Preliminary Forecasts
Issued by NWS

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000
FXUS02 KWNH 200650
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
249 AM EDT FRI OCT 20 2017

VALID 12Z MON OCT 23 2017 - 12Z FRI OCT 27 2017

...OVERVIEW...

GUIDANCE AGREES WELL ON A MEAN PATTERN FEATURING AN AMPLIFIED
EAST-CENTRAL U.S. TROUGH BETWEEN STRONG RIDGES OVER EASTERN
PACIFIC/WEST COAST AS WELL AS WEST-CENTRAL ATLANTIC.  HOWEVER
THERE ARE MEANINGFUL DIFFERENCES FOR MULTIPLE EMBEDDED FEATURES
WITHIN THIS FLOW.  FROM THE SENSIBLE WEATHER PERSPECTIVE THE
PRIMARY THEMES WILL BE VERY WARM TEMPS OVER THE WEST AND THE
SOUTHERN HALF OF THE REGION IN PARTICULAR, AND FARTHER EAST A
PERIOD OF HEAVY RAIN POTENTIAL FOLLOWED BY A PRONOUNCED COOLING
TREND.


...GUIDANCE EVALUATION/PREFERENCES...

AT THE START OF THE PERIOD THERE IS ALREADY SIGNIFICANT SPREAD
WITH MS VALLEY TROUGH ENERGY THAT MAY CLOSE OFF AN UPPER LOW OVER
THE SOUTHERN HALF OF THE REGION.  ASIDE FROM THE 00Z CMC THAT HAS
A PROGRESSIVE TROUGH, THERE IS AT LEAST CONSENSUS ON FLOW
SEPARATION AS OF EARLY DAY 3 MON BUT THEN WIDENING SPREAD FOR HOW
QUICKLY THIS FEATURE GETS ABSORBED BY RAPIDLY AMPLIFYING NORTHERN
STREAM FLOW.  WHAT MODEL TRENDS EXIST SEEM TO BE LEANING FASTER,
ESPECIALLY IN THE CASE OF THE 12Z ECMWF WHICH OFFERS A FAST
EXTREME.  EVEN IF NOT FROM THE SOUTHERN U.S. WAVE AS IN THE ECMWF
EVOLUTION, THERE IS A GOOD ENSEMBLE MEAN AGREEMENT TOWARD DEEPEST
LOW PRESSURE EVOLVING OVER AND JUST NORTHEAST OF THE GREAT LAKES
BY DAY 4 TUE IN RESPONSE TO THE NORTHERN STREAM DYNAMICS.  THE 00Z
GFS HAS TRENDED A BIT FASTER FROM ITS PREVIOUS COUPLE RUNS WHILE
THE UKMET CONTINUES TO BE ONE OF THE SLOWER SOLUTIONS.  OVERALL
THE GUIDANCE HAS BEEN SUFFICIENTLY INCONSISTENT OVER RECENT DAYS
TO HAVE FAIRLY LOW CONFIDENCE IN ANY PARTICULAR SCENARIO THUS FAR,
FAVORING A MODEL/ENSEMBLE MEAN COMPROMISE FOR THIS ASPECT OF THE
FORECAST.  AS THE LARGER SCALE TROUGH REACHES THE EAST COAST THE
12Z-18Z GEFS/12Z ECMWF MEANS ARE SIMILAR IN SUGGESTING A SURFACE
WAVE TRACKING FROM THE WESTERN ATLANTIC INTO NEW ENGLAND LATE IN
THE WEEK WHILE INDIVIDUAL MODEL RUNS VARY CONSIDERABLY FOR
STRENGTH/TIMING.  THE MEANS PROVIDE THE MOST APPEALING
LATER-PERIOD OPTION WHILE WAITING FOR BETTER AGREEMENT IN THE
MODELS.

THE NEXT AREA OF UNCERTAINTY INVOLVES FLOW SEPARATION LIKELY TO
OCCUR OVER THE PACIFIC SOUTH OF ALASKA AROUND LATE MON-TUE WITH
THE NORTHERN PORTION OF THE TROUGH ENERGY QUICKLY HEADING INTO
NORTH AMERICA TO REINFORCE THE MEAN TROUGH.  THE 00Z ECMWF IS
SOMEWHAT TOWARD THE AGGRESSIVE SIDE OF THE ENSEMBLE SPREAD IN HOW
QUICKLY IT BRINGS IN THE HEIGHT FALLS.  HOWEVER MULTIPLE
ECMWF/ECMWF MEAN RUNS HAVE BEEN DEPICTING THIS FEATURE AND THE
GFS/GEFS MEAN HAVE MADE A NOTICEABLE TREND IN THAT DIRECTION OVER
THE PAST 12 HOURS OF RUNS, SO CONFIDENCE IS NOW INCREASING IN THE
IDEA OF A STRONGER SHORTWAVE BY WED ONWARD.

