Quantitative Precipitation Forecast
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FOUS11 KWBC 171953
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
252 PM EST MON NOV 17 2014

VALID 00Z TUE NOV 18 2014 - 00Z FRI NOV 21 2014


DAYS 1 THROUGH 3...


...GREAT LAKES...

A VERY STRONG MID LEVEL CLOSED LOW (WITH 500 MB HEIGHTS BETWEEN
THREE AND FOUR STANDARD DEVIATIONS BELOW NORMAL) WILL CROSS THE
GREAT LAKES DURING DAYS 1 AND 2...SETTING THE STAGE FOR A MAJOR
LAKE EFFECT SNOW EVENT. AFTER A BIT OF A LULL IN THE ACTIVITY LATE
ON DAY 2...ANOTHER  SHORT WAVE DROPS INTO PLACE OVER THE GREAT
LAKES...WITH LAKE EFFECT ACTIVITY INCREASING AGAIN DURING DAY 3.
FOR THE MOST PART...THERE WAS GOOD MODEL AGREEMENT CONCERNING THE
OVERALL SYNOPTIC SETUP. WITH THIS IN MIND...THE QPF AND THERMAL
PORTIONS OF THE FORECAST WAS BASED ON A BLEND OF THE 12Z NAM/09Z
SREF MEAN...CHOSEN MAINLY FOR THEIR ABILITY TO BETTER RESOLUTION
THE LAKE EFFECT ACTIVITY.

DAY 1...AS THE VERY COLD AIR OVERSPREAD THE GREAT LAKES (WITH THE
LOWER LAKES THE LAST TO HAVE THE PIECES FOR A MAJOR LAKE EFFECT
EVENT FALL INTO PLACE)...THE LOW LEVEL FLOW BECOMES UNIDIRECTIONAL
(ALONG THE LONG AXIS OF THE LAKE) UNDER A DEEPENING CAPPING
INVERSION. MODEL SOUNDINGS ACROSS THE LOWER GREAT LAKES (ERIE AND
ONTARIO) SHOWED THE CAPPING INVERSION RISING TO AROUND 700
MB...LOW LEVEL LAPSE RATES STEEPENING TO GREATER THAN 20 C (LAKE
TEMPERATURE - 850 MB TEMPERATURE) AND A PEAK IN VERTICAL MOTION
BETWEEN 18/06Z AND 18/18Z. DURING THIS TIME...INTENSE SINGLE BAND
FEATURES ARE EXPECTED OFF BOTH LOWER LAKES. DUE TO THE INSTABILITY
EXPECTED (WHICH COULD RESULT IN THUNDER AND LIGHTNING)...AS WELL
AS NEARLY IDEAL LAKE EFFECT CONDITIONS...AXES OF 18 T0 24 INCHES
WERE STRETCHED EAST OF EACH LAKE...WITH LOCALLY HIGHER AMOUNTS
WHERE THE BAND STAYS IN PLACE THE LONGEST.

ACROSS THE UPPER GREAT LAKES...THE COLDEST AIR CROSSES THE REGION
DURING THE FIRST PART OF THE DAY...RESULTING IN THE BEST LOW LEVEL
LAPSE RATES. THE LOW LEVEL NORTHWEST FLOW PERSISTS UNDER THE
CAPPING INVERSION...WHICH GETS DISRUPTED LATER IN THE DAY AHEAD OF
A SYNOPTIC SCALE SYSTEM. THE LOW LEVEL FETCH FAVORS THE UP OF MI
OFF LAKE SUPERIOR AND THE NORTHERN PORTION OF THE LP OF MI...AS
WELL AS SOUTHWEST MI OFF LAKE MI. IN THE FAVORED LOCATIONS...6 TO
10 INCHES OF SNOW IS EXPECTED...WITH LOCALLY HIGHER AMOUNTS ACROSS
THE UP OF MI.

DAY 2...THE NEARLY IDEAL LAKE EFFECT CONDITIONS (DEEP CAPPING
INVERSION...UNIDIRECTIONAL FLOW UNDER THE INVERSION...PEAK IN
VERTICAL MOTION) WILL CONTINUES ACROSS THE LOWER LAKES FOR AT
LEAST THE FIRST PORTION OF THE DAY. A CLIPPER LIKE SURFACE SYSTEM
(AND ITS MID LEVEL SHORT WAVE) CROSSING THE UPPER LAKES WILL START
TO DISRUPT THE LAKE EFFECT CIRCULATIONS FROM WEST TO EAST DURING
THE PERIOD. THE LAKE EFFECT ENHANCEMENT REMAINS IN PLACE LONGEST
OVER LAKE ONTARIO...WHERE THE SINGLE BAND SETUP SHOULD RESULT IN
AN AXIS OF 8 TO 12 INCHES (WITH LOCALLY HIGHER AMOUNTS IN PLACES
WHERE THE BAND REMAINS IN PLACE THE LONGEST). ACROSS LAKE
ERIE...THE SINGLE BAND BECOMES DISRUPTED EARLIER BY THE SYNOPTIC
SCALE FLOW...BUT THE BAND SHOULD DROP BETWEEN 4 AND 8 INCHES OF
SNOW BEFORE THAT OCCURS. ADDITIONAL LIGHT SNOWFALL AMOUNTS ARE
POSSIBLE IN THE SYNOPTIC SCALE WARM AIR ADVECTION LATE IN THE DAY.

