Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8
000
FXUS02 KWBC 181558
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1157 AM EDT WED OCT 18 2017

VALID 12Z SAT OCT 21 2017 - 12Z WED OCT 25 2017

...HEAVY RAINFALL POTENTIAL IN THE EASTERN U.S. WITH RECORD HEAT
IN THE SOUTHWESTERN STATES NEXT WEEK...


...PATTERN OVERVIEW/MODEL GUIDANCE/FORECAST PREFERENCES...

DURING THE UPCOMING FORECAST PERIOD...A HIGHLY AMPLIFIED SYNOPTIC
PATTERN WILL FEATURE SOME IMPRESSIVE 500-MB HEIGHT ANOMALIES WITH
A PAIR OF NOTABLE FEATURES. A CLOSED LOW POTENTIALLY PEELING OFF
FROM THE PREVAILING WESTERLIES MAY AFFECT THE GULF COAST REGION
EARLY NEXT WEEK. GIVEN POTENTIAL PHASING WITH A NORTHERN STREAM
TROUGH...A RESULTANT NEGATIVELY-TILTED TROUGH COULD LEAD TO 500-MB
HEIGHT DEPARTURES ON THE ORDER OF 3 TO 4 STANDARD DEVIATIONS BELOW
AVERAGE BY THE MIDDLE OF NEXT WEEK ACROSS THE EAST COAST. WHILE A
SHARP NEGATIVE HEIGHT ANOMALY ENCOMPASSES THE EASTERN THIRD OF THE
NATION...PRONOUNCED RIDGING OVER THE WEST WILL BRING WARM/DRY
CONDITIONS TO THE REGION. EQUALLY IMPRESSIVE...THE 00Z GFS FAVORS
500-MB HEIGHT ANOMALIES IN THE 1.5 TO 2 STANDARD DEVIATION RANGE.

CONSIDERING THE EVOLUTION OF THE LONGWAVE PATTERN...THE INITIAL
BAND OF HEIGHT FALLS WILL BE TRAVERSING THROUGH THE HIGH PLAINS
EARLY SATURDAY WHILE SWINGING EASTWARD IN TIME. THE CORE OF LOWER
HEIGHTS ARE FORECAST TO LIFT TOWARD SOUTHERN MANITOBA INTO WESTERN
ONTARIO OVER THE WEEKEND WHILE THERE ARE INCREASING INDICATIONS OF
A SPLIT IN THE JET STREAM TAKING SHAPE. BY SUNDAY...A MAJORITY OF
THE AVAILABLE SOLUTIONS FAVOR A SOUTHERN STREAM SYSTEM UNFOLDING
OVER THE SOUTHERN PLAINS WITH GRADUAL DOWNSTREAM PROGRESSION. THE
LARGEST UNCERTAINTY IS HOW QUICKLY THE SHORTWAVE ENERGY WILL TRACK
TOWARD THE GULF COAST AS OPERATIONAL MODELS CONTINUE TO WAVER
BETWEEN RUNS AND AMONGST THEMSELVES. THERE HAS BEEN PLENTY OF
ENSEMBLE SUPPORT VIA SPAGHETTI PLOTS TO SUPPORT A GENERAL SLOWING
AND CLOSING OFF OF AN UPPER LOW. THE OVERALL PATTERN IS CONDUCIVE
TO SUCH A FORECAST GIVEN AMPLE SEPARATION BETWEEN THE NORTHERN AND
SOUTHERN STREAMS. HOWEVER...THE POOR RUN-TO-RUN MODEL CONTINUITY
LEAVES A GREAT DEAL OF UNCERTAINTY MOVING FORWARD. MOST
NOTABLY...THE 00Z CMC/UKMET/GFS DEPICT A POWERFUL UPPER LOW IN THE
VICINITY OF THE UPPER TX/LA COAST ON MONDAY EVENING WHILE THE 06Z
GFS AND 00Z/12Z ECMWF ARE MORE PROGRESSIVE. THE 16/1200Z CYCLE OF
THE ECMWF WAS THE MOST RECENT IN THE SUITE TO SUPPORT A CLOSED LOW
AS SLOW AS THE 00Z CMC/UKMET/GFS ARE SUGGESTING. WHILE WITHIN THE
REALM OF POSSIBILITIES...IT HAS BEEN A FEW RUNS SINCE THE ECMWF
INDICATED SUCH A FORECAST. EVENTUALLY THIS SOUTHERN STREAM FEATURE
WILL BECOME ABSORBED BY A LARGER-SCALE TROUGH ACROSS THE GREAT
LAKES WITH A RESULTANT NEGATIVE TILT EXPECTED. AN ABUNDANT SURGE
OF TROPICAL MOISTURE IS LIKELY TO ADVECT POLEWARD BRINGING A
THREAT FOR HEAVY RAINFALL ALONG AND AHEAD OF THE COLD FRONT.
EAST/WEST TIMING DIFFERENCES ARE NOTED WITH THE WEAKER SOUTHERN
STREAM SOLUTIONS FAVORING A QUICKER EASTWARD PROGRESSION.
ELSEWHERE...THERE IS NOT MUCH TO COMMENT ON ACROSS THE WESTERN
U.S. WITH BROAD RIDGING ESTABLISHING ITS PRESENCE OVER THE REGION.
THE CENTER OF THE UPPER RIDGE SHOULD BE FOCUSED OVER THE
SOUTHWESTERN STATES LEADING TO A POTENTIAL HEAT WAVE.

