Prognostic Meteorological Discussion
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FXUS10 KWNH 211903
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
303 PM EDT MON AUG 21 2017

VALID AUG 21/1200 UTC THRU AUG 25/0000 UTC

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

...AN EVALUATION OF THE 12Z GUIDANCE ALONG WITH FINAL
PREFERENCES/FORECAST CONFIDENCE...


...TUTT CELL DRIFTING ACROSS THE NORTHERN GULF OF MX...
...TROPICAL DISTURBANCE MOVING TOWARD THE BAY OF CAMPECHE BY
MID-WEEK...

FINAL PREFERENCE: A NON-12Z NAM MODEL BLEND
FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE

WATER VAPOR/RADAR IMAGERY SHOW A WELL-DEFINED TUTT CELL TRACKING
WESTWARD ACROSS THE CENTRAL GULF OF MX CONTINUES TO BE A FOCUS FOR
CONVECTION ON THE EASTERN SIDE OF THE CIRCULATION. DURING THE NEXT
24 TO 36 HOURS...MODELS SHOW A GREAT DEAL OF SHEARING TAKING PLACE
ALTHOUGH THE PRIMARY AXIS OF VORTICITY SHOULD CONGREGATE AROUND
THE UPPER TX COAST. SOME RESIDUAL SHORTWAVE ENERGY LINGERS INTO
THE MIDDLE OF THE WEEK ALTHOUGH IT GETS COMPLICATED WITH A
TROPICAL DISTURBANCE PUSHING TOWARD THE BAY OF CAMPECHE. A NOTABLE
OUTLIER IS THE 12Z NAM WHICH IS WELL TO THE NORTHEAST OF THE
GENERAL CONSENSUS. OTHERWISE...THE 12Z GUIDANCE HAS BEGUN TO
STRONGLY CLUSTER WHICH COINCIDES WELL WITHIN THE NHC TROPICAL
OUTLOOK PATH NOTED. WILL MOVE TOWARD A NON-12Z NAM MODEL BLEND.


...COMPLEX OF VORTICITY CENTERS EJECTING OUT OF THE
SOUTHERN/CENTRAL ROCKIES...
...SHEARING FEATURES EVENTUALLY ABSORBED IN EASTERN U.S. LONGWAVE
TROUGH...

FINAL PREFERENCE: A NON-00Z ECMWF ENSEMBLE MEAN
FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE

MULTIPLE VORTICITY CENTERS SPINNING ACROSS THE FOUR CORNERS REGION
WILL GRADUALLY SHIFT THEIR FOCUS TOWARD THE NORTHEAST. ONE SUCH
IMPULSE HAS ALREADY MOVED INTO SOUTHWESTERN KS WHICH HAS SHIFTED
INTO A VERY MOIST/UNSTABLE ENVIRONMENT...THUS SPREADING CONVECTION
INTO THE CENTRAL PLAINS. BASED ON THE CURRENT POSITION OF THE
VORTICITY CENTER...IT APPEARS THE 12Z GUIDANCE HAVE HANDLED THE
SYSTEM MORE READILY. NOW JUST THE 00Z ECMWF ENSEMBLE MEAN IS A
SLOWER OUTLIER SO WILL SHIFT THE PREFERENCE ACCORDINGLY.


...LONGWAVE TROUGH AFFECTING CENTRAL TO EASTERN NORTH AMERICA...
...ASSOCIATED DEEP SURFACE LOW/COLD FRONT...
...TRAILING SHORTWAVE PUSHING THROUGH THE UPPER MIDWEST 24/0000Z...

