Prognostic Meteorological Discussion
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FXUS02 KWBC 161559
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1158 AM EDT MON OCT 16 2017

VALID 12Z THU OCT 19 2017 - 12Z MON OCT 23 2017

...OVERVIEW...

A COOL AND STORMY PATTERN FOR THE NORTHERN HALF OF THE WEST WILL
GIVE WAY TO DRIER CONDITIONS EARLY NEXT WEEK AS A RIDGE ALOFT
BUILDS OVER THE WEST, WHILE EASTWARD PROGRESSION OF INITIAL
EASTERN PACIFIC/WESTERN U.S. ENERGY WILL INCREASE RAINFALL
POTENTIAL OVER THE CENTRAL U.S./GULF COAST STATES BY NEXT SUN-MON.
 D+8 MULTI-DAY MEANS SHOW GOOD AGREEMENT THAT THE OVERALL PATTERN
WILL SETTLE INTO AN AMPLIFIED EAST-CENTRAL PACIFIC TROUGH/WESTERN
U.S. RIDGE/EASTERN TROUGH BY NEXT WEEK.  THE LARGE SCALE OF THIS
EXPECTED PATTERN SHOULD TYPICALLY LEAD TO ABOVE AVERAGE FORECAST
PREDICTABILITY THOUGH AS THIS FLOW IS IN THE EARLY STAGES OF
BECOMING ESTABLISHED THERE ARE SOME MEDIUM-SMALLER SCALE DETAIL
UNCERTAINTIES OVER THE CENTRAL/SOUTHERN AREAS OF THE LOWER 48 BY
SUN-MON.


...GUIDANCE/UNCERTAINTY ASSESSMENT...

FOR DAYS 3-5 THU-SAT GUIDANCE IS IN VERY GOOD AGREEMENT FOR
INDIVIDUAL FEATURES ALOFT.  A FAIRLY VIGOROUS EASTERN PACIFIC
TROUGH WILL COME ASHORE EARLY FRI AND REACH THE ROCKIES/HIGH
PLAINS BY SAT WHILE A RIDGE CROSSES THE EAST.  ALSO EARLY IN THE
WEEKEND AN UPSTREAM MID LEVEL JET WILL BE AIMED AT WESTERN CANADA
AND THE NORTHERN PACIFIC NORTHWEST, SUPPORTING ANOTHER EPISODE OF
ENHANCED PRECIP.  FOR THIS PERIOD A BLEND OF THE 06Z GFS/00Z ECMWF
AND TO A LESSER EXTENT THEIR ENSEMBLE MEANS REPRESENTED THE
CURRENT CONSENSUS QUITE WELL.

AS TROUGHING MOVES INTO THE CENTRAL U.S. AHEAD OF THE BUILDING
WESTERN RIDGE, MODELS AND ENSEMBLES DIVERGE OVER THE EXTENT AND
LOCATION OF POTENTIAL FLOW SEPARATION DAYS 6-7 SUN-MON.  BEYOND
THE CURRENT SPREAD IN GUIDANCE THERE HAVE ALSO BEEN SIGNIFICANT
TRENDS OVER RECENT DAYS.  THE AVERAGE OF ALL GUIDANCE HAS
GENERALLY SHIFTED A BIT EASTWARD WITH THE OVERALL TROUGH.  TWO
DAYS AGO MANY ECMWF-BASED ENSEMBLE MEMBERS/MODEL RUNS WANTED TO
PULL OFF ENERGY OVER OR JUST WEST OF THE SOUTHERN ROCKIES AS OF
SUN WHILE THE GEFS/GFS RUNS WERE FLATTER AND PROGRESSIVE.  THE TWO
GUIDANCE SYSTEMS TRENDED TOWARD EACH OTHER YESTERDAY AND NOW
SOMEWHAT MORE ECMWF MEMBERS OUTPACE GEFS MEMBERS.  HOWEVER
DIFFERENCES ARE MORE PRONOUNCED IN THE 00Z-06Z OPERATIONAL MODELS
WITH THE 00Z ECMWF STRAYING AHEAD OF ITS MEAN AND THE 06Z GFS
CLOSING OFF ITS UPPER LOW IN THE SOUTHWESTERN PART OF THE GEFS
MEAN TROUGH.  TELECONNECTIONS RELATIVE TO UPSTREAM FLOW APPEAR TO
SUPPORT THE ENSEMBLE MEANS THAT ARE MORE MODEST IN THEIR FLOW
SEPARATION.  THIS CONSIDERATION ALONG WITH THE DESIRE TO MAINTAIN
BETTER CONTINUITY GIVEN CURRENT MODEL/ENSEMBLE SPREAD, LED TO
PREFERRING INCREASED WEIGHT ON THE 06Z GEFS/00Z ECMWF MEANS
ESPECIALLY FROM LATE SUN ONWARD.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

