Prognostic Meteorological Discussion
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000
FXUS02 KWBC 191551
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1150 AM EDT THU OCT 19 2017

VALID 12Z SUN OCT 22 2017 - 12Z THU OCT 26 2017

...HEAVY RAINFALL POSSIBLE ACROSS THE EASTERN U.S. WHILE RECORD
HEAT RETURNS TO THE DESERT SOUTHWEST NEXT WEEK...


...PATTERN OVERVIEW/MODEL GUIDANCE/WPC PREFERENCES...

AN ACTIVE PERIOD AHEAD ACROSS THE NATION WITH AMPLIFIED FLOW
EXPECTED AS WELL BELOW AVERAGE HEIGHTS CONCENTRATE TO THE EAST OF
THE GREAT PLAINS. AS THE UPPER TROUGH FURTHER DIGS INTO THE
SOUTHEASTERN U.S...THE 00Z GFS SUGGESTS 500-MB HEIGHT ANOMALIES
MAY EXCEED 4 STANDARD DEVIATIONS BELOW CLIMATOLOGY. A COMPLEX
SPLIT-FLOW SET UP EXISTS NEXT WEEK AS A POTENT SOUTHERN STREAM
SYSTEM BARRELS ACROSS THE GULF COAST WITH A NUMBER OF MODEL
UNCERTAINTIES AT HAND. THIS IS FURTHER COMPLICATED BY THE NORTHERN
STREAM CARRYING A BROAD LONGWAVE TROUGH INTO THE EASTERN U.S. FROM
DAY 5-7...OCTOBER 24-26. POTENTIAL INTERACTION WITH THESE TWO
STREAMS IN CONJUNCTION WITH EVENTUAL PLACEMENT OF SUCH FEATURES
RAISES A NUMBER OF QUESTIONS WITHIN THE FORECAST. ACROSS THE
WESTERN STATES...STRONG UPPER RIDGING WILL BE THE NORM FOR THE
MOST PART WITH 500-MB HEIGHT DEPARTURES ON THE ORDER OF 2 STANDARD
DEVIATIONS BY THE EARLY/MIDDLE PART OF NEXT WEEK. GRADUAL EROSION
OF THE RIDGE IS EXPECTED AS HEIGHT FALLS DIP THROUGH THE NORTHERN
TIER BY LATE IN THE PERIOD.

MUCH OF THE FORECAST WAS DRIVEN BY THE HANDLING OF THE SPLIT FLOW
ACROSS THE CENTRAL/EASTERN U.S. WHICH ROUGHLY COVERS THE SUNDAY
THROUGH TUESDAY TIMEFRAME. OPERATIONAL MODELS CONTINUE TO WAVER
BACK AND FORTH BETWEEN A MORE WOUND UP CLOSED LOW DRIFTING
EASTWARD ACROSS THE CENTRAL GULF COAST VERSUS A LESS SUBSTANTIAL
BUT MORE PROGRESSIVE SYSTEM. THE 00Z UKMET AND MANY OF ITS PRIOR
RUNS HAVE FAVORED A MORE ROBUST UPPER LOW WHILE BEING THE SLOWEST
IN THE PACK. THE 00Z CMC HAS BEEN IN A SIMILAR POSITION AS THE
UKMET...BUT THE LATEST RUN IS DISPLACED NORTHWARD AND BECOMES
ABSORBED MORE READILY INTO THE NORTHERN STREAM. CONSIDERING THE
GFS/ECMWF...RUN-TO-RUN CONTINUITY IS NOT GREAT ALTHOUGH THE
06Z/00Z GFS STICK WITH THE WEAKER MID-LEVEL REFLECTION AS
SUPPORTED BY THE PREVIOUS 12Z ECMWF. THE 00Z ECMWF WENT WITH THE
MORE AGGRESSIVE SOLUTION AS IT HAS SHOWN FROM TIME TO TIME.
EVENTUALLY THIS SYSTEM LIFTS NORTHWARD ALONG THE EASTERN SEABOARD
IN ADVANCE OF THE NORTHERN STREAM TROUGH. TOWARD THE DAY
6/7...OCTOBER 25/26 WINDOW...AN EXPANSIVE CYCLONIC CIRCULATION IS
EVIDENT AMONG ALL SOLUTIONS BUT WITH VARYING DEGREES OF STRENGTH
ACCOMPANIED BY A MYRIAD OF POSITIONS. SOME FEATURE A NEGATIVE TILT
TO THE TROUGH LEADING TO ADDITIONAL WAVES OF LOW PRESSURE.

