Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 200656
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
155 AM EST SAT JAN 20 2018

VALID 12Z TUE JAN 23 2018 - 12Z SAT JAN 27 2018

...OVERVIEW...

A FAIRLY PROGRESSIVE FLOW REGIME WILL BE IN PLACE ACROSS THE CONUS
DURING MUCH OF THE MEDIUM RANGE PERIOD. A BUILDING UPPER RIDGE
EXTENDING FROM THE NORTH CENTRAL PACIFIC OCEAN TO THE BERING SEA
THROUGH THE PERIOD WILL PROMOTE CONTINUED TROUGHING IN THE GULF OF
ALASKA AND OFF THE NORTH AMERICAN WEST COAST THROUGH MIDWEEK. THIS
PERSISTENT TROUGH WILL EJECT UPPER LEVEL SHORTWAVES EASTWARD
ACROSS NORTH AMERICA, SOME OF WHICH WILL AMPLIFY AS THEY ENCOUNTER
INCREASING BAROCLINICITY ACROSS THE CONUS. LARGER SCALE EAST
PACIFIC TROUGHING WILL LIKELY MOVE INTO THE WEST LATER THIS WEEK.
THE END RESULT WILL BE A PATTERN MORE REMINISCENT OF EARLY SPRING
THAN ONE OF MID-WINTER, WITH FREQUENT AND PERHAPS RELATIVELY
INTENSE SURFACE CYCLONES ACROSS THE CENTRAL U.S., CHARACTERIZED BY
STRONG THERMAL GRADIENTS ACROSS THEM.


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

MODEL CONSENSUS HAS IMPROVED COMPARED TO THIS TIME LAST NIGHT
THROUGHOUT THE EXTENDED FORECAST PERIOD. LOW PRESSURE SYSTEM
CROSSING THE GREAT LAKES ON DAY 3 (TUE) SHOWS IMPROVED CONSENSUS
AMONG THE GUIDANCE WITH THE ECMWF AND GFS BOTH CLUSTERED TIGHTLY
WITHIN THEIR RESPECTIVE ENSEMBLE SPREADS. ALONG THE WEST COAST,
SHORTWAVE EXPECTED TO REACH THE PAC NW TUE-TUE NIGHT ALSO SHOWS AN
IMPROVEMENT IN CONSENSUS COMPARED TO LAST NIGHT. A SECOND
SHORTWAVE WILL FOLLOW QUICKLY IN ITS HEELS, REACHING THE NW LATE
WED. WHILE MODELS SHOWED SOME DIFFERENCES WITH RESPECT TO TIMING
OF THESE FEATURES AND THEIR ASSOCIATED SURFACE LOWS, A CONSENSUS
APPROACH IS PREFERRED AT THIS TIME. THUS, A MAJORITY DETERMINISTIC
BLEND OF THE ECMWF/GFS WAS USED AS A FORECAST STARTING POINT
DURING DAYS 3-4.

FROM DAY 5 ONWARD, THE AFOREMENTIONED SHORTWAVE REACHING THE PAC
NW EARLIER IN THE PERIOD WILL AMPLIFY AS IT CROSSES THE FOUR
CORNERS/ROCKIES AND MOVES TOWARD THE CENTRAL U.S. BY LATE FRI INTO
SAT. WHILE MODELS SHOW DIFFERENCES WITH RESPECT TO THE DETAILS
SURROUNDING THE EVOLUTION OF THIS FEATURE, THEY SHOW ABOVE AVERAGE
CONSENSUS ON THE LARGE SCALE, TAKING THE TIME RANGE INTO
CONSIDERATION. SURFACE CYCLOGENESIS IS EXPECTED TO OCCUR BY LATE
FRI SOMEWHERE ACROSS THE CENTRAL OR NORTHERN PLAINS. WHILE
DETERMINISTIC SOLUTIONS DIFFER ON EXACTLY WHERE THIS WILL OCCUR,
THE ECENS/GEFS MEANS SHOWED BETTER AGREEMENT. THUS, THE FORECAST
WAS GRADUALLY TRENDED TOWARD HEAVIER ECENS/GEFS MEAN WEIGHTING
FROM DAY 5 ONWARD, WITH CONTINUED MINORITY COMPONENTS OF THE ECMWF
AND GFS INCLUDED.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

SNOW ALONG WITH GUSTY WINDS WILL BE POSSIBLE ACROSS PORTIONS OF
THE GREAT LAKES TUE ON THE NORTHWEST SIDE OF THE SURFACE LOW
PRESSURE SYSTEM MOVING THROUGH THE REGION. RAIN AND PERHAPS A FEW
THUNDERSTORMS WILL BE POSSIBLE ALONG/AHEAD OF THE SURFACE COLD
FRONT ON THU ALONG MUCH OF THE EAST COAST. AREAS OF HEAVY RAIN
WILL BE POSSIBLE ACROSS SOUTHERN NEW ENGLAND WHILE A WINTRY MIX OF
PRECIPITATION IS POSSIBLE FOR NORTHERN NEW ENGLAND. MEANWHILE, THE
WESTERN U.S. WILL SEE A PROLONGED PERIOD OF ONSHORE FLOW WITH
SEVERAL UPPER-LEVEL IMPULSES AND SURFACE FRONTAL SYSTEMS ENHANCING
PRECIPITATION AT TIMES. LOW ELEVATION RAIN AND MOUNTAIN SNOWS ARE
EXPECTED THROUGH MUCH OF THE PERIOD, WHICH COULD BE HEAVY. SNOW
LEVELS SHOULD LOWER BY MID-WEEK AS HEIGHTS FALL IN RESPONSE TO THE
AMPLIFYING UPPER TROUGH. THE SOUTH CENTRAL AND SOUTHEASTERN STATES
WILL SEE INCREASING POTENTIAL FOR SHOWERS AND THUNDERSTORMS BY
LATE NEXT WEEK AS SURFACE CYCLOGENESIS OCCURS ACROSS THE CENTRAL
U.S. AND SOUTHERLY FLOW BEGINS TO SPREAD MOISTURE AND INSTABILITY
FROM THE GULF OF MEXICO NORTHWARD. CONFIDENCE IN THE SPECIFICS OF
WHERE/WHEN ANY POTENTIAL PRECIPITATION WOULD OCCUR REMAINS
RELATIVELY LOW AT THIS TIME, WITH THE GFS REMAINING SLOWER TO
SPREAD MOISTURE AND PRECIPITATION NORTHWARD RELATIVE TO THE ECMWF.

TEMPERATURES ARE EXPECTED TO BE WELL ABOVE AVERAGE ON TUE ALONG
THE EAST COAST IN THE WARM SECTOR AHEAD OF THE INCOMING COLD
FRONT. MAX TEMPS MAY REACH 10-15 DEG F ABOVE AVERAGE. A BUILDING
SUBTROPICAL RIDGE OFF THE SOUTHEAST COAST AND RETURNING WARM AIR
ADVECTION AHEAD OF THE LATE WEEK WESTERN U.S. TROUGH WILL KEEP
MUCH OF THE CENTRAL/EASTERN U.S. ABOVE AVERAGE THROUGH NEXT WEEK,
WITH MAX TEMPS 10-20 DEG F ABOVE AVERAGE ACROSS MUCH OF THE
CENTRAL/EASTERN U.S. BY LATE IN THE WEEK. MEANWHILE COLDER AIR
WILL SPREAD INTO THE WEST AS A THE UPPER TROUGH AMPLIFIES
OVERHEAD, AND MAX TEMPS OF 10-15 DEG F BELOW AVERAGE ARE EXPECTED
BY LATE NEXT WEEK.

RYAN

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