Quantitative Precipitation Forecast
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FOUS11 KWBC 010917
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
417 AM EST SUN MAR 01 2015

VALID 12Z SUN MAR 01 2015 - 12Z WED MAR 04 2015


DAYS 1 THROUGH 3...


...OH VALLEY/MID ATLANTIC/NORTHEAST...

TWO ROUNDS OF WINTRY WEATHER ARE EXPECTED ACROSS PORTIONS OF THE
REGION. THE FIRST ONCE OCCURS DURING THE DAY 1 PERIOD...WITH COLD
AIR REMAINING FIRMLY IN PLACE WHILE WARM AIR OVERRUNS IT. THE
SECOND ROUND OCCURS DURING DAY 3...AS COLD AIR RETREATS ACROSS THE
MID ATLANTIC INTO NEW ENGLAND...AS STRONG UPGLIDE OCCURS OVER THE
RETREATING COLD AIR. FOR DAY 1...THE THERMAL PROFILES LEANED MORE
HEAVILY TOWARD THE 00Z NAM/21Z SREF MEAN...AS THIS COMBINATION
APPEARS TO BETTER CAPTURE THE COLD AIR REMAINING IN PLACE LONGER
THAN THE 00Z GFS. FOR DAY 3...THE THERMAL FIELDS WERE BASED ON A
MULTI MODEL BLEND...AS THE COLD AIR IN PLACE AT THE START OF THE
EVENT SHOULD NOT BE AS DIFFICULT TO DISLODGE OVER THE MID ATLANTIC
STATES. FOR BOTH DAYS...THE QPF PORTION OF THE FORECAST WAS BASED
ON THE MOST RECENT WPC QPF.

DAY 1...
STRONG WARM AIR ADVECTION OVER THE MIDWEST AND OH VALLEY IS
FEEDING MOISTURE FROM THE WESTERN GULF OF MEXICO ACROSS THE
INTERIOR PORTIONS OF THE MID ATLANTIC EARLY ON DAY 1. COLD AIR IS
IN PLACE HERE (AS EVIDENCED BY THE 00Z SOUNDINGS IN THE MID
ATLANTIC AND NORTHEAST)...SO MOISTENING COLUMNS SHOULD RESULT IN
SNOW AND SLEET AT THE ONSET FOR JUST ABOUT ALL AREAS.

ACROSS OH/WESTERN PA/NORTHERNMOST WV...THE STRONGEST LIFT IN THE
WARM AIR ADVECTION OCCURS DURING THE FIRST PART OF THE
PERIOD...WHEN THE COLUMN REMAINS COLD. THERE IS A MULTI MODEL
SIGNAL FOR MOISTURE AND LIFT IN THE DENDRITIC GROWTH ZONE...WHICH
SHOULD RESULT IN A SNOW TO LIQUID RATIO CLOSER TO 12:1 OR SO. THIS
SHOULD PRODUCE AN AREA OF 6 TO 9 INCHES OF SNOW ACROSS THE
ABOVEMENTIONED AREAS. THE BAND OF LIFT AND MOISTURE IN THE
DENDRITIC GROWTH ZONE EXTENDS ACROSS NORTHERN PA AND THE SOUTHERN
TIER OF NY STATE INTO SOUTHERN NEW ENGLAND. THE ABOVEMENTIONED
AREAS SHOULD SEE A GENERAL 4 TO 6 INCH SNOWFALL...AND THIS IS
SUPPORTED BY MEMBERS OF BOTH THE 21Z SREF AND LATEST ECMWF
ENSEMBLE OUTPUT.

