846
FXUS07 KWBC 312021
PMD30D
PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT THU AUG 31 2023
30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2023
THE CURRENT STATE OF THE EL NIñO SOUTHERN OSCILLATION (ENSO) REMAINS IN THE
POSITIVE PHASE WITH THE LATEST WEEKLY OBSERVED NIñO 3.4 INDEX VALUE STANDING AT
+1.5 DEGREES CELSIUS. MEANWHILE, THE MAGNITUDE OF THE REAL-TIME MULTIVARIATE
(RMM) BASED MADDEN JULIAN OSCILLATION (MJO) INDEX HAS WEAKENED FOLLOWING A
PERIOD OF ENHANCEMENT OVER THE WESTERN HEMISPHERE, BUT IS FORECAST TO
POTENTIALLY QUICKLY TRANSLATE AND EMERGE OVER THE MARITIME CONTINENT DURING THE
NEXT TWO WEEKS. ANY INTRASEASONAL SIGNAL OVER THE WESTERN HEMISPHERE OR INDIAN
OCEAN WOULD DESTRUCTIVELY INTERFERE WITH THE EL NIñO BASE STATE. NATURAL ANALOG
COMPOSITES DERIVED FROM THE CURRENT STATES OF THE MJO AND ENSO ARE REFLECTIVE
OF THIS DESTRUCTIVE INTERFERENCE, AS THE RESULTANT 500-HPA PATTERN ACROSS NORTH
AMERICA BASED ON ENSO AND THE MJO INPUTS ARE ALMOST COMPLETELY OUT OF PHASE
WITH EACH OTHER. NATURAL ANALOGS BASED ON THE CURRENT EVOLUTION OF THE ARCTIC
OSCILLATION (AO), NORTH ATLANTIC OSCILLATION (NAO), AND THE PACIFIC NORTH
AMERICA (PNA) INDEX, GENERALLY ALIGN MORE CLOSELY WITH THE MJO COMPOSITES FOR
MUCH OF THE FORECAST DOMAIN. COMPOSITES FROM THE COMBINED ENSO/MJO/AO/NAO/PNA
STATE FOR THE MONTH OF SEPTEMBER DEPICT ABOVE-NORMAL 500-HPA HEIGHTS OVER THE
EASTERN AND SOUTHWESTERN CONTIGUOUS UNITED STATES (CONUS) AND OVER THE SOUTHERN
TWO-THIRDS OF ALASKA. ANOMALOUS MID-LEVEL TROUGHING IS DEPICTED OFF THE COAST
OF THE PACIFIC NORTHWEST.
AS WE CLOSE OUT THE MONTH OF AUGUST, SHORT TERM DYNAMICAL MODEL GUIDANCE IS
INCREASINGLY RELIED UPON WITH THE OFFICIAL SHORT AND MEDIUM RANGE OUTLOOKS FROM
THE WEATHER PREDICTION CENTER (WPC), 8-14 DAY OUTLOOKS FROM THE CLIMATE
PREDICTION CENTER (CPC), AND WEEKS 3-4 OUTLOOKS FROM CPC FORMING THE BACKBONE
OF THE UPDATED SEPTEMBER OUTLOOKS. DYNAMICAL MODEL GUIDANCE FROM THE CLIMATE
FORECAST SYSTEM VERSION 2 (CFSV2), THE GLOBAL FORECAST SYSTEM (GFS), THE
EUROPEAN CENTRE FOR MEDIUM RANGE WEATHER FORECASTS (ECMWF), AND ENVIRONMENT AND
CLIMATE CHANGE CANADA (ECCC) WERE UTILIZED TO SUPPLEMENT THE OFFICIAL OUTLOOKS.
THESE INPUTS PREDICT ANOMALOUS RIDGING OVER THE EASTERN TWO-THIRDS OF THE CONUS
AND SOUTHEAST ALASKA DURING THE BEGINNING OF THE MONTH. ANOMALOUS TROUGHING IS
DEPICTED ACROSS MUCH OF NORTHERN ALASKA AND THE WESTERN CONUS. BY MID-MONTH,
ABOVE NORMAL HEIGHTS ARE FORECAST TO BE FIRMLY ESTABLISHED ACROSS THE CENTRAL
CONUS WITH CYCLONIC FLOW PERSISTING ACROSS MUCH OF ALASKA. THE LATEST CPC WEEK
3-4 OUTLOOK INDICATED THE POTENTIAL FOR PERSISTENT RIDGING ACROSS THE CENTRAL
CONUS FOR MUCH OF THE MONTH WITH WEAK TROUGHING INDICATED ACROSS THE WEST AND
EAST COASTS OF THE CONUS AND ACROSS WESTERN ALASKA.
