FOUS11 KWBC 172132
QPFHSD
 
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
430 PM EST THU JAN 17 2008
 
VALID 00Z FRI JAN 18 2008 - 00Z MON JAN 21 2008
 
DAY 1...

UPPER GREAT LAKES...

A STRONG SHRTWV WILL BE ADVANCING NEWD OUT OF THE MIDWEST THIS
EVENING AND WILL BE DRIVING AN INTENSIFYING SFC LOW ACRS THE U.P.
OF MI AND INTO ONTARIO BY FRI MORN. A WELL-DEFINED MID LVL
DEFORMATION ZONE/COMMA HEAD SHOULD ENHANCE SNOWFALL RATES
OVERNIGHT ACRS MAINLY THE U.P. OF MI WHICH WILL BE FURTHER
COMPLEMENTED BY AN INCREASINGLY STG AND MOIST FETCH OFF LAKE
SUPERIOR ON THE BACK SIDE OF THE DEPARTING SFC LOW. THE 12Z NAM
AND GFS SOLNS ARE IN GOOD AGREEMENT WITH THEIR RESPECTIVE QPF
TOTALS AND HPC PREFERRED A BLEND OF THESE SOLNS OVERALL. A MDT TO
HIGH THREAT FOR 4 INCH PLUS AMTS IS INDICATED THRU MUCH OF UPPER
MI WHICH WILL ACCOUNT FOR BOTH THE DEPARTING SFC AND THE EXPECTED
LAKE EFFECT COMPONENT WHICH WILL BEGIN. 

NRN MID-ATLANTIC INTO NEW ENGLAND...

A COMPLICATED MIXTURE OF PTYPES WILL BE ADVANCING NEWD OVERNIGHT
AND FRIDAY ACRS THIS REGION AS LOW PRESSURE ALONG THE MID-ATL
COAST THIS EVENING WILL BE ADVANCING RATHER RAPIDLY NWD TO A PSN
NR LONG ISLAND BY 12Z FRI AND THEN INTO SERN CANADA BY FRI EVE.
ALREADY SEEING A MODERATELY FAIRLY STG LLJ TRANSPORTING COPIOUS
AMTS OF ATLANTIC  MOISTURE INTO THE NRN MID-ATL AND THIS WILL
OVERSPREAD THE REMAINDER OF NEW ENGLAND OVERNIGHT. SUFFICIENTLY
COLD/DRY AIR WILL IN PLACE ACRS NEW ENGLAND FOR MAINLY SNOW...BUT
THERE WILL BE A WARM LAYER AT 85H ASSOC WITH THE LLJ ENERGY WHICH
SHOULD NOSE INTO AT LEAST SOME OF THE INTERIOR SECTIONS OF CNTRL
AND NRN NEW ENGLAND BEFORE THE PCPN EXITS FRI...AND THIS WILL
ALLOW FOR SOME CHANGEOVER TO SLEET AND FREEZING RAIN ALTHOUGH THE
MAIN PCPN TYPE SHOULD BE SNOW. AREAS CLOSE TO THE COAST ARE LIKELY
TO SEE A TOTAL CHANGEOVER TO RAIN BEFORE THE PCPN ENDS. USING
ESSENTIALLY A BLEND OF THE 12Z NAM AND GFS WARRANTS A MDT TO HIGH
RISK OF 4 INCHES ACRS PARTS OF CNTRL AND ESP NRN NEW ENGLAND WITH
A LOW RISK OF 8 INCHES IN ME.
 
MT/WY...

A CLIPPER SYS DROPPING SEWD WILL DRIVE AN ARCTIC COLD FRONT DOWN
THE ERN SLOPES OF THE NRN ROCKIES AND ACRS THE HIGH PLAINS. A
COMBINATION OF ENHANCED UPSLOPE FLOW AND NOTABLE FEED OF MOISTURE
IN THE MID LVLS BEHIND THE FRONTAL PASSAGE SHOULD ENCOURAGE
LOCALLY HVY SNOW SHOWER ACTIVITY. THE HEAVIEST FOCUS SHOULD BE
INVOF THE TETONS...BIG HORNS AND POSSIBLY THE BLACK HILLS AREA.
LOCAL 8 INCH OR GREATER SNOWFALL AMTS ARE POSSIBLE OVER THE MOST
FAVORED TERRAIN WITH THE BIG HORNS MOST LIKELY TO SEE THIS.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


DAY 2...

GREAT LAKES...

AN ARCTIC BNDRY WILL CROSS THE GREAT LAKES BY EARLY FRI
NIGHT...BRINGING A SURGE OF FRIGIDLY COLD TEMPS. INSTABILITY WILL
BE MAXIMIZED ACRS ESP LAKES SUPERIOR AND MICHIGAN WITH A POOL OF
85H TEMPS DOWN TO AS LOW AS -25C AND LOCALLY COLDER. EXPECT BANDS
OF VERY HVY LAKE EFFECT SNOW ACROSS FAVORED PARTS OF THE MI UPPER
PENINSULA AND WRN LOWER MI WITHIN THE DEEPER LYR WNW FLOW.

