Quantitative Precipitation Forecast
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FOUS11 KWBC 142012
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
312 PM EST THU DEC 14 2017

VALID 00Z FRI DEC 15 2017 - 00Z MON DEC 18 2017

DAYS 1 THROUGH 3...

...GREAT LAKES REGION...

THE HEAVIEST SNOW ACROSS THIS REGION WILL LIKELY BE DUE TO LAKE
EFFECT...GENERALLY FOCUSED IN THE NEXT 36 HOURS. LOW-MID LEVEL
WARM AIR ADVECTION COMMENCING SATURDAY SHOULD SUBSTANTIALLY REDUCE
LAKE-INDUCED INSTABILITY AND SHUT DOWN NOTEWORTHY LAKE EFFECT
SNOWS. FORECAST SNOWFALL WAS TRENDED UP SLIGHTLY IN THE FAVORED
SNOW BELTS DOWNSTREAM OF LAKE SUPERIOR AND LAKE MICHIGAN BASED ON
STRONG SIGNALS FROM HI-RES MODEL GUIDANCE FOR LOCALIZED QPF AT OR
ABOVE 0.5 INCHES. 12Z HREF BLENDED MEAN HAS THE FAVORED AREAS IN:
(1) THE LOWER KEWEENAW PENINSULA INTO THE PORCUPINE MOUNTAINS OF
THE UP...(2) NEAR PICTURED ROCKS NATIONAL LAKE SHORE IN THE
UP...AND (3) JUST EAST OF GRAND TRAVERSE BAY IN NRN MI. MODELS
SHOW MORE PERSISTENT FETCH AND BOUNDARY LAYER WIND DIRECTIONS OVER
LAKE SUPERIOR AND NRN LAKE MICHIGAN THAN ELSEWHERE IN THE
REGION...AND 850MB TEMPERATURES DIPPING INTO THE -15 TO -20C RANGE
SHOULD SUPPORT STRONG LAKE-INDUCED INSTABILITY WITH EQUILIBRIUM
LEVELS OVER 7000 FEET AT TIMES.

SNOWFALL TOTALS ARE ALSO EXPECTED TO BE SUBSTANTIAL DOWNSTREAM OF
LAKE ERIE AND LAKE ONTARIO...WITH LAKE-INDUCED INSTABILITY EVEN
STRONGER THAN ON LAKES FURTHER TO THE NORTHWEST. MODELS SHOW THAT
THE EQUILIBRIUM LEVELS MAY BRIEFLY EXCEED 10000 FEET NEAR THE
EASTERN END OF LAKE ERIE. THIS IS VERY LIKELY TO DRIVE SIGNIFICANT
SNOW RATES IN ANY BAND THAT DEVELOPS. HOWEVER THE SNOW SHOULD BE
DISTRIBUTED OVER A LARGER AREA WITH MORE VARIABLE WIND
DIRECTIONS...DRIVEN BY THE DEVELOPING LOW-LEVEL CYCLONE TO THE
NORTHWEST. THIS SHOULD PREVENT LONG-DURATION PERSISTENCE OF HEAVY
SNOW BANDS IN ANY SPECIFIC LOCATION.

AHEAD OF THE DIGGING SHORTWAVE ON FRIDAY...MODELS DO AGREE ON A
BROADER AREA OF LIGHT QPF FALLING IN THE FORM OF SNOW. DESPITE LOW
QPF AMOUNTS...FORECAST SNOW SQUALL PARAMETER VALUES SUGGEST THAT
THE PRECIP MAY BE MORE CONVECTIVE IN NATURE WITH HIGHER SNOW RATES
DESPITE FORECAST LOW AMOUNTS (GENERALLY AROUND 1 INCH IN LOWER
MICHIGAN AWAY FROM THE LAKE EFFECT BELTS).

BROADER LIGHT TO MODERATE SNOW IS LIKELY TO OCCUR IN A SWATH FROM
THE RED RIVER VALLEY OF ND/MN...INTO THE NORTHWOODS OF NRN
WI...AND INTO WEST-CENTRAL MICHIGAN FROM FRIDAY EVENING INTO
SATURDAY AFTERNOON. THIS WILL BE ASSOCIATED WITH A BAND OF
LOW-LEVEL FRONTOGENESIS AND STRONG LOW-MID LEVEL WAA IN ADVANCE OF
A LOW-AMPLITUDE SHORTWAVE. DESPITE THE FAST-MOVING NATURE OF THE
EVENT (SNOW DURATION SHOULD BE LESS THAN 12HR)...THE LOW-LEVEL
FORCING SHOULD SUPPORT A SWATH OF AROUND A COUPLE INCHES OF
SNOW...ESPECIALLY FROM NRN WI INTO WRN MI.


