THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


EXTENDED FORECAST DISCUSSION




EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
151 AM EST TUE JAN 17 2017

VALID 12Z FRI JAN 20 2017 - 12Z TUE JAN 24 2017

...OVERVIEW...

LATEST MODEL AND ENSEMBLE RUNS ARE CONSISTENT WITH THE IDEA OF A
VERY STRONG CORE OF POSITIVE HGT ANOMALIES NEAR THE ERN SHORE OF
HUDSON BAY... LIKELY SUPPORTING BELOW AVG HGTS ALOFT OVER MOST OF
THE LOWER 48 ASIDE FROM THE NRN PLAINS TO NORTHEAST.  MULTI-DAY
MEANS BY D+8 DIFFER THE MOST NEAR THE WEST COAST WHERE
TELECONNECTIONS RELATIVE TO THE POSITIVE HGT ANOMALY CENTER WOULD
FAVOR SOLNS THAT HAVE BELOW NORMAL HGTS.  THE RELATIVELY LOW MEAN
HGTS OVER MOST OF THE LOWER 48 WILL BE REFLECTED IN THE DAILY
FCSTS AS A SERIES OF SYSTEMS BRINGING HVY RAIN/STRONG WINDS TO THE
WEST COAST WITH ACTIVE WEATHER ALSO EXTENDING WELL EWD.  FOR THE
MOST PART LATEST SOLNS IDENTIFY INDIVIDUAL SYSTEMS FAIRLY WELL
WITH DECENT CONTINUITY BUT THERE ARE SOME DETAIL UNCERTAINTIES
REQUIRING MORE TIME TO BE RESOLVED.


...GUIDANCE EVALUATION/PREFERENCES...

THE DOMINANT FEATURE AFFECTING THE LOWER 48 DURING THE PERIOD
STARTS NEAR THE WEST COAST EARLY DAY 3 FRI WITH A VERY STRONG LOW
SFC/ALOFT OFF THE PAC NW COAST AND FRONTAL SYSTEM EXTENDING WELL
SWD.  THE 12Z UKMET WAS THE ODD MODEL OUT IN TRACKING THE SFC LOW
SEWD INSTEAD OF KEEPING IT OFFSHORE.  THE NEW 00Z RUN HAS JOINED
THE MAJORITY CLUSTER.  ENERGY IN THE SRN PART OF THE UPR TROUGH
WILL STREAM ACROSS NRN MEXICO AND LIKELY SUPPORT A CLOSED LOW OVER
THE SRN HALF OF THE PLAINS BY EARLY DAY 5 SUN.  THE OVERALL SYSTEM
SHOULD THEN TRACK NEWD.  ONCE E OF THE PLAINS THE ENSEMBLE SPREAD
FOR SFC LOW TRACKS HAS NARROWED FROM 24 HRS AGO WITH A SIMILAR
CENTROID... SO CONSENSUS FCST MAINTAINS GOOD CONTINUITY.  THE 18Z
GEFS MEAN WAS SOMEWHAT SLOWER/SWD THAN MOST OTHER SOLNS. 
REVIEWING TIME PERIODS CENTERED ON COMPOSITE ANALOG DATES THAT
WERE CLOSEST TO THE FCST D+8 PATTERN REVEALED A JAN 19-21,1995
SYSTEM THAT HAD A DIFFERENT ORIGIN BUT DURING THAT WINDOW FROM THE
MS VLY INTO THE ERN STATES HAD THE COMMON TRAITS OF ERN CANADA
RIDGING AND A TRACK SIMILAR TO CURRENT CONSENSUS.  BASED ON THAT
SYSTEM IT WOULD APPEAR REASONABLE THAT THE CURRENT SYSTEM MAY
REACH AT LEAST AS FAR NWD AS THE OH VLY/NRN MID ATLC AND PSBLY SRN
NEW ENGLAND.

THE NEXT STRONG ERN PAC UPR TROUGH SHOULD APPROACH THE WEST COAST
BY SUN AND THEN CONTINUE GRADUALLY EWD INTO NEXT WEEK.  RECENT GFS
RUNS HAVE BEEN DIFFERING FROM CONSENSUS FOR THIS SYSTEM... IN
VARYING WAYS DEPICTING FASTER PROGRESSION OF LEADING HGT FALLS. 
IN THE NEW 00Z GFS THIS TRAIT LEADS TO TRACKING A SFC WAVE THROUGH
THE NWRN STATES ON SUN INSTEAD OF HAVING A CONSOLIDATED AND DEEPER
SYSTEM OFFSHORE AT THAT TIME.  ECMWF/CMC RUNS AND THE ENSEMBLE
MEANS AGREE IN PRINCIPLE THROUGH DAY 7 TUE AS THE UPR TROUGH
REACHES THE INTERIOR WEST.

