Preliminary Forecasts
Issued by NWS

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FXUS02 KWNH 140708
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
208 AM EST THU DEC 14 2017

VALID 12Z SUN DEC 17 2017 - 12Z THU DEC 21 2017

...OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT...

MODELS AND ENSEMBLES OFFER GREATER THAN NORMAL FORECAST SPREAD AND
UNCERTAINTY FOR THE 3-7 DAY FORECAST PERIOD. THERE ARE TWO MAIN
FORECAST CLUSTER SCENARIOS EVIDENT. THE MAIN FORECAST PROBLEM
EMINATES FROM THE DIFFERENT HANDLING OF SHORT RANGE BASED TROUGH
ENERIES DIGGING ALOFT OVER THE WRN US. RECENT GFS/CANADIAN RUNS
AND TO A LESSER EXTENT GEFS/NAEFS MEANS DEVELOP A PHASED/AMPLIFIED
SWRN US UPPER TROUGH THIS WEEKEND. THIS SRN STREAM SPLIT FEATURE
AND ASSOCIATED SURFACE SYSTEM EJECTS ACROSS THE US SRN TIER
MON-FRI TO FOCUS MODERATE TO HEAVY RAINS/THUNDERSTORMS OVER THE
GULF COAST STATES/SERN US/SRN MID-ATLANTIC AS GULF MOISTURE IS
ADVECTS INLAND. MEANWHILE IN THIS SCENARIO...SEPARATE NRN STREAM
FLOW MID-UPPER LEVEL TROUGH ENERGIES AND ASSOCIATED AND PCPN
SWATHS WITH MORE COLD AIR TO WORK WITH PROGRESS OVERTOP THEN TEND
TO REMERGE TO SOME DEGREE WITH EJECTING SRN STREAM FLOW OVER THE
E-CENTRAL STATES EARLY-MID NEXT WEEK BEFORE FULLY REDOMINATING
LATER WEEK. IN STARK CONTRAST...RECENT ECMWF RUNS AND TO A LESSER
EXTEND ECMWF ENSEMBLE MEANS SHOW MORE WEEKEND SPLIT FLOW OVER THE
WEST AND SPLIT SOME TROUGHING BACK OVER THE ERN PACIFIC AND ALLOWS
MORE PROGRESSIVE TROUGHING/PCPN SWATH SPLIT RAPIDLY EASTWARD
CENTRAL AND ERN US MID-LATITUDES MON-TUE. IT DOES NOT SEEM
REASONABLE AMID UNCERTAINTY TO FAVOR EITHER CAMP OF THOUGHT. A
BLEND OF SOMEWHAT MORE COMPATABLE GEFS/NAEFS/ECMWF ENSEMBLES
PROVIDES A COMPROMISE SOLUTION THAT SEEMS TO TAKE INTO ACCOUNT
SOME OF EACH CAMP AND MAINTAINS MAX WPC FORECAST CONTINUITY. THE
WPC PRODUCT SUITE WAS DERIVED FROM THAT BLEND. LATEST 00 UTC
GFS/ECMWF GUIDANCE DOES SEEM TO BETTER SOLUTION CONVERGE/TIMING
WED-THU SHOWING INCREASINGLY WET FLOW INTO/ACROSS THE NWRN
STATES...INCLUDING SOME HEAVIER PAC NW RAINS...WITH ALSO BOLSTERED
FORECAST CONFIDENCE IN A GENERAL SUBSEQUENT INLAND WINTERY THREAT
TRACK ACROSS THE WRN US TO THE ROCKIES BY NEXT THU.

SCHICHTEL

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