Prognostic Meteorological Discussion
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FXUS02 KWBC 161559
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST TUE JAN 16 2018

VALID 12Z FRI JAN 19 2018 - 12Z TUE JAN 23 2018

...OVERVIEW...

DURING THE COURSE OF THE PERIOD THE LONGWAVE PATTERN OVER THE
LOWER 48 WILL SETTLE INTO BROAD MEAN TROUGH WHOSE AXIS WILL TEND
TO BE ALIGNED OVER THE WEST-CENTRAL STATES.  THIS PATTERN WILL
FAVOR PERIODS OF ACTIVE WEATHER OVER THE WEST-- WITH HEAVY PRECIP
POTENTIAL OVER THE CENTRAL/NORTHERN WEST COAST IN PARTICULAR--
WHILE AN EMBEDDED WESTERN SYSTEM LATE THIS WEEK WILL EMERGE OVER
THE PLAINS THIS WEEKEND AND TRACK NORTHEASTWARD.  THIS SYSTEM WILL
BRING THE POSSIBILITY OF A BAND OF SIGNIFICANT SNOWFALL AND WIND
TO SOME LOCATIONS FROM THE CENTRAL PLAINS THROUGH UPPER
MIDWEST/UPPER GREAT LAKES DURING THE WEEKEND INTO EARLY NEXT WEEK.


...GUIDANCE ASSESSMENT/UNCERTAINTIES/PREFERENCES...

BROADLY SPEAKING THE 00Z GFS/ECMWF, THEIR ENSEMBLE MEANS PLUS THE
06Z GEFS MEAN, AS WELL AS THE 00Z CMC/UKMET, REPRESENT THE PATTERN
FAIRLY WELL.  ONE AREA OF RELATIVE CONTENTION IS WITH FLOW OVER
THE NORTHEAST U.S./EASTERN CANADA EARLY-MID PERIOD, WITH LOWER
HEIGHTS ALOFT IN THE ECMWF-BASED GUIDANCE BRINGING A SURFACE
FARTHER SOUTHEAST INTO NEW ENGLAND THAN A MAJORITY OF OTHER
SOLUTIONS.  A COMPROMISE APPROACH LOOKS BEST WITH THIS ASPECT OF
THE FORECAST.  ELSEWHERE THE 06Z GFS APPEARS TO BE A LOWER
PROBABILITY/FAST SCENARIO RELATIVE TO CONSENSUS.  THIS INCLUDES
THE FORECAST OF THE SHORTWAVE TRACKING OUT OF THE RIO GRANDE
VALLEY, THE WESTERN U.S. THROUGH GREAT LAKES SYSTEM, AND EASTERN
PACIFIC/WESTERN U.S. FLOW LATE IN THE PERIOD.  AS A RESULT THE
UPDATED FORECAST EMPHASIZED A 00Z OPERATIONAL MODEL BLEND DAYS 3-5
FRI-SUN WHILE ADDING SOME 06Z GEFS/00Z ECMWF ENSEMBLE MEAN
GUIDANCE BUT MAINTAINING SOME 00Z GFS/ECMWF/CMC INPUT BY DAYS 6-7
MON-TUE.

TAKING A CLOSER LOOK AT THE WEST COAST THROUGH GREAT LAKES/EASTERN
CANADA SYSTEM, ASIDE FROM THE AFOREMENTIONED FAST 06Z GFS THERE
HAS BEEN SOMEWHAT BETTER THAN AVERAGE AGREEMENT/STABILITY GIVEN
THE TIME FRAME INVOLVED.  DETAIL DIFFERENCES/ADJUSTMENTS ALOFT
BECOME NOTICEABLE OVER THE PLAINS/MS VALLEY BY SUN-MON, LEADING TO
UNCERTAINTY OVER EXACTLY HOW STRONG THE THE SYSTEM WILL BE.  MOST
RECENT OPERATIONAL RUNS HAVE BEEN SHOWING A FAIRLY STRONG SYSTEM,
WHICH AT LEAST ARGUES FOR LEANING AWAY FROM GEFS MEANS WHICH IN
SOME RUNS HAVE BEEN CONSIDERABLY WEAKER THAN THE ECMWF MEAN.
ENSEMBLE LOW CLUSTERING IN THE 00Z CYCLE HAS IMPROVED COMPARED TO
24 HOURS AGO WITH, WITH THE GREATEST DENSITY OF SURFACE LOWS OVER
KS/NORTHWEST OK AS OF 12Z SUN AND IA INTO THE UPPER GREAT LAKES BY
12Z MON.  THE PREFERRED GUIDANCE BLEND YIELDS A SOLUTION WELL
WITHIN THE SPREAD AND CLOSE TO OR A FRACTION SLOWER THAN
CONTINUITY.  REGARDLESS OF HOW WELL GUIDANCE APPEARS TO BE
CLUSTERED, IT MAY STILL TAKE UNTIL MUCH CLOSER TO THE VALID TIME
FOR IMPORTANT DETAILS OF THIS STORM TO BE RESOLVED.

