Prognostic Meteorological Discussion
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FXUS10 KWNH 151847
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
146 PM EST FRI DEC 15 2017

VALID DEC 15/1200 UTC THRU DEC 19/0000 UTC

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


12Z MODEL EVALUATION...WITH PRELIMINARY PREFERENCES AND CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...SHORTWAVE KICKING EAST TODAY FROM THE GREAT LAKES AND OHIO
VALLEY AND BECOMING NEGATIVE TILT WITH OFFSHORE CYCLOGENESIS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-GFS BLEND
CONFIDENCE: ABOVE AVERAGE

MODELS HAVE A FAIR DEGREE OF SIMILARITY WITH MASS FIELDS ALOFT AS
THE SHORTWAVE KICKS EAST OVER THE NEXT 24-48 HOURS...BUT STILL
SHOW SOME SPREAD WITH THE DEEPENING RATE AND LOCATION OF THE
SURFACE LOW. THE OVERWHELMING TREND OVER THE PAST FEW MODEL CYCLES
HAS BEEN FOR A STRONGER LOW THAT IS POSITIONED A BIT FURTHER NW.
FOR THIS REASON...WILL PREFER A BLEND OF THE 00Z ECMWF...00Z
UKMET...AND 00Z ECMWF ENSEMBLE MEAN WHICH ARE GENERALLY SITUATED
ON THAT END OF MODEL SPREAD. OVERALL THE DIFFERENCES ARE FAIRLY
SMALL FOR THIS SYSTEM.

18Z UPDATE: THE 12Z CMC HAS TRENDED CLOSER TO THE ECMWF...AND THE
12Z ECMWF IS RELATIVELY CONSISTENT WITH ITS PREVIOUS RUN. THE 12Z
GFS IS FURTHEST EAST WITH THE SURFACE LOW...SO THE CHANGE WILL BE
TO A NON-GFS BLEND. DIFFERENCES WITH THIS SYSTEM REMAIN SMALL.


...UPPER LEVEL LOW CURRENTLY OVER THE GULF OF CALIFORNIA...
...OPENING INTO A WAVE AND EJECTING NORTHEAST INTO THE SOUTHERN
PLAINS ON SATURDAY...DEAMPLIFYING ON SUNDAY AS IT PHASES INTO THE
NORTHERN STREAM JET OVER THE OHIO VALLEY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-NAM BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

THE 12Z NAM INITIALIZED ABOUT 20 METERS LOWER WITH ITS 500MB
HEIGHTS OVER NRN MEXICO THAN THE SOUNDING FROM NEAR CHIHUAHUA
(MMCU) OBSERVED THIS MORNING. THIS RESULTS IN A TROUGH THAT IS
SLIGHTLY DEEPER AND MORE EXPANSIVE...AND EVENTUALLY LEADS TO A
STRONGER SHORTWAVE THAT EJECTS NORTHEAST INTO THE PLAINS. THIS
PRESENTED SOME DIFFERENCES RELATIVE TO THE OTHER MODELS WHICH
TENDED TO BE MORE TIGHTLY CLUSTERED. ALTHOUGH OTHER SMALLER
DIFFERENCES EXISTED BETWEEN THE GLOBAL MODELS...THEY WERE NOT
CONSISTENT OR LARGE ENOUGH TO EXCLUDE. THEREFORE...THE PREFERENCE
IS FOR A NON-NAM BLEND.

18Z UPDATE: NO CHANGE TO PRELIMINARY PREFERENCE. MODEL SPREAD WAS
REDUCED ON THE 12Z MODEL CYCLE WITH THE EXCEPTION OF THE NAM...SO
THE PRELIMINARY PREFERENCE OF A NON-NAM BLEND IS MAINTAINED.


...SHORTWAVE APPROACHING THE PACIFIC NORTHWEST TODAY...
...DIGGING SHARPLY SOUTH ALONG THE WEST COAST ON SATURDAY...
...POSSIBLY DEVELOPING INTO CLOSED LOW NEAR THE DESERT SOUTHWEST...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 65 PERCENT 12Z ECMWF/UKMET
            35 PERCENT 12Z GFS/CMC
CONFIDENCE: SLIGHTLY BELOW AVERAGE

MODELS SHOW QUITE A BIT OF SPREAD OVER THE SOUTHWEST WITH THIS
TROUGH...AND IT LARGELY RELATES TO IF AND HOW A CLOSED LOW FORMS
ALOFT...AND (RELATED) HOW QUICKLY THE TROUGH EJECTS TO THE EAST.
THE 12Z NAM SHOWS A SUBSTANTIALLY DIFFERENT SOLUTION WITH TWO
DISTINCT WAVES OR VORT CENTERS...ONE FURTHER NORTH NEAR THE FOUR
CORNERS AND THE OTHER MUCH FURTHER SOUTH NEAR THE SOUTHERN TIP OF
BAJA CALIFORNIA. THE GLOBAL MODELS SHOW A PRIMARY VORT CENTER
LYING SOMEWHERE IN BETWEEN AND GIVEN THE LACK OF SUPPORT...WILL
EXCLUDE THE 12Z NAM FROM THE PREFERENCE.

