Preliminary Forecasts
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FXUS02 KWNH 170710
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
310 AM EDT TUE OCT 17 2017

VALID 12Z FRI OCT 20 2017 - 12Z TUE OCT 24 2017

...GUIDANCE/UNCERTAINTY ASSESSMENT AND SENSIBLE WEATHER
HIGHLIGHTS/THREATS...

MODELS AND ENSEMBLES REMAIN IN REASONABLE AGREEMENT INTO THE
WEEKEND AND FORECAST CONFIDENCE IS ABOVE NORMAL DURING THIS PERIOD
WITH GOOD RUN TO RUN GUIDANCE CONTINUITY.
IN THIS PERIOD THE MAIN FORECAST FOCUS WILL BE ACROSS THE WRN US.
DEEP PACIFIC TROUGH RELATED HEIGHT FALLS SLAM INTO THE WEST COAST
FRI THAT WORK INLAND ACROSS MUCH OF THE WRN US AND ROCKIES INTO
SAT. EXPECT HEAVY PCPN OVER THE NWRN US INTO CA WITH INITIAL
MOISTURE FEED THAT COULD PRODUCE SOME RUNOFF ISSUES. DYNAMICALLY
DRIVEN/TERRAIN ENHANCED AMOUNTS THEN PROGRESS ACROSS THE N-CENTRAL
INTERMOUNTAIN WEST/ROCKIES. COOLING TEMPS WITH FRONTAL PASSAGE AND
HIGH PRESSURE SURGE WILL SUPPORT SOME HEAVY TERRAIN ENHANCED SNOWS
IN A PERIOD WITH LOWERING SNOW LEVELS. ADDITIONAL UPSTREAM PACIFIC
ENERGIES PUNCHING INTO THE NWRN US SHOULD ADD ANOTHER SHOT OF
MODERATE PCPN INTO THE REGION OVER THE WEEKEND. MID-UPPER LEVEL
RIDGE BUILDING SHOULD THEN USHER IN A DRY PATTERN WITH MODIFYING
TEMPS NEXT WEEK.

FORECAST SPREAD AND UNCERTAINTY INCREASES SIGNIFICANTLY BY LATER
WEEKEND INTO EARLY NEXT WEEK DOWNSTREAM ACROSS THE CENTRAL AND ERN
US. GUIDANCE HAS NOT OFFERED STELLAR RUN TO RUN CONSISTENCY WITH
THE FORECAST OF SUBSEQUENT STREAM SEPARATION AS AFOREMENTIONED WRN
US TROUGH ENERGIES PROGRESS INTO THE CENTRAL US AND LATER THE ERN
US. RECENT RUNS OF THE ECMWF HAS BEEN SHOWING DEEP CLOSED LOW
DEVELOPMENT THAT HAS SHIFTED OVER THE PAST FEW DAYS FROM AS FAR
WEST AS THE SRN ROCKIES TO THE SRN PLAINS THEN LOWER MS VALLEY.
ENSEMBLE SUPPORT FOR THIS HAS BEEN MARGINAL...AND THE LATEST 00
UTC ECMWF RUN CONTINUES TRENDING TO A MUCH WEAKER LOW SHIFTED
PROGRESIVELY EASTWARD UNDERNEATH INCREASINGLY DOMINANT AND NRN
STREAM TROUGH ENERGIES DIGGING SEWD FROM THE N-CENTRAL US MON TO
THE E-CENTRAL US TUE. ACCORDINGLY...THE NOTION THAT THE HIGHER
RESOLUTION ECMWF MIGHT BE CATCHING ONTO A MORE SIGNIFICANT SRN
STREAM SYSTEM NOT SHOWN WELL IN OTHER GUIDANCE HAS DWINDLED.
INSTEAD EXPECT THERE MAY WELL BE STREAM SEPARATION TO SOME
DEGREE...BUT OVERALL EXPECT A POTENT AND WAVY COLD FRONT AND
EMERGING DEEP INFLOW FROM THE GULF OF MEXICO INLAND AND NORTHWARD
TO FUEL A PROGRESSIVE AND WIDESPREAD SWATH OF MODERATE TO HEAVY
RAINS ACROSS THE CENTRAL THEN ERN US NEXT WEEK WITH THE EMERGING
NRN STREAM AMPLITUDE LIFTING ACTIVITY RAPIDLY NORTHWARD ACROSS THE
ERN THIRD OF THE NATION WITH ACTIVITY ADDING WRN ATLANTIC INFLOW
OEVR TIME. MOST GUIDANCE NOW SUPPORTS THIS SCENARIO AND WPC PROGS
WERE PRIMARILY DERIVED WITH A COMPOSITE MODEL AND ENSEMBLE BLEND
THAT DE-EMPHASIZED MUCH OF THE EARLIER ECMWF POTENTIAL.

SCHICHTEL

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