ACROSS PORTIONS OF VA/MD/NORTHERN DE/SOUTHEAST PA AND PORTIONS OF
NJ ALONG THE I 95 CORRIDOR...THE PROBLEM BECOMES ICE. COLD AIR IS
IN PLACE...AND THERE IS GOOD MODEL AGREEMENT CONCERNING THE
ENTRENCHMENT OF THE SHALLOW COLD AIR ALONG THIS AXIS. DRY AIR IN
PLACE SUGGESTS A PERIOD OF
SNOW/SLEET...BUT WARMING IN THE COLUMN (CENTERED NEAR 875 MB)
CHANGES THE PRECIPITATION TYPE TO FREEZING RAIN RELATIVELY
QUICKLY. MODEL SOUNDINGS SHOWED THAT THE COLD AIR REMAINS IN PLACE
FOR MUCH OF...IF NOT ALL...OF THE REMAINDER OF THE EVENT. IF THIS
DOES OCCUR...MUCH OF THE QPF COULD FALL AS FREEZING SURROUNDING
THE I 95 CORRIDOR FROM NORTH OF KDCA THROUGH KTTN IN
NJ...PRODUCING AN AXIS OF 0.25 TO 0.50 INCHES OF ICE (THOUGH MOST
PLACES PROBABLY SEE CLOSER TO THE 0.25 INCH ICE AMOUNTS). THESE
AMOUNTS ARE WELL SUPPORTED BY THE 00Z NAM/GFS/21Z SREF MEAN...SO
THE CONFIDENCE IN ICE AMOUNTS APPROACHING 0.25 INCHES IS
INCREASING. A LARGER AREA OF 0.10 TO 0.25 INCHES OF ICE COVERS
MUCH OF THE MID ATLANTIC THROUGH A PORTION OF LONG ISLAND NY...AS
THE CHANGEOVER TO FREEZING RAIN OCCURS FROM SOUTHWEST TO NORTHEAST.

DAY 3...
STRONG WARM AIR ADVECTION (WITH 45 TO 55 KNOT SOUTH SOUTHWEST
WINDS RIDING UP THE 290 K ISENTROPIC SURFACE ACROSS THE MID
ATLANTIC INTO NEW ENGLAND) TRANSPORTS MOISTURE ACROSS THE REGION
DURING DAY 3 FROM SOUTHWEST TO NORTHEAST. A BRIEF INTERLUDE OF
ARCTIC AIR BEHIND THE SYSTEM ON DAY 1 SETS THE STAGE FOR ICE AGAIN
ACROSS MUCH OF THE MID ATLANTIC INTO SOUTHERN NEW ENGLAND. THE
COLD AIR DOES NOT APPEAR TO BE AS PERSISTENT AS THE DAY 1
EVENT...ESPECIALLY ACROSS THE INTERIOR MID ATLANTIC STATES. MODEL
SOUNDINGS SHOWED THE POTENTIAL FOR THE SHALLOW COLD AIR TO LAST
LONGER ACROSS MUCH OF WESTERN AND CENTRAL PA AND THE SOUTHERN TIER
OF NY STATE...AND THESE AREAS COULD SEE 0.10 TO 0.25 INCHES OF ICE.

THE MAIN AREA OF LIFT AND MOISTURE IS FOCUSED ACROSS NORTHERN NY
STATE AND CENTRAL AND NORTHERN NEW ENGLAND...WHERE THE COLD AIR
SHOULD BE SUFFICIENT FOR A MAINLY SNOW EVENT...AT LEAST DURING THE
BEST PERIOD OF LIFT. THE HIGHEST SNOWFALL AMOUNTS (BETWEEN 4 AND 8
INCHES) IS EXPECTED ACROSS NORTHERN VT/NORTHERN NH AND NORTHERN
AND CENTRAL ME...BUT EVEN HERE A MIX WITH FREEZING RAIN IS
POSSIBLE AS THE WARM AIR ROBS THE COLUMN OF ITS ABILITY TO PRODUCE
SNOWFLAKES.


...MIDWEST/GREAT LAKES...

