Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 162049
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
348 PM EST SAT DEC 16 2017

VALID 00Z SUN DEC 17 2017 - 00Z WED DEC 20 2017


DAYS 1 THROUGH 3...


...CASCADES AND NORTHERN ROCKIES...

SIGNIFICANT SNOWFALL IS EXPECTED FROM THE WASHINGTON CASCADES EAST
INTO THE MOUNTAINOUS REGIONS OF NORTHERN IDAHO AND NORTHWEST
MONTANA OVER THE NEXT FEW DAYS. MODELS ARE IN GOOD AGREEMENT IN
SHOWING PERSISTENT ZONAL FLOW FROM TONIGHT THROUGH MONDAY
NIGHT...WITH AN ASSOCIATED PLUME OF DEEPER MOISTURE EXTENDING BACK
INTO THE CENTRAL PACIFIC AROUND 30N AND THE INTERNATIONAL DATE
LINE. ALTHOUGH THIS MAY NOT ATTAIN TRADITIONALLY DEFINED
ATMOSPHERIC RIVER THRESHOLDS...THE PERSISTENT W-SW FLOW
INTERCEPTING PRIMARILY N-S ORIENTED TERRAIN FEATURES SHOULD
PROVIDE SEVERAL DAYS WORTH OF MODERATE TO OCCASIONALLY HEAVY
PRECIPITATION VIA ENHANCED OROGRAPHIC ASCENT. A DIGGING TROUGH AND
ASSOCIATED COLD FRONT WILL ARRIVE IN THE PACIFIC NORTHWEST ON DAY
3 (TUESDAY)...WHICH SHOULD PROVIDE BOTH AN ENHANCEMENT TO
PRE-FRONTAL SW WINDS (AND THUS PERHAPS OROGRAPHIC ASCENT) AND
SYNOPTIC-SCALE LIFTING MECHANISMS.

IN ADDITION TO BEING TIED TO UPSLOPE REGIONS OF PROMINENT TERRAIN
FEATURES...THE HEAVIER SNOW TOTALS ARE LIKELY TO OCCUR FURTHER
NORTH GIVEN THE ZONAL FLOW AND RELATED SYNOPTIC WEST-EAST LOW-MID
LEVEL TEMPERATURE GRADIENT. POSITIVE THETAE ADVECTION ON SUNDAY IS
LIKELY TO PUSH SNOW LEVELS A BIT HIGHER...BUT THESE SHOULD
STABILIZE ON MONDAY AND THEN BEGIN TO FALL ON TUESDAY WITH THE
APPROACH OF THE DIGGING SHORTWAVE. NORTH OF 48N ALONG AND TO THE
EAST OF THE CASCADES...SNOW LEVELS ARE LIKELY TO BE AROUND 4000
FEET OR LOWER FOR MOST THE DURATION OF THE EVENT. ALTHOUGH THEY
MAY FLUCTUATE FURTHER SOUTH...THEY SHOULD ALSO BE BELOW 6000 FEET
FROM SUNDAY NIGHT INTO TUESDAY THROUGH MUCH OF THE NORTHWEST BASED
ON A GENERAL MODEL BLEND AND THE NBM.

FOR THE COLDER (NEAR OR NORTH OF 48N) LOCATIONS ALONG THE MORE
SIGNIFICANT MOUNTAIN RANGES (CASCADES EAST TO THE CONTINENTAL
DIVIDE IN NRN ID AND NWRN MT)...IT IS VERY LIKELY OVER 18 INCHES
OF SNOW WILL FALL THROUGH 00Z WEDNESDAY. WPC PROBABILISTIC
SNOWFALL HAS OVER A 70 PERCENT CHANCE OF EXCEEDING 18 INCHES IN
THE HIGHER ELEVATION PORTIONS OF THESE MOUNTAIN RANGES. FOR THE
HIGHEST ELEVATIONS OF THE NORTHERN WASHINGTON CASCADES...THE
PROBABILITY OF EXCEEDING 30 INCHES OF SNOW IS OVER 80 PERCENT AND
MEDIAN VALUES ARE CLOSER TO 4 FEET. LOWER AMOUNTS ARE EXPECTED
FURTHER SOUTH WHERE SNOW LEVELS MAY COME UP A LITTLE MORE.


...HIGHER ELEVATIONS OF ARIZONA AND NEW MEXICO ESPECIALLY FROM THE
WHITE MOUNTAINS INTO THE GILA REGION...

