Quantitative Precipitation Forecast
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FOUS11 KWBC 230900
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
359 AM EST THU FEB 23 2017

VALID 12Z THU FEB 23 2017 - 12Z SUN FEB 26 2017


DAYS 1 THROUGH 3...


...WEST COAST...

SHORT WAVE ENERGY DROPPING SOUTH ACROSS THE WEST COAST ON DAY 2
FOCUSES MOISTURE ON THE TERRAIN...AND LOWERING SNOW LEVELS SUPPORT
LOCALLY HEAVY SNOWFALL OVER PORTIONS OF OR AND NORTHWEST CA.
ADDITIONAL SHORT WAVE ENERGY WILL PROVIDE LIFT FOR SNOWFALL DURING
DAY 3 ACROSS THE PACIFIC NORTHWEST AND PORTIONS OF CA. THERE WAS
GENERALLY GOOD MODEL AGREEMENT OVER THIS AREA...SO THE THERMAL
FIELDS WERE BASED PRIMARILY ON THE 00Z GFS/12Z ECMWF. THE QPF
PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

AS THE SHORT WAVE ENERGY DROP SOUTH ALONG THE PACIFIC NORTHWEST
COAST INTO NORTHERN CA DURING DAY 2...THE LOW LEVEL FLOW AHEAD IT
FOCUSES MOISTURE ON THE TERRAIN. AS THE SHORT WAVE PASSES...SNOW
LEVELS DROP TO BETWEEN 4500 AND 5500 FEET...AND THERE IS
SUFFICIENT QPF TO SUPPORT AXES OF 4 TO 8 INCHES OF SNOWFALL ACROSS
THE CENTRAL OR CASCADES...AS WELL AS THE SISKIYOU RANGE IN FAR
SOUTHWEST OR AND NORTHERN CA. AS THE SHORT DROPS SOUTH ACROSS THE
COASTAL RANGE OF CA ON DAY 3...SNOW LEVELS DROP ENOUGH TO ALLOW
LOCAL 3 TO 6 INCHES SNOWFALL AMOUNTS OVER SHASTA AND TRINITY
MOUNTAINS.


...ROCKIES/SOUTHWEST...

LIFT ASSOCIATED WITH A SHORT WAVE CROSSING THE GREAT BASIN AND
CENTRAL AND SOUTHERN ROCKIES DURING DAY 1 WILL COMBINE WITH
STRENGTHENING UPSLOPE FLOW TO PRODUCE HEAVY SNOWFALL OVER PORTIONS
OF THE HIGHER TERRAIN IN WY AND FAR NORTHERN CO. AFTER
THAT...SHORT WAVE ENERGY DROPPING OUT OF WESTERN CANADA INTO A
DEVELOPING LONG WAVE TROUGH OVER THE ROCKIES AND SOUTHWEST WILL
PROVIDE LIFT FOR SNOWFALL ACROSS THE HIGHER TERRAIN ON DAYS 2 AND
3. THERE WAS GENERALLY GOOD MODEL AGREEMENT WITH THE OVERALL
SYNOPTIC SETUP...SO THE THERMAL PORTION OF THE FORECAST WAS BASED
ON A BLEND OF THE 00Z GFS/12Z ECMWF. THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 1...
SHORT WAVE ENERGY TRACKING OUT OF THE GREAT BASIN AND THE SOUTHERN
ROCKIES DURING THE FIRST PART OF DAY 1 WILL COMBINE WITH
STRENGTHENING UPSLOPE FLOW BEHIND LOW PRESSURE FORMING AND
DEEPENING OVER THE FRONT RANGE OF THE ROCKIES TO PRODUCE BANDS OF
HEAVY SNOWFALL OVER PORTIONS OF WY INTO THE NORTHERN AND CENTRAL
CO ROCKIES. THE STRONGEST LOW LEVEL EAST NORTHEAST LOW LEVEL FLOW
BECOMES UPSLOPE ACROSS THE LARAMIE AND SOUTHERN EXTENT OF THE WIND
RIVER MOUNTAINS IN WY. THERE IS A MULTI MODEL SIGNAL FOR 12 TO 18
INCHES OF SNOWFALL IN THE FAVORED UPSLOPE AREAS OF EACH
RANGE...WITH LOCALLY HIGHER AMOUNTS POSSIBLE WHERE THE UPSLOPE
REMAINS PERSISTENT. THESE SNOWFALL AMOUNTS SUPPORT A MODERATE
PROBABILITY OF 12+ INCHES OF SNOW ACROSS WY INTO THE WESTERN
PANHANDLE OF NE. THE UPSLOPE EAST NORTHEAST FLOW FOCUSES MOISTURE
ACROSS THE NORTHERN AND CENTRAL CO ROCKIES. HOWEVER...THE UPSLOPE
FLOW THIS FAR SOUTH IS NOT AS ROBUST...SO THE SNOWFALL AMOUNTS
HERE ARE EXPECTED TO TOP OUT BETWEEN 4 AND 8 INCHES.

