933
FXUS10 KWNH 170621
PMDHMD
MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
220 AM EDT WED JUN 17 2020
VALID JUN 17/0000 UTC THRU JUN 20/1200 UTC
...SEE NOUS42 KWNO (ADMSDM) FOR THE STATUS OF THE UPPER AIR
INGEST...
00Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
EASTERN U.S. CLOSED LOW AND COASTAL FRONT...
RETURNING FRONTAL ZONE/MOISTURE CONFLUENCE AXIS ACROSS FL...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND UNTIL 20.00,
00Z NAM/ECMWF/UKMET THEREAFTER.
CONFIDENCE: ABOVE AVERAGE EXCEPT IN FL (AVERAGE)
07Z UPDATE: INCREASING SPREAD AS THE UKMET HAS TRENDED A BIT
FASTER TOWARD WEAKENING AND MELDING INTO THE APPROACHING WESTERLY
FLOW ON LATE FRIDAY INTO SAT. WHILE THE ECMWF STARTS LIFTING
FASTER, THE CMC LANGUISHES A BIT MORE CENTERING NEAR VA BY THE END
OF DAY 3. SO WILL FAVOR A 00Z NAM/ECMWF/UKMET COMPROMISE TO
REMOVE THE OUTLIERS IN THE FIELD AND ACCOUNT FOR THE KNOWN TIMING
BIASES OF THE GFS/CMC AFTER 20.00Z.
---PRIOR DISCUSSION---
THE MODELS CONTINUE TO SHOW VERY GOOD AGREEMENT WITH THE CLOSED
LOW THROUGH THE BULK OF THE FORECAST PERIOD, WITH MESOSCALE
CONVECTIVE ALIGNMENT RESULTING IN THE GREATEST UNCERTAINTY.
HOWEVER, AS THE LOW STARTS TO WEAKEN AND FEEL THE AFFECTS OF THE
APPROACHING NORTHERN PLAINS LOW, THE 00Z GFS SHOWS A TYPICAL
NEGATIVE BIAS OF WEAKENING TOO EARLY AND LIFTING OUT BY THE END OF
DAY 3, CONTRADICTORY TO EVEN THE BULK OF 18Z GEFS MEMBERS. SO
WILL SUPPORT A GENERAL MODEL BLEND THROUGH ABOUT 72 HRS (20.00Z),
HEDGING TOWARD A NON-GFS BLEND REMAINING AT ABOVE AVERAGE
CONFIDENCE.
THE GREATEST UNCERTAINTY MAY NOT BE REALLY AFFECTED BY THE CLOSED
LOW ITSELF, BUT BY ITS WEAKENING INFLUENCE AND THE STRENGTH OF
RETURNING FLOW AROUND THE BERMUDA HIGH. THIS RESULTS IN A
NORTHWARD ADVANCEMENT OF THE OLD "FRONTAL" ZONE WHICH IS MORE OF A
MOISTURE DISCONTINUITY BOUNDARY/AXIS. THIS WILL INCREASE THE
THREAT OF THUNDERSTORMS ACROSS/NEAR THE COASTAL ZONES OF FL BY THE
END OF THE FORECAST PERIOD. AS THE GFS IS LIFTING OUT A BIT
FASTER WITH THE CLOSE LOW, THE RETURN MAY BE A BIT TOO FAST HERE,
AS WELL. YET, THE UNCERTAINTY IS GREATER THAN FURTHER NORTH BUT
THE SAME PREFERENCE OF GENERAL MODEL BLEND SHADING AWAY FROM THE
GFS AFTER 72 HOURS IS PREFERRED.
CLOSED LOW MOVING INTO THE NORTHWEST TUESDAY INTO THE NORTHERN
HIGH PLAINS BY THURSDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-CMC BLEND (HEDGING MORE GFS/NAM IN CENTRAL PLAINS
D3)
CONFIDENCE: SLIGHTLY ABOVE AVERAGE FOR MOST MASS FIELDS
BELOW AVERAGE IN CENTRAL PLAINS FRI NIGHT-SAT
(QPF/OUTFLOW BOUNDARY)
07Z UPDATE: WHILE THE 00Z CMC TRENDED INITIALLY A BIT FASTER, AND
THEREFORE TRENDS NORTH-EAST RELATIVE TO THE 12Z RUN, AFTER 19.00Z
IT ONCE AGAIN STARTS WOBBLING WEST OF THE GROWING CONSENSUS TO THE
EAST WITH RESPECT TO THE CLOSED LOW AND FRONTAL ZONE. THE
ECMWF/UKMET CONTINUE TO BE SIMILAR TO THEIR PRIOR RUNS WITH THE
MASS FIELDS. YET THE UKMET SEEMS TO SPLIT THE DIFFERENCE EVOLVING
A STRONG/HEAVY RAINFALL COMPLEX DROPPING SOUTH INTO OK, BUT THEN
REDEVELOPING FURTHER NORTH ALONG THE COLD FRONT LIKE THE ECMWF.
AS SUCH WILL CONTINUE TO KEEP INITIAL PREFERENCE/THINKING OF A
NON-CMC BLEND, BUT HEDGE MORE TOWARD THE GFS/UKMET/NAM IN THE QPF
SENSE ON DAY 3.
