Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 192025
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
324 PM EST MON JAN 19 2015

VALID 00Z TUE JAN 20 2015 - 00Z FRI JAN 23 2015


...DAYS 2/3...

...SOUTHERN ROCKIES/SOUTHERN HIGH PLAINS...

A BROAD UPPER TROUGH OVER THE SOUTHWEST/SOUTHERN GREAT BASIN ON
TUESDAY BEGINS TO BECOME MORE POSITIVELY TILTED AS THE NORTHERN
PORTIONS MOVES EAST FASTER THAN THE SOUTHERN PORTION OF THE
TROUGH.  UPPER DIVERGENCE MAXIMA DEVELOP IN THE RIGHT ENTRANCE
REGION OF THE UPPER JET ACROSS SOUTHEAST CO AND NORTHEAST NM.
AREAS OF 700 MB CONVERGENCE DEVELOP IN ADVANCE OF THE 700 MB
FRONT/POSSIBLE EMBEDDED CIRCULATION. THESE WILL COMBINE WITH
UPSLOPE FLOW TO PRODUCE ASCENT...AS THE LOW LEVEL RIDGE ALONG THE
HIGH PLAINS IN CO PRODUCES RETURN EASTERLY COMPONENT FLOW AND
MOISTURE ADVECTION ACROSS THE TX PANHANDLE INTO EASTERN NM.

THE PRIMARY DIFFERENCE IN MODEL SOLUTIONS IS TIMING OF THE UPPER
TROUGH.  WHILE MODELS ARE IN PRETTY GOOD AGREEMENT ON DAY
2...DIFFERENCES DEVELOP IN DAY 3 AS THE GFS AND NAM HAVE HIGHER
SNOW AMOUNTS IN NORTHEAST NM THAN THE 00-12Z ECMWF/12Z UKMET/09Z
SREF MEAN.
MORE WEIGHTING WAS GIVEN THE LATTER CLUSTER OF SOLUTIONS...WHICH
WOULD RESULT IN A LARGE AREA OF 6-12 INCHES OF SNOW IN NORTHEAST
TO EAST CENTRAL NM.

UPPER DIVERGENCE MAXIMA ON WED NIGHT STREAK OUT OF NM INTO THE TX
PANHANDLE...SO SNOWS ARE EXPECTED TO DEVELOP THERE.  THE TIMING OF
THE FRONT IS ANOTHER COMPONENT OF UNCERTAINTY AS INITIAL
TEMPERATURES IN SOUTHWEST TX ARE TOO WARM FRONT FOR AND THE
FRONTAL PASSAGE RESULTS IN COOLING NEEDED TO TRANSITION THE PRECIP
TYPE TO SNOW.

...DAYS 1 TO 3...

...NORTHERN PLAINS/UPPER MISSISSIPPI VALLEY/GREAT LAKES/MIDDLE
ATLANTIC/SOUTHERN NEW ENGLAND...

A SERIES OF LOW-MID LEVEL SHORTWAVE DISTURBANCES MOVE ACROSS THE
NORTHERN PLAINS TO UPPER MS VALLEY ON DAY 1 AND OHIO VALLEY TO MID
ATLANTIC ON DAY 2.  THE UPPER LEVEL CYCLONIC SHEAR AXIS AND WEAK
CIRCULATION RESULTS IN MODEST FRONTOGENESIS AND LOW LEVEL
CONVERGENCE...SO SNOW AMOUNTS ARE EXPECTED TO BE LIGHT. A MODEST
ENHANCEMENT IS POSSIBLE WHERE UPSLOPE FLOW OCCURS IN SOUTHWEST PA.

THE NORTHERN WAVE MAY WEAKEN AS IT APPROACHES SOUTHEAST
NY/SOUTHERN NEW ENGLAND DAY 3/THU AS A SECOND WAVE DEVELOPS OFF
THE COAST AND THE SURFACE LOW CENTER DEVELOPS AND PROGRESSES OUT
TO SEA.  THIS SUPPORTS THE MID LEVEL FRONTOGENESIS BAND SHIFTING
SOUTH OFF THE COAST THU...ACCORDING TO THE 12Z NAM/GFS/ECMWF/UKMET.

THERE IS A LOW PROBABILITY OF GREATER THAN 4 INCHES OF SNOW FROM
EASTERN PENNSYLVANIA AND NORTHERN NEW JERSEY ACROSS SOUTHERN NEW
ENGLAND. THE NORTHWARD EXPANSION OF THE PROBABILITIES NORTH INTO
NEW ENGLAND ARE LIKELY A REFLECTION OF THE CONTRIBUTION OF THE
SREF MEMBERS...WHICH MAY BE SKEWING A MORE NORTHWARD SOLUTION
SINCE THE SREF MEAN HAS A HIGHER AMPLITUDE/SHORTER WAVELENGTH
TROUGH THAT RESULTS IN SNOW EXTENDING FURTHER NORTH THAN THE
OPERATIONAL MODELS. SINCE THE OPERATIONAL MODEL CLUSTER BETTER
WITH LOWER AMPLITUDE/LONGER WAVELENGTH SOLUTION...THE LEAST
WEIGHTING WAS GIVEN TO THE SREF FORECAST.

THE PROBABILITY OF 0.25 INCHES OF ICE IS LESS THAN 10 PERCENT
ACROSS THE CONTIGUOUS US.

PETERSEN

$$





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