STRONG OVERRUNNING OCCURS AHEAD OF SHORT WAVE DIVING OUT OF THE
NORTHERN STREAM...WHICH SPINS UP A LOW THAN CROSSES THE REGION ON
DAYS 2 AND 3. NORTH OF THE TRACK OF THE LOW...THE COLUMN APPEARS
TO REMAIN COLD ENOUGH FOR A MOSTLY SNOW EVENT. CLOSER TO THE TRACK
OF THE SURFACE LOW...LOW LEVEL THERMAL FIELDS SUPPORT SNOW TO
SLEET AND FREEZING RAIN. THE THERMAL PORTION OF THE FORECAST WAS
BASED ON A MULTI MODEL BLEND...AS THE POSITION OF THE LOW LEVEL
THERMAL RIBBON IS FAIRLY CLOSE AMONG MODEL SOLUTIONS. THE QPF
PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

DAY 2...
A DEEP SOUTHWEST FLOW IN THE MID LEVEL HELPS TO TRANSPORT MOISTURE
FROM THE WESTERN GULF OF MEXICO AND THE PACIFIC UP AND OVER COLD
AIR IN PLACE ACROSS THE MIDWEST AND GREAT LAKES ON DAY 2. SURFACE
LOW PRESSURE FORMS ON A LOW LEVEL BOUNDARY ACROSS THE CENTRAL
PLAINS AND MIDWEST LATE IN THE PERIOD. MODEL SOUNDINGS ALONG THE
BOUNDARY SHOWED THE POTENTIAL FOR A PERIOD OF FREEZING RAIN ACROSS
PORTIONS OF IA/MO/IL DURING THIS PERIOD...AS THE 00Z NAM IS MUCH
MORE AGGRESSIVE WITH THE MID LEVEL WARMING.

SINCE THERE IS SOME MODEL SPREAD HERE...A LARGE AREA OF 0.01 TO
0.10 INCHES OF ICE WAS PLACED OVER SOUTHERN IA...NORTHEAST MO AND
MUCH OF IL...AS 00Z GFS MODEL SOUNDINGS WOULD SUPPORT MORE SNOW
THAN ICE. FURTHER NORTH ACROSS NORTHERN IA...WHERE MODEL SOUNDINGS
SUPPORTED A COLD COLUMN...1 TO 3 INCHES OF SNOWFALL IS EXPECTED.

DAY 3...
THE SURFACE LOW DEEPENS AS IT CROSSES THE MIDWEST EARLY IN THE
PERIOD...REACHES SOUTHERN MI BY 04/00Z...BEFORE IT TRACKS INTO
SOUTHERN ONTARIO. THE STRONG ISENTROPIC LIFT (WITH 40 TO 50 KNOT
SOUTHWEST WINDS RIDING UP THE 290 K SURFACE) SPREADS MOISTURE
ACROSS THE GREAT LAKES. THE WARM NOSE REACHES A LINE STRETCHING
ACROSS CENTRAL IA/SOUTHERN WI/SOUTHERN MI. SOUTH OF THAT
LINE...MODEL SOUNDINGS SHOWED A WARM NOSE BETWEEN 775 AND 725
MB...WITH MAXIMUM TEMPERATURES NEAR 3 DEGREES CELSIUS. THE
PROFILES SUPPORT FREEZING RAIN ACROSS MUCH OF THE GREAT LAKES AND
MIDWEST...WITH LOCAL 0.10 TO 0.25 INCH QPF AMOUNTS EXTENDING
ACROSS NORTHERN IL AND NORTHERN I INTO SOUTHERN MI.

NORTH OF THAT LINE...MODEL SOUNDINGS SUPPORT A MAINLY SNOW EVENT.
THE LIFT AND MOISTURE COMBINE FOR A LARGE AREA OF 4 TO 8 INCHES
OVER CENTRAL WI AND THE UP OF MI. THE SPEED OF MOVEMENT OF THE
SYSTEM SHOULD PRECLUDE HIGHER SNOWFALL AMOUNTS...AND THE LATEST
ECMWF ENSEMBLE OUTPUT DOES NOT HAVE ANY MEMBERS WITH 8+ INCHES OF
SNOWFALL WITH THIS SYSTEM.


...SOUTHERN PLAINS...

