Prognostic Meteorological Discussion
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000
FXUS02 KWBC 181536
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1136 AM EDT WED APR 18 2018

VALID 12Z SAT APR 21 2018 - 12Z WED APR 25 2018

...SOUTHERN PLAINS TO SOUTHEAST U.S. HEAVY RAIN THREAT...


15 UTC UPDATE...

A SLIGHT NORTHWARD TREND WAS NOTED AMONG THE GUIDANCE THIS MORNING
WITH RESPECT TO THE UPPER LOW FORECAST TO CROSS THE SOUTHERN
PLAINS/LOWER MS VALLEY. THE UKMET REMAINED THE SOLUTION MORE
SIMILAR TO THE CONSENSUS OF PREVIOUS CYCLES, KEEPING THE UPPER LOW
CLOSER TO THE GULF COAST ON DAY 5 (MON) RELATIVE TO THE GFS/ECMWF
WHICH WERE CLOSER TO THE TN VALLEY. AT THIS TIME, A BLEND OF THESE
SOLUTIONS IS PREFERRED, WHICH RESULTS IN AT LEAST SOMEWHAT OF A
NORTHWARD SHIFT COMPARED TO THE OVERNIGHT FORECAST. ELSEWHERE, THE
FORECAST REMAINED VERY CLOSE TO CONTINUITY WITH NO SIGNIFICANT
CHANGES MADE. MODELS CONTINUE TO SHOW SOME TIMING DIFFERENCES WITH
A VIGOROUS SHORTWAVE CROSSING THE CANADIAN PRAIRIES AND ASSOCIATES
SFC FRONT CROSSING THE NORTHERN PLAINS/MIDWEST SUN ONWARD, WITH
THE CMC ON THE FAST SIDE. FINALLY, UNCERTAINTY IN THE FORECAST
INCREASES QUICKLY BY DAY 6-7 (TUE-WED) ACROSS THE WEST COAST AND
ADJACENT EASTERN PACIFIC. AMPLIFYING NORTHERN STREAM ENERGY OFF
THE WEST COAST ON TUE IS FORECAST BY SOME SOLUTIONS (ECMWF/GFS) TO
CUT OFF ALONG OR OFF THE WEST COAST, ALTHOUGH EVEN THESE TWO
SOLUTIONS DIFFER SUBSTANTIALLY ON THE LOCATION. MEANWHILE, THE CMC
IS QUITE DIFFERENT, WEAKER WITH THE ENERGY AND MOVING IT INLAND
MUCH MORE QUICKLY. PREFERRED TO GO SUBSTANTIALLY TOWARD ENSEMBLE
MEANS (00Z ECENS/NAEFS) BY THIS TIME PERIOD, AS THE MEANS SHOWED
MUCH SMALLER DIFFERENCES THAN THE DETERMINISTIC RUNS. THIS WOULD
SHOW A TROUGH NEAR THE WEST COAST ON DAY 7, WITH A SURFACE FRONT
PUSHING INTO THE WEST COAST STATES.


RYAN


PREVIOUS DISCUSSION (ISSUED AT 0654 UTC)...


...OVERVIEW...

THE GREATEST ATTENTION IN TERMS OF SENSIBLE WEATHER IMPACT
CONTINUES TO BE ON THE CLOSED UPPER LOW FORECAST TO TRACK OUT OF
THE SOUTHERN HIGH PLAINS ON SAT AND REACH THE SOUTHEAST BY AROUND
TUE.  THE COMBINATION OF UPPER DYNAMICS ALONG WITH LOW PRESSURE
TRACKING ACROSS THE SOUTHERN TIER/GULF COAST WILL LIKELY PRODUCE
AREAS OF HEAVY RAINFALL THAT WILL PROGRESS FROM WEST TO EAST
DURING THE PERIOD.  NORTHERN STREAM FLOW WILL GENERALLY BE ON THE
PROGRESSIVE SIDE--BRINGING SHORTWAVE ENERGY ACROSS THE
NORTHEAST/MID ATLANTIC AROUND SUN, A TRAILING RIDGE STILL EXPECTED
TO TRAVEL ALONG TO THE NORTH OF THE SOUTHERN STREAM SYSTEM FOR A
PERIOD OF TIME, AND A PACIFIC NORTHWEST/BC UPPER TROUGH THAT WILL
PUSH A COLD FRONT SOUTHEASTWARD INTO/THROUGH THE WESTERN-CENTRAL
STATES.  WITH A FAIR DEGREE OF UNCERTAINTY THE MOST COMMON SIGNAL
FARTHER UPSTREAM IS FOR SOME DECELERATION/AMPLIFICATION OF TROUGH
ENERGY OFF THE WEST COAST TOWARD NEXT TUE-WED.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

