Preliminary Forecasts
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14
957
FXUS02 KWNH 230620
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
120 AM EST FRI FEB 23 2018

VALID 12Z MON FEB 26 2018 - 12Z FRI MAR 02 2018

...PATTERN OVERVIEW...

THE SUBTROPICAL RIDGE IS EXPECTED TO SHIFT A BIT WESTWARD DURING
THE MEDIUM RANGE. THE STRONGEST POSITIVE HEIGHT ANOMALIES
ASSOCIATED WITH THE FEATURE, INITIALLY CENTERED OFF THE EAST
COAST, WILL SHIFT INTO THE GULF OF MEXICO BY EARLY NEXT WEEK, WITH
A RIDGE AXIS ATTEMPTING TO EXERT INFLUENCE FARTHER NORTH ACROSS
THE SOUTHERN U.S. AND PERHAPS THE TN/OH VALLEYS. MEANWHILE,
PERSISTENT UPPER RIDGING ACROSS THE NORTH PACIFIC WILL CONTINUE TO
FAVOR TROUGHING ACROSS THE WEST. THE COMBINATION OF CONTINUED
WESTERN TROUGHING AND GULF OF MEXICO RIDGING WILL FAVOR A
RELATIVELY ACTIVE WEATHER PATTERN ACROSS THE CENTRAL AND EASTERN
U.S., WITH THE POTENTIAL FOR ADDITIONAL AREAS OF HEAVY RAINFALL.


...MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES...

IN GENERAL, THE MODELS/ENSEMBLES SHOWED DECENT AGREEMENT WITH
RESPECT TO THE LARGE SCALE PATTERN THROUGHOUT THE PERIOD. FOR THE
MOST PART, SIGNIFICANT DIFFERENCES AMONG THE GUIDANCE WERE MATTERS
OF TIMING. FIRST SIGNIFICANT TIMING ISSUE ARISES BY TUE-WED, AND
RESULTS FROM UNCERTAINTY SURROUNDING HOW QUICKLY THE TROUGH/UPPER
LOW OVER THE SOUTHWEST OPENS UP AND MOVES EAST. THE ECMWF HAS
SHOWN A FAIR DEGREE OF RUN-TO-RUN VARIABILITY WITH THE 00Z RUN
LAST NIGHT SHOWING A CLOSED UPPER LOW LINGERING FOR LONGER ACROSS
THE WEST, RESULTING IN MUCH SLOWER TIMING FROM TUE-WED ONWARD. THE
12Z ECMWF, ON THE OTHER HAND, TRANSITIONS THE FEATURE INTO AN OPEN
WAVE, AND MOVES IT EASTWARD MORE QUICKLY RELATIVE TO THE PREVIOUS
RUN. THE GFS HAS BEEN A BIT MORE CONSISTENT WITH WEAKENING THE
UPPER LOW/MOVING THE SYSTEM EASTWARD A BIT MORE QUICKLY. ENSEMBLE
SPREAD IS RATHER HIGH, AND COVERS THE ENTIRE RANGE OF
POSSIBILITIES. THESE DIFFERENCES TRANSLATE DOWNSTREAM TO THE
MIDWEST/OH VALLEY BY THU, AND IMPACT THE DEGREE OF PHASING THAT
BEGINS TO OCCUR WITH AN AREA OF RELATIVELY WEAK NORTHERN STREAM
ENERGY. THE GFS HAS BEEN CONSISTENT IN DEVELOPING A RELATIVELY
STRONG SURFACE LOW ACROSS THE GREAT LAKES/OH VALLEY BY WED
NIGHT/THU, WHILE THE ECMWF HAS VARIED SIGNIFICANTLY FROM
RUN-TO-RUN, WITH THE LATEST RUN DEVELOPING A MUCH WEAKER LOW
RELATIVE TO THE GFS AND KEEPING A MORE OPEN FRONTAL WAVE.
UNCERTAINTY INCREASES FURTHER BY FRI AS ADDITIONAL PHASING COULD
OCCUR ACROSS THE NORTHEASTERN U.S., WITH SOME SOLUTIONS SUGGESTING
SURFACE LOW DEVELOPMENT NEAR OR OFF THE EAST COAST. BY THIS TIME
PERIOD BOTH THE ECMWF AND GFS HAVE VARIED SUBSTANTIALLY FROM
RUN-TO-RUN, AND CONFIDENCE IN THE FORECAST DETAILS IS LOW. FARTHER
WEST, MODELS ARE IN GENERAL AGREEMENT ON ANOTHER SHORTWAVE DIGGING
SOUTHWARD ALONG THE WEST COAST AND AMPLIFYING INTO A LARGE TROUGH
OR PERHAPS ANOTHER UPPER LOW.

THE WPC FORECAST WAS INITIALLY BASED HEAVILY ON DETERMINISTIC
GUIDANCE DURING DAYS 3-4, WITH THE 12Z ECMWF/GFS/CMC MAKING UP THE
MAJORITY OF THE BLEND. AFTER THAT TIME, DUE TO INCREASING
UNCERTAINTY, ENSEMBLE MEAN (12Z ECENS/NAEFS) WEIGHTING WAS
GRADUALLY BOOSTED THROUGH TIME, WITH MAJORITY ENSEMBLE MEANS
DURING DAYS 6-7.


...WEATHER HIGHLIGHTS/THREATS...

WIDESPREAD CONVECTION AND POTENTIALLY HEAVY RAINS WILL ONCE AGAIN
POSE A THREAT ACROSS PORTIONS OF THE LOWER MS VALLEY, TN VALLEY,
AND SOUTHEAST FROM LATE TUE THROUGH THU NIGHT AS LOW-LEVEL
SOUTHERLY FLOW SPREADS AMPLE MOISTURE NORTHWARD, AND A WEAK
REMNANT SURFACE BOUNDARY PROVIDES A FOCUS FOR CONVECTIVE
DEVELOPMENT. FARTHER NORTH, ANY POTENTIAL LOW PRESSURE SYSTEMS
TRACKING ACROSS THE CENTRAL PLAINS/MIDWEST/GREAT LAKES WOULD BRING
CHANCES OF SNOWFALL TO PORTIONS OF THOSE REGIONS, ALTHOUGH
CONFIDENCE IS LOW IN THE SPECIFICS AT THIS POINT GIVEN THE TIME
FRAME AND THE DESCRIBED MODEL SPREAD. PERSISTENT TROUGHING ALONG
THE WEST COAST WILL KEEP CONDITIONS UNSETTLED THERE, WITH THE
THREAT FOR HEAVY PRECIPITATION INCREASING BY MID TO LATE NEXT WEEK
AS THE NEXT TROUGH/UPPER LOW DIGS ALONG THE WEST COAST. HEAVY
PRECIPITATION IS POSSIBLE FROM THE CASCADES SOUTHWARD THROUGH MUCH
OF CALIFORNIA AS A COLD FRONT PUSHES INLAND. BROAD POSITIVE HEIGHT
ANOMALIES WILL KEEP TEMPERATURES ABOVE AVERAGE FOR MUCH OF THE
CENTRAL/EASTERN U.S., WITH HIGHS REACHING DOUBLE DIGITS ABOVE
AVERAGE ACROSS PORTIONS OF THE MID-MS VALLEY/MIDWEST BY TUE.
MEANWHILE, PERSISTENT TROUGHING AND PRECIPITATION WILL KEEP
TEMPERATURES BELOW AVERAGE ACROSS MUCH OF THE WEST THROUGH NEXT
WEEK.

RYAN

$$





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.