EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1131 AM EDT WED MAY 15 2013

VALID 12Z SAT MAY 18 2013 - 12Z WED MAY 22 2013

...SEVERE WEATHER POSSIBLE FOR THE CENTRAL PLAINS REGION THIS
WEEKEND...

...HEAVY RAIN POSSIBLE FOR THE UPPER MIDWEST NEXT WEEK...


...OVERVIEW...

THE AMPLIFIED PATTERN SEEN LAST WEEK OVER THE CONUS SHOULD RESUME
IN EARNEST BY NEXT WEEK AFTER A BRIEF RESPITE. THE EXIT OF THE
POLAR VORTEX FROM FAR NORTHERN CANADA SHOULD ALLOW RIDGING TO
BUILD OVER THE NW TERRITORIES AND NUNAVUT AS WELL AS OVER SE
CANADA. THIS SHOULD ALLOW A CLOSED LOW TO FORM OVER THE UPPER
MIDWEST AND MOVE SLOWLY EASTWARD TOWARD THE GREAT LAKES NEXT WEEK.
OVER THE PAC NW... ANOTHER VIGOROUS CLOSED LOW IS FORECAST TO DROP
SOUTHEASTWARD OUT OF THE GULF OF ALASKA AS IT SHOULD BE BLOCKED
FROM MOVING EASTWARD INTO CANADA BY THE UPPER RIDGING.


...MODEL PREFERENCES...

THE MODELS/ENSEMBLES HAVE BEEN TRENDING TOWARD THIS
DEEPER/SLOWER/AMPLIFIED SOLUTION OVER THE PAST FEW DAYS... AND
GIVEN THE AGREEMENT OF THE DETERMINISTIC RUNS RECENTLY WPC HAS
TRENDED STRONGLY TOWARD A MORE ROBUST UPPER PATTERN AND A SLOWER
SFC PATTERN INTO NEXT WEEK. THE MODELS TEND TO BE TOO QUICK WITH
CLOSED FEATURES AND TOO EAGER TO BREAK DOWN AN ONGOING PATTERN...
THUS OUR PREFERENCE TODAY FOR A 06Z GFS AND 00Z ECMWF BLEND
THROUGH TUE/D6. THIS HAS SUPPORT FROM THE 00Z UKMET AND SOMEWHAT
THE CANADIAN /THOUGH THE CANADIAN BECOMES AN EXTREME OUTLIER IN
THE WEST AS IT IS MUCH FARTHER EAST WITH A STRONG UPPER TROUGH IN
THE EAST PACIFIC WHICH BRINGS IN STRONG RIDGING TO THE WEST COAST
NEXT WEEK/. THE ENSEMBLES SHOW A TYPICAL AMOUNT OF SPREAD BUT TEND
TO WASH OUT THE UPPER LOW IN THE MIDWEST AS THERE REMAIN SEVERAL
MEMBERS THAT SHOW A PROGRESSIVE TROUGH BUT AN INCREASING AMOUNT
SHOWING A CLOSED LOW.

IN THE WEST... THE ENSEMBLES ARE IN MUCH BETTER AGREEMENT THAT
THERE WILL BE A CLOSED LOW THAT DROPS SOUTHWARD TOWARD THE WA
COAST BY WED/D7... BUT THE GEFS MEMBERS ARE GENERALLY A BIT
FARTHER NORTHEAST THAN THE EC MEMBERS. OPTED TO BELIEVE THE EC
MEMBERS OVER THE GEFS MEMBERS GIVEN THE RELATIVELY SHARPER
DOWNSTREAM PATTERN FORECAST BY THE ECENS OVER THE GEFS MEAN.

THE SFC PATTERN WILL FEATURE A SLOWLY MOVING FRONT THROUGH THE
CENTRAL PLAINS WITH A BOUNDARY EXTENDING EASTWARD THROUGH THE OH
VALLEY AND INTO NORTH CAROLINA... WHERE THAT /BACKDOOR/ FRONT MAY
HAVE A TOUGH TIME LIFTING NORTHWARD EAST OF THE APPALACHIANS. SFC
LOW PRESSURE SHOULD TRAVEL THROUGH SD INTO MN AND EVENTUALLY THE
GREAT LAKES AND ONLY DRAG THE COLD FRONT EAST TOWARD THE MS VALLEY
OVER SEVERAL DAYS. STRONG UPPER LOW IN THE PAC NW SHOULD DRIVE A
COLD FRONT INTO THE INTERIOR NW.


...SENSIBLE WEATHER HIGHLIGHTS...

SLOWLY MOVING UPPER LOW AND SFC BOUNDARY COUPLED WITH PROLONGED
SOUTHERLY FLOW OUT OF THE GULF COULD SET THE STAGE FOR A MULTI-DAY
SEVERE WEATHER AND HEAVY RAINFALL EVENT. SPC HAS HIGHLIGHTED
SAT/D3 THROUGH MON/D5 FROM THE HIGH PLAINS SOUTHEASTWARD FOR THE
THREAT OF SEVERE WEATHER. SEE THEIR PRODUCT FOR MORE INFO. NEAR
THE SFC LOW OVER SD/MN... THE ENSEMBLES CONTINUE TO SUGGEST THE
POSSIBILITY OF SEVERAL INCHES OF RAIN NEXT WEEK /SLOWER ECENS
MEMBERS MORE BULLISH WITH QPF THAN THE QUICKER GEFS MEMBERS/.
WETTEST ENSEMBLE MEMBERS FORECAST OVER FIVE INCHES OF RAIN NEAR
ND/SD/MN. EVEN THE ECENS MEAN SHOWS OVER TWO INCHES DAYS 4-7 WHICH
IS IMPRESSIVE FOR THAT LEAD TIME. HOWEVER... THIS IS CONTINGENT
UPON A SLOWLY MOVING SYSTEM WHICH IS NOT A GUARANTEE.

TEMPERATURES SHOULD CLIMB ABOVE NORMAL EAST OF THE COLD FRONT FROM
THE PLAINS TOWARD THE APPALACHIANS. BACKDOOR FRONT INITIALLY OVER
NC THIS WEEKEND MAY HAVE A TOUGH TIME PUSHING NORTHWARD /MODELS
CAN BE TOO QUICK TO WARM UP THE AREA/... SO WPC TENDED TO RELY ON
THE SLOWER GUIDANCE NEAR THE EAST COAST NORTH OF THE FRONT.
EVENTUALLY THE WARM AIR SHOULD WIN OUT AS THE COLD FRONT MAKES
PROGRESS TOWARD THE OH VALLEY BUT PERHAPS NOT UNTIL TUE OR WED
NEXT WEEK.


FRACASSO