IN TERMS OF THE GUIDANCE AVERAGE, SOME ASPECTS OF TIMING AND TRACK
SEEM TO BE STARTING THE PROCESS OF STABILIZING A BIT FOR THE
SOUTHERN TIER SYSTEM.  RECENT DAYS HAVE TENDED TO FEATURE A SLOWER
AND MORE SUPPRESSED TREND IN THE GUIDANCE BUT THE 12Z ECMWF
ADJUSTED BACK TO THE NORTH.  THIS WAS IN RESPONSE TO TONING DOWN
THE STRENGTH OF THE UPPER RIDGE TO THE NORTH WHICH HAD BECOME A
FAIRLY STRONG EXTREME IN THE OLD 00Z RUN BY MON-TUE.  MEANWHILE
THE NEW 00Z UKMET IS A LITTLE FASTER THAN THE 12Z RUN AND THE CMC
IS NO FARTHER SOUTH THAN ITS PREVIOUS RUN.  THE GFS/GEFS STILL
LEAN A BIT TO THE FASTER SIDE OF THE ENVELOPE BY THE LATTER HALF
OF THE PERIOD BUT THE 00Z RUN HAS SLOWED A BIT FROM ITS 18Z/12Z
RUNS.  BY TUE-WED THE PRIMARY UNCERTAINTIES INVOLVE HOW DIFFUSE
SOUTHERN STREAM ENERGY APPROACHING FROM THE WEST COULD AFFECT THE
INITIAL UPPER LOW, AND SPECIFICS OF THE NORTHERN STREAM TROUGH
THAT SHOULD REACH EAST-CENTRAL NORTH AMERICA BY NEXT WED AND MAY
BEGIN TO HAVE AN INFLUENCE.  INDIVIDUAL ENSEMBLE SURFACE LOW
SPREAD FROM ALONG/OFFSHORE THE SOUTHEAST COAST THROUGH THE
NORTHERN MID ATLANTIC AS OF EARLY WED DOES NOT INSPIRE CONFIDENCE
IN A SPECIFIC SCENARIO.  HOWEVER THE MEANS HAVE BEEN CONSISTENT IN
DEPICTING A GENERAL AREA OF LOWER PRESSURES/INVERTED TROUGHING
AROUND MIDWEEK.

AS FOR THE TROUGH PROGRESSING ACROSS THE NORTHERN TIER U.S. AND
SOUTHERN CANADA, GUIDANCE AGREES FAIRLY WELL FOR THE LEADING
ENERGY INTO EARLY NEXT WEEK.  BY TUE-WED MODEL/ENSEMBLE SOLUTIONS
DIVERGE FOR IMPORTANT DETAILS THAT WOULD AFFECT TIMING/WAVINESS OF
THE LEADING SURFACE FRONT.  AT THAT TIME THE 12Z ECMWF/CMC WERE
SOMEWHAT CLOSER TO THE MEANS IN PRINCIPLE WHILE THE 12Z GFS BECAME
MORE SHALLOW/PROGRESSIVE WITH THE UPPER TROUGH AND THE 18Z RUN
DEVELOPED A MORE NEUTRAL TO NEGATIVE TILT LEADING TO A PRONOUNCED
UPPER GREAT LAKES/ONTARIO FRONTAL WAVE.

THE FORECAST PROBLEM ALONG THE WEST COAST BY THE LATTER HALF OF
THE PERIOD IS WHAT PROPORTION OF APPROACHING TROUGH ENERGY WILL
SEPARATE AND POSSIBLY CLOSE OFF A LOW CLOSE TO CALIFORNIA VERSUS
PROGRESS INTO THE NORTHERN U.S./SOUTHERN CANADA.  WITHIN A FAIRLY
BROAD RANGE OF MODEL/ENSEMBLE SOLUTIONS THE MOST COMMON IDEA IS
FOR AN UPPER LOW TO CLOSE OFF SOMEWHERE BETWEEN THE ECMWF/ECMWF
MEAN AND FASTER GFS/GEFS MEAN.  THE NEW 00Z GFS/GEFS MEAN HAVE
TRENDED SLOWER.  THE 12Z GFS DIFFERED FROM MOST OTHER SOLUTIONS
WITH THE STRENGTH/SHARPNESS OF ITS SHORTWAVE CROSSING THE
NORTHWEST.

