Prognostic Meteorological Discussion
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FXUS10 KWNH 161634
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1233 PM EDT MON OCT 16 2017

VALID OCT 16/1200 UTC THRU OCT 20/0000 UTC

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

12Z PRELIMINARY MODEL EVALUATION WITH PREFERENCES/CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


...SHORTWAVE MOVING QUICKLY EAST IN ZONAL FLOW ALONG THE
U.S.-CANADA BORDER TUESDAY THROUGH THURSDAY...
...ASSOCIATED DEEPENING SURFACE LOW AND COLD FRONT...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 00Z ECMWF / CMC / UKMET
CONFIDENCE: ABOVE AVERAGE

A SHORTWAVE EMBEDDED IN ZONAL FLOW WILL MOVE RAPIDLY EAST ALONG OR
JUST NORTH OF THE CANADIAN BORDER IN THE NEXT 48 HOURS...MOVING
ABOUT 50 DEGREES LONGITUDE (150W TO 100W) BY WEDNESDAY MORNING.
MODELS ALSO SHOW RELATIVELY STRONG LEE CYCLOGENESIS TO THE EAST OF
THE CANADIAN ROCKIES (DEEPENING RATE OF SURFACE LOW NOT QUITE 1
BERGERON BUT CLOSE) AND OCCLUDE THE CYCLONE IN THE VICINITY OF
NORTHERN MANITOBA AND HUDSON BAY BY WEDNESDAY NIGHT. THE 12Z GFS
SHOWS A SLIGHTLY STRONGER WAVE APPROACHING THE COAST...AND HAS A
MID-LEVEL VORT MAX DISPLACED ABOUT 2 DEGREES LATITUDE FURTHER
SOUTH THAN THE OTHER MODELS. THIS EVENTUALLY LEADS TO A SURFACE
LOW ON THE STRONGER SIDE OF MODEL SPREAD...AND DISPLACED FURTHER
TO THE WEST (SLOWER PROGRESSION). GIVEN THE ZONAL FLOW...PREFER
THE CONSENSUS WITH A SLIGHTLY LOWER AMPLITUDE AND FASTER WAVE. THE
12Z NAM HAS A SIMILAR AMPLITUDE TO THE OTHER MODELS...BUT IS
SLOWER BY A FEW HOURS. A STRONGER VORT MAX EMBEDDED NORTH OF THE
PRIMARY WAVE ALSO LEADS TO SURFACE LOW DEVELOPMENT FURTHER NORTH
THAN THE OTHER MODELS. OVERALL THE 12Z NAM AND GFS DIFFERENCES ARE
NOT LARGE...BUT ENOUGH AT SUCH SHORT TIME RANGES THAT A
ECMWF...CMC...UKMET BLEND IS RECOMMENDED.


...TROUGH DIGGING INTO THE PACIFIC NORTHWEST LATE WEDNESDAY INTO
THURSDAY ASSOCIATED WITH AN ATMOSPHERIC RIVER EVENT...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-CMC BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

A DEEPER TROUGH (COMPARED TO THE TUESDAY SYSTEM) WILL DIG INTO THE
PACIFIC NORTHWEST LATE WEDNESDAY INTO THURSDAY...WITH A STRONG
SURFACE CYCLONE APPROACHING BRITISH COLUMBIA BY THE END OF THE
WORK WEEK. AN ATMOSPHERIC RIVER WILL BE PUSHING INTO THE PACIFIC
NORTHWEST JUST AHEAD OF THE TROUGH. MODEL DIFFERENCES WITH THIS
SYSTEM ARE FAIRLY SMALL. NORMALIZED STANDARD DEVIATION OF MSLP AND
500MB HEIGHTS ON THE 00Z ECMWF ENSEMBLES SHOWED GENERALLY NEAR OR
SLIGHTLY BELOW NORMAL AMOUNT OF SPREAD OVER THE NORTHWEST FROM
WEDNESDAY INTO THURSDAY. ONE EXCEPTION WAS THE 00Z CMC WHICH
SHOWED A MUCH FLATTER WAVE ACROSS THE EASTERN PACIFIC WITH HIGHER
HEIGHTS THROUGH MOST OF THE MID-LATITUDES (30-60N)...AS MUCH AS
60M HIGHER THAN ENSEMBLE MEANS AT 500MB IN SOME LOCATIONS.
THEREFORE...A GENERAL MODEL BLEND THAT EXCLUDES THE 00Z CMC IS
PREFERRED.


...PROGRESSIVE TROUGH MOVING FROM THE EASTERN GREAT LAKES TODAY TO
THE CANADIAN MARITIMES TUESDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

A BROAD TROUGH OVER THE NORTHEASTERN CONUS WILL GRADUALLY
DEAMPLIFY AND QUICKLY EXIT OVER THE ATLANTIC DURING THE NEXT 24
HOURS...WITH THE SURFACE COLD FRONT GENERALLY ALREADY OFFSHORE.
MODELS SHOW NO SUBSTANTIAL DISAGREEMENT IN THE MASS FIELDS WITH
THIS SYSTEM SO A GENERAL MODEL BLEND IS RECOMMENDED WITH HIGH
CONFIDENCE.


...LINGERING BROAD TROUGH OVER THE SOUTHEAST STATES...
...STALLING COLD FRONT IN THE VICINITY OF FLORIDA...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 12Z GFS / 00Z ECMWF
CONFIDENCE: AVERAGE

ALTHOUGH THE COMPONENT OF THE EASTERN CONUS WAVE TIED MORE CLOSELY
TO THE NORTHERN STREAM JET SHOULD BE PROGRESSIVE...A BROAD TROUGH
IS LIKELY TO LINGER OVER THE SOUTHEAST THROUGH THE REMAINDER OF
THE WEEK. INITIALLY THIS WILL LIKELY BE RELATED TO THE SOUTHERN
PORTION OF THE INITIAL TROUGH...HOWEVER SEVERAL REINFORCING
SHORTWAVES CURRENTLY NEAR THE NORTHWESTERN U.S. ARE LIKELY TO
ARRIVE OVER THE NEXT SEVERAL DAYS. AT THE SURFACE...A COLD FRONT
WILL STALL OUT NEAR THE SOUTHERN PERIPHERY OF THE LOW-LEVEL MID
ATLANTIC ANTICYCLONE.

IN GENERAL...THE 00Z ECMWF AND 12Z GFS ARE CLOSEST TO THE ENSEMBLE
MEANS IN THEIR DEPICTION OF THE MASS FIELDS. BOTH MAINTAIN
SLIGHTLY LOWER HEIGHTS ALOFT THROUGH THE REGION...WHICH IS
GENERALLY PREFERRED GIVEN THE SOUTHWARD EXTENSION OF THE TROUGH
NOTED ON WATER VAPOR SATELLITE AT THE MOMENT...AS WELL AS SOME
CONVECTION OVER THE CENTRAL GULF THAT MAY BE TIED TO A WEAK
MID-LEVEL LOW. THE CMC...UKMET...AND NAM (TO A SLIGHTLY LESSER
EXTENT) TEND TO DEAMPLIFY THE TROUGH AND BUILD HEIGHTS
FASTER...AND DEPICT A STRONGER SURFACE HIGH ACROSS THE EASTERN
CONUS. GIVEN THE SUPPORT FROM THE 00Z ENSEMBLE MEANS...PREFER A
BLEND OF THE 12Z GFS AND 00Z ECMWF IN THIS REGION.


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

LAMERS

$$





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