DAY 2...
SHORT WAVE TRACKING OUT OF WESTERN CANADA WILL CROSS THE NORTHERN
AND CENTRAL ROCKIES DURING DAY 2...PROVIDING SYNOPTIC SCALE ASCENT
OVER A BROAD AREA. HOWEVER...THE COLUMN IS FAIRLY DRY...AND
UPSLOPE FLOW AIDS IN MOISTURE CONVERGENCE...RESULTING IN AREAS OF
2 TO 4 INCHES OF SNOWFALL ACROSS THE HIGHER TERRAIN.

DAY 3...
SHORT WAVE ENERGY DROPPING INTO A DEVELOPING LONG WAVE TROUGH OVER
THE ROCKIES THROUGH THE SOUTHWEST STATES ON DAY 3 PROVIDES
SUFFICIENT LIFT FOR PRECIPITATION ACROSS THE ROCKIES INTO THE
HIGHER TERRAIN ACROSS NORTHERN
AZ/NM. AGAIN...THE COLUMN IS FAIRLY DRY (WITH PRECIPITABLE WATER
VALUES NEARLY UNDER 0.25 INCHES)...SO SNOWFALL AMOUNTS ARE
GENERALLY EXPECTED TO BE 4 INCHES OR LESS OVER THE HIGHER TERRAIN.


...CENTRAL PLAINS INTO THE UPPER MS VALLEY AND UPPER GREAT LAKES...

SHORT WAVE ENERGY TRACKING OUT OF THE GREAT BASIN EARLY ON DAY 1
SPINS UP LOW PRESSURE OVER THE FRONT RANGE OF THE ROCKIES. THE MID
LEVEL AND SURFACE SYSTEMS MOVE ACROSS THE CENTRAL PLAINS INTO THE
MID TO UPPER MS VALLEY DURING DAY 2...REACHING A POSITION OVER THE
UPPER GREAT LAKES TOWARD THE END OF DAY 2. AN AREA OF STRONG MID
LEVEL LIFT NORTHWEST OF THE LOWS WILL PRODUCE A SWATH OF HEAVY
SNOWFALL FROM NE ACROSS PORTIONS OF SOUTHEAST
SD/NORTHWEST IA INTO SOUTHERN MN AND PORTIONS OF NORTHWEST WI.
THERE IS STILL SOME SPREAD ON THE TRACK OF THE MID LEVEL AND
SURFACE LOWS...BUT THERE HAS BEEN AN OVERALL TREND FOR A SOLUTION
THAT IS FURTHER SOUTHEAST AND FASTER...SO THE THERMAL FIELDS WERE
BASED PRIMARILY ON A BLEND OF THE 00Z GFS/12Z ECMWF.

DAY 1...
THE WATER VAPOR LOOP SHOWED SHORT WAVE ENERGY EXITING THE GREAT
BASIN FOR THE CENTRAL AND SOUTHERN ROCKIES... PROVIDING LIFT
ACROSS THE CENTRAL PLAINS. THE LIFT WITH THE SHORT WAVE ENERGY
WILL AID IN SPINNING UP A SURFACE LOW OVER SOUTHEAST CO/SOUTHWEST
KS...WHICH TRACKS ACROSS THE MID MS VALLEY BY THE END OF DAY 1.
AHEAD OF THE SURFACE LOW AND CLOSING MID LEVEL SYSTEM...A LOW
LEVEL SOUTHERLY FLOW TRANSPORTS MOISTURE OVER THE CENTRAL PLAINS
INTO THE MID AND UPPER MS VALLEY. THE MOISTURE IS ENTRAINED INTO
AN AXIS OF DEVELOPING MID LEVEL LIFT OVER WESTERN AND CENTRAL NE
BEFORE 24/00Z...AND BANDED PRECIPITATION IS EXPECTED TO FORM HERE.
MODEL SOUNDINGS INDICATED THAT THE COLUMN ACROSS NORTHERN AND
CENTRAL NE WILL BE COLD ENOUGH FOR PRIMARILY SNOW...AND A BROAD
STRIPE OF 8 TO 14 INCHES OF SNOW WAS PLACED ACROSS THIS PORTION OF
NE. LOCALLY HIGHER AMOUNTS ARE POSSIBLE ACROSS NORTHWEST
NE...WHERE UPSLOPE FLOW COULD ENHANCE SNOWFALL RATES.

