL/SRN ROCKIES BY SUNDAY MORNING. THE OTHER IS
FORECAST TO GENERALLY WEAKEN WHILE MOVING THROUGH THE MEAN RIDGE
FROM THE SRN HIGH PLAINS INTO OZARK PLATEAU.
IN THE LOW LEVELS...LOW PRESSURE INITIALLY OVER THE GREAT BASIN WILL
DEVELOP NEWD INTO THE NRN HIGH PLAINS IN RESPONSE TO FORMER LEAD
SHORT WAVE TROUGH MENTIONED ABOVE. AS THIS OCCURS...SECONDARY LOW
PRESSURE /ASSOCIATED WITH PRIMARY UPPER TROUGH/ WILL REFORM ALONG
PACIFIC FRONT OVER THE GREAT BASIN DURING THE LATTER HALF OF THE
FORECAST PERIOD. FARTHER E...A LEE TROUGH WILL STRENGTHEN OVER THE
HIGH PLAINS WITH A COUPLE OF WEAK WARM FRONTS LIFTING NWD THROUGH
THE NRN AND CNTRL PLAINS.
...WRN STATES...
A PLUME OF VERY STEEP MIDLEVEL LAPSE RATES /8-9 C PER KM/ WILL
RESIDE FROM THE GREAT BASIN EWD THROUGH THE ROCKIES IN ADVANCE OF
EVOLVING ERN PACIFIC/WEST COAST TROUGH. WITH THE EXCEPTION OF
ONSHORE FLOW REGIME OVER NRN/CNTRL CA...AIR MASS AHEAD OF PACIFIC
COLD FRONT IS EXPECTED TO REMAIN RATHER DRY WITH PW VALUES LESS THE
0.5 INCH. THIS MARGINAL MOISTURE CONTENT WILL LIKELY PRECLUDE ANY
SEVERE WEATHER THREAT. NONETHELESS...INFLUENCES OF PRIMARY UPPER
TROUGH AND LEAD IMPULSE SHOULD SERVE TO FOCUS SCATTERED STORMS ALONG
PACIFIC FRONT AND IN AREAS OF FAVORED TERRAIN SATURDAY AFTERNOON AND
NIGHT.
...GREAT PLAINS...
PREVAILING SWLY FLOW REGIME IN THE MIDLEVELS WILL MAINTAIN THE ERN
EXTENSION OF EML/STEEP LAPSE RATE ENVIRONMENT OVER THE REGION
THROUGH THE PERIOD. HOWEVER...DESPITE PERSISTENT SLY FLOW IN THE
LOW LEVELS...BOUNDARY LAYER E OF LEE TROUGH WILL REMAIN RELATIVELY
DRY...EFFECTIVELY LIMITING THE DEGREE OF CONVECTIVE INSTABILITY.
CLUSTERS OF ELEVATED TSTMS MAY BE ONGOING SATURDAY MORNING OVER
PORTIONS OF KS PERHAPS INTO MO...DRIVEN LARGELY BY WAA/ISENTROPIC
ASCENT ALONG NOSE OF NOCTURNAL LLJ...IN PART INDUCED BY APPROACH OF
WEAK IMPULSE FROM THE SRN HIGH PLAINS. SOME SMALL HAIL MAY
ACCOMPANY THE MORE INTENSE STORMS...BUT THE GENERALLY WEAK
CONVECTIVE INSTABILITY SHOULD LIMIT ANY ORGANIZED SEVERE WEATHER
THREAT.
POTENTIALLY STRONGER INSTABILITY IS FORECAST TO DEVELOP LATE IN DAY
ALONG LEE TROUGH OVER PORTIONS OF THE NRN AND CNTRL HIGH PLAINS
WHERE MLCAPE MAY APPROACH 500-800 J/KG. MODEL FORECAST SOUNDINGS
SUGGEST THAT ENVIRONMENT MAY REMAIN WEAKLY CAPPED...AND GIVEN THE
ABSENCE OF ANY APPRECIABLE FORCING MECHANISMS...THERE IS SOME
UNCERTAINTY AS TO WHETHER DIURNAL STORMS WILL INITIATE AND BECOME
SUSTAINED. CURRENT THINKING IS THAT ISOLATED STORMS WILL BE
POSSIBLE IN THE 21/21Z-22/03Z TIME FRAME WITH SOME POTENTIAL FOR
HAIL OWING TO THE STEEP LAPSE RATE ENVIRONMENT. WHILE LOW-LEVEL
HODOGRAPHS WILL TEND TO ENLARGE WITH TIME...MID AND HIGH-LEVEL WIND
FIELDS ARE EXPECTED TO REMAIN RATHER WEAK WHICH SHOULD LIMIT THE
POTENTIAL FOR STORM ORGANIZATION.
FINALLY...CLUSTERS OF ELEVATED TSTMS WILL BE POSSIBLE SATURDAY NIGHT
OVER THE NRN AND CNTRL PLAINS ALONG STRENGTHENING...NOCTURNAL LLJ.
HERE TOO...THE PRESENCE OF STEEP MIDLEVEL LAPSE RATES MAY PROMOTE A
FEW STRONGER UPDRAFTS CAPABLE OF SOME HAIL.
..MEAD.. 03/20/2009
CLICK TO GET WUUS02 PTSDY2 PRODUCT
NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 1730Z
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