...PACIFIC NORTHWEST AND CASCADIA INTO THE INTERMOUNTAIN WEST AND
NRN/CTRL ROCKIES...

A DIGGING TROUGH SHOULD ARRIVE AT THE PACIFIC NORTHWEST COAST ON
FRIDAY AND THEN PROGRESS EAST TO NEAR THE CONTINENTAL DIVIDE THIS
WEEKEND...SLOWING DOWN WITH TIME AS BROADER CYCLOGENESIS COMMENCES
OVER THE FOUR CORNERS REGION. A BAND OF PRECIPITATION IS LIKELY TO
BE FOCUSED ALONG THE ASSOCIATED INITIAL COLD FRONT. CONTINUED
WESTERLY FLOW INTO COASTAL WA AND NRN OR THIS WEEKEND SHOULD KEEP
THE CASCADES ACTIVE WITH SNOW AT HIGHER ELEVATIONS.
MEANWHILE...SNOW MAY CONTINUE AT HIGHER ELEVATIONS IN THE
INTERMOUNTAIN WEST AND NRN/CTRL ROCKIES BEHIND THE INITIAL FRONT
AS INCREASING EASTERLY COMPONENT TO THE LOW-LEVEL FLOW IS LIKELY
THIS WEEKEND TO THE NORTH OF THE AFOREMENTIONED DEVELOPING
CYCLONE. AS THIS UPSLOPE FLOW CONTINUES IN THE PRESENCE OF A MOIST
LOW-MID LEVEL ATMOSPHERE...SNOW IS LIKELY TO CONTINUE WITH A BROAD
AREA OF SNOW IN EXCESS OF 4 INCHES LIKELY FROM NRN ID AND WRN MT
SOUTHEAST INTO WYOMING. BEHIND THE FRONT...SNOW LEVELS ARE LIKELY
TO FALL BELOW 5000 FEET OVER MUCH OF THE REGION...AND POTENTIALLY
AS LOW AS 2000 FEET IN SOME AREAS NEAR THE TAIL END OF THE SYSTEM.

LIGHT FREEZING RAIN ALSO APPEARS INCREASINGLY LIKELY IN THE
COLUMBIA BASIN ON FRIDAY. THE SIGNAL FOR LIGHT ZR (AT LEAST 0.01
INCHES) SHOWS UP ON THE MAJORITY OF MODELS AND ENSEMBLE MEMBERS.
HI-RES MODELS SUGGEST THE COLD AIR (TEMPS LESS THAN 0C) SHOULD BE
FAIRLY RESILIENT...PARTICULARLY IN ERN/NRN PORTIONS OF THE
BASIN...WITH THE 12Z HREF KEEPING THE DOMINANT PRECIPITATION TYPE
AS ZR THROUGH THE ENTIRE DAY. INCLUDED HIGHER AMOUNTS (JUST UNDER
0.10 INCHES) IN THE EASTERN PORTION OF THE BASIN...OR IN EC/SE WA
AT LOWER ELEVATIONS.


...HIGHER ELEVATIONS OF AZ/NM INCLUDING THE MOGOLLON RIM...

THE MODEL TREND HAS BEEN FOR A STRONGER TROUGH DIGGING INTO THE
DESERT SOUTHWEST. A CORRESPONDING INCREASE IN QPF AMOUNTS AND
DECREASE IN MODEL FORECAST THICKNESSES IS ALSO ADVERTISED...AND
THUS SOME LIGHT SNOW TOTALS AROUND 1-2 INCHES HAVE BEEN ADDED TO
HIGHER ELEVATIONS (GENERALLY ABOVE 6500 FEET) OF AZ/NM FROM THE
MOGOLLON RIM INTO THE GILA REGION OF WC NM. DEPENDING ON
ADDITIONAL MODEL TRENDS...ITS POSSIBLE THAT SNOW TOTALS COULD BE
TRENDED UP SLIGHTLY IN THE FUTURE...PARTICULARLY AT THE HIGHEST
ELEVATIONS ABOVE 8000 FEET.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAYS
1-3.


LAMERS

$$





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