314
FXUS07 KWBC 171231
PMD30D
PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
830 AM EDT THU AUG 17 2023
30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2023
THE SEPTEMBER 2023 OUTLOOKS ARE ISSUED AGAINST THE BACKDROP OF AN ONGOING EL
NIñO SIGNAL. THE LATEST NIñO 3.4 WEEKLY SEA SURFACE TEMPERATURE (SST)
DEPARTURES STOOD AT +1.2 DEGREES CELSIUS AND POSITIVE SUBSURFACE TEMPERATURE
ANOMALIES DOMINATE THE EQUATORIAL PACIFIC OCEAN. MEANWHILE, THE MAGNITUDE OF
THE REAL-TIME MULTIVARIATE (RMM) BASED MADDEN JULIAN OSCILLATION (MJO) INDEX
HAS STRENGTHENED DURING EARLY AUGUST, WITH THE AMPLITUDE OF THE SIGNAL RISING
ABOVE THE RMM UNIT CIRCLE. HOWEVER, THERE IS LITTLE EVIDENCE OF EASTWARD
PROPAGATION AT THIS TIME WHICH IS REFLECTIVE OF DISORGANIZED INTRASEASONAL
ACTIVITY. THE MJO IS GENERALLY FAVORED TO WEAKEN DURING THE NEXT SEVERAL WEEKS
WITH A POSSIBLE RE-EMERGENCE LATER IN AUGUST OVER THE WESTERN PACIFIC OR
WESTERN HEMISPHERE. FARTHER TO THE NORTH, ACROSS THE EXTRATROPICS OF THE
NORTHERN HEMISPHERE, BOTH THE ARCTIC OSCILLATION (AO) AND THE NORTH ATLANTIC
OSCILLATION (NAO) HAVE GENERALLY BEEN NEGATIVE DURING EARLY AUGUST WHILE THE
PACIFIC NORTH AMERICA (PNA) INDEX HAS EXHIBITED A POSITIVE SIGNAL IN RECENT
DAYS. COMBINED NATURAL ANALOG COMPOSITES DERIVED FROM THE RECENT EVOLUTION OF
EQUATORIAL PACIFIC, MJO, AND EXTRATROPICAL INDICES DEPICT A 500-HPA FLOW
PATTERN DOMINATED BY BELOW NORMAL HEIGHTS ACROSS THE BERING SEA AND WESTERN
ALASKA AS WELL AS OVER THE NORTHEASTERN PACIFIC OFF THE COASTS OF SOUTHEAST
ALASKA AND THE PACIFIC NORTHWEST. ANOMALOUS RIDGING IS GENERALLY FAVORED ACROSS
THE WESTERN AND NORTH-CENTRAL CONTIGUOUS UNITED STATES (CONUS). FARTHER TO THE
EAST, NATURAL ANALOG COMPOSITES DEPICT NEAR NORMAL HEIGHTS ACROSS THE SOUTHERN
PLAINS, SOUTHEAST, AND NORTHWARD TO MUCH OF THE GREAT LAKES AND NORTHEAST.
MEANWHILE, TRENDS DURING THE LAST 15 YEARS FAVOR ABOVE NORMAL HEIGHTS ACROSS
MOST OF THE WESTERN AND NORTHERN TWO-THIRDS OF THE CONUS AS WELL AS THE
ALEUTIANS AND PARTS OF EASTERN MAINLAND ALASKA DURING THE MONTH OF SEPTEMBER,
WITH THE STRONGEST POSITIVE ANOMALIES OVER THE NORTH-CENTRAL CONUS. NATURAL
ANALOG COMPOSITES, TRENDS, AND DYNAMICAL GUIDANCE FROM THE COPERNICUS CLIMATE
CHANGE SERVICE (C3S), NORTH AMERICAN MULTI-MODEL ENSEMBLE (NMME), THE COUPLE
FORECAST SYSTEM VERSION 2 (CFSV2), AND THE FORECAST CONSOLIDATION TOOL (CON)
(WHICH INCLUDES BOTH DYNAMICAL MODEL INPUT AND STATISTICAL GUIDANCE) FORM THE
BASIS OF THE SEPTEMBER OUTLOOKS. THE LATEST CPC WEEKS 3-4 OUTLOOKS AND
ASSOCIATED ENSEMBLE GUIDANCE FROM THE EUROPEAN CENTRE FOR MEDIUM RANGE WEATHER
FORECASTS (ECMWF), GLOBAL ENSEMBLE FORECAST SYSTEM (GEFS), AND THE CFS ALSO
CONTRIBUTED. ANTECEDENT CONDITIONS SUCH AS EXTRATROPICAL SSTS, SEA ICE EXTENT,
AND SOIL MOISTURE CONDITIONS WERE ALSO CONSIDERED WHERE APPROPRIATE.
ABOVE NORMAL TEMPERATURES ARE FAVORED FOR MOST OF THE WESTERN CONUS, CONSISTENT
WITH ANOMALOUS RIDGING. THE STRONGEST PROBABILITIES (GREATER THAN 50 PERCENT)
OF ABOVE NORMAL TEMPERATURES ARE INDICATED NEAR THE FOUR CORNERS REGION, WHERE
DYNAMICAL AND STATISTICAL GUIDANCE SHOW THE STRONGEST AGREEMENT. HOWEVER,
PROBABILITIES OF WARMER THAN NORMAL CONDITIONS ARE TEMPERED ACROSS MUCH OF THE
NORTHWESTERN CONUS, DUE TO COLD SIGNALS DEPICTED BY THE ECMWF AND CFS ENSEMBLES
FOR THE WEEKS 3-4 PERIOD, WHICH INCLUDES MOST OF THE FIRST HALF OF SEPTEMBER.