THE NEXT FEATURE UPSTREAM THAT REQUIRES MONITORING IS THE
COMBINATION OF TYPHOON LAN AND NORTHERN STREAM ENERGY LEADING TO A
VERY DEEP SYSTEM REACHING THE BERING SEA BY AROUND MIDWEEK.
GUIDANCE APPEARS TO SHOW LESS SPREAD THAN HAD BEEN THE CASE OVER
RECENT DAYS BUT TYPICAL DIFFICULTY THAT MODELS CAN HAVE IN
PROPERLY DEPICTING TROPICAL-EXTRATROPICAL TRANSITIONS OFFERS
POTENTIAL FOR DOWNSTREAM ADJUSTMENTS NOT YET APPARENT.

THE UPDATED FORECAST INCORPORATES VARIOUS WEIGHTS OF THE 12Z-18Z
GFS/12Z ECMWF AND 18Z GEFS/12Z ECMWF MEANS WITH SLIGHTLY MORE
WEIGHT GIVEN TO THE ECMWF/ECMWF MEAN VERSUS GFS/GEFS MEAN,
ESPECIALLY WITH CONTINUITY/TRENDS FOR THE NORTHERN PACIFIC
SHORTWAVE MID-LATE PERIOD.  THE BLEND INCREASES EMPHASIS OF THE
ENSEMBLE MEANS LATE IN THE PERIOD GIVEN THEIR RELATIVE AGREEMENT
VERSUS THE SPREAD OF OPERATIONAL RUNS.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

MULTIPLE AREAS OF HEAVY RAINFALL ARE POSSIBLE TO THE EAST OF THE
MS RIVER AS A WAVY FRONT GRADUALLY PROGRESSES ACROSS THE EASTERN
STATES.  FLOW AHEAD OF AN INITIAL MS VALLEY UPPER TROUGH/LOW
SHOULD SUPPORT AN AXIS OF HEAVIEST ACTIVITY FROM THE EAST-CENTRAL
GULF COAST NORTHWARD EARLY IN THE WEEK WITH ATLANTIC INFLOW
EVENTUALLY SUPPORTING ENHANCED AMOUNTS OVER THE NORTHEAST MID-LATE
WEEK AS THE FRONT CONTINUES ONWARD.  DETAILS/TIMING OF INDIVIDUAL
FEATURES ALOFT ARE STILL UNRESOLVED, TEMPERING CONFIDENCE IN
SPECIFICS OF EXACTLY WHERE/WHEN HEAVIEST RAINFALL WILL OCCUR.
TOWARD FRI ANOTHER AREA OF PRECIP MAY DEVELOP WITH ANOTHER FRONT
REACHING THE EAST-CENTRAL U.S..  MEANWHILE AN AREA OF MOSTLY LIGHT
PRECIP SHOULD EXTEND FROM THE NORTHWEST INTO THE CENTRAL
ROCKIES/HIGH PLAINS AROUND WED-THU.

THE AMPLIFIED PATTERN ALOFT WILL LEAD TO SOME SIGNIFICANT
TEMPERATURE ANOMALIES.  LOCATIONS OVER CALIFORNIA AND THE
SOUTHWEST/GREAT BASIN MAY SEE ONE OR MORE DAYS WITH HIGHS OF
10-20F ABOVE NORMAL WITH A FEW DAILY RECORDS BEING CHALLENGED.
THIS HEAT ALONG WITH DRY/GUSTY WINDS ASSOCIATED WITH OFFSHORE
GRADIENTS WILL LIKELY ENHANCE WILDFIRE DANGER ONCE AGAIN.  A VERY
BRIEF EPISODE OF WARMTH MAY OCCUR OVER THE NORTHERN-CENTRAL PLAINS
AROUND MIDWEEK.  THE EASTERN U.S. WILL ALSO BE QUITE WARM AT THE
START OF THE WEEK BEFORE FRONTAL PASSAGE, BUT THEN TREND BELOW
NORMAL WITH A DAY OR TWO OF MINUS 5-15F ANOMALIES BY WED-THU.

RAUSCH

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