OVER THE UPPER GREAT LAKES...THE LAKE EFFECT CIRCULATION IS
DISRUPTED BY A SYNOPTIC SCALE SYSTEM EARLY IN THE DAY.
HOWEVER...BEHIND THE STRONG SHORT WAVE (AND ITS SURFACE
REFLECTION)...THE LOW LEVEL WEST NORTHWEST FLOW INCREASES UNDER A
DEEPENING CAPPING INVERSION. THE LOW LEVEL FLOW FAVORS THE UP OF
MI...THE NORTHERN PORTION OF LP OF MI AND PORTIONS OF SOUTHWEST MI
WITH 4 TO 6 INCHES OF SNOW.

DAY 3...THE LAKE EFFECT CIRCULATION IS RESTORED OVER ALL OF THE
LAKES AS THE MID LEVEL FLOW BECOMES BROADLY CYCLONIC. THE AIR IS
NOT QUITE AS COLD AS THE SHOT FOR DAY 1...BUT THE LOW LEVEL FLOW
BECOMES MORE UNIDIRECTIONAL AND DEEPENS UNDER THE RISING CAPPING
INVERSION. ACROSS THE UPPER GREAT LAKES...THE LOW LEVEL FLOW AGAIN
FAVORS THE UP OF MI AND THE NORTHERN PORTION OF THE LP OF MI FOR 4
TO 8 INCHES. OF SNOW. OVER THE LOWER GREAT LAKES...THE BOUNDARY
LAYER FLOW IS MORE ALONG THE LONG AXIS OF BOTH LAKES...WHICH
SHOULD RESULT IN SWATHES OF 4 TO 8 INCHES OF SNOWFALL (WITH
LOCALLY HIGHER AMOUNTS IN PLACES WHERE THE BANDS REMAIN IN PLACE
THE LONGEST).


...PACIFIC NORTHWEST/NORTHERN ROCKIES.

A SHORT WAVE COMING ASHORE ACROSS OR/NORTHERN CA LATE ON DAY 3
WILL FUNNEL MOISTURE INTO THE REGION...SETTING THE STAGE FOR SOME
WINTRY PRECIPITATION...MAINLY ACROSS THE HIGHER TERRAIN. THERE WAS
SOME TIMING ISSUES CONCERNING THE SHORT WAVE AS IT APPROACHES THE
PACIFIC NORTHWEST...SO THE QPF PORTION OF THE FORECAST WAS BASED
ON A BLEND OF THE 12Z ECMWF/GFS. THE THERMAL PORTION OF THE
FORECAST WAS BASED ON A BLEND OF THE 12Z GFS/NAM/09Z SREF MEAN.

MOISTURE STREAMING AHEAD OF A SHORT WAVE APPROACHING THE
OR/NORTHERN CA COASTS DURING THE SECOND HALF OF DAY 3 WILL BECOME
FOCUSED ON THE PACIFIC NORTHWEST AND THE NORTHERN ROCKIES. IN THE
BROAD WARM AIR ADVECTION...SNOW LEVELS RISE TO ABOVE 8000 FEET
ACROSS THE CASCADES IN WA AND THE BITTERROOTS IN ID. IN THESE
LOCATIONS...LOCAL 2 TO 4 INCHES OF SNOWFALL WAS PLACED. IN
ADDITION...THERE IS SOME INDICATION IN THE LOW LEVEL THERMAL
FIELDS FROM BOTH THE 12Z NAM/GFS THAT COLD AIR COULD BECOME
TRAPPED IN SOME OF THE VALLEYS IN THE CASCADES OF WA...AS WELL AS
PORTION OF EASTERN OR AND EXTREME SOUTHWEST ID. AS THIS
POINT...LESS THAN 0.10 INCHES OF ICE WAS PLACED IN THE
ABOVEMENTIONED LOCATIONS...AND THERE IS NOT A STRONG MODEL SIGNAL
FOR MUCH MORE THAN THAT.


HAYES

$$





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