FINAL PREFERENCE: BLEND OF THE 12Z GFS/ECMWF
FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE

AMPLIFIED FLOW WITHIN THE ACTIVE NORTHERN STREAM WILL USHER IN A
RATHER PRONOUNCED BAND OF HEIGHT FALLS INITIALLY THROUGH THE UPPER
MIDWEST/GREAT LAKES REGION. THE SYSTEM SHOULD CONTINUE TO DIG AS
IT PROGRESSES DOWNSTREAM TOWARD THE LOWER GREAT LAKES/NEW ENGLAND
THE FOLLOWING COUPLE OF DAYS. WHILE THERE IS GENERAL
SYNOPTIC-SCALE AGREEMENT WITH THE TRACK OF THIS SYSTEM...MUCH OF
THE UNCERTAINTY LIES WITH THE SOUTHWESTERN EXTENT OF THE FRONTAL
ZONE ACROSS THE RED RIVER. OF NOTE...BY 23/1200Z...THE 12Z ECMWF
DEPICTS THE BOUNDARY ACROSS CENTRAL TX TOWARD THE BIG BEND WHILE
THE 12Z GFS FAVORS THE TAIL END CLOSER TO THE TX PANHANDLE. SUCH
DIFFERENCES CONTINUE THROUGH THE END OF THE FORECAST PERIOD.
FARTHER NORTH...THERE ARE TIMING ISSUES WITH THE TRAILING
SHORTWAVE PUSHING THROUGH THE UPPER MIDWEST WITH THE 12Z CMC/UKMET
BEING QUITE THE SLOW OUTLIERS. GIVEN THE UNCERTAINTY IN THE
FORECAST ACROSS THE SOUTHERN PLAINS...WILL SPLIT THE DIFFERENCE
BETWEEN THE 12Z GFS/ECMWF


...QUASI-STATIONARY UPPER LOW IN THE VICINITY OF CA/DESERT
SOUTHWEST...

FINAL PREFERENCE: BLEND OF THE 12Z NAM/GFS/ECMWF
FORECAST CONFIDENCE: AVERAGE

A BROAD CIRCULATION ACROSS THE EASTERN PACIFIC INTO THE
SOUTHWESTERN U.S. WILL CONTINUE TO AFFECT THE REGION THROUGH
THURSDAY. DURING THE FIRST 24 HOURS OR SO...THE SYSTEM SHOULD
RETROGRADE BACK TOWARD THE OFFSHORE WATERS BEFORE EVENTUALLY
ACCELERATING EASTWARD IN TIME BACK TOWARD THE DESERT SOUTHWEST. IT
APPEARS THE 12Z CMC IS OUT OF THE PHASE FROM THE CONSENSUS
SOLUTION BEING A BIT QUICKER BY MID-WEEK. TOWARD DAY
3/THURSDAY...THE 12Z UKMET DOES NOT DEPICT MUCH EASTWARD
PROGRESSION AS ADVERTISED AMONG MOST OF THE ENSEMBLE MEMBERS. WILL
FAVOR A COMBINATION OF THE 12Z NAM/GFS/ECMWF HERE.


...STRONG HEIGHT FALLS MOVING INTO WESTERN CANADA/PACIFIC
NORTHWEST BY THURSDAY...

FINAL PREFERENCE: MODEL COMPROMISE BETWEEN THE 12Z GFS/ECMWF/CMC
FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE

LATE IN THE FORECAST PERIOD...A PRONOUNCED LONGWAVE TROUGH BEGINS
TO PUSH THROUGH BRITISH COLUMBIA DOWN INTO THE PACIFIC NORTHWEST.
COMPARED TO THE WELL CLUSTERED ENSEMBLE MEMBERS VIA THE SPAGHETTI
PLOT...THE 12Z UKMET IS FLATTER AND QUICKER. EVENTUALLY THE 12Z
NAM JOINS THIS MORE PROGRESSIVE SOLUTION BY LATE WEDNESDAY/EARLY
THURSDAY. GIVEN SOLID AGREEMENT BETWEEN THE 12Z
GFS/ECMWF/CMC...WILL FAVOR A MODEL COMPROMISE AMONG THESE
SOLUTIONS.



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

RUBIN-OSTER

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