Preliminary Forecasts
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FXUS02 KWNH 130659
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM EDT SUN JUL 13 2014

VALID 12Z WED JUL 16 2014 - 12Z SUN JUL 20 2014

...OVERVIEW AND MODEL PREFERENCES...
THE MEDIUM RANGE FORECAST REQUIRED VERY LITTLE CHANGE EAST OF THE
DIVIDE THROUGH DAY 5 UTILIZING THE 12/12Z ECENS/NAEFS MEANS AND
ASPECTS OF THE DETERMINISTIC ECMWF. THE 12/12Z GEFS COULD BE
INCLUDED IN THE MIX...BUT OPTED TO USE THE PREVIOUS WPC GUIDANCE
IN PLACE OF THE GEFS TO COMPILE THE WPC 500MB AND SURFACE
GRAPHICS. THIS ALLOWS THE CANADIAN COLD FRONT AND ITS LOW-LEVEL
DRY AIRMASS TO REACH THE WESTERN ATLANTIC AND SOUTHERN TIER
STATES...AND MODIFY AFTER DAY 3 INTO A STATIONARY FRONT FOR DAY 4
INTO DAY 7. THE MEDIUM RANGE PROGS THEN UTILIZE A NAEFS-ECENS MEAN
APPROACH AT THE 500MB LEVEL TO CAPITALIZE ON THE WEAKNESS IN
PACIFIC NORTHWEST-NORTHERN DIVIDE PORTION OF THE UPPER-LEVEL RIDGE
AND CREATE A SECOND OPPORTUNITY FOR ENERGY IN THE CANADIAN ROCKIES
TO MIGRATE DOWNWIND INTO THE NORTHERN PLAINS.

FROM A PURELY DETERMINISTIC PERSPECTIVE...SOMETHING CLOSE TO THE
12/12Z ECMWF AND 12/18Z GFS AT 18/06Z GIVES THIS GENERAL
IMPRESSION...AND DEPICTS A MID-LEVEL DISTURBANCE INVOF THE LOWER
MS VALLEY THAT WILL EVENTUALLY TRACK EASTWARD ACROSS THE
APPALACHIANS AROUND THE 19/00Z-19/12Z TIME STEPS (LATE DAY 5
THROUGH MUCH OF DAY 6). FROM THIS ILLUSTRATION...IT MIGHT BECOME
CLEARER AS TO WHY I DIDN`T THINK THE FORECAST EAST OF THE DIVIDE
NEEDED MUCH CHANGE THROUGH DAY 5. BUT ALSO WHY DAY 6-7 BACKED OFF
USING ANY DETERMINISTIC 12/12Z ECMWF GUIDANCE.

...UNCERTAINTY ASSESSMENT...
THE 12/12Z DETERMINISTIC GUIDANCE HAS NOT WAVERED MUCH WITH THIS
COOL...CANADIAN AIRMASS DESCENT INTO THE LOWER 48. AND WHAT
DIFFERENCES THERE WERE...SEEMED TO BE TIED TO THE PRECIPITATION
DISTRIBUTIONS ALONG AND AHEAD OF THE FRONT ON DAY 3 ACROSS THE
NORTHEAST COAST LINE---AND IN GENERALLY-SPEAKING TERMS---THE DAY 4
THROUGH DAY 6 PERIOD ALONG THE SOUTHERN EDGE OF THE AIRMASS INVOF
THE I-10/I-20 CORRIDORS.

THIS UNCERTAINTY IS TIED TO THE ESTABLISHMENT OF WEST TO NORTHWEST
MID-LEVEL FLOW  IMPINGING UPON THE BOUNDARY...AND GRADUALLY
DEVELOPING A SERIES OF SURFACE WAVES ALONG THE STATIONARY BOUNDARY
`DRAPE` INVOF THE TX/OK PANHANDLE-NORTH TEXAS REGION...EASTWARD
ACROSS THE SOUTHERN OZARKS TO THE OUTER BANKS/DELMARVA REGION. THE
FIRST APPRECIABLE WAVE ENERGY TO INTERACT WITH (IMPINGE ON) THE
BOUNDARY IS THE MID-LEVEL WAVE EXITING MT/WYO PORTION OF THE
DIVIDE ON DAY 3...AND MIGRATING INTO NERN CO/NWRN KS ON DAY 4. BY
17/00Z...THE 12/12Z DETERMINISTIC CANADIAN...GFS AND ECMWF
DIVERGE...AS THE SHEAR AND DYNAMICAL LIFT BETWEEN 700MB-500MB
QUICKLY RACES EAST-SOUTHEASTWARD WHILE THE FRONT CONTINUES TO
SPREAD SOUTH-SOUTHWESTWARD INTO CENTRAL TEXAS. WHAT THEY DO AGREE
UPON IS THAT SOME OF THE LIFT CONTINUES INTO SOUTHERN
ARKANSAS...AND CLINGS TO THE BASE OF THE DOWNSTREAM TROUGH
MIGRATING THROUGH THE LOWER THIRD OF THE MS VALLEY ON THURSDAY
MORNING (MID-POINT DAY 4).

DAY 5 AND DAY 6...THE EAST CENTRAL STATES SHOULD BE UNSETTLED AS
MID-LEVEL ENERGY INTERACTS WITH THE LOW-LEVEL BOUNDARY AND THE
BULK OF THE UPPER-LEVEL TROUGH LOOKS TO BE LIFTING OUT INTO
ATLANTIC CANADA.

UNCERTAINTY IN THE WEST BEYOND DAY 5 DID NOT LOOK AS CHAOTIC THIS
12/12Z CYCLE VERSUS THE 11/12Z CYCLE GIVEN THE GFS DID BACK OFF
ITS SOLUTION TO DROP A SIZABLE AMOUNT OF ENERGY TO 40N LATITUDE
RATHER THAN CONTINUE SENDING THE ENERGY DOWNSTREAM...AND ALLOW THE
LONGWAVE PATTERN OVER CANADA TO SHIFT AWAY FROM THE HUDSON/ST
JAMES AREA. NOT THAT THE LONGWAVE WILL ANCHOR ALONG
100W-105W...BUT LIKE YESTERDAY...IT DOES SERVE TO BRIEFLY DISPLACE
THE "UNSEASONABLE COOLNESS" OUT AND AWAY FROM THE GREAT LAKES AND
NORTHERN PLAINS BY THE END OF THIS MEDIUM RANGE PERIOD.

...SENSIBLE WEATHER HIGHLIGHTS...
HEAVY RAINFALL THREAT WILL BE ON THE INCREASE FROM THE COLORADO
FRONT RANGE AND SOUTHERN ROCKIES/SOUTHERN HIGH
PLAINS---EAST-SOUTHEASTWARD INTO THE SOUTHERN PLAINS AND
SOUTHEAST/PIEDMONT---ALONG THE LEADING EDGE OF THE MODIFIED
CANADIAN COLD FRONT.

THE COOL AND DRY CANADIAN AIRMASS DOES MODIFY...AND TEMPERATURES
DO SLOWLY MODIFY BACK TO MORE SEASONAL VALUES IN THE
PLAINS...MIDWEST AND SOUTHERN STATES DURING THIS MEDIUM RANGE
PERIOD...ONCE THE RAINS DIMINISH IN INTENSITY AND AREAL COVERAGE.

VOJTESAK

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