BASED ON DECENT AGREEMENT DURING THE FIRST COUPLE DAYS OR SO OF
THE PERIOD AND THEN A FAIRLY RAPID DIVERGENCE IN GUIDANCE FOR SOME
DETAILS THEREAFTER, THE FORECAST STARTED WITH A 12Z ECMWF/18Z
GFS/12Z CMC BLEND (IN ORDER OF DECREASING WEIGHT) FOLLOWED BY A
TRANSITION TOWARD SOMEWHAT MORE 18Z GEFS/12Z ECMWF MEAN INPUT.
COMPONENTS OF THE 18Z-12Z GFS/12Z ECMWF WERE INCLUDED LATE IN THE
PERIOD BUT IN LOW ENOUGH WEIGHT TO DOWNPLAY ASPECTS THAT DIFFERED
SIGNIFICANTLY FROM THE CONSENSUS/AVERAGE.


...WEATHER/THREATS HIGHLIGHTS...

EXPECT THE THREAT FOR HEAVY RAINFALL AND STRONG TO SEVERE
CONVECTION TO BE FOCUSED OVER THE SOUTHERN PLAINS ON SAT.  CONSULT
SPC OUTLOOKS FOR LATEST INFO ON SEVERE THREATS.  PROGRESSION OF
THE UPPER LOW AND SURFACE SYSTEM SHOULD BRING HEAVIEST RAIN INTO
THE LOWER MS VALLEY BY SAT NIGHT/POSSIBLY SUN WITH ACTIVITY
REACHING THE SOUTHEAST THEREAFTER.  SOME AREAS ALONG THE EAST
COAST WILL LIKELY SEE RAINFALL EXTEND INTO MIDWEEK THOUGH LIKELY
WITH LESS INTENSITY THAN WHAT IS EXPECTED EARLIER IN THE PERIOD.
UNCERTAINTIES WITH SYSTEM EVOLUTION LATE IN THE PERIOD TEMPER
CONFIDENCE IN THE NORTHWARD EXTENT OF PRECIP TOWARD THE LATTER
PART OF THE PERIOD.

THE COLD FRONT INITIALLY OVER THE NORTHWEST WILL BRING LIGHT
PRECIP TO EXTREME NORTHERN AREAS ON SAT.  FARTHER EAST THE FRONT
SHOULD BE FAIRLY DRY UNTIL LATE WEEKEND INTO NEXT WEEK WHEN
TRAILING SHORTWAVE ENERGY/FRONTAL WAVE(S) SUPPORT DEVELOPMENT OF
SOME ACTIVITY FROM THE NORTH-CENTRAL ROCKIES
EASTWARD/NORTHEASTWARD.  SOME OF THIS RAIN/HIGH ELEVATION SNOW MAY
DROP SOUTHWARD OVER THE ROCKIES WITH TIME.

EXPECT THE MAJORITY OF THE WEST TO SEE ABOVE NORMAL TEMPS DURING
THE PERIOD WITH PLUS 5-15F ANOMALIES.  THE FRONT CROSSING NORTHERN
AREAS EARLY IN THE PERIOD WILL BRING A BRIEF PERIOD OF NEAR TO
BELOW NORMAL READINGS TO THAT PART OF THE REGION BEFORE A WARMING
TREND NEXT WEEK.  THE SOUTHERN TIER STORM WILL BRING MUCH BELOW
NORMAL HIGHS (MINUS 15-25F ANOMALIES) TO THE CENTRAL/SOUTHERN
PLAINS DURING THE WEEKEND AND LESS EXTREME COOL DAYTIME HIGHS
THROUGH THE SOUTHEAST INTO NEXT WEEK.  CHILLY TEMPS OVER THE
NORTHERN HALF OF THE EAST OVER THE WEEKEND WILL MODERATE BY
MON-WED.


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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