Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2
000
FXUS02 KWBC 241554
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1053 AM EST FRI NOV 24 2017

VALID 12Z MON NOV 27 2017 - 12Z FRI DEC 01 2017

...PATTERN ASSESSMENT AND MODEL CHOICES...

WE CONTINUE WITH RELATIVELY HIGH CONFIDENCE ON THE LARGEST
SCALES...THAT DURING DAYS 3-7 A PROGRESSIVE WAVE TRAIN WILL SET UP
ALL THE WAY FROM THE WESTERN PACIFIC ACROSS NORTH AMERICA...THEN
SLOWING AND SPLITTING AROUND A PERSISTENT BLOCK OVER THE MIDDLE
ATLANTIC OCEAN. GIVEN THE SEASON AND STRENGTH OF THE OVERALL POLE
TO EQUATOR GRADIENTS THERE IS POTENTIAL FOR EMBEDDED NOTEWORTHY OR
EVEN STRONG LOWS TO SPIN UP WITHIN THE PROGRESSIVE FLOW...ALTHOUGH
THE DETERMINISTIC AND ENSEMBLE GUIDANCE SUGGESTS CYGLOGENESIS WILL
FOR THE MOST PART REMAIN NORTH OF THE LOWER 48 STATES THROUGH NEXT
FRIDAY. TO BEGIN THE MEDIUM RANGE PERIOD MONDAY MORNING...A TROUGH
WILL BE EXITING THE EAST COAST OF NORTH AMERICA. THE D3-7 FORECAST
THEN FEATURES A SUCCESSION OF THREE LARGE SCALE TROUGHS...THE
FIRST OF WHICH WILL LIKELY INCLUDE A PRONOUNCED SOUTHERN STREAM
COMPONENT...AND POTENTIALLY A CLOSED MID LEVEL CIRCULATION.

TELECONNECTIONS BASED ON THE ATLANTIC BLOCK FAVOR AN EVENTUAL
CONFLUENCE OF FLOW OVER THE EASTERN U.S. WHICH WOULD SHEAR AND
WEAKEN THE SOUTHERN STREAM TROUGH...AND THIS DOES OCCUR IN THE
GUIDANCE D5-6. MODELS STILL VARY GREATLY WITH THE DETAILS OF THE
INITIAL STRENGTH OF THE SYSTEM OVER THE SOUTHWEST STATES...ITS
FORWARD SPEED...AND RATE OF WEAKENING. THE ECMWF IS
CHARACTERISTICALLY SLOWER/STRONGER...AND THE GFS IS
CHARACTERISTICALLY FAST. ASIDE FROM THE VERY FLAT CANADIAN
ENSEMBLE MEMBERS WHICH LEAN TOWARD A WEAKER REFLECTION...THERE IS
A GROWING CONSENSUS OF OTHER GUIDANCE SOMEWHERE IN THE
MIDDLE...AND SIMILAR TO THE 00Z UKMET THROUGH DAY 5.

THIS FIRST WAVE IS FOLLOWED VERY QUICKLY BY A MORE CONSOLIDATED
TROUGH THAT ENTERS THE NORTHWEST U.S. ON TUESDAY...FALLS INTO THE
HEIGHT DEPRESSION LEFT BEHIND BY THE SHEARING WAVE IN THE PLAINS
ON THURSDAY...AND SWEEPS TO THE EAST COAST NEXT FRIDAY.

THE THIRD TROUGH OF INTEREST NEARS THE WEST COAST LATE NEXT WEEK.
ECMWF/ECMWF ENSEMBLE MEAN RUNS HAVE GENERALLY BEEN STEADIEST OVER
THE PAST COUPLE DAYS. GEFS MEANS HAVE MADE A NOTABLE TREND TOWARD
THE ECMWF MEAN FOR THE DEPTH OF THE TROUGH...HELPING TO INCREASE
CONFIDENCE SOMEWHAT. OCCASIONAL RUNS OF THE CMC ARE SIMILAR IN
PRINCIPLE AS WELL.

ONCE AGAIN...IN AN OVERALL SENSE...THE 00Z AND 06Z GFS WERE OUT OF
PHASE WITH THE MEANS AT SEVERAL TIMES...ON SEVERAL PARTS OF THE
MAP...WHICH DID NOT INSPIRE CONFIDENCE...BUT SEEING GOOD AGREEMENT
BETWEEN THE GEFS MEAN AND ECMWF ENSEMBLE MEAN GIVES US A SIMPLE
PATH FORWARD...AND ONE THAT STICKS CLOSE TO WPC CONTINUITY. OUR
BLENDS FOR THE WEATHER ELEMENTS ACROSS THE CONUS INCLUDED 20 TO 30
PERCENT EACH OF THE OPERATIONAL ECMWF AND UKMET THROUGH DAY
5.5...THEN TRANSITIONING TO A 60/40 BLEND OF THE ECMWF ENSEMBLE
MEAN/GEFS MEAN AS WAS DONE DURING THE PREVIOUS FORECAST CYCLE.


...WEATHER HIGHLIGHTS...

RECORD AND NEAR-RECORD WARMTH IS FORECAST ON DAY 3 FROM THE
SOUTHWEST STATES INTO THE PLAINS...WITH HIGHS IN THE UPPER 60S TO
MID 80S DEPENDING ON ELEVATION AND LATITUDE. TUESDAY WILL STILL
SEE STRONGLY ANOMALOUS HIGHS AHEAD OF THE FRONT ENTERING THE
SOUTHERN PLAINS...ALTHOUGH RECORDS THERE WILL BE LESS WITHIN
REACH. THE SUCCESSION OF FULL LATITUDE TROUGHS THEN KNOCKS MUCH OF
THE NATION BACK DOWN CLOSER TO AVERAGE...ALTHOUGH ANY ARCTIC AIR
REMAINS TIED TO A BROAD AND UNCHANGING LOW ACROSS CENTRAL CANADA.

ACROSS THE WEST...PROGRESSIVE EARLY-MID WEEK SYSTEMS MAY GENERATE
A COUPLE PERIODS OF RAIN/HIGH ELEVATION SNOW. LATE PERIOD EASTERN
PACIFIC TROUGHING ALOFT MAY BRING ANOTHER AREA OF MOISTURE TO THE
NORTHWEST TOWARD THE END OF THE WEEK. THE WASHINGTON
OLYMPICS/CASCADES HAVE THE GREATEST PROBABILITY OF SEEING THE
HIGHEST 5-DAY PRECIP TOTALS BUT UNCERTAINTY IN SHORTWAVE DETAILS
ALOFT ALLOW POTENTIAL FOR OTHER AREAS TO SEE FOCUSED ACTIVITY AS
WELL. THE LEADING SOUTHERN STREAM SYSTEM EMERGING FROM THE WEST
MAY GENERATE AN AXIS OF MEANINGFUL RAINFALL ACROSS THE
CENTRAL/SOUTHERN PLAINS AND LOWER MISSOURI VALLEY AROUND MIDWEEK.
AMOUNTS WILL DEPEND ON STRENGTH/TIMING OF THE UPPER SYSTEM. THE
NORTHEAST QUADRANT OF THE LOWER 48 SHOULD SEE LIGHT TO PERHAPS
LOCALLY MODERATE PRECIP IN ASSOCIATION WITH FRONTAL PASSAGES.
EXPECT SNOW CHANCES AT LOWER ELEVATIONS TO BE CONFINED TO NEAR THE
CANADIAN BORDER.

BURKE/RAUSCH

$$





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.