THE AIRMASS AHEAD OF THE SURFACE AND MID LEVEL SYSTEMS IS FAIRLY
MILD EARLY THIS MORNING...AND THIS WILL RESULT IN A PHASE TYPE
PROBLEM...ESPECIALLY ACROSS PORTIONS SOUTHERN NE INTO SOUTHEAST
IA...STRETCHING INTO SOUTHERNMOST WI. AS THE SURFACE LOW
DEVELOP...IT BEGINS TO DRAW COLDER AIR FROM THE NORTHERN PLAINS
ACROSS FAR SOUTHEAST SD...NORTHWEST IA INTO SOUTHERN MN...WITH THE
CHANGE TO ALL SNOW IS THIS AREA OCCURRING DURING THE AFTERNOON. AS
THE MID LEVEL SYSTEM CLOSES OFF...THE MOISTURE ON THE SOUTH
SOUTHEAST LOW LEVEL INFLOW BECOMES ENTRAINED INTO A DEVELOPING AND
STRENGTHENING MID LEVEL FRONTOGENETIC BAND OF FORCING EXTENDING
ACROSS THE ABOVEMENTIONED AREAS (ON THE NORTHWEST SIDE OF THE
IMPINGING MID LEVEL DRY SLOT). MODEL SOUNDINGS SHOWED MOISTURE AND
LIFT IN THE DENDRITIC GROWTH ZONE...ENHANCING SNOWFALL RATES AND
RESULTING IN AN ISOTHERMAL PROFILE...WHICH REINFORCES THE COOLING
IN THE COLUMN. AN AXIS OF 6 TO 10 INCHES OF SNOWFALL WAS PLACED
HERE ON DAY 1...WITH A SHARP GRADIENT EXPECTED ON THE NORTHERN
EDGE OF THE HEAVY SNOWFALL. THESE SNOWFALL AMOUNTS SUPPORT A HIGH
PROBABILITY OF 8+ INCHES OF SNOWFALL ACROSS CENTRAL NE...AND A
MODERATE PROBABILITY OF 8+ INCHES EASTWARD INTO FAR NORTHWEST
IA/FAR SOUTHERN MN.

MODEL SOUNDINGS IN THE STRONG UPGLIDE AHEAD OF THE MID LEVEL
SYSTEM SHOWED A WARM NOSE (WITH MAXIMUM TEMPERATURES NEAR +3
CELSIUS AT ABOUT 750 MB) EXTENDING FROM NORTHEAST IA ACROSS SOUTH
CENTRAL WI INTO THE NORTHERN LP OF MI. THIS SHOULD BE A TRANSITION
ZONE...BEFORE THE BEST MID LEVEL FRONTOGENETIC FORCING
DEVELOPS...AND THE MODEL SOUNDINGS SHOWED A FREEZING RAIN PROFILE
IN THIS STRIPE. THERE WAS A MULTI MODEL SIGNAL FOR 0.01 TO 0.15
INCHES OF FREEZING RAIN IN THIS ZONE.

