Quantitative Precipitation Forecast
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FOUS11 KWBC 192040
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
340 PM EST WED NOV 19 2014

VALID 00Z THU NOV 20 2014 - 00Z SUN NOV 23 2014


DAYS 1 THROUGH 3...


...GREAT LAKES...

LAKE EFFECT ENHANCEMENT IS REINVIGORATED WITH THE PASSAGE OF
STRONG SHORT WAVE DURING THE FIRST PART OF DAY 1. LAKE EFFECT
CONDITIONS REMAIN FAVORABLE THROUGH MUCH OF DAY 2...AND START
EASING FROM WEST TO EAST ON DAY 3. THERE WAS GOOD MODEL AGREEMENT
CONCERNING THE SYNOPTIC PATTERN OVER THE GREAT LAKES...SO THE QPF
AND THERMAL PORTIONS OF THE FORECAST WERE BASED ON A BLEND OF THE
12Z NAM/09Z SREF MEAN (DUE TO THEIR HIGHER RESOLUTION AND BETTER
DEPICTION OF LAKE SNOW BANDING).

A SYNOPTIC SCALE SYSTEM DISRUPTED THE INTENSE LAKE EFFECT
CIRCULATIONS ACROSS THE GREAT LAKES...BUT BEHIND THE SYSTEM THE
LAKE EFFECT CIRCULATION IS BECOMING REESTABLISHED. THE LOW LEVEL
FLOW BECOMES WEST NORTHWEST OVER THE UPPER GREAT LAKES...CROSSING
THE SHORT AXIS OF THE LAKES. THE CIRCULATION TAKES LONGER TO
BECOME REESTABLISHED ACROSS THE LOWER LAKES...AND IT COULD TAKE
UNTIL BETWEEN 20/0Z AND 20/12Z TO BECOME FULLY ENGAGED OVER LAKE
ONTARIO. MODEL SOUNDINGS SHOWED A DEEPENING CAPPING INVERSION
(REACHING TO BETWEEN 700 AND 600 MB AT ITS
DEEPEST)...UNIDIRECTIONAL WINDS IN THE LAYER...AND MOISTURE AND
LIFT IN THE DENDRITIC GROWTH ZONE. THE LOW LEVEL FLOW OVER THE
SHORT AXIS OF THE UPPER LAKES SUPPORTS MULTI BAND
FEATURES...DEPOSITING OF 4 TO 8 INCHES OF SNOW (WITH LOCALLY
HIGHER AMOUNTS WHERE THE BANDS LINGER LONGEST) ACROSS THE UP OF MI
AND THE NORTHERN PORTION OF THE LP OF MI

ACROSS THE LOWER LAKES...THE LAKE-850 TEMPERATURE DIFFERENCES ARE
NOT EXPECTED TO BE AS STEEP AS THE PREVIOUS EVENT...BUT STRONG
INSTABILITY IS INDICATED (ALONG WITH THUNDER AND LIGHTNING EAST OF
LAKE ERIE). THE LOW LEVEL FLOW PARALLELS THE LONG AXIS OF THE
LOWER LAKES...SUGGESTING SINGLE SNOW BANDS DOMINATING BOTH ERIE
AND ONTARIO...RESULTING IN 6 TO 12 INCH SNOWFALL AMOUNTS (WITH
HIGHER AMOUNTS WHERE BANDS REMAINS THE LONGEST).

ON DAY 2...THE LAKE EFFECT ENHANCEMENT REMAINS IN PLACE ACROSS THE
LOWER LAKES...AS THE LOW LEVEL WINDS REMAIN PARALLEL TO THE LONG
AXIS OF THE LAKE. THE WIND PROFILE SUGGESTS THAT SINGLE SNOW BANDS
CONTINUE ACROSS ERIE AND ONTARIO FOR MUCH OF THE DAY. AS THE LOW
LEVEL FLOW VEERS TO THE NORTHWEST DURING THE SECOND HALF OF THE
PERIOD...THE SINGLE BAND REGIME TRANSITIONS TO MORE MULTI BAND
ACROSS THE SHORTER LAKE AXIS. THE SINGLE BAND AXIS REMAINS IN
PLACE LONGEST ACROSS LAKE ONTARIO...WHERE 4 TO 8 INCHES ARE
EXPECTED...WITH HIGHER AMOUNTS WHERE THE BAND REMAINS IN PLACE THE
LONGEST.