---PRIOR DISCUSSION---
THE CLOSED LOW ACROSS OR/ID IS STARTING TO OPEN/STRETCH INTO A
MORE POSITIVE TILT ORIENTATION AS IT CROSSES THE ROCKIES TONIGHT
INTO TOMORROW. THE GFS IS A BIT FAST, MORE ELONGATED THAN THE
OTHER GUIDANCE, BUT NOT RESULTING TOO MUCH OFFSET TO THE OVERALL
EVOLUTION. THIS INCLUDES UP TO AND INCLUDING THE INTERACTION WITH
THE ARCTIC/NORTHERN STREAM IN CANADA DRAWING IT SOUTHWARD BY
THURSDAY RESULTING IN A NEW CLOSED LOW ACROSS THE SOUTHERN
CANADIAN PRAIRIES. THE 12Z CMC IS SLOW WITH THIS AND DOES NOT
DRAW ANY NORTHERN STREAM ENERGY SOUTH, LEADING TO A DIFFERENT
EVOLUTIONARY PATH, MUCH FURTHER SOUTH AND WEST WITH THE CLOSE LOW
AND NOT FAVORED.
UPSTREAM JET/SHORTWAVE ENERGY WILL ROUND THE BASE AND FILL OUT A
WEAKLY FORMED TROF ACROSS THE CENTRAL ROCKIES BY FRIDAY, RESULTING
IN SOLID UPSLOPE FLOW REGIME. FROM A SYNOPTIC POINT OF VIEW, THE
MASS FIELDS REMAIN IN GOOD AGREEMENT, THE UNCERTAINTY LIES IN THE
PLACEMENT OF THE OUTFLOW BOUNDARY FROM THE CONVECTION THE PRIOR
NIGHT ACROSS THE CENTRAL PLAINS. THE GFS AND NAM ARE
STRONGER/FASTER WITH PRESSING THE RESULTING OUTFLOW BOUNDARY
SOUTHWARD, WHICH LEADS TO A SOUTHWARD SHIFT IN THE FRIDAY
EVENING/SAT MORNING CONVECTIVE COMPLEX COMPARED TO THE
UKMET/ECMWF, GIVEN BETTER DIFFERENTIAL HEATING, REINFORCING THE
BOUNDARY. SYNOPTICALLY, THE MASS FIELDS WOULD YIELD A SIMILAR
RESULT IN A BLEND TO SUPPORT A NON-CMC BLEND OVERALL.
HOWEVER, THE QPF DIFFERENCES ARE SIGNIFICANT BUT ALSO HIGHLY
CONTINGENT ON THE PLACEMENT OF THE BOUNDARY; TRADITIONALLY, THE
FASTER, SOUTHWARD OUTFLOW PROPAGATION FROM THE PRIOR NIGHT WOULD
FAVOR THE GFS/NAM, BUT IF THE CONVECTION IS STRONGLY FORCED
BECAUSE OF SLOWER EXITING OF THE WAVE THE PRIOR NIGHT AS THE ECMWF
WOULD BE APT TO DO, THE RESULTANT CONVECTIVE COMPLEX WOULD DELAY
WITH STRONGER MEAN FLOW TOWARD THE NE AND NOT RESULT IN FASTER
SOUTHWARD PROPAGATION, NOT DROPPING AS FAR SOUTH. SO OVERALL A
NON-CMC BLEND IS SUPPORTED AT SLIGHTLY ABOVE AVERAGE CONFIDENCE,
YET THE QPF UNCERTAINTY LATE DAY 2 INTO DAY 3 YIELDS A BELOW
AVERAGE CONFIDENCE SCENARIO.
SURFACE FRONT/ASSOCIATED MOISTURE APPROACHING THE PACIFIC
NORTHWEST BY LATE FRIDAY NIGHT INTO SAT...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 00Z NAM/GFS/ECMWF BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE
07Z UPDATE: THE ECMWF AND UKMET BOTH TRENDED A BIT FASTER, BUT
THIS MEANS THE ECMWF IS CLOSER TO THE INITIAL PREFERENCE OF THE
00Z NAM/GFS WHILE THE UKMET IS IN STEP WITH THE SLOWER/FLATTER,
EASTWARD SHIFTING CMC. SO, WILL SUPPORT A NAM/GFS/ECMWF BLEND AT
SLIGHTLY ABOVE AVERAGE CONFIDENCE.
---PRIOR DISCUSSION---
POSITIVE TILT RIDGING JUST OFF-SHORE THE PACIFIC NW WILL CONTINUE
TO ERODE/FLATTEN WITH THE APPROACH OF A LARGER SCALE CLOSED LOW
THAT WILL SLOWLY MEANDER IN THE NW GULF OF AK. A SHORT-WAVE LOBE
WILL PRESS SOUTHEAST AND MOVE NEAR HAIDA GWAII LATE FRIDAY. THIS
WILL HELP TO PRESS THE FRONTAL ZONE TOWARD VANCOUVER ISLAND AND
INCREASE SOUTHWESTERLY MOIST FLOW AHEAD OF IT. THE UKMET IS THE
ONLY CLEAR OUTLIER BEING VERY SLOW WITH THE SHORTWAVE SPURRING THE
WEAK SURFACE WAVE ALONG THE FRONT. THE 12Z CMC IS A BIT WEAKER
BUT ALSO BROADER WITH THE GYRE, SUPPRESSING THE RIDGING A BIT MORE
THAN THE GFS/NAM/ECMWF TO AT LEAST PRESS THE FRONTAL ZONE MORE
SOUTHEAST AND NEARER THE COAST. SO IT HELPS BUILD CONFIDENCE IN
THIS EVOLUTION BUT MAY BE A BIT TOO FAR EAST/BROADER/FLATTER FOR
INCLUSION TO A 00Z GFS/NAM AND 12Z ECMWF BLEND. CONFIDENCE IS
AVERAGE TO SLIGHTLY ABOVE AVERAGE.
GALLINA
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