THE UPDATED SEPTEMBER TEMPERATURE OUTLOOK INDICATES INCREASED PROBABILITIES OF
ABOVE NORMAL TEMPERATURES ACROSS THE EASTERN TWO-THIRDS OF THE CONUS. THE
GREATEST CONFIDENCE FOR ABOVE NORMAL TEMPERATURES IS ACROSS PARTS OF THE
SOUTHERN PLAINS, WHERE PROBABILITIES EXCEED 60 PERCENT. THIS REPRESENTS AN
EASTWARD SHIFT OF THE REGION OF FAVORED ABOVE NORMAL TEMPERATURES RELATIVE TO
THE MID-MONTH OUTLOOK, DUE TO THE LIKELIHOOD FOR STRONG, ANOMALOUS RIDGING OVER
THE CENTRAL CONUS THROUGHOUT MOST OF THE MONTH. ABOVE NORMAL TEMPERATURES ARE
ALSO FAVORED FOR THE NORTH SLOPE AND SOUTH COAST OF ALASKA, THE EASTERN
MAINLAND, AND SOUTHEAST ALASKA, SIMILAR TO THE MID-MONTH OUTLOOK. THIS TILT
TOWARD MILDER THAN NORMAL CONDITIONS IS CONSISTENT WITH CFSV2 GUIDANCE ACROSS
SOUTHERN ALASKA, AS WELL AS ABOVE NORMAL SEA SURFACE TEMPERATURES (SSTS) AND
BELOW NORMAL SEA ICE EXTENT ADJACENT TO THE NORTH SLOPE OF ALASKA. HOWEVER,
PERSISTENT CYCLONIC FLOW IS ANTICIPATED ACROSS MUCH OF THE STATE, WHICH MAY
PROMOTE BOUTS OF COLDER THAN NORMAL CONDITIONS, PARTICULARLY ACROSS THE WESTERN
MAINLAND, WHERE A SLIGHT TILT TOWARD BELOW NORMAL TEMPERATURES IS INDICATED.
EQUAL CHANCES (EC) IS INDICATED ACROSS THE WESTERN CONUS, WHERE COOLER THAN
NORMAL CONDITIONS ANTICIPATED EARLY IN THE MONTH MAY GIVE WAY TO A MODERATION
BY MID-MONTH.
THE UPDATED SEPTEMBER PRECIPITATION OUTLOOK FAVORS BELOW NORMAL PRECIPITATION
ACROSS MOST OF THE PLAINS, UPPER AND MIDDLE MISSISSIPPI VALLEY, AND GREAT LAKES
DUE TO ANTICIPATED PERSISTENT ANOMALOUS RIDGING ACROSS THESE REGIONS.
CONVERSELY, CHANCES OF ABOVE NORMAL PRECIPITATION ARE ELEVATED ACROSS MOST OF
MAINLAND ALASKA DUE TO THE POTENTIAL FOR TROUGHING NEAR WESTERN PARTS OF THE
STATE FOR MUCH OF THE MONTH. HOWEVER, A TILT TOWARD BELOW NORMAL PRECIPITATION
IS INDICATED ACROSS SOUTHEAST ALASKA, CONSISTENT WITH CFSV2 GUIDANCE. FARTHER
TO THE SOUTH, ABOVE NORMAL PRECIPITATION IS FAVORED ACROSS THE PACIFIC
NORTHWEST. THIS WET SIGNAL IS CONSISTENT WITH THE LATEST CPC 6-10 AND 8-14 DAY
OUTLOOKS AS WELL AS WITH NATURAL ANALOG COMPOSITES, WHICH FAVOR ANOMALOUS
TROUGHING OFF THE COAST. ABOVE NORMAL PRECIPITATION IS ALSO FAVORED FOR PARTS
OF THE SOUTHWEST, AS MOIST MONSOONAL FLOW IS ANTICIPATED DURING THE FIRST
COUPLE OF DAYS OF THE MONTH. HOWEVER, THE MAGNITUDES OF THESE ENHANCED ABOVE
NORMAL PRECIPITATION PROBABILITIES ARE VERY MODEST, AS A DRYING TREND IS
FAVORED ACROSS THE SOUTHWEST THEREAFTER. FARTHER TO THE EAST, EC IS INDICATED
FOR THE SOUTHEAST AND ALONG THE EASTERN SEABOARD DUE TO WEAK OR CONFLICTING
SIGNALS FROM DYNAMICAL AND STATISTICAL GUIDANCE. MOISTURE ASSOCIATED WITH
IDALIA IS ANTICIPATED TO TIME OFF ACROSS THE SOUTHEAST PRIOR TO THE START OF
THE MONTH, AND A DRYING TREND IS ANTICIPATED ACROSS THE REGION IN ITS WAKE.
THEREFORE, THE TILT TOWARD ABOVE NORMAL PRECIPITATION THAT HAD BEEN INDICATED
IN THE MID-MONTH OUTLOOK HAS BEEN REMOVED IN FAVOR OF EC IN THIS UPDATED
OUTLOOK.
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****** PREVIOUS MID-MONTH DISCUSSION BELOW ******
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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.
THE NEXT MONTHLY OUTLOOK...FOR OCT ... WILL BE ISSUED ON THU SEP 21 2023
THESE OUTLOOKS ARE BASED ON DEPARTURES FROM THE 1991-2020 BASE PERIOD.
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