Prognostic Meteorological Discussion
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333
FXUS02 KWBC 210554
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1254 AM EST SUN JAN 21 2018

VALID 12Z WED JAN 24 2018 - 12Z SUN JAN 28 2018

...OVERVIEW...

WAVY AND PROGRESSIVE UPPER FLOW IS EXPECTED TO PERSIST ACROSS THE
CONUS THROUGH THE MEDIUM RANGE. A BLOCKING UPPER RIDGE EXPECTED TO
DEVELOP FROM THE NORTH CENTRAL PACIFIC OCEAN TO THE BERING SEA
WILL PROMOTE CONTINUED NEGATIVE HEIGHT ANOMALIES ACROSS THE GULF
OF ALASKA. AN ENERGETIC SHORTWAVE IS FORECAST BY ALL GUIDANCE TO
EJECT FROM THIS AREA OF PERSISTENT TROUGHING AND MOVE EASTWARD
INTO THE CONUS. THE WAVE WILL AMPLIFY QUICKLY AS IT CROSSES THE
ROCKIES AND MOVES INTO THE CENTRAL U.S., AS IT ENCOUNTERS
INCREASING LOW-LEVEL BAROCLINICITY. THUS, EXPECT ANOTHER CENTRAL
U.S. LOW PRESSURE SYSTEM BY LATE THIS WEEK INTO THE WEEKEND,
CHARACTERIZED BY AN INTENSE THERMAL GRADIENT AND A WIDE ARRAY OF
SENSIBLE WEATHER IMPACTS. MEANWHILE, SUBTROPICAL RIDGING IS
EXPECTED TO BUILD NORTH ALONG THE EASTERN SEABOARD THROUGH THE
WEEK, AND ALONG THE WEST COAST BY LATE IN THE WEEK.


...GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES...

MODELS/ENSEMBLES SHOWED BETTER THAN AVERAGE LARGE SCALE AGREEMENT,
AND THIS ALLOWED FOR SIGNIFICANT USE OF DETERMINISTIC SOLUTIONS
THROUGH MOST OF THE FORECAST PERIOD. A BLEND OF THE 18Z GFS/12Z
ECMWF SERVED AS A BASIS FOR THE FORECAST THROUGH DAY 5 (WITH THE
ADDITION OF THE 12Z UKMET ON DAY 3). THESE SOLUTIONS REPRESENTED
CONSENSUS WILL FOR THE DEEP CYCLONE AFFECTING NEW ENGLAND EARLY
WED. SPREAD WAS SOMEWHAT HIGHER WITH RESPECT TO THE SURFACE LOW
OFF THE PAC NW LATE WED-EARLY THU, BUT THE DESCRIBED BLEND ALSO
REPRESENTED THE GUIDANCE CONSENSUS WELL.

THE GFS/ECMWF AND THEIR RESPECTIVE ENSEMBLE MEANS CONTINUE TO SHOW
RELATIVELY GOOD AGREEMENT AS THE UPPER TROUGH AMPLIFIES ACROSS THE
FOUR CORNERS/ROCKIES THROUGH FRI, AND WITH THE DEVELOPMENT OF A
SURFACE CYCLONE ACROSS THE HIGH PLAINS FRI. THE ECMWF WAS SLIGHTLY
FASTER THAN THE GFS WITH MOVING THE SURFACE LOW NORTHEASTWARD INTO
WESTERN ONTARIO. BUT THE ECENS/GEFS MEANS REMAINED VERY CLOSE.
THUS, BY DAY 6 (SAT) A 50/50 BLEND OF DETERMINISTIC/ENSEMBLE MEANS
WAS USED, TIPPING TO A SLIGHT MAJORITY ECENS/GEFS WEIGHTING BY DAY
7 (SUN). ONE POINT OF CONTENTION WAS HOW STRONG THE EVENTUAL
RIDGING ALONG THE WEST COAST WILL BE, AND HOW FAR NORTH THE AXIS
EXTENDS. AT THIS TIME, A BLENDED/COMPROMISE SOLUTION APPEARS BEST.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

THE APPROACH OF A COUPLE FRONTAL SYSTEMS ALONG WITH CONTINUED
ONSHORE FLOW WILL KEEP THE PACIFIC NORTHWEST AS WELL AS THE GREAT
BASIN AND PORTIONS OF THE NORTHERN ROCKIES WET THROUGH THE PERIOD.
HEAVY RAIN AND MOUNTAIN SNOW WILL BE POSSIBLE. FARTHER EAST, A
COLD FRONT WILL BRING SHOWERS AND THUNDERSTORMS TO SOUTH FLORIDA
WED-THU. AS THE UPPER TROUGH EXITS THE ROCKIES AND A NEW SURFACE
CYCLONE TAKES SHAPE ACROSS THE CENTRAL U.S. LATE IN THE WEEK,
EXPECT INCREASING GULF OF MEXICO MOISTURE TO EVENTUALLY PRODUCE
WIDESPREAD PRECIPITATION FROM THE GULF COAST AND SOUTHEAST TO THE
GREAT LAKES BY LATE FRI INTO THE WEEKEND. MODELS SUGGEST
SUFFICIENT MOISTURE RETURN TO POTENTIALLY SUPPORT THUNDERSTORMS
AND HEAVY RAINFALL FROM PORTIONS OF THE TN/OH VALLEYS TO THE GULF
COAST AND SOUTHEAST. ON THE NORTHERN SIDE OF THE SYSTEM, ANOTHER
ROUND OF SNOW IS POSSIBLE ACROSS THE MIDWEST AND UPPER GREAT LAKES.

THE BUILDING SUBTROPICAL RIDGE ALONG THE EAST COAST ALONG WITH
WARM AIR ADVECTION BY MID/LATE WEEK AHEAD OF THE DEVELOPING
CENTRAL U.S. LOW PRESSURE WILL RESULT IN ABOVE AVERAGE
TEMPERATURES ACROSS MUCH OF THE CENTRAL/EASTERN U.S., CONTINUING
TO RISE THROUGH LATE IN THE WEEK. HIGH TEMPS OF 10-20 DEG F WILL
BE POSSIBLE BY MID/LATE WEEK ACROSS A LARGE AREA. MEANWHILE, BELOW
AVERAGE TEMPS (5-10 DEG F) WILL ACCOMPANY THE UPPER TROUGH ACROSS
THE WESTERN U.S. THU-FRI.

RYAN

$$





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