THE LOW LEVEL FLOW STARTS EASING DURING THE SECOND HALF OF DAY 2
ACROSS THE UPPER LAKES...AND THE INTENSITY OF THE MULTI BAND
FEATURES DIMINISHES. THIS OCCURS FIRST ACROSS LAKE
SUPERIOR...WHERE 2 TO 4 INCHES OF SNOWFALL IS EXPECTED ACROSS THE
UP OF MI. LESSER AMOUNTS ARE EXPECTED ACROSS THE NORTHERN PORTION
OF THE LP OF MI...WHERE THE FLOW LEVEL FLOW EASES A BIT LATER.

THE SYNOPTIC ENVIRONMENT BECOMES LESS FAVORABLE FOR LAKE EFFECT
SNOWFALL ON DAY 3...AS THE MID LEVEL FLOW BACKS TO THE SOUTHWEST.
MOISTURE STREAMING NORTHEAST WILL OVERRUN THE COLD AIR AND SNOW IN
PLACE...RESULTING IN THE POTENTIAL FOR LIGHT ICING...MAINLY ACROSS
WI AND THE LP OF MI LATER IN THE DAY.


...MIDWEST/OH VALLEY...

MOISTURE AHEAD OF A DEVELOPING MID LEVEL LONG WAVE TROUGH OVER THE
CENTER OF THE COUNTRY WILL OVERRIDE RETREATING LOW LEVEL COLD AIR
DURING DAY 3...RESULTING IN THE POTENTIAL FOR ICING ACROSS THE
MIDWEST AND OH VALLEY. THERE WAS GENERALLY GOOD MODEL AGREEMENT
WITH THE SYNOPTIC SETUP...SO THE QPF PORTION OF THE FORECAST WAS
BASED ON A MULTI MODEL BLEND. THE THERMAL PORTION OF THE FORECAST
WAS BASED ON A BLEND OF THE 12Z GFS/NAM/09Z SREF MEAN.

MOISTURE STREAMING NORTHEAST ON A DEEP MID LEVEL SOUTHWEST FLOW
WILL OVERRUN RESIDUAL ARCTIC AIR IN PLACE ACROSS PORTIONS OF THE
MIDWEST AND OH VALLEY LATE ON DAY 2 AND DURING DAY 3. THE COLUMN
IS DRY AT THE START OF THE LOW LEVEL WARM AIR PROCESS...BUT
MOISTENS LATE ON DAY 2 ACROSS PORTIONS OF THE MIDWEST. MODEL
SOUNDINGS ACROSS NORTHERN IL...IN...AND NORTHERN OH SHOW A CLASSIC
FREEZING RAIN PROFILE...WITH A MAXIMUM TEMPERATURE IN THE WARM
NOSE OF 4 CELSIUS AT ABOUT 850 MB. PRECIPITATION AMOUNTS SHOULD BE
FAIRLY LIGHT...SO ICE ACCUMULATIONS OF 0.10 INCHES OR LESS ARE
EXPECTED.


...PACIFIC NORTHWEST/NORTHERN ROCKIES/GREAT BASIN...

SHORT WAVE ENERGY COMING ASHORE FROM THE PACIFIC WILL BEGIN TO
CHOP DOWN MID LEVEL RIDGING ALONG THE WEST COAST DURING DAY 1.
ADDITIONAL SHORT WAVE ENERGY WILL TAP PACIFIC MOISTURE AND
INCREASE THE THREAT FOR HEAVY SNOW ACROSS THE HIGHER TERRAIN OF
THE PACIFIC NORTHWEST AND NORTHERN ROCKIES...ESPECIALLY ON DAY 3.
WHILE THERE IS GENERALLY GOOD MODEL AGREEMENT WITH RESPECT TO THE
OVERALL SYNOPTIC SETUP...THERE IS SOME SPREAD ON THE TIMING OF THE
SHORT WAVES ON DAYS 2 AND 3. WITH THIS IN MIND...THE QPF PORTION
OF THE FORECAST WAS BASED ON A BLEND OF THE 12Z GFS/00Z
ECMWF...WITH SOME 09Z SREF MEAN USED TO BETTER DELINEATE HIGHER
TERRAIN QPF MAXIMA. THE THERMAL PORTION OF THE FORECAST WAS BASED
ON A BLEND OF THE 12Z NAM/GFS/00Z ECMWF.