THE NORTHWEST U.S. AND NORTHERN CA WILL SEE A RATHER STORMY/WET
PERIOD IN THE LATE WEEK-WEEKEND TIME FRAME.  EXPECT HIGHEST PRECIP
TOTALS FOR THE PERIOD OVER FAVORED TERRAIN ALONG THE WA/OR COAST
AND WINDWARD FACING SLOPES OF THE CASCADES.  THERE SHOULD BE
ANOTHER RELATIVE MAXIMUM OVER THE NORTHERN ROCKIES.  SYNOPTIC
SCALE ASCENT IN THE VICINITY OF THE UPPER JET, A SERIES OF
EMBEDDED SHORTWAVES TAPPING AVAILABLE PACIFIC MOISTURE, AND
TOPOGRAPHIC LIFT WILL COMBINE TO BRING HEAVY/INTENSE PRECIP AT
TIMES TO AFOREMENTIONED FAVORED AREAS.  INCOMING COLDER AIR, WITH
A DAY OR SO OF HIGHS AT LEAST 10F BELOW NORMAL AT SOME LOCATIONS,
WILL SUPPORT LOWERING SNOW LEVELS OVER MOUNTAIN AREAS.  THE UPPER
RIDGE BUILDING OVER THE WEST LATE IN THE PERIOD WILL LEAD TO A
LIGHTER/NORTHWARD TREND FOR PRECIP THOUGH SOME ACTIVITY COULD
LINGER OVER EXTREME NORTHERN LOCATIONS, ALONG WITH WARMER TEMPS.

INCREASING MOISTURE RETURN AHEAD OF THE COLD FRONT REACHING THE
PLAINS BY SAT WILL LEAD TO AN EXPANDING AREA OF LOCALLY HEAVY
RAINFALL MOST LIKELY OVER THE CENTRAL-SOUTHERN PLAINS DURING THE
WEEKEND.  SOME ENHANCED PRECIP MAY EXTEND FARTHER NORTH ALONG THE
FRONT AS WELL.  BY NEXT MON A FAIRLY LARGE AREA ENCOMPASSING THE
SOUTHEAST PLAINS TO SOUTHEAST HAS POTENTIAL FOR SOME HEAVY
RAINFALL BUT WITH LOW CONFIDENCE IN SUCH ACTIVITY AT ANY
PARTICULAR LOCATION DUE TO WIDENING SPREAD FOR FLOW ALOFT AND
CORRESPONDING SURFACE PATTERN.  EARLIER IN THE PERIOD, LOCATIONS
ALONG THE WESTERN GULF COAST MAY SEE AREAS OF RAIN WITH AN
INVERTED SURFACE TROUGH.

MUCH OF THE EAST SHOULD SEE DRY WEATHER FOR MOST OF THE PERIOD
WITH HIGH PRESSURE DOMINATING.  TO THE SOUTH OF THIS HIGH, MOIST
EASTERLY FLOW MAY BRING PERIODS OF RAIN TO THE FL PENINSULA.

WITH AN UPPER RIDGE CROSSING THE EAST AND WARM FLOW AHEAD OF THE
FRONT INITIALLY MOVING INTO THE NORTHWEST, THERE WILL BE A BROAD
AREA OF ABOVE TO MUCH ABOVE NORMAL TEMPS OVER THE LOWER 48 AS OF
THU WITH STEADY EROSION FROM THE WEST AS THE COLD FRONT PROGRESSES
EASTWARD.  AS FOR GREATEST ANOMALIES, THE NORTHERN PLAINS MAY SEE
HIGHS OF AT LEAST 15-20F ABOVE NORMAL THU-FRI WHILE PLUS 10-20F
ANOMALIES EXTEND ACROSS THE UPPER MS VALLEY INTO THE GREAT
LAKES/NORTHEAST FOR MULTIPLE DAYS.  MANY AREAS WILL SEE PLUS
10-20F ANOMALIES FOR MIN TEMPS AS WELL WITH DEPARTURES FROM NORMAL
EXCEEDING PLUS 20F MOST LIKELY OVER THE UPPER MIDWEST ON SAT.

RAUSCH

$$





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