Prognostic Meteorological Discussion
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FXUS02 KWBC 171600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST FRI NOV 17 2017

VALID 12Z MON NOV 20 2017 - 12Z FRI NOV 24 2017

...RECORD WARMTH LIKELY FOR THE DESERT SOUTHWEST ON THANKSGIVING...


...OVERVIEW...

AN AMPLIFIED PATTERN WILL BRIEFLY BUILD ACROSS THE LOWER 48 NEXT
WEEK AS AN UPPER HIGH EXPANDS JUST SOUTHWEST OF BAJA CALIFORNIA.
NORMALIZED 500MB HEIGHT ANOMALIES NEAR +2 TO EVEN +3 OVER THE
BERING SEA WILL CONTINUE TO PERSIST THOUGH PUNCTUATED BY NORTHERN
STREAM INTRUSIONS THROUGH WESTERN ALASKA. DOWNSTREAM, AN UPPER
HIGH IS FORECAST TO REMAIN IN PLACE NEAR THE DAVIS STRAIT (+2.5 TO
+3.5 STND DEV). THE COMBINATION OF ALL THREE POSITIVE ANOMALIES
SUPPORTS WETTER THAN AVERAGE CONDITIONS FOR THE WEST WITH
SOUTHWEST FLOW ALOFT AND WETTER THAN AVERAGE FOR THE LOWER MS
VALLEY AND N GULF COAST. THE TEMPERATURE SIGNAL WAS A BIT MIXED
BUT GENERALLY SUPPORTS MILDER TEMPERATURES IN THE WEST AND COOLER
THAN AVERAGE TEMPERATURES IN THE EAST.


...GUIDANCE/UNCERTAINTY PREFERENCES......

THE MODELS/ENSEMBLES GENERALLY AGREE ON THE LONGWAVE EVOLUTION
WITH THE GFS MOSTLY IN LINE WITH THE ENSEMBLES. THE 00Z CANADIAN
WAS FAVORED TO START THE PERIOD ESPECIALLY IN THE WEST WITH A
POORLY RESOLVED INFLUX OF STRETCHED VORTICITY MOVING ASHORE. THE
MODELS HAVE HAD A TOUGH TIME FOR SEVERAL DAYS RESOLVING THIS
FEATURE BUT THE 00Z ECMWF APPEARED TOO SLOW TO MOVE IT BODILY
EASTWARD GIVEN THE UP/DOWNSTREAM PATTERN AND THE GFS PERHAPS A BIT
TOO FAST. BY NEXT WEDNESDAY, THE ECMWF CONVERGED WITH THE GFS IN
THE EAST AND A BLEND OF THE TWO WITH THE ENSEMBLE MEANS OFFERED A
GOOD STARTING POINT, DESPITE THE TIMING DIFFERENCES BY NEXT FRIDAY
ACROSS THE NORTHERN TIER. THE ENSEMBLES SHOW CONTINUED SUPPORT FOR
A CLOSED LOW (OR NEARLY CLOSED LOW) TO FORM IN THE GULF OF MEXICO
ON THE TAIL END OF A COLD FRONT. THIS MAY HELP A SFC LOW TO FORM
ALONG THE BOUNDARY AND SLOWLY LIFT NORTHEASTWARD NEXT THU/FRI AS A
SFC HIGH BUILD EASTWARD THROUGH THE OHIO VALLEY.

PRIOR TO THAT, NORTHERN STREAM WILL BE ACTIVE ACROSS THE
US/CANADIAN BORDER WITH A RATHER STRONG SYSTEM MOVING THROUGH
ONTARIO INTO QUEBEC TUESDAY. ECMWF/GFS AND EVEN THE 00Z CANADIAN
CLUSTERED FAIRLY WELL WHILE THE UKMET CONTINUED TO LAG BEHIND. TO
THE WEST, SFC COLD FRONT WILL PUSH EASTWARD INTO WA/OR ON
WEDNESDAY, THROUGH THE INTERIOR NORTHWEST ON THU/THANKSGIVING,
THEN INTO THE UPPER MIDWEST AND GREAT LAKES NEXT FRIDAY WHILE
STRETCHED BACK THROUGH THE CENTRAL ROCKIES AND GREAT BASIN/NORCAL.

...WEATHER HIGHLIGHTS...

WET PATTERN WILL CONTINUE FOR THE PAC NW WITH MODEST TO
OCCASIONALLY HEAVY DAILY RAINS AND LOCALLY HEAVY ELEVATION SNOWS.
UPPER RIDGING TO THE SW OF CALIFORNIA WILL SUPPORT MILDER THAN
AVERAGE TEMPERATURES FOR THE SOUTHWEST THROUGH THE ROCKIES INTO
THE PLAINS BUT COOLER THAN AVERAGE IN THE EAST ESPECIALLY BEHIND
THE COLD FRONT. BEST CHANCE OF APPRECIABLE PRECIPITATION IN THE
EAST WILL BE ALONG THE GULF COAST WITH ONLY LIGHTER RAIN/SNOW
AROUND THE GREAT LAKES AND GENERALLY NO PRECIPITATION FROM THE
SOUTHWEST TO THE SOUTHERN PLAINS AND MS VALLEY.


...THANKSGIVING FORECAST...

GENERALLY QUIET FOR MOST OF THE LOWER 48. PRECIPITATION WILL BE
MOST CONCENTRATED IN THE PACIFIC NORTHWEST INTO NORTHERN
CALIFORNIA AS WELL AS NORTHERN IDAHO/WESTERN MONTANA/NW WYOMING.
ADDITIONAL RAIN IS QUITE POSSIBLE ALONG THE GULF COAST FROM TEXAS
EASTWARD TO FLORIDA WHERE A FRONT WILL LINGER ACROSS THE STATE.
RECORD HIGH TEMPERATURES ARE POSSIBLE IN THE SOUTHWEST WITH
WIDESPREAD 80S AND PERHAPS 90 DEGREES IN SOME AREAS.


FRACASSO

$$





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