INCREASING UPGLIDE OF MOISTURE OVER SHALLOW COLD AIR IN PLACE ON
DAYS 1 AND 2 RAISES THE THREAT FOR FREEZING RAIN ACROSS PORTIONS
OF NORTH CENTRAL TX. THERE WAS GOOD AGREEMENT AMONG 00Z MODEL
SOLUTION CONCERNING THE PLACEMENT AND EROSION OF THE LOW LEVEL
COLD AIR...SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON A
BLEND OF THE 00Z NAM/GFS/21Z SREF MEAN. THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

MOISTURE AND MARGINAL INSTABILITY OVER THE WESTERN GULF OF MEXICO
RIDES UP AND OVER THE SHALLOW COLD AIRMASS IN PLACE ACROSS THE
SOUTHERN PLAINS DURING DAY 1. THE ISENTROPIC LIFT IS FAIRLY
WEAK...SO THE OVERALL QPF AMOUNTS SHOULD BE FAIRLY LIGHT.
HOWEVER...MODEL SOUNDINGS SHOWED THE POTENTIAL FOR FREEZING
RAIN...AS THE WARM NOSE PEAKS AT ABOUT 4 CELSIUS NEAR 800 MB. THIS
SHOULD BE MORE THAN ENOUGH FOR AN AREA OF 0.10 TO 0.05 INCHES OF
ICE ACCUMULATION ACROSS A PORTION OF NORTH TX.

THE POTENTIAL FOR LIGHT ICING LINGERS INTO THE FIRST PART OF DAY 2
ACROSS NORTH CENTRAL TX. AFTER THAT...MODELS SHOWS THAT THE
BOUNDARY LAYER TEMPERATURES WARM ENOUGH TO PRECLUDE ADDITIONAL ICE
CONCERNS.


...GREAT BASIN/SOUTHERN ROCKIES...

MOISTURE RIDING NORTH AHEAD OF A LONG WAVE TROUGH ACROSS THE
SOUTHWEST STATES SETS THE STAGE FOR LOCALLY HEAVY SNOWFALL ACROSS
THE REGION EACH OF THE NEXT THREE DAYS. THERE WAS A FAIR AMOUNT OF
MODEL SPREAD CONCERNING THE EVOLUTION OF THE CLOSED LOW INTO A
LONG WAVE TROUGH AS IT APPROACHES THE REGION...SO THE THERMAL
PORTION OF THE FORECAST WAS BASED PRIMARILY ON A BLEND OF THE 12Z
ECMWF/00Z GFS. THE QPF PORTION OF THE FORECAST WAS BASED ON A THE
MOST RECENT WPC QPF.

DAY 1...
THE CLOSED MID LEVEL SYSTEM OVER SOUTHERN CA SLOWLY OPENS UP
DURING THE PERIOD. MOISTURE IS DRAWN NORTHEAST FROM THE
PACIFIC...AND THIS COMBINED WITH THE LIFT FROM THE MID LEVEL
SYSTEM (AS WELL AS MARGINAL INSTABILITY DUE TO STEEPENING MID
LEVEL LAPSE RATES) IS EXPECTED TO PRODUCE 6 TO 10 INCHES OF
SNOWFALL ACROSS THE SOUTHERN WASATCH IN UT AND NORTHERN AZ.
UPSLOPE FLOW OVER A FRONTAL ZONE BANKED UP AGAINST THE ROCKIES IN
CO AID ADDITIONAL LIFT AND MOISTURE...RESULTING IN 8 TO 16 INCHES
OF SNOWFALL...WITH THE HIGHEST AMOUNTS ACROSS THE SAN JUAN
MOUNTAINS IN SOUTHWEST CO. THESE AMOUNTS ARE SUPPORTED BY MEMBERS
OF THE MOST RECENT ECMWF ENSEMBLE OUTPUT...AS WELL AS THE 21Z SREF
OUTPUT.