DECIDED TO MAINTAIN CONTINUITY FAVORING A MORE PROGRESSIVE
SOUTHERN STREAM SOLUTION. THE 06Z GFS AND PREVIOUS 12Z ECMWF
SUPPORT THIS NOTION THROUGHOUT THE PERIOD. UTILIZED 20 PERCENT OF
THE OVERNIGHT SHIFT FORECAST BLEND IN COMBINATION WITH THESE TWO
OPERATIONAL MODELS. GRADUALLY INCORPORATED THEIR ENSEMBLE
COUNTERPARTS FROM DAY 5/MONDAY ONWARD GIVEN INCREASING UNCERTAINTY
ACROSS THE EASTERN U.S. GIVEN THE HIGH AMPLITUDE PATTERN IN
PLACE...THE FORECAST CONFIDENCE IS AROUND AVERAGE BUT MUCH OF THIS
FOCUSES ON THE EVOLVING UPPER LOW.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

MILD AIR PUMPING NORTHWARD AHEAD OF THE SLOW MOVING UPPER TROUGH
WILL POTENTIALLY LEAD TO A NUMBER OF DAILY TEMPERATURE RECORDS
BEING BROKEN. WHILE THIS IS NOT EXPECTED FOR THE AFTERNOON
HIGHS...THE OVERNIGHT WARMTH MAY LEAD TO SOME RECORDS ANYWHERE
FROM THE SOUTHERN/CENTRAL PLAINS INTO THE UPPER MIDWEST ON
SATURDAY MORNING. READINGS AROUND 20 TO 25 DEGREES ABOVE NORMAL
WILL BE COMMONPLACE FROM EASTERN KS UP INTO IA/MN/WI. WHILE THE
COLD FRONTAL PASSAGE WILL BRING COOLER WEATHER WITH IT...THE LACK
OF ARCTIC AIR SHOULD ONLY LEAD TO TEMPERATURES RETURNING CLOSER TO
CLIMATOLOGY. HOWEVER...SOME BELOW NORMAL TEMPERATURES ARE LIKELY
GIVEN ACTIVE PRECIPITATION ACROSS THE SOUTHERN TIER STATES.
ELSEWHERE...A STRONG UPPER RIDGE BUILDING ACROSS THE SOUTHWESTERN
U.S. SHOULD BRING SOME RECORD WARMTH TO THE REGION EARLY NEXT
WEEK. CURRENT FORECAST HIGHS ABOVE THE 100 DEGREE MARK OVER
SOUTHERN CA MAY BREAK SOME DAILY RECORDS. OFFSHORE GRADIENTS WILL
LEAD TO HOT/DRY/BREEZY CONDITIONS POTENTIALLY AIDING IN A FIRE
WEATHER RISK DURING THE PERIOD.

OVER THE WEEKEND...ZONAL FLOW ACROSS THE PACIFIC NORTHWEST WILL
LEAD TO HEAVY PRECIPITATION OVER THE OROGRAPHICALLY FAVORED
LOCATIONS OF THE OLYMPICS AND CASCADES. THIS WESTERLY FLOW REGIME
SHOULD BE DEVOID OF ANY APPRECIABLE COLD AIR WHICH MAINTAINS SNOW
OVER THE HIGHER ELEVATIONS. WITH HEIGHTS BUILDING IN TIME...THE
ACTIVE STORM TRACK WILL LIFT UP INTO BRITISH COLUMBIA NEXT WEEK.
FARTHER EAST...WIDESPREAD RAINFALL SHOULD BE ONGOING ALONG AND
AHEAD OF THE COLD FRONT MOVING THROUGH THE CENTRAL U.S. ON
SATURDAY. THE ACTIVITY WILL LIKELY PICK UP IN COVERAGE AND
INTENSITY AS THE SOUTHERN STREAM SHORTWAVE DEVELOPS WITH HEAVIER
AMOUNTS POSSIBLE IF THE SYSTEM SLOWS DOWN. A HEAVY RAINFALL SIGNAL
EXISTS OVER THE ENTIRE EASTERN SEABOARD FROM OCTOBER 23-25 ONCE
THE UPPER TROUGH STRENGTHENS AND A SERIES OF SURFACE LOWS FORM.
SOME OPERATIONAL MODELS PRODUCE 2 TO 4 INCHES IN A 24-HOUR PERIOD
SUGGESTING THE POTENTIAL FOR FLASH FLOODING IN CERTAIN LOCATIONS.
IT STILL IS TOO FAR OUT IN TIME TO DETERMINE ANY SPECIFIC THREATS.


RUBIN-OSTER

$$




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