DAY 2...
THE MID LEVEL FRONTOGENETIC FORCED BANDED SNOWFALL (ON THE
NORTHWEST SIDE OF THE MID LEVEL DRY SLOT) IS EXPECTED TO EXTEND
FROM WESTERN IA ACROSS SOUTHEAST MN INTO NORTH CENTRAL WI AND THE
UP OF MI. MODEL SOUNDINGS SHOWED MOISTURE AND INSTABILITY IN THE
DENDRITIC GROWTH ZONE...ENHANCING SNOWFALL RATES IN THE BANDING.
AS MENTIONED EARLIER...THERE HAS BEEN AN OVERALL SHIFT IN THE
GUIDANCE WITH RESPECT TO THE PLACEMENT OF THE MID LEVEL AND
SURFACE SYSTEM...WITH THE 00Z NAM PROBABLY TOO FAR NORTHWEST WITH
THE BANDED SNOWFALL. THUS...THE STRIPE OF 6 TO 10 INCHES OF
SNOWFALL WAS MOVED SOUTHEAST COMPARED TO THE PREVIOUS FORECAST
(THESE SNOWFALL AMOUNTS SUPPORT A MODERATE PROBABILITY OF 8+
INCHES OF SNOWFALL ACROSS NORTHERN IA INTO SOUTHEAST MN AND WEST
CENTRAL WI). THE BEST LIFT OCCURS BEFORE 25/00Z...AS THE MID LEVEL
SYSTEM LIFTS OUT AND THE BEST DEFORMATION WEAKENS IN PLACE.
SOUTHEAST OF THE BEST BANDED SNOWFALL...MODEL SOUNDINGS INDICATED
ANOTHER TRANSITION ZONE...IN WHICH MID LEVEL TEMPERATURES MAY
REACH +3 CELSIUS NEAR 750 MB...RESULTING IN A CLASSIC FREEZING
RAIN PROFILE. THERE WAS A MULTI MODEL SIGNAL FOR 0.10 TO 0.15
INCHES OF RAINFALL IN AN AXIS FROM FAR SOUTHEAST WI ACROSS THE
NORTHERN LP OF MI (WITH THE HIGHEST QPF AMOUNTS EXPECTED OVER THE
APX COUNTY WARNING AREA).


...LOWER GREAT LAKES/NORTHEAST...

A NEGATIVELY TILTED LONG WAVE TROUGH CRESTS OVER THE NORTHEAST
STATES DURING DAY 3...AS LOW PRESSURE TRACKS INTO QUEBEC. A COLD
FRONT TRAILING THE SURFACE LOW FOCUSES MOISTURE AND LIFT ACROSS
NEW ENGLAND...WHERE COLD AIR ARRIVES IN TIME FOR SNOWFALL...MAINLY
ACROSS THE HIGHER TERRAIN. THERE WAS GENERALLY GOOD MODEL
AGREEMENT WITH THE OVERALL SYNOPTIC SETUP...SO THE THERMAL PORTION
OF THE FORECAST WAS BASED ON A BLEND OF THE 00Z GFS/12Z GEFS
MEAN/12Z ECMWF/12Z ECMWF ENSEMBLE MEAN. THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 2...
HEIGHTS RISE AHEAD OF THE LONG WAVE TROUGH TAKING ON A NEGATIVE
TILT ACROSS THE LOWER GREAT LAKES AND NORTHEAST DURING DAY 2. THE
SOUTHERLY FLOW AHEAD OF THE MID LEVEL SYSTEM WILL WARM THE COLUMN
THROUGH NORTHERN NEW ENGLAND...BUT A LOW LEVEL EASTERLY FLOW
ACROSS NORTHERN ME WILL MAINTAIN A COLD BOUNDARY LAYER FLOW. MODEL
SOUNDINGS ACROSS NORTHERN ME SHOWED A PRONOUNCED WARM NOSE (WITH
MAXIMUM TEMPERATURES NEAR +7 CELSIUS NEAR 800 MB).
EVENTUALLY...THE LOW LEVEL COLD AIR IS ERODED...BUT NOT BEFORE
0.01 TO 0.10 INCHES OF QPF CAN OCCUR ACROSS PORTIONS OF NORTHERN
ME DURING DAY 2.

DAY 3...
AS THE NEGATIVELY TILTED LONG WAVE TROUGH CRESTS OVER NORTHERN NEW
ENGLAND ON DY 3...IT PUSHES A COLD FRONT ACROSS NY STATE AND NEW
ENGLAND. AHEAD OF THE COLD FRONT...THE COLUMN IS TOO WARM TO
SUPPORT SNOWFALL... BUT AS HEIGHTS CRASH...MODEL SOUNDINGS
INDICATED PROFILES BECOMING ISOTHERMAL WHILE THERE IS STILL LIFT
IN THE COLUMN. THE RAIN CHANGES TO SNOW IN WHAT BECOMES AN
ELEVATION SNOWFALL EVENT...FROM WEST TO EAST BETWEEN 25/06Z TO
25/12Z. AN AREA OF 3 TO 6 INCHES OF SNOWFALL WAS STRETCHED FROM
THE ADIRONDACKS IN NORTHERN NY STATE ACROSS THE CENTRAL AND
NORTHERN GREEN MOUNTAINS IN VT INTO FAR NORTHERN NH...WITH LESSER
AMOUNTS IN ELEVATIONS ABOVE 1500 FEET. THESE SNOWFALL AMOUNTS
SUPPORT A MODERATE PROBABILITY OF 4+ INCHES OF SNOWFALL IN THIS
AREA.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


HAYES

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