Preliminary Forecasts
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FXUS02 KWNH 190649
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
248 AM EDT THU APR 19 2018

VALID 12Z SUN APR 22 2018 - 12Z THU APR 26 2018

...OVERVIEW...

THE SYSTEM EXPECTED TO BRING THE THREAT OF HEAVY RAINFALL TO SOME
LOCATIONS ACROSS THE SOUTHEAST AND POSSIBLY INTO THE SOUTHERN MID
ATLANTIC SUN-TUE WILL MOST LIKELY LIFT NORTHWARD ALONG THE EAST
COAST THEREAFTER IN RESPONSE TO THE APPROACH OF SHORTWAVE ENERGY
THAT SHOULD BE REACHING WESTERN NORTH AMERICA AROUND THE START OF
THE PERIOD.  THIS ENERGY TRAILS A LEADING VIGOROUS SHORTWAVE THAT
INITIALLY PUSHES A COLD FRONT INTO THE NORTHERN
PLAINS/NORTH-CENTRAL WESTERN STATES.  THERE ARE SOME
LOW-PREDICTABILITY ASPECTS TO THE EASTERN U.S. EVOLUTION BY THE
LATTER HALF OF THE PERIOD BUT AGREEMENT IS STILL MUCH BETTER IN
PRINCIPLE THAN FOR THE EASTERN PACIFIC/WESTERN U.S. FORECAST BY
NEXT TUE-THU, AS GUIDANCE DIVERGES IN A WIDE VARIETY OF WAYS FOR
ENERGY WITHIN A TROUGH THAT SHOULD BE NEAR 150W LONGITUDE AS OF
EARLY SUN.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

THE BROADENING ENVELOPE OF ENSEMBLE MEMBERS MORE THAN ACCOUNTS FOR
THE WIDE SPECTRUM OF CURRENT AND RECENT OPERATIONAL MODEL
SOLUTIONS FOR THE PACIFIC TROUGH ENERGY THAT MAY OR MAY NOT REACH
THE WESTERN U.S.  CONSISTENT WITH THEIR HISTORICAL TIMING BIASES,
RECENT ECMWF RUNS HAVE BEEN PULLING OFF A CLOSED LOW THAT STAYS
WELL OFFSHORE WHILE THE MAJORITY OF GFS RUNS BRING MUCH OF THE
TROUGH ENERGY AT LEAST TO THE COAST IF NOT WELL INLAND.  THE 00Z
GFS FROM 24 HOURS AGO (00Z/18 CYCLE) WAS THE CLOSEST TO THE ECMWF
SCENARIO.  ON THE OTHER HAND THE 18Z AND NEW 00Z GFS RUNS ARE
QUITE PROGRESSIVE WITH A FAIRLY STRONG TROUGH REACHING THE FOUR
CORNERS/ROCKIES BY DAY 7 THU.  UKMET RUNS SO FAR HAVE BEEN
GRAVITATING TOWARD THE ECMWF AND THE PAST COUPLE CMC RUNS ARE
BETWEEN THE TWO EXTREMES.  WHILE THEY GET THERE IN DIFFERENT WAYS,
GEFS/ECMWF/CMC ENSEMBLE MEANS PROVIDE A MORE SIMILAR AND
INTERMEDIATE SOLUTION WITH GREATER RUN TO RUN STABILITY THAN SOME
OPERATIONAL GUIDANCE.  THUS FOR THE PURPOSES OF PROVIDING A SINGLE
DETERMINISTIC FORECAST THE PREFERENCE GOES TO THE MEANS ACROSS THE
EASTERN PACIFIC/WESTERN U.S. LATE TUE ONWARD UNTIL GUIDANCE
CLUSTERING PROVIDES ENOUGH CONFIDENCE TO SUPPORT AN ALTERNATIVE
SCENARIO.

FARTHER EAST THERE IS GOOD AGREEMENT WITH A VIGOROUS SOUTHERN
CANADA TROUGH/UPPER LOW THAT INITIALLY PUSHES A COLD FRONT INTO
THE NORTHERN PLAINS AND BACK INTO THE NORTH-CENTRAL WESTERN U.S.
HOWEVER DETAIL DIFFERENCES ARISE WITH IMMEDIATELY TRAILING ENERGY
THAT ORIGINATES FROM THE NORTH PACIFIC, WITH SOME RECENT MODEL
RUNS SHOWING AMPLIFICATION/SHARPENING OF SOUTHERN CANADA ENERGY TO
INTERACT WITH A SEPARATE NORTHERN U.S. SHORTWAVE.  THESE
LOWER-PREDICTABILITY DETAILS AFFECT SPECIFICS OF WAVINESS ALONG
THE INITIAL PLAINS/WESTERN U.S. FRONT WITH A COMPROMISE APPROACH
PREFERRED TO RESOLVE DIFFERENCES.  EVEN WITH THE UNCERTAINTY
INVOLVED, THERE IS A FAIR CONSENSUS SUGGESTING THAT THE OVERALL
SHORTWAVE SHOULD REACH THE EAST-CENTRAL U.S. BY WED-THU.  IT
REMAINS TO BE SEEN WHETHER AN EMBEDDED UPPER LOW CLOSES OFF AND
WHERE IT TRACKS (RECENT GFS/CMC RUNS, 00Z/18 ECMWF), OR THE TROUGH
REMAINS OPEN (12Z ECMWF, ENSEMBLE MEANS).

