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FXUS07 KWBC 201330
PMD30D
PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD
830 AM EDT THU MAR 20 2008
30-DAY OUTLOOK DISCUSSION FOR APRIL 2008
STRONG LA NINA CONDITIONS CONTINUE THROUGHOUT THE TROPICAL PACIFIC AND WILL
CONTINUE TO FORM THE UNDERLYING BASIS FOR THIS MONTHLY FORECAST. WELL BELOW
AVERAGE SEA SURFACE TEMPERATURES (SSTS) ARE OBSERVED ACROSS THE CENTRAL AND
EAST-CENTRAL EQUATORIAL PACIFIC. SSTS MORE THAN 2 DEGREES CENTIGRADE BELOW
AVERAGE WERE OBSERVED BETWEEN 165E AND 120W AND THE NINO 3.4 REGION INDEX VALUE
HAS BEEN BETWEEN -1.6 AND -2.1 DEGREES C FOR THE PAST MONTH. IN ADDITION THE
LOW LEVEL EQUATORIAL EASTERLY WINDS HAVE BEEN STRONGER THAN AVERAGE ACROSS THE
CENTRAL EQUATORIAL PACIFIC... STRONGER THAN AVERAGE UPPER LEVEL WESTERLY WINDS
HAVE BEEN OBSERVED... AND CONVECTION HAS BEEN SUPPRESSED OVER THE CENTRAL
PACIFIC. COLLECTIVELY, THESE ATMOSPHERIC AND OCEANIC CONDITIONS ARE SIMILAR TO
THOSE ACCOMPANYING THE LAST STRONG LA NINA EPISODE IN 1998-2000. SST FORECASTS
FROM THE CFS AND A SUITE OF OTHER STATISTICAL AND DYNAMICAL MODELS INDICATE
THAT LA NINA SHOULD REMAIN MODERATELY STRONG THROUGH BOREAL SPRING. THEREFORE,
THE APRIL TEMPERATURE AND PRECIPITATION FORECASTS ARE STILL BASED TO A
SIGNIFICANT DEGREE ON LA NINA COMPOSITES FOR THE MARCH - APRIL - MAY SEASON.
INTRASEASONAL (MJO) ACTIVITY HAS BEEN RATHER PERSISTENT SINCE MID-NOVEMBER, AND
HAS MODULATED LA NINA IMPACTS ACROSS THE CONUS AT VARIOUS TIMES DURING THE
WINTER. DURING MID-MARCH, THE MJO SIGNAL WEAKENED CONSIDERABLY WITH ANOMALOUS
CONVECTION FOCUSED OVER THE EASTERN INDIAN OCEAN AND INDONESIA, AND SUPPRESSED
CONVECTION AT THE DATE LINE. VARIOUS MODEL FORECASTS ARE INDICATING INCREASING
UNCERTAINTY REGARDING THIS MJO, ESPECIALLY DURING THE TWO TO THREE WEEK TIME
FRAME. THEREFORE, THE MJO IS UNLIKELY TO HAVE A SIGNIFICANT INFLUENCE ON THIS
MONTHLY FORECAST.
THE TEMPERATURE OUTLOOK FOR APRIL FAVORS ABOVE AVERAGE TEMPERATURES ACROSS MOST
OF THE SOUTH FROM ARIZONA EASTWARD ACROSS THE SOUTHERN AND CENTRAL PLAINS...
THE SOUTHERN AND CENTRAL MISSISSIPPI VALLEY... THE OHIO AND TENNESSEE
VALLEYS... THE ENTIRE SOUTHEAST... THE CENTRAL AND SOUTHERN APPALACHIANS AND
THE MID-ATLANTIC REGION. BELOW-AVERAGE TEMPERATURES ARE MOST LIKELY IN THE
PACIFIC NORTHWEST AND ALONG THE SOUTHERN CALIFORNIA COAST, AS WELL AS IN
SOUTHERN ALASKA AND THE ALASKAN PANHANDLE. THE OBSERVED BELOW-AVERAGE SSTS
ALONG THE CALIFORNIA COAST FAVOR A CONTINUATION OF BELOW AVERAGE TEMPERATURES
IN SOUTHERN CALIFORNIA. THE TEMPERATURE FORECAST OVER THE CONUS AND ALASKA IS
CONSISTENT WITH OBSERVATIONS FROM PREVIOUS STRONG LA NINA EPISODES. THE BELOW
AVERAGE SURFACE TEMPERATURES FORECAST FOR MOST OF SOUTHERN ALASKA IS ATTRIBUTED
TO THE CCA AND SMLR TOOLS.
THE PRECIPITATION FORECAST FOR APRIL USED MARCH - APRIL - MAY TREND-ADJUSTED LA
NINA COMPOSITES AS THE STARTING POINT, WHICH INDICATE MEAN DRY CONDITIONS
ACROSS PRACTICALLY THE ENTIRE SOUTHERN U.S. AND WETTER THAN AVERAGE CONDITIONS
IN THE PACIFIC NORTHWEST, UPPER MIDWEST AND OHIO VALLEY. THE PACIFIC NORTHWEST
WET SIGNAL DERIVES FROM THE FEW STRONG LA NINA COMPOSITE CASES (ONI RANGE -1.0
TO -1.4), WHILE THE UPPER MIDWEST WET SIGNAL COMES FROM ALL CASES REGARDLESS OF
THE INTENSITY OF THE COLD EVENT. ALTHOUGH THE OHIO VALLEY WET SIGNAL IS ALSO
PRESENT FOR ALL CASES, IT IS BOTH VERY WEAK AND VERY SMALL AND THEREFORE WAS
LEFT OUT OF THE OFFICIAL PRECIPITATION FORECAST. THE CAS TOOL ALSO SUPPORTS
THESE TWO MAIN WET AREAS. THE COMPOSITES ALSO REVEAL A FAIRLY STRONG SIGNAL FOR
MEAN DRY CONDITIONS FROM TEXAS NORTHWARD INTO EASTERN NEBRASKA. MODEST SUPPORT
FOR DRIER-THAN-AVERAGE CONDITIONS IS INDICATED IN SOUTHERN ALASKA ... BASED ON
THE CCA TOOL AND THE UNMASKED CFS TOOL.
FORECASTER: ANTHONY ARTUSA
NOTES: AREAS OF UNCERTAINTY THAT FORMERLY WERE LABELED CL (CLIMATOLOGICAL
PROBABILITIES) ARE NOW LABELED AS EC (EQUAL CHANCES).
AN UPDATED MONTHLY OUTLOOK... FOR APR WILL BE ISSUED ON MON MARCH 31 2008
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1971-2000 BASE PERIOD.
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