SRN APPALACHIANS/SRN MID-ATLANTIC...

A VERY HIGH DEGREE OF UNCERTAINTY EXISTS FOR SATURDAY ACRS THIS
REGION AS THE 12Z MDL GUIDANCE SHOWS A LARGE SPREAD IN POSSIBLE
SOLNS CONCERNING THE DEVELOPMENT OF LOW PRESSURE OVER THE NRN GULF
OF MEXICO AND ITS PATH TO OFF THE MID-ATL BY SAT EVE. HPC FOLLOWED
A SOLN CLOSEST TO THE 00Z ECMWF WHICH WILL ALLOW FOR A STRIPE OF
ENHANCED WARM ADVECTION AND MOISTURE TO OVERSPREAD THE SRN
APPALACHIANS AND INTO PARTS OF THE MID-ATL COASTAL PLAIN AS THE
SFC LOW EJECTS VERY QUICKLY ACRS NRN FL AND TO OFF THE EAST COAST.
MEANWHILE...MUCH COLDER AIR FILTERING SEWD FRM THE OH/MS VLYS MAY
BE IN PLACE IN TIME TO INTERACT WITH THIS MOISTURE...AND SUPPORTS
AT LOW SOME THREAT OF ACCUMULATING SNOWFALL. GIVEN THE LARGE
SPREAD IN THE MDL GUIDANCE WHICH RANGES FRM AN ALL RAIN EVENT...TO
A STRIPE OF HVY SNOW...AND TO NOTHING AT ALL...HPC WILL REFLECT A
LOW RISK OF 4 INCHES ATTM FRM FAR NRN GA NEWD INTO THE LWR
DELMARVA.

NRN ROCKIES...

DEEP LYR NWLY FLOW WILL ALLOW ADDITIONAL SHRTWV ENERGY AND A
STRONGER CLIPPER SYS TO DROP DOWN FRM WRN CANADA ON SAT. EXPECT
SUFFICIENT MID LVL MOISTURE TO REMAIN IN PLACE FOR ADDITIONAL MDT
TO LOCALLY HVY OROGRAPHICALLY FOCUSED SNOWFALL. THE MAIN THREAT
SHOULD BE FRM THE BITTERROOTS SEWD INTO THE TETONS WHERE A LOW TO
MDT RISK OF 4 INCH AMTS IS INDICATED.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


DAY 3...

GREAT LAKES...

THE REINFORCING SURGE OF ARCTIC AIR WILL REACH INTO THE LOWER
LAKES ON SUN...BRINGING STG COLD ADV AND BANDS OF LAKE EFFECT
SNOW.  VERY COLD 85H TEMPS HOVERING BELOW -20C WILL PERSIST
THROUGHOUT THE GREAT LAKES REGION INTO SUNDAY AND SO XPCT
PERSISTENT BANDS OF SNOW SHOWERS AND SQUALLS AT LEAST FOR THIS PD
DOWNWIND FROM EACH OF THE LAKES. 

NORTHEAST COAST...

LOW PRESSURE SHOULD BE ADVANCING OUT TO SEA OFF THE EAST COAST ON
SUN...BUT THERE IS POTENTIAL IT COULD BE CLOSE ENOUGH TO NEW
ENGLAND FOR SOME ACCUMULATING SNOWFALL NR THE COAST. AGAIN THIS IS
A HIGHLY UNCERTAIN FCST AS ANY FURTHER EWD TRAJECTORY COULD EASILY
RESULT IN A NON-EVENT. THE BEST THREAT FOR 4 INCH PLUS AMTS
APPEARS TO BE ACROSS CAPE COD AND POSSIBLY DOWNEAST ME. THIS IS
CONSISTENT WITH THE 00Z ECMWF SOLNS AND NOT NEARLY AS AGGRESSIVE
AS THE 12Z NAM AND GFS SOLNS.

NRN ROCKIES/HIGH PLAINS...

THE AFOREMENTIONED SHRTWV DROPPING INTO THE NRN ROCKIES ON DAY 2
WILL DIG MORE TO THE SOUTH TWD THE GREAT BASIN ON SUN. THIS WILL
SUPPORT ADDITIONAL SNOWFALL...ESPECIALLY ACROSS THE MORE FAVORED
OROGRAPHIC LOCATIONS INCLUDING THE SAWTOOTH...TETONS...BIG HORNS
AND SOUTH INTO NRN CO NEAR THE FLAT TOPS. A TIGHT PRES GRADIENT
WITH STRONG LOW LEVEL ELY UPSLOPE FLOW IS FCST TO SET UP BY THE
GFS AND ECMWF THIS PD AS STG ARCTIC HIGH PRESSURE FILTERS SOUTH
FRM WRN CANADA. A LOW TO MDT RISK OF 4 INCHES IS INDICATED ATTM
WITH SOME OF THE SNOWFALL LIKELY TO SPREAD OUT INTO THE IMMEDIATE
HIGH PLAINS TOO.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

ORRISON
$$