THE SHAPE AND TIMING OF THIS APPROACHING ENERGY WILL BECOME
INCREASINGLY IMPORTANT IN DETERMINING THE EVOLUTION/TRACK OF LOW
PRESSURE INITIALLY MOVING ALONG OR JUST NORTH OF THE GULF COAST IN
ASSOCIATION WITH A SOUTHERN STREAM UPPER LOW TRACKING FROM THE
EASTERN PLAINS INTO THE SOUTHEAST/TN VALLEY DURING THE FIRST HALF
OF THE PERIOD.  FOR MOST OF THE PERIOD THE UPDATED FORECAST BLEND
GAVE ONLY MINORITY WEIGHT TO RECENT GFS RUNS AS THEY HAVE BEEN ON
THE FASTER SIDE OF THE ENVELOPE.  THE NEW 00Z GFS SEEMS TO BE
TRENDING SLOWER FOR AT LEAST PORTIONS OF THE FORECAST.  BY WED-THU
THERE IS DECENT CONFIDENCE IN LOW PRESSURE TRACKING NORTHWARD NEAR
THE EAST COAST BUT POSSIBLE SCENARIOS RANGE BETWEEN A
STRONGER/WESTWARD SYSTEM IF GREAT LAKES ENERGY CLOSES OFF AND A
WEAKER COASTAL WAVE IN THE EVENT OF AN OPEN TROUGH ALOFT.  A BLEND
OF MOSTLY OPERATIONAL GUIDANCE EARLY TRANSITIONING TO THE ENSEMBLE
MEANS (MORE 12Z ECMWF MEAN THAN 18Z GEFS/12Z NAEFS) BY DAYS 6-7
WED-THU PROVIDED GOOD CONTINUITY FOR SYSTEM DEPICTION OVER THE
SOUTH AND EAST THROUGH THE PERIOD, EVEN IF CONFIDENCE IN THE EXACT
FORECAST IS NOT PARTICULARLY HIGH BY WED-THU.


...WEATHER/THREATS HIGHLIGHTS...

THE SYSTEM TRACKING ALONG THE SOUTHERN TIER/GULF COAST EARLY IN
THE WEEK WILL BRING AN EPISODE OF HEAVY RAINFALL TO LOCATIONS FROM
THE LOWER MS VALLEY INTO THE SOUTHEAST DURING THE LATE
WEEKEND/EARLY NEXT WEEK TIME FRAME.  THEN THE MOISTURE SHIELD WILL
LIKELY SPREAD NORTHWARD WITH TIME AS LOW PRESSURE BEGINS TRACKING
ALONG/NEAR THE EAST COAST.  RAINFALL AMOUNTS OVER THE
CENTRAL/NORTHERN PARTS OF THE EAST SHOULD TEND TO BE LOWER THAN
OVER THE SOUTH, BUT THERE ARE POSSIBLE SYSTEM EVOLUTIONS AT THE
SURFACE AND ALOFT THAT COULD LEAD TO SOME AREAS OF ENHANCED
RAINFALL.  THERE MAY ALSO BE A PERIOD OF STRONG WINDS ALONG THE
EAST COAST GIVEN THE GRADIENT BETWEEN THE SURFACE LOW AND STRONG
RIDGE TO THE NORTH.

THE COMBINATION OF SHORTWAVE ENERGY ALOFT AND WAVY FRONT PUSHING
SOUTHEAST FROM THE NORTHERN PLAINS/NORTH-CENTRAL WEST WILL BRING
AREAS OF PRECIP TO PORTIONS OF THE ROCKIES/PLAINS AND POSSIBLY
INTO THE MIDWEST/GREAT LAKES BEFORE POSSIBLY MERGING WITH THE
EASTERN SYSTEM.  THE NORTH-CENTRAL ROCKIES/HIGH PLAINS APPEAR TO
HAVE BEST POTENTIAL FOR HIGHEST AMOUNTS WITHIN THIS AREA OF
MOISTURE.  SOME SNOW IS POSSIBLE OVER HIGH ELEVATIONS.

UNCERTAINTY WITH THE LARGE SCALE EVOLUTION BY THE LATTER HALF OF
THE PERIOD LOWERS CONFIDENCE IN COVERAGE/INTENSITY OF ANY PRECIP
THAT MAY OCCUR OVER THE WEST BY MIDWEEK.  THE PREFERRED ENSEMBLE
MEAN SCENARIO ALOFT WOULD AT LEAST SUPPORT SOME LIGHT/SCATTERED
ACTIVITY.

ASIDE FROM SOME NEAR TO SLIGHTLY BELOW NORMAL READINGS OVER THE
NORTHWEST EARLY IN THE PERIOD, EXPECT MUCH OF THE WESTERN U.S. TO
SEE ABOVE NORMAL TEMPS DURING THE PERIOD WITH ANOMALIES REACHING
PLUS 10-15F AT SOME LOCATIONS.  ISOLATED DAILY RECORDS FOR
HIGHS/WARM MINS MAY BE POSSIBLE.  CLOUDS/PRECIP WILL KEEP HIGHS
WELL BELOW NORMAL OVER THE SOUTH AND THEN EAST AND THE FRONT
PROGRESSING SOUTHEAST FROM THE NORTHERN PLAINS WILL BRING
REINFORCING COOL AIR TO THE CENTRAL/EAST BY TUE-THU.


ASSOCIATED WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS...SURFACE
SYSTEMS...WEATHER GRIDS...QUANTITATIVE PRECIPITATION FORECASTS
(QPF)...AND WINTER WEATHER OUTLOOK SNOW/SLEET PROBABILITIES CAN BE
FOUND AT...

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4

RAUSCH

$$





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