BASED ON GUIDANCE COMPARISONS THE 12Z GEFS/ECMWF MEANS AND
DEPENDING ON THE TIME FRAME PAST 1-2 ECMWF RUNS WERE INCLUDED IN
THE UPDATED FCST THROUGH THE PERIOD.  THE 18Z GFS COMPARED
ACCEPTABLY TO CONSENSUS FOR ABOUT THE FIRST HALF OF THE PERIOD SO
THAT PART OF THE FCST IS CLOSER TO AN 18Z GFS/12Z ECMWF COMPROMISE
VS THE LATTER HALF THAT HAS MORE ENSEMBLE MEAN AND NO GFS INPUT.


...WEATHER HIGHLIGHTS/THREATS...

DETAILS WILL STILL BE SUBJECT TO CHANGE GIVEN THE TIME FRAME...
BUT CONFIDENCE REMAINS ABOVE AVG FOR AT LEAST A COUPLE EPISODES OF
SIGNIFICANT RNFL/HIGH ELEV SNOW AND STRONG WINDS ALONG THE WEST
COAST WITH THIS ENHANCED ACTIVITY EXTENDING INTO THE SIERRA NEVADA
RANGE AND SOUTHWEST.  CONTINUE TO EXPECT AREAL AVERAGE AMOUNTS TO
EXCEED 5 INCHES ALONG SOME COASTAL LOCATIONS AND OVER THE SIERRA
NEVADA WITH HIGHEST TOTALS ALONG WINDWARD FACING SLOPES.  FOR SRN
CA/AZ THE HEAVIEST PRECIP SHOULD OCCUR AROUND FRI-FRI NIGHT AND
AGAIN AROUND SUN-MON.  MUCH OF THE REMAINDER OF THE WEST SHOULD
SEE LESS INTENSE RAIN AND INLAND/HIGHER ELEV SNOW.  THE UNSETTLED
WEATHER WILL TEND TO KEEP TEMPERATURE RANGES MORE NARROW THAN
USUAL WITH BELOW NORMAL HIGHS... ON A LOCALIZED BASIS BY GREATER
THAN 10F... AND NEAR TO ABOVE NORMAL LOWS. 

PORTIONS OF THE EAST/MIDWEST WILL SEE SOME RAIN ON FRI FOLLOWED BY
A BRIEFLY DRIER PERIOD.  LOCALLY MDT/HVY RNFL MAY DEVELOP NEAR THE
GULF COAST AROUND FRI NIGHT-SAT ALONG A LINGERING FRONT.  AS
ENERGY FROM THE INITIAL WEST COAST STORM CROSSES THE
PLAINS/SOUTHEAST... MORE HVY RNFL MAY DEVELOP AND SPC IS
MONITORING THE POTENTIAL FOR SEVERE CONVECTION NEAR THE ERN HALF
OF THE GULF COAST DURING THE WEEKEND.  CONSULT LATEST SPC PRODUCTS
FOR FURTHER INFO.  THE PLAINS TO OH VLY/MID ATLC SYSTEM WILL
SPREAD A BROAD AREA OF PCPN MOSTLY IN THE FORM OF RAIN ACROSS THE
ERN HALF OF THE COUNTRY.  THERE IS STILL A FAIRLY GOOD SIGNAL FOR
A PERIOD OF STRONG LOW LVL INFLOW FROM THE ATLC THAT WOULD ENHANCE
RNFL FOR A TIME ALONG THE EAST COAST/ERN SLOPES OF THE APLCHNS. 
IT STILL APPEARS THAT ANY SNOW THAT DOES OCCUR WOULD BE LIMITED TO
NEAR THE UPR LOW AND AT THE NRN FRINGE OF THE MSTR SHIELD IF IT
REACHES CLOSE TO THE CANADIAN BORDER.  WITHIN A PATTERN OF ABOVE
TO MUCH ABOVE NORMAL TEMPS OVER MOST AREAS E OF THE ROCKIES... THE
UPR MS VLY CONTINUES TO BE THE EXPECTED FOCUS FOR HIGHEST
ANOMALIES... AVERAGING PLUS 25-35F FOR MINS OVER THE 5-DAY PERIOD.
 THERE MAY BE MULTIPLE DAYS WITH MORNING LOWS NEAR OR ABOVE RECORD
WARM VALUES.

RAUSCH





Last Updated: 151 AM EST TUE JAN 17 2017