ACUS01 KWNS 200514
SWODY1
SPC AC 200511
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1211 AM CDT SAT SEP 20 2008
VALID 201200Z - 211200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
MODELS INDICATE THAT A STRONGER AND MORE AMPLIFIED NORTHERN BRANCH
OF SPLIT POLAR WESTERLIES WILL REMAIN MOSTLY NORTH OF THE CANADIAN
/U.S. BORDER THROUGH THIS FORECAST PERIOD. BUT...A LARGE UPPER
TROUGH WITHIN THIS STREAM IS PROGGED SHIFT ACROSS THE NORTHEAST
PACIFIC TOWARD BRITISH COLUMBIA COASTAL AREAS. A SMALLER SCALE
IMPULSE PROGRESSING THROUGH THE BASE OF THIS FEATURE...COUPLED WITH
AN IMPULSE EMBEDDED WITHIN THE SOUTHERN STREAM...WILL LIKELY
APPROACH THE PACIFIC NORTHWEST COAST...CONTRIBUTING TO THE
NORTHEASTWARD ACCELERATION OF A SIGNIFICANT DOWNSTREAM SOUTHERN
BRANCH CLOSED LOW/TROUGH ACROSS THE NORTHERN INTERMOUNTAIN REGION.
WITHIN A WEAKER FLOW REGIME FARTHER DOWNSTREAM...A BROADLY CYCLONIC
FLOW WITH SEVERAL SMALLER SCALE EMBEDDED IMPULSES IS PROGGED TO
LINGER FROM PARTS OF THE CENTRAL/SOUTHERN PLAINS INTO THE
CENTRAL/SOUTHERN APPALACHIANS REGION.
...NORTHERN GREAT BASIN INTO NORTHERN ROCKIES...
STRONGER MID-LEVEL COOLING/LIFT ASSOCIATED WITH THE SOUTHERN BRANCH
IMPULSE IS PROGGED TO SHIFT ACROSS THE REGION DURING OR SHORTLY
AFTER THE PEAK HEATING HOURS. DESPITE SOMEWHAT LIMITED AVAILABLE
MOISTURE...SUFFICIENT DESTABILIZATION IS EXPECTED FOR CONVECTIVE
DEVELOPMENT...IN THE PRESENCE OF AT LEAST MODERATE DEEP LAYER SHEAR
BENEATH A MID/UPPER JET AXIS. THERMODYNAMIC PROFILES APPEAR
SUPPORTIVE OF HAIL WHICH COULD APPROACH OR BRIEFLY EXCEED SEVERE
CRITERIA. AND...SUB-CLOUD TEMPERATURE/DEW POINT SPREADS MAY BE
CONDUCIVE TO LOCALIZED DOWNBURSTS...BEFORE STORMS WEAKEN WITH THE
LOSS OF HEATING DURING THE EVENING HOURS.
...EAST OF THE ROCKIES...
SCATTERED THUNDERSTORMS APPEAR POSSIBLE TODAY ACROSS PARTS OF THE
GREAT LAKES/UPPER MISSISSIPPI VALLEY REGION...ALONG A FRONTAL ZONE
ASSOCIATED WITH THE EASTERN CANADIAN POLAR TROUGH. OTHERWISE...
SIMILAR TO FRIDAY...WIDELY SCATTERED THUNDERSTORMS ARE POSSIBLE IN A
LINGERING MOIST EAST NORTHEASTERLY LOW-LEVEL FLOW REGIME ACROSS THE
FLORIDA PENINSULA...AND BENEATH THE LOWER MISSISSIPPI VALLEY UPPER
TROUGH AXIS...WHERE A LIGHT SOUTHERLY LOW-LEVEL FLOW WILL MAINTAIN
ELEVATED MOISTURE LEVELS AND POTENTIAL INSTABILITY. MODELS ARE
SUGGESTIVE THAT DIVERGENT UPPER FLOW ASSOCIATED WITH HIGH LEVEL
SPEED MAXIMA PROGRESSING THROUGH THE BASE OF THE BROAD TROUGH MAY
FOCUS HIGHEST THUNDERSTORM PROBABILITIES ACROSS PARTS OF
SOUTHEASTERN LOUISIANA/SOUTHWESTERN MISSISSIPPI.
..KERR/SMITH.. 09/20/2008
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