326
FXUS10 KWNH 171835
PMDHMD
MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
234 PM EDT SUN MAY 17 2020
VALID MAY 17/1200 UTC THRU MAY 21/0000 UTC
...SEE NOUS42 KWNO (ADMSDM) FOR THE STATUS OF THE UPPER AIR
INGEST...
12Z MODEL EVALUATION INCLUDING FINAL PREFERENCES AND CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
18Z UPDATE: THERE WERE NO MAJOR CHANGES NOTED WITH THE ARRIVAL OF
THE 12Z UKMET, AND THE CMC WAS ALSO CONSISTENT AND NOW MORE IN
LINE ACROSS THE ROCKIES AND SLIGHTLY TO THE WEST OF ITS PREVIOUS
RUN WITH THE UPPER LOW DROPPING SOUTH ACROSS ILLINOIS. THE ECMWF
TRENDED A LITTLE FASTER IN BRINGING THE CLOSED LOW SOUTHWARD TO
WESTERN TENNESSEE BY MIDDAY TUESDAY, AND CLOSE TO ITS PREVIOUS
POSITION BY 84 HOURS. THE 12Z NAM AND A FEW OF THE 12Z GEFS
MEMBERS REMAIN WELL TO THE WEST OF THE MODEL CONSENSUS REGARDING
THE TRACK OF ARTHUR ONCE IT PASSES CAPE HATTERAS, NC, BUT
OTHERWISE GOOD MODEL CONTINUITY. OUT WEST, THE 12Z CMC CAN NOW BE
USED AS PART OF THE MODEL BLEND WITH THE BUILDING TROUGH OVER THE
INTERMOUNTAIN WEST.
TROPICAL STORM ARTHUR
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NATIONAL HURRICANE CENTER OFFICIAL TRACK
BEST MODEL PROXY: 12Z ECMWF/12Z GFS BLEND
CONFIDENCE: MODERATE
TROPICAL STORM ARTHUR FORMED LATE YESTERDAY EVENING OFF THE EAST
COAST OF FLORIDA, AND IS FORECAST TO TRACK NORTHWARD TOWARD THE
OUTER BANKS BY EARLY MONDAY BEFORE TURNING NORTHEASTWARD AWAY FROM
THE MID-ATLANTIC. THE INTERACTION OR LACK THEREOF WITH AN
APPROACHING MID-LEVEL CLOSED LOW WILL HELP DETERMINE THE NORTHWARD
OR NORTHEASTWARD STEERING MOTION OF ARTHUR. THE 12Z NAM BEGINS TO
DIVERGE FROM THE CONSENSUS BY MONDAY MORNING, WITH A WESTWARD
TRACK OVER THE OUTER BANKS OF NC, AND THIS DIFFERENCE BECOMES MORE
NOTICEABLE BY TUESDAY WITH THE STORM LINGERING OFF THE NEW JERSEY
COAST, WHEREAS THE OTHER MODELS ARE IN DECENT AGREEMENT THAT A
SUSTAINED TRACK TO THE NORTHEAST WILL TAKE PLACE. THERE ARE ALSO
A COUPLE OF GEFS MEMBERS THAT INDICATE A SIMILAR PATH AND EVEN
BRING THE STORM BACK TO THE WEST AGAIN TOWARDS NEW JERSEY, PERHAPS
OWING TO THE BUILDING RIDGE TO THE NORTH AND THE UPPER LOW TO THE
WEST. THE ECMWF/EC MEAN CONTINUE TO FAVOR A TRACK TO THE
SOUTHEAST OF THE GFS/GEFS MEAN, AND THIS IS SIMILAR TO YESTERDAY.
THE BEST PROXY TO THE OFFICIAL NHC TRACK IS ABOUT HALFWAY BETWEEN
THE GFS AND ECMWF, CLOSE TO THE MODEL CONSENSUS. FOR THE LATEST
AND OFFICIAL FORECAST INFORMATION, SEE UPDATES FROM THE NATIONAL
HURRICANE CENTER.