ACROSS THE WESTERN/CENTRAL U.S...THE UPPER RIDGE APPEARS TO BE A
WELL ESTABLISHED FEATURE WHICH IS AGREED UPON...ALBEIT WITH MINOR
DIFFERENCES. THE CENTER OF THE UPPER RIDGE WILL BE LOCATED OVER
THE SOUTHWESTERN U.S. WHERE SOME OF THE MOST ANOMALOUS HEAT WILL
BE SITUATED. GRADUAL AMPLIFICATION ACROSS THE NORTHEASTERN PACIFIC
SHOULD USHER IN JET ENERGY INTO WESTERN CANADA WHICH WILL
ULTIMATELY DENT THE NORTHERN EXTENT OF THE RIDGE. THIS LEADS TO
LOWERING HEIGHTS DOWNSTREAM ACROSS THE NORTH-CENTRAL U.S. WITH THE
STRONGEST HEIGHT FALLS APPROACHING THE REGION BY NEXT THURSDAY. AT
THIS POINT...LONGWAVE TROUGHING WILL LIKELY EXTEND OVER THE
CENTRAL/EASTERN U.S. WITH A SHORTWAVE RIDGE SEPARATING THE TWO
MAJOR FEATURES DISCUSSED.

WITH THE OVERNIGHT CONTINUITY FAVORING A LOWER AMPLITUDE/MORE
PROGRESSIVE SOUTHERN STREAM...DECIDED TO STICK CLOSER TO THAT
BEFORE A CONSENSUS DEVELOPS. THUS...THE FORECAST WAS LED BY THE
00Z GFS/GEFS MEAN AND PREVIOUS 12Z ECMWF/ECMWF ENSEMBLE MEAN IN
CONJUNCTION WITH ABOUT 20 PERCENT OF THE INHERITED CONTINUITY.
GRADUALLY INCREASED ENSEMBLE INFLUENCES IN THE FORECAST BEFORE
DEFAULTING TO ALL OPERATIONAL SOLUTIONS BY MID-WEEK.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

GIVEN THE AMPLIFIED FLOW IN PLACE...SOME FAIRLY IMPRESSIVE
TEMPERATURE ANOMALIES ARE NOTED DURING THE PERIOD. WITH A DEARTH
OF ANY APPRECIABLE ARCTIC AIR TO WORK WITH...ALL OF THE POTENTIAL
DAILY TEMPERATURE RECORDS WILL BE ON THE WARM SIDE OF THE
EQUATION. SOME OF THE WARMEST TEMPERATURE ANOMALIES SHOULD BE OVER
THE SOUTHWESTERN STATES WITH DEPARTURES POSSIBLY REACHING THE 15
TO 20 DEGREE RANGE BY MONDAY. A NUMBER OF DAILY RECORDS MAY BE
BROKEN AS HIGHS SOAR ABOVE THE CENTURY MARK ACROSS SOUTHERN CA. ON
TUESDAY...SOME COULD BE SHATTERED WITH BURBANK AND LONG BEACH CA
EXPECTING HIGHS 5 DEGREES ABOVE THEIR CURRENT RECORDS.
UNFORTUNATELY THIS HEAT WILL ALSO ENHANCE THE WILDFIRE DANGER
GIVEN EXPECTED OFFSHORE GRADIENTS LEADING TO DRY/GUSTY WINDS.
ELSEWHERE...IN ADVANCE OF THE SLOW-MOVING UPPER TROUGH...SOME
RECORD WARMTH MAY BE EXPERIENCED OVERNIGHT WITH LOWS IN THE 60S
FROM THE LOWER MS VALLEY UP INTO MO/IL ON SUNDAY.

WHILE CONDITIONS BEGIN TO IMPROVE ACROSS THE PACIFIC NORTHWEST AS
HEIGHTS BEGIN TO RISE...THE FOCUS SHIFTS TO THE EASTERN THIRD OF
THE NATION. IT SHOULD BE A RATHER WET PERIOD OVER THE REGION ALONG
AND AHEAD OF THE ADVANCING FRONTAL ZONE. THE SOUTHERN STREAM UPPER
LOW WILL ALSO PROVIDE AN ADDITIONAL FOCUS FOR HEAVY
RAINFALL...PARTICULARLY IF IT ENDS UP MOVING AS SLOW AS SOME
MODELS SUGGEST. BY TUESDAY THROUGH THURSDAY...THE FOCUS SHOULD
SHIFT INTO NEW ENGLAND AS A SERIES OF WAVES OF LOW PRESSURE LIFT
THROUGH THE REGION. WHILE SOME MODELS SHOW SOME RATHER COOL
1000-500 MB THICKNESSES...THERE DO NOT APPEAR TO BE ANY MAJOR
WINTRY PRECIPITATION IMPLICATIONS AT THIS TIME.


RUBIN-OSTER

$$





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