Prognostic Meteorological Discussion
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FXUS10 KWNH 180451
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1151 PM EST FRI NOV 17 2017

VALID NOV 18/0000 UTC THRU NOV 21/1200 UTC

...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR
INGEST...

00Z MODEL ASSESSMENT INCLUDING PREFERENCES AND CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
MODEL INITIALIZATION ERRORS APPEAR NEGLIGIBLE WITH RESPECT TO
THEIR FORECASTS

LARGE STORM SYSTEM OVER THE EASTERN U.S. THIS WEEKEND
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 00Z GFS/00Z NAM/12Z ECMWF BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

A LARGE FULL LATITUDE UPPER LEVEL TROUGH CURRENTLY OVER THE
CENTRAL U.S. WILL CONTINUE TO REMAIN AMPLIFIED AS IT REACHES THE
EAST COAST BY SUNDAY.  A SURGE OF SHORTWAVE ENERGY BEHIND THE
TROUGH IN NORTHWEST FLOW WILL LIKELY RESULT IN THIS FEATURE
ACQUIRING NEGATIVE TILT AS IT APPROACHES THE NORTHEAST U.S.  THIS
WILL SUPPORT THE DEVELOPMENT OF A STRONG SURFACE LOW TRACKING FROM
THE OHIO VALLEY TO SOUTHERN QUEBEC.  A SECONDARY TROUGH THEN
CROSSES THE MID-ATLANTIC REGION SUNDAY NIGHT BEFORE HEIGHTS RISE
AGAIN.  IN TERMS OF THE MODELS, THE GFS AND THE GEFS MEAN ARE BOTH
FASTER WITH THE COLD FRONT REACHING THE EAST COAST, AND THE NAM
AND ECMWF ON THE SLOWER SIDE OF THE GUIDANCE.  THE CMC ALSO
BECOMES PROGRESSIVE ONCE THE SYSTEM EXITS THE COAST.  THE NAM IS
NOW MUCH CLOSER TO THE MODEL CONSENSUS COMPARED TO ITS RUN LAST
NIGHT.  A BLEND OF THE GFS, NAM, AND ECMWF WOULD YIELD A SUITABLE
TIMING OF THE FRONTAL PASSAGE.


NORTHERN PORTION OF TROF APPROACHING PACIFIC NORTHWEST SUNDAY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 12Z ECMWF/00Z GFS BLEND
CONFIDENCE: AVERAGE

PART OF THE ENERGY EMERGING FROM THE GULF OF ALASKA IS EXPECTED TO
BREAK OFF AND TRACK EASTWARD ACROSS SOUTHERN CANADA SUNDAY AND
INTO MONDAY WITHIN THE NORTHERN STREAM FLOW.  A RELATIVELY DEEP
SURFACE WAVE JUST NORTH OF US BORDER WITH A FRONT ENTERING THE
NORTHERN PLAINS LATE MONDAY IS PREDICTED BY THE MODEL GUIDANCE.  A
BLEND OF THE FASTER GFS AND SLOWER ECMWF SHOULD YIELD THE BEST
POSITION OF THE SURFACE LOW.


BASE OF TROUGH APPROACHING THE PAC NW SUNDAY...CONSOLIDATING INTO
WA/OR MON
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 12Z ECMWF/12Z UKMET/12Z EC MEAN
CONFIDENCE: SLIGHTLY BELOW AVERAGE

A LARGE UPPER LEVEL TROUGH OVER THE GULF OF ALASKA WILL SLOWLY
SETTLE SOUTHWARD OFF THE BRITISH COLUMBIA COAST AS A POSITIVELY
TILTED FEATURE, WITH A SURFACE LOW TRACKING INLAND JUST NORTH OF
VANCOUVER ISLAND.  THE TRAILING COLD FRONT IS EXPECTED TO REACH
WESTERN WASHINGTON BY SUNDAY EVENING.  AN ATMOSPHERIC RIVER OF
DEEP PACIFIC MOISTURE IS EXPECTED TO ACCOMPANY THIS FEATURE.  THE
OVERALL MASS FIELD DIFFERENCES ARE RELATIVELY MINOR FOR THIS
SYSTEM THROUGH 48 HOURS AND A GENERAL MODEL BLEND SHOULD WORK WELL
THROUGH SUNDAY AFTERNOON.  BEYOND THAT TIME, MODEL DIFFERENCES
BECOME MUCH MORE NOTICEABLE, WITH THE CMC INDICATING A DEEPER AND
MORE PROGRESSIVE TROUGH, AND THE NAM/GFS INDICATING MORE ZONAL
FLOW.  WITH THE EXPECTATION THAT THE TROUGH WILL PERSIST BEYOND
SUNDAY EVENING, IN CONTRAST TO THE GFS/NAM, THE UKMET AND ECMWF
ALONG WITH THE EC MEAN, ARE PREFERABLE SOLUTIONS FOR NOW.


www.wpc.ncep.noaa.gov/html/model2.shtml
www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

HAMRICK

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