DAY 2...
THE CLOSED LOW OVER SOUTHERN CA OPENS UP AND LIFTS ACROSS THE
GREAT BASIN AND AZ DURING DAY 2. DEEP MOISTURE CONTINUES TO BE
DRAWN NORTHEAST FROM THE PACIFIC AHEAD OF THE TROUGH ACROSS THE
GREAT BASIN AND SOUTHERN ROCKIES. MORE LIFT AND MOISTURE MEANS
MORE SNOWFALL THAN DAY 2 ACROSS THIS REGION...AND THE UPSLOPE FLOW
ACROSS THE SOUTHERN ROCKIES ADDS EVEN MORE MOISTURE AND LIFT. A
LARGE AREA OF 1 TO 2 FEET OF SNOWFALL IS EXPECTED ACROSS THE
ROCKIES OF CO...WITH LESSER AMOUNTS (BETWEEN 4 AND 8 INCHES)
REACHING INTO THE SANGRE DE CRISTO MOUNTAINS OF NORTHERN NM.
FURTHER WEST...WHERE SNOW LEVEL EQUALIZE BETWEEN 6000 AND 700O
FEET..A LARGE AREA OF 6 TO 10 INCHES OF SNOW IS EXPECTED ACROSS
EAST CENTRAL NV...THE WASATCH RANGE IN UT...AND NORTH CENTRAL AZ.
AGAIN...THESE AMOUNTS ARE WELL SUPPORTED BY THE LATEST ECMWF AND
21Z SREF ENSEMBLE OUTPUT.

DAY 3...
THE LONG WAVE TROUGH PATTERN REMAINS IN PLACE ACROSS THE SOUTHWEST
STATES. HOWEVER...THE MOISTURE SOURCE ON DAY 3 IS NOT NEARLY AS
ROBUST AS THE PREVIOUS DAYS. THE UPSLOPE IS NOT QUITE AS ROBUST IN
CO EITHER. BOTH OF THESE FACTORS RESULT IN LOWER SNOWFALL TOTALS
ON DAY 3...WITH 4 TO 8 INCHES OF SNOWFALL EXPECTED ACROSS THE CO
ROCKIES...WITH A GENERAL 2 TO 4 INCH SNOWFALL OVER THE WASATCH
RANGE.


...NORTHERN AND CENTRAL ROCKIES...

SHORT WAVE DROPPING OUR OF WEST CENTRAL CANADA PUSHES AN ARCTIC
FRONT INTO THE NORTHERN ROCKIES DURING DAY 2. LIFT WITH THE SHORT
WAVE AND UPSLOPE FLOW BEHIND THE ARCTIC FRONT COMBINE FOR SNOWFALL
ACROSS THE REGION. FOR THE MOST PART...THERE WAS DECENT MODEL
AGREEMENT CONCERNING THE SEQUENCE OF EVENTS...SO THE THERMAL
PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 00Z
GFS/NAM/12Z ECMWF. THE QPF PORTION OF THE FORECAST WAS BASED ON
THE MOST RECENT WPC QPF.

AS THE SHORT WAVE DIVES OUT OF WEST CENTRAL CANADA ON DAY 2...IT
PUSHES AN ARCTIC FRONT INTO THE NORTHERN ROCKIES AND NORTHERN
PLAINS. WHILE THE LIFT WITH THE SHORT WAVE IS IMPRESSIVE...THE
MOISTURE IN THE COLUMN IS FAIRLY SPARSE (WITH PRECIPITABLE WATER
VALUES REMAINING AT OR BELOW 0.25 INCHES). HOWEVER...UPSLOPE
BEHIND THE ARCTIC FRONT THAT BECOMES BANKED UP AGAINST THE
NORTHERN ROCKIES ACROSS WESTERN MT AND NORTHERN WY COMBINES WITH
THE LIFT TO MAKE THE MOST OF THE MOISTURE IN THE COLUMN. THE
RESULT SHOULD BE AN AREA OF 2 TO 4 INCHES OF SNOWFALL ACROSS THIS
REGION...WITH LOCAL 4 TO 8 INCH AMOUNTS POSSIBLE OVER THE ROCKY
MOUNTAIN FRONT RANGE IN WESTERN MT.


HAYES


$$





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