AN UPSTREAM SYSTEM WILL TRACK TOWARD THE BC COAST/VANCOUVER ISLAND
AROUND SUN-MON WITH SHORTWAVE ENERGY TO THE SOUTH AND WEST PUSHING
INTO THE WEST EARLY NEXT WEEK.  ENSEMBLES SHOW A FAIR AMOUNT OF
SPREAD BUT THE 00Z GFS/ECMWF/CMC ALL FIT INTO THE ENSEMBLE MEAN
PATTERN SO SOME OF THEIR SOLUTIONS COULD BE MAINTAINED THROUGH THE
END OF THE PERIOD.


...WEATHER HIGHLIGHTS/THREATS...

EXPECT THE HEAVIEST PRECIP DURING THE PERIOD TO OCCUR FROM THE
PACIFIC NORTHWEST INTO NORTHERN CA WITH FAVORED TERRAIN POSSIBLY
SEEING SEVERAL INCHES LIQUID IN LOWER ELEVATION RAIN/MOUNTAIN
SNOW.  SOME OF THIS MOISTURE WILL EXTEND INTO THE WEST WITH THE
NORTHERN-CENTRAL ROCKIES SEEING A RELATIVE MAXIMUM OF PRECIP
ALBEIT WITH LOWER ABSOLUTE TOTALS THAN SOME LOCATIONS ALONG THE
WEST COAST.  LEADING SYSTEM EMERGING FROM THE WEST AND THEN
TRACKING NORTHEAST AS IT STRENGTHENS WILL BRING POTENTIAL FOR A
BAND OF MODERATE/HEAVY SNOW FROM THE CENTRAL PLAINS THROUGH UPPER
GREAT LAKES.  IF THE STORM AND ASSOCIATED WINDS BECOME
SUFFICIENTLY STRONG THERE MAY BLOWING/DRIFTING OF SNOW POSSIBLY TO
THE EXTENT OF BLIZZARD CONDITIONS.  WARM SECTOR RAINFALL OVER THE
EASTERN HALF OF THE COUNTRY SHOULD BE MOSTLY ON THE LIGHT-MODERATE
SIDE BUT WITH ISOLATED POCKETS OF HEAVIER ACTIVITY POSSIBLE.  THE
UPPER SHORTWAVE TRACKING OUT OF THE RIO GRANDE VALLEY MAY SUPPORT
SOME CONVECTION ALONG THE TX COAST LATE THIS WEEK.

THE MOST PRONOUNCED ANOMALIES FOR TEMPS WILL BE IN THE WARM SECTOR
OF THE WESTERN U.S. THROUGH GREAT LAKES SYSTEM.  LOW TEMPS IN
PARTICULAR MAY BE 15-25F ABOVE NORMAL OVER THE NORTH CENTRAL
ROCKIES/NORTHERN PLAINS FRI WITH THIS AXIS OF WARMTH PROGRESSING
EAST/SOUTHEAST WITH TIME, REACHING THE EAST COAST STATES BY NEXT
MON.  HIGH TEMPS WILL A BIT LESS EXTREME BUT STILL 10-20F ABOVE
NORMAL OVER SOME AREAS.  THE SOUTHEAST WILL START OUT QUITE CHILLY
ON FRI, WHILE COOL AIR WILL SPREAD ACROSS MUCH OF THE WEST AND TO
SOME EXTENT THE NORTHERN PLAINS.

RAUSCH

$$





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