FARTHER TO THE SOUTH AND EAST, ENHANCED ABOVE NORMAL TEMPERATURE PROBABILITIES
ARE INDICATED ACROSS THE SOUTHERN PLAINS, LOWER MISSISSIPPI VALLEY, AND PARTS
OF THE SOUTHEAST, DUE TO GOOD AGREEMENT AMONG DYNAMICAL MODEL GUIDANCE.
PROBABILITIES OF ABOVE NORMAL TEMPERATURES EXCEED 50 PERCENT FOR PARTS OF THE
GULF COAST REGION, WHERE SSTS IN ADJACENT WATERS ARE MUCH WARMER THAN NORMAL.
EQUAL CHANCES (EC) OF ABOVE, NEAR, AND BELOW NORMAL TEMPERATURES ARE INDICATED
FOR THE REMAINDER OF THE EASTERN CONUS DUE TO WEAK SIGNALS EXHIBITED BY NMME
AND CFSV2 OUTPUT AS WELL AS DUE TO CONFLICTING STATISTICAL GUIDANCE. ABOVE
NORMAL TEMPERATURES ARE FAVORED FOR SOUTHERN, EASTERN, AND NORTHERN ALASKA,
CONSISTENT WITH THE CONSENSUS OF DYNAMICAL MODEL GUIDANCE. ABOVE NORMAL SSTS
ALSO CONTRIBUTED TO INCREASED ABOVE NORMAL TEMPERATURE PROBABILITIES ALONG THE
SOUTH COAST AND NORTH SLOPE OF ALASKA. HOWEVER, EC IS INDICATED FOR MUCH OF
WESTERN MAINLAND ALASKA DUE TO CONFLICTING SIGNALS BETWEEN GENERALLY WARMER
DYNAMICAL MODEL GUIDANCE AND COLDER NATURAL ANALOGS AS WELL AS THE PRESENCE OF
BELOW NORMAL SSTS ADJACENT TO SOUTHWESTERN MAINLAND ALASKA.
ABOVE NORMAL PRECIPITATION IS FAVORED ACROSS THE SOUTHEAST NORTHWARD TO THE
MID-ATLANTIC AND WESTWARD TO PARTS OF THE TENNESSEE AND OHIO VALLEYS DUE TO
GOOD AGREEMENT AMONG THE CON, EL NIñO SOUTHERN OSCILLATION (ENSO)-BASED NATURAL
ANALOGS, AND THE LATEST CPC WEEKS 3-4 OUTLOOKS. CONVERSELY, BELOW NORMAL
PRECIPITATION IS FAVORED FARTHER TO THE NORTH AND WEST FOR THE WESTERN GREAT
LAKES AND UPPER MISSISSIPPI VALLEY AS THE CON, CFSV2, AND NATURAL ANALOG
COMPOSITES TILT DRY ACROSS THESE REGIONS. LATEST WEEKS 3-4 GUIDANCE FROM THE
ECMWF AND GEFS DEPICT A WET SIGNAL ACROSS THE BAJA PENINSULA AND THE GULF OF
CALIFORNIA, SUGGESTIVE OF POTENTIAL TROPICAL CYCLONE (TC) ACTIVITY. THIS
POTENTIAL TC ACTIVITY COUPLED WITH A WET SIGNAL DEPICTED BY THE CON LEADS TO A
SLIGHT TILT TOWARD ABOVE NORMAL PRECIPITATION FOR PARTS OF THE SOUTHWESTERN
CONUS. FARTHER TO THE NORTH, ABOVE NORMAL PRECIPITATION IS FAVORED FOR WESTERN
ALASKA DUE TO GOOD AGREEMENT BETWEEN NMME AND CON GUIDANCE. A WEAK TILT TOWARD
BELOW NORMAL PRECIPITATION IS INDICATED FOR SOUTHEAST ALASKA DUE TO MONTHLY
GUIDANCE FROM THE CON, CFSV2, AND WEEKS 3-4 GUIDANCE FROM THE ECMWF AND GEFS.
FORECASTER: SCOTT HANDEL
THE CLIMATIC NORMALS ARE BASED ON CONDITIONS BETWEEN 1991 AND 2020, FOLLOWING
THE WORLD METEOROLOGICAL ORGANIZATION CONVENTION OF USING THE MOST RECENT 3
COMPLETE DECADES AS THE CLIMATE REFERENCE PERIOD. THE PROBABILITY ANOMALIES
FOR TEMPERATURE AND PRECIPITATION BASED ON THESE NEW NORMALS BETTER REPRESENT
SHORTER TERM CLIMATIC ANOMALIES THAN THE FORECASTS BASED ON OLDER NORMALS.
AN UPDATED MONTHLY OUTLOOK... FOR SEP WILL BE ISSUED ON THU AUGUST 31 2023
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1991-2020 BASE PERIOD.
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