A SHARPLY DIGGING SHORTWAVE ALONG THE PACIFIC COAST TODAY WILL
QUICKLY CLOSE OFF A LOW ALOFT NEAR THE NORTHERN TIP OF THE GULF OF
CALIFORNIA BY SUNDAY MORNING. MODEL CONSENSUS THEN SLOWLY DRIFTS
THE LOW TO THE EAST TOWARD THE NM-AZ BORDER REGION BY MONDAY
BEFORE GRADUALLY DEVOLVING BACK INTO AN OPEN WAVE AS IT CONTINUES
EAST. PRECIPITATION WILL BE LIKELY IN THE VICINITY OF THE CLOSED
LOW AND SOME DYNAMIC COOLING IS LIKELY TO SUPPORT ENOUGH COLD AIR
FOR SNOW AT THE HIGHEST ELEVATIONS OF EC/SE ARIZONA AND SW/WC NEW
MEXICO. BLENDED SNOW LEVELS ARE FORECAST TO BE AROUND 6000-7000
FEET OVER MUCH OF THE REGION WHICH SHOULD SUPPORT SNOW
PARTICULARLY FROM THE WHITE MOUNTAINS IN ERN AZ INTO THE GILA
REGION OF WRN NEW MEXICO. THE MOGOLLON RIM MAY ALSO BE
AFFECTED...BUT FORECAST QPF IS LIGHTER THAT FAR NORTHWEST...SO
SNOW ACCUMULATIONS IN THOSE AREAS WILL LIKELY BE TIED MORE TO
POSITIONAL UNCERTAINTY OF THE PRECIPITATION PLUME. OTHER SMALLER
RANGES WITH PEAKS ABOVE 6000 FEET IN SOUTHEAST ARIZONA (LIKE THE
PINALENOS OR CHIRICACUAS) AND SOUTHERN NEW MEXICO MAY BE AFFECTED
AS WELL.

THROUGH THE ENTIRE REGION...THERE IS FAIRLY LARGE SPREAD ON
FORECAST SNOW ACCUMULATIONS. WPC PROBABILITIES AND SNOWFALL
PERCENTILES BEAR THIS OUT. FOR EXAMPLE...IN THE WHITE MOUNTAINS
ABOVE 8000 FEET...MEDIAN (50TH PERCENTILE) 24-HOUR SNOW
ACCUMULATIONS ENDING 12Z MONDAY ARE IN THE 4-6 INCH RANGE. HOWEVER
25TH PERCENTILE VALUES ARE CLOSER TO 1 INCH AND 75TH PERCENTILE
VALUES ARE 8-12 INCHES (BOTH REASONABLE POSSIBILITIES IN THE
PROBABILITY DISTRIBUTION). THE WIDE SPREAD IN SNOWFALL IS LIKELY
DUE TO MODEL UNCERTAINTIES RELATED TO TIMING OF THE WAVE AS THE
CLOSED LOW SLOWLY PUSHES EAST...AS WELL AS TEMPERATURE PROFILES.
THEREFORE...WHILE SOME ACCUMULATING SNOW DOES APPEAR LIKELY AT
HIGHER ELEVATIONS...HIGHER IMPACT ACCUMULATIONS SHOULD NOT BE
DISCOUNTED AS A POSSIBILITY IN SOME AREAS.


...NORTHERN INTERIOR NEW ENGLAND...

SEVERAL LOW-AMPLITUDE WAVES IN THE NORTHERN STREAM JET ARE LIKELY
TO PASS QUICKLY THROUGH THE GREAT LAKES AND NORTHEAST ON DAYS 2
AND 3 (MONDAY AND TUESDAY)...WHICH SHOULD PROVIDE A COUPLE CHANCES
OF ACCUMULATING SNOW IN INTERIOR PORTIONS OF NORTHERN NEW ENGLAND.
THE FIRST CHANCE ON MONDAY WILL RESULT FROM A RAPIDLY DEAMPLIFYING
WAVE FROM THE PLAINS PHASING WITH THE NORTHERN STREAM AND DRAWING
SOME MOISTURE FURTHER NORTH IN THE PROCESS. A SECOND SHORTWAVE
FOLLOWING CLOSELY BEHIND SHOULD PROVIDE ENOUGH LARGE SCALE FORCING
FOR SOME PRECIPITATION ACROSS THE REGION. THE SECOND CHANCE ON
TUESDAY WILL RESULT FROM A STRONGER DIGGING WAVE AND AN ASSOCIATED
COLD FRONT.

THE HIGHEST PROBABILITIES OF AT LEAST 1 INCH OF SNOW STRETCH FROM
UPSTATE NEW YORK AND THE ADIRONDACKS NORTHEAST THROUGH VERMONT AND
NRN NEW HAMPSHIRE...AND INTO THE WHITE MOUNTAINS AND NORTH WOODS
OF INTERIOR MAINE. THE TWO ROUNDS OF SNOW COMBINED MAY PRODUCE
SOME GREATER ACCUMULATIONS (4+ INCHES) IN THE ADIRONDACKS.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

LAMERS


$$





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