CONSOLIDATION OF MIDWEST AND GULF COAST SHORTWAVES INTO CUT-OFF
UPPER LOW
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 12Z NAM, 12Z ECMWF, 12Z UKMET BLAND
CONFIDENCE: MODERATE
THIS FEATURE WILL BE THE MOST IMPACTFUL IN TERMS OF SENSIBLE
WEATHER ACROSS THE NATION FOR THE SHORT RANGE PERIOD. TWO
DISTINCT SHORTWAVES, ONE NEAR THE GULF COAST, AND ANOTHER
CURRENTLY OVER IOWA, ARE EXPECTED TO MERGE AND EVOLVE INTO A
PRONOUNCED AND CUT-OFF UPPER LEVEL LOW. THE NORTHERN SHORTWAVE
WILL BE THE DOMINANT FEATURE AND GROW AS IT SETTLES TOWARDS THE
SOUTHEAST ACROSS ILLINOIS AND THEN THE MID-SOUTH BY TUESDAY NIGHT.
MODEL FORECAST GUIDANCE OVER THE PAST SEVERAL CYCLES HAS
STRUGGLED WITH THE EVOLUTION OF THIS SYSTEM, AND THIS IS NOT
UNUSUAL GIVEN CUT-OFF LOW EVOLUTION. IT NOW APPEARS TO BE IN
BETTER AGREEMENT ON THE MAIN FEATURES WITH THE ARRIVAL OF THE 12Z
GFS AND NAM. THERE HAS BEEN A TREND TO THE WEST TO COMPARED TO
PREVIOUS MODEL RUNS, AND NOW SHOW THE CENTER OF THE LOW REACHING
TENNESSEE BY THE END OF THE PERIOD, WITH THE GFS SLIGHTLY TO THE
EAST. TO ACCOUNT FOR THESE TRENDS, A BLEND OF THE FARTHER WEST
ECMWF, NAM, AND UKMET WOULD SERVE WELL AS A STARTING POINT IN THE
FORECAST PROCESS.
DEEP UPPER LEVEL TROUGH BUILDING IN ACROSS THE WESTERN U.S.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: MODERATE-HIGH
WITH THE AMPLIFYING UPPER LEVEL PATTERN ACROSS THE CONTINENTAL
U.S., AN AMPLIFIED UPPER LEVEL TROUGH IS FORECAST TO BECOME
ESTABLISHED OVER THE WEST COAST AND THEN THE INTERMOUNTAIN WEST BY
THE MIDDLE OF THE WEEK AS A BIG UPPER LOW FROM THE NORTHEAST
PACIFIC TRACKS SOUTHEASTWARD. THIS WILL ALSO BE IN RESPONSE TO A
BUILDING DOWNSTREAM RIDGE AXIS ACROSS THE PLAINS AS AN OMEGA BLOCK
PATTERN DEVELOPS. THE NCEP GUIDANCE IS A LITTLE WEAKER WITH THE
DOWNSTREAM RIDGE, AND THE NAM BECOMES SLIGHTLY STRONGER BY TUESDAY
MORNING WITH THE UPPER LOW. TOWARDS THE END OF THE FORECAST
PERIOD, THE CMC IS SLOWER IN ROTATING SHORTWAVE ENERGY FROM THE
DESERT SOUTHWEST TO THE CENTRAL ROCKIES COMPARED TO THE OTHER
GUIDANCE, AND ALSO SOUTHWEST OF THE MODEL CONSENSUS WITH THE
DEVELOPING SURFACE LOW ACROSS MONTANA BY TUESDAY NIGHT.
MODEL TRENDS AT WWW.WPC.NCEP.NOAA.GOV/HTML/MODEL2.SHTML
500 MB FORECASTS AT WWW.WPC.NCEP.NOAA.GOV/H5PREF/H5PREF.SHTML
HAMRICK
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