THE FIRST SHORT WAVE CRASHES THROUGH THE MID LEVEL RIDGE POSITION
ALONG THE WEST COAST BETWEEN 20/0Z AND 20/0Z ACROSS OR AND
NORTHERN CA. COLD AIR REMAINING TRAPPED UNDER THE STRONG INVERSION
IN PLACE UNDER THE MID LEVEL RIDGE (AS CLEARLY SHOWN ON 12Z
OBSERVED SOUNDINGS) WILL BE SLOW TO BECOME DISLODGED...RAISING THE
SPECTER FOR FREEZING RAIN. MODEL SOUNDINGS SHOWED A CLASSIC
FREEZING RAIN PROFILE...WITH A MAXIMUM TEMPERATURE IN THE WARM
NOSE BETWEEN 3 AND 4 CELSIUS. THE LIFT AND MOISTURE WITH THE
INITIAL SHORT WAVE IS RELATIVELY WEAK...RESULTING IN AN EXPANSIVE
AREA OF 0.01 TO 0.10 INCHES OF ICE ACROSS THE COLUMBIA GORGE AND
HOOD RIVER VALLEY.

A SECOND STRONGER SHORT WAVE CROSSES WA AND OR LATE ON DAY
2...WITH A LARGE AREA OF LIFT AND MOISTURE ACROSS THE REGION. SNOW
LEVELS WILL BE ON THE RISE IN THE DIFFERENTIAL WARM AIR ADVECTION
AHEAD OF THE SHORT WAVE...REACHING LEVELS BETWEEN 6000 AND 8000
FEET BY THE END OF THE PERIOD. THE LIFT AND MOISTURE SHOULD
SUPPORT 4 TO 8 INCHES OF SNOWFALL ACROSS THE NORTHERN CASCADES OF
WA...AND THESE VALUES ARE SUPPORTED BY MEMBERS OF THE LATEST ECMWF
ENSEMBLE OUTPUT INDICATING THE POTENTIAL FOR 8+ INCHES OF SNOW
HERE.

THE STRONG SHORT WAVE CROSSES THE PACIFIC NORTHWEST AND NORTHERN
ROCKIES ON DAY 3...PUSHING A SURFACE FRONT TO THE ROCKIES BY LATE
IN THE DAY. THE SHORT WAVE ALSO AIDS IN LOW LEVEL CYCLOGENESIS
OVER WESTERN CANADA...AND THE INCREASING FLOW BEHIND THE
DEVELOPING LOW SUPPORTS UPSLOPE FLOW IN THE HIGHER TERRAIN OF THE
NORTHERN ROCKIES. THE SHORT WAVE TAPS PACIFIC MOISTURE AS IT COMES
ASHORE...AND THE LOW LEVEL FLOW FOCUSES THE MOISTURE ON THE HIGHER
TERRAIN. SNOW LEVELS STABILIZE BETWEEN 6000 AND 8000 FEET...WITH
THE HIGHEST VALUES ACROSS THE CASCADES OF WA AND OR.

THE COMBINATION OF DEEP MOISTURE AND UPSLOPE FLOW SHOULD SUPPORT
AXES OF 8 TO 16 INCHES OF SNOWFALL ACROSS THE CASCADES...WITH THE
HIGHEST AMOUNTS ACROSS THE NORTHERN CASCADES OF WA. WHILE THE
MOISTURE SOURCE IS NOT AS DEEP ACROSS THE NORTHERN ROCKIES...THE
SYNOPTIC SCALE LIFT...AIDED BY UPSLOPE FLOW...SHOULD MAKE THE MOST
OF THE AVAILABLE MOISTURE. SNOWFALL AMOUNTS OF 8 TO 14 INCHES ARE
EXPECTED ACROSS THE BITTERROOTS OF ID...WITH 6 TO 10 INCHES OF
SNOW STRETCHING FROM THE NORTHERN WASATCH OF UT INTO THE GRAND
TETONS OF NORTHWEST WY. ECMWF ENSEMBLE MEMBERS SHOWING THE
POTENTIAL FOR 12+ INCHES OF SNOWFALL HERE SUPPORT A MODERATE RISK
FOR 12+ INCHES OF SNOW.


HAYES





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