OTHERWISE...THE 00Z ECMWF AND 00Z ECMWF ENSEMBLE CLOSE OFF A LOW
QUICKLY OVER NRN BAJA / FAR NW MEXICO AND ARE THE SLOWEST WITH
EASTWARD PROGRESSION. SOME ECMWF ENSEMBLE MEMBERS EVEN BEGIN TO
CUT OFF THE LOW AND LINGER IT IN THE VICINITY OF AZ/NM INTO THE
MEDIUM RANGE PORTION OF THE FORECAST. THE TREND ON ALL THE GLOBAL
MODELS HAS GENERALLY BEEN TOWARD A SLOWER PROGRESSION...SO THIS
DOES LEND CREDENCE TO THE IDEA OF A CLOSED LOW DEVELOPING ALOFT
OVER THE SOUTHWEST. THE TOP FIVE 500MB ANALOGS FOR THE SHORTWAVE
AS IT DIVES SOUTH ALONG THE WEST COAST PER CIPS GUIDANCE (00Z
CYCLE) ALL SHOW A CLOSED LOW DEVELOPING IN A SIMILAR AREA (AND
SOME EVEN RETROGRADING SLIGHTLY). THE 12Z GFS SLOWED DOWN SLIGHTLY
AND IS NOW FAIRLY SIMILAR IN TIMING TO THE 00Z UKMET AND 00Z CMC.
GIVEN THE RELATIVELY GOOD AGREEMENT...THIS SEEMS LIKE A REASONABLE
SOLUTION. HOWEVER THE PATTERN...SOME ENSEMBLE MEMBERS...AND
ANALOGS SUGGEST THAT THE ECMWF ALSO CANNOT BE DISCOUNTED. FOR THIS
REASON...A NON-NAM BLEND IS THE PREFERENCE FOR NOW.

18Z UPDATE: GREATER WEIGHT PLACED ON THE 12Z ECMWF/UKMET
SOLUTIONS. THERE CONTINUES TO BE A CONSIDERABLE AMOUNT OF SPREAD
WITH HOW THE MODELS HANDLE THE TROUGH BEGINNING ABOUT 24-36 HOURS
FROM NOW. THE 12Z ECMWF CONTINUED TO SHOW A CLOSED LOW...AND EVEN
SHOWED A DEEPER SOLUTION AND A STALLED MOTION RELATIVE TO THE
PREVIOUS MODEL CYCLE. THE 12Z UKMET HAS JOINED THIS IDEA NOW WHILE
THE 12Z CMC HAS REMAINED RELATIVELY UNCHANGED. THIS LEADS TO A
DIVERGENCE BETWEEN THE 12Z ECMWF/UKMET CAMP AND THE 12Z GFS/CMC
CAMP. AS MENTIONED IN THE PRELIMINARY DISCUSSION...WITH A
SHORTWAVE DIVING SHARPLY SOUTH...WOULD EXPECT A LOW TO CLOSE OFF
AND TO HAVE A GREATER LIKELIHOOD FOR SLOWER MOTION.
THEREFORE...WILL ADJUST THE PREFERENCE TO PLACE A GREATER WEIGHT
ON THE SLOWER SOLUTIONS NOW.


...NORTHERN STREAM PORTION OF THE SHORTWAVE APPROACHING THE
PACIFIC NORTHWEST TODAY SHIFTING TO THE GREAT LAKES BY MONDAY...
...SOUTHWARD SHIFT IN THE JET OVER THE NORTHWEST...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-NAM BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

AS THE 12Z NAM SPLITS THE TROUGH IN THE WEST...IT HAS A STRONGER
VORT MAX SITUATED AT HIGHER LATITUDES -- IN CLOSER PROXIMITY TO
THE NORTHERN STREAM JET. AS A RESULT...ITS DEPICTION OF THE
EVENTUAL TROUGH OVER THE NORTHERN PLAINS AND GREAT LAKES IS MUCH
DEEPER AND STRONGER THAN THE OTHER MODELS. OTHERWISE...THE OTHER
GLOBAL MODELS ARE IN FAIRLY GOOD AGREEMENT ON THE TIMING AND
STRENGTH OF THE INITIAL LOW AMPLITUDE WAVE AND SHOW SIMILARITY AS
THE NORTHERN STREAM JET BUILDS SOUTH INTO THE NORTHWESTERN CONUS.
THEREFORE A NON-NAM BLEND IS PREFERRED.

18Z UPDATE: NO CHANGE TO THE PRELIMINARY PREFERENCE.


MODEL TRENDS AT www.wpc.ncep.noaa.gov/html/model2.shtml
500 MB FORECASTS AT www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

LAMERS



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