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FXUS07 KWBC 312029
PMD30D
PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD
300 PM EDT SUN AUG 31 2008
30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2008
NWP GUIDANCE FOR THE FIRST COUPLE OF WEEKS OF SEPTEMBER INDICATES ANOMALOUS
TROUGHING DOMINATING THE CENTRAL PART OF THE NATION... WIGTH A RIDGE ALONG THE
WEST COAST. THIS PATTERN HAS BEEN PRETTY WELL INDICATED BY THE ALL MODEL
GUIDANCE FOR THE PAST FEW DAYS... WITH THE TROUGH SLOWLY SHIFTING EASTWARD
DURING THE PERIOD. THIS EXPECTED CIRCULATION PATTERN WILL RESULT IN CHANGES TO
THE 0.5 MONTH LEAD OUTLOOK ISSUED ON 19 AUGUST. THAT FORECAST WAS LARGELY
BASED ON LONG TERM TRENDS... WHICH WERE STRONGEST IN THE WESTERN HALF OF THE
NATION. THE EVOLVING PATTERN DOES NOT CONFLICT WITH THAT FORECAST... AND THE
BULK OF THAT FORECAST WAS LEFT UNTOUCHED.
THE UPDATED TEMPERATURE FORECAST MAINTAINS EXPECTATIONS FOR ABOVE AVERAGE
TEMPERATURES IN THE SOUTHWEST AND BELOW NORMAL TEMPERATURE ALONG SOUTHERN
ALASKA. HOWEVER, SEA SURFACE TEMPERATURES HAVE WARMED TO NEAR NORMAL LEVELS
ALONG THE PACIFIC NORTHWEST COAST... SO THE TILT TOWARD BELOW AVERAGE
TEMPERATURES IN THAT REGION WAS REMOVED. ENHANCED PROBABILITIES FOR BELOW
AVERAGE TEMPERATURES WERE ADDED TO A LARGE PART OF THE CENTRAL PART OF THE
NATION... EXTENDING FROM THE NORTHERN PLAINS SOUTHEASTWARD THROUGH PARTS OF THE
SOUTHEAST. THIS OUTLOOK IS CONSISTENT WITH THE EXPECTED TROUGHING OVER THE
REGION DURING THE EARLY AND MIDDLE PART OF THE MONTH... WITH SOIL MOISTURE ALSO
PLAYING A ROLE IN THE SOUTHEAST AS THIS REGION IS EXPECTED TO SEE VERY WET
CONDITIONS DURING THE EARLY PART OF THE MONTH IN ASSOCIATION WITH HURRICANE
GUSTAV.
THE OUTLOOK FOR PRECIPITATION DURING SEPTEMBER HAS ALSO SEEN SOME FAIRLY
SIGNIFICANT REVISIONS, MAINLY WITH REGARDS TO INCREASED PROBABILITIES FOR ABOVE
MEDIAN RAINFALL IN PARTS OF THE SOUTHEAST. HURRICANE GUSTAV IS EXPECTED TO
DROP AT LEAST 10-15 INCHES OF RAIN ACROSS PARTS OF LOUISIANA AND ARKANSAS
DURING THE FIRST FEW DAYS OF THE MONTH, LEADING TO A SHARP INCREASE IN THE
PROBABILITIES FOR ABOVE AVERAGE PRECIPITATION IN THIS REGION. THE AREA WITH AN
ENHANCED LIKELIHOOD OF ABOVE MEDIAN PRECIPITATION WAS ALSO EXTENDED FARTHER
NORTH IN THIS REGION TO ACCOUNT FOR THE EFFECTS OF GUSTAV. ELSEWHERE, DRIER
THAN AVERAGE CONDITIONS IN THE PACIFIC NORTHWEST WERE EXTENDED TO THE
WASHINGTON AND OREGON COASTS, AS THE BUILDING RIDGE INTO THAT REGION WILL FAVOR
GENERALLY DRY CONDITIONS DURING THE FIRST HALF OF THE MONTH.
THE MESSAGE FROM AUGUST 21 FOLLOWS BELOW:
ENSO-NEUTRAL CONDITIONS CONTINUED DURING JULY 2008, AS SEA SURFACE TEMPERATURES
(SSTS) IN THE CENTRAL EQUATORIAL PACIFIC OCEAN REMAINED NEAR-AVERAGE. SSTS IN
THE EASTERN PACIFIC ARE ABOVE AVERAGE... BUT THE ACTUAL SSTS ARE NOT WARM
ENOUGH TO SUPPORT CONVECTION. THE ATMOSPHERIC CIRCULATION OVER THE WESTERN AND
CENTRAL TROPICAL PACIFIC CONTINUES TO REFLECT SOME ASPECTS OF LA NINA. ENHANCED
LOW-LEVEL EASTERLY WINDS AND UPPER-LEVEL WESTERLY WINDS PERSIST IN THIS REGION,
WHILE CONVECTION REMAINS GENERALLY SUPPRESSED OVER THE CENTRAL PACIFIC. THE
CONSOLIDATION OF NUMERICAL AND STATISTICAL NINO 3.4 FORECASTS INDICATES ENSO
NEUTRAL CONDITIONS THROUGH THE NORTHERN HEMISPHERE FALL. THE JUST CONCLUDED LA
NINA IS NOT EXPECTED TO HAVE ANY IMPACTS ON U.S. TEMPERATURE OR PRECIPITATION
DURING SEPTEMBER.
BELOW NORMAL SST ANOMALIES ALONG THE PACIFIC COAST OF NORTH AMERICA FROM OREGON
TO ALASKA TILT THE ODDS TOWARD BELOW AVERAGE AIR TEMPERATURES FOR THE COASTAL
AREAS FROM THE SOUTHERN COAST OF ALASKA TO WESTERN WASHINGTON AND OREGON. AN
ENHANCED PROBABILITY OF ABOVE NORMAL TEMPERATURES IS FORECAST FOR THE
SOUTHWESTERN U.S. THIS IS DUE MOSTLY TO LATE SUMMER-EARLY FALL TEMPERATURE
TRENDS IN THESE REGIONS AS INDICATED IN THE CAS AND OCN TOOLS.
LITTLE DEFINITIVE GUIDANCE IS PROVIDED BY THE TRADITIONAL STATISTICAL TOOLS FOR
THE SEPTEMBER PRECIPITATION FORECAST WITH THE EXCEPTION OF A LARGE PART OF THE
PACIFIC NORTHWEST... WHERE THE OCN FAVORS BELOW NORMAL PRECIPITATION. IN
ADDITION, THE EXPECTED ACTIVE HURRICANE SEASON INCREASES THE PROBABILITY OF
ABOVE MEDIAN PRECIPITATION ALONG BOTH THE GULF COAST AND THE SOUTH ATLANTIC
COAST DURING THE MONTH.
FORECASTER: MIKE HALPERT
NOTES: AREAS OF UNCERTAINTY THAT FORMERLY WERE LABELED CL (CLIMATOLOGICAL
PROBABILITIES) ARE NOW LABELED AS EC (EQUAL CHANCES).
THE NEXT MONTHLY OUTLOOK...FOR OCTOBER 2008...WILL BE ISSUED ON THU SEP 18 2008
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1971-2000 BASE PERIOD.
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