Smithsonian, 1997

1
Winter Weather 2004-2005
Naval Atlantic Meteorology Oceanography Center
Smithsonian, 1997
2
ALONG CAME A SEASON

• BIG PICTURE JET INFLUENCE AND THE LONGWAVE
  PATTERN
• December through March

WINTER WEATHER
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2004
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JET INFLUENCE
STORMY WEATHER PATTERNS LIKE THE ONE ARE THE
RESULT OF HIGH-AMPLITUDE, JET STREAM PATTERNS.
THIS SIMPLY MEANS THERE ARE HUGE UNDULATIONS IN
THE UPPER AIR FLOW. THE RESULT IS A VAST AMOUNT
OF NORTH TO SOUTH AND SOUTH TO NORTH AIR
MOVEMENT, WHICH RESULTS IN LARGE TEMPERATURE
CONTRASTS. THESE EXTREMES FUEL BIG WINTER STORMS.
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JET INFLUENCE
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JET INFLUENCE
A STORMY WEATHER WILL UNFOLD ACROSS THE CENTRAL
AND EASTERN UNITED STATES WITH THIS PATTERN. THE
REASON IS A SPLIT JET STREAM. THE NORTHERN BRANCH
WILL SUPPLY THE COLD AIR, WHILE THE SOUTHERN
BRANCH WILL SUPPLY THE MOISTURE. THE CONFLICT
BETWEEN AIR MASSES WILL GENERATE EXTENSIVE AREAS
OF SNOW AND RAIN.
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JET INFLUENCE

• DUE TO THE ORIENTATION OF THE JET IN THIS PATTERN
  AND THE LACK OF HIGH AMPLITUDE VARIATIONS,
• WARM WEATHER WOULD DOMINATE THE SOUTHEAST IN THIS
  SET UP WHILE ARCTIC AIR WOULD REMAIN IN PLACE

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JET INFLUENCE
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JET INFLUENCE
ONE NOTABLE FEATURE OF WINTER IS THE GREENLAND
BLOCK. THIS IS A STATIONARY RIDGE OF HIGH
PRESSURE ALOFT NEAR GREENLAND. THE BLOCK FORCES
COLD, ARCTIC AIR INTO THE NORTHEASTERN UNITED
STATES, AND PREVENTS IT FROM MOVING INTO EASTERN
CANADA OR THE ATLANTIC.
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JET INFLUENCE
BLOCKING PATTERN OVER THE EASTERN PACIFIC OCEAN
CAN SOMETIMES CAUSE THE JET STREAM TO SPLIT
DURING THE WINTER MONTHS. THIS RESULTS IN ONE
BRANCH OF THE JET STREAM GOING OVER THE TOP AND
PUSHING COLD AIR INTO THE NATION. THE OTHER
BRANCH OF THE JET STREAM CUTS UNDERNEATH THE
BLOCK AND DIPS INTO THE TROPICS, BRINGING
MOISTURE INTO THE SOUTHERN PART OF THE U.S.
OCCASIONALLY, THE TWO BRANCHES MERGE, BRINGING
TOGETHER THE INGREDIENTS FOR STORMY WEATHER.
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JET INFLUENCE
THE LARGEST COLD ZONE IN THIS PATTERN IS THE
EAST, WITH ITS FREQUENT BLASTS OF ARCTIC AIR.
THE SECOND AREA IS UNDER THE GREAT BASIN. A
STRONG TEMPERATURE INVERSION WILL KEEP COLD AIR
TRAPPED IN THE LOWLEVELS.
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JET INFLUENCE
"IT'S TOO COLD TO SNOW" IS A WEATHER SAYING THAT
WAS ADOPTED LONG AGO. PEOPLE NOTICED THAT IN
EXTREMELY COLD WEATHER, SELDOM DID A LOT OF SNOW
FALL. MAJOR WINTER STORMS HAVE OCCURRED IN
BITTERLY COLD ARCTIC AIR MASSES, BUT THEY ARE THE
EXCEPTION RATHER THAN THE RULE. WHEN THE ARCTIC
BRANCH TAKES OVER, THE FLOW FROM CANADA DOWN
ACROSS THE UNITED STATES CONTAINS VERY LITTLE
MOISTURE. ALSO, BIG MOISTURE SOURCES LIKE THE
ATLANTIC AND THE GULF OF MEXICO ARE BLOCKED.
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JET INFLUENCE
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ARCTIC OUTBREAK
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ARCTIC OUTBREAK
IN THE EASTERN UNITED STATES, THE COLDEST WEATHER
OCCURS WHEN ARCTIC AIR TAKES A DIRECT ROUTE INTO
THAT PART OF THE COUNTRY. WHEN COLD AIR MOVES
SOUTHWARD THROUGH THE PLAINS FIRST, SOME
MODIFICATION OCCURS BEFORE THE AIR REACHES THE
EASTERN STATES.
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ARCTIC OUTBREAK
DURING JANUARY, INTENSELY COLD AIR MASSES REACH
MATURITY OVER THE FROZEN SNOWFIELDS OF CENTRAL
AND NORTHERN CANADA. AT THESE NORTHERN LATITUDES,
THERE IS VIRTUALLY NO HEATING FROM THE SUN. IN
JANUARY, THE JETSTREAM IS TYPICALLY DISPLACED
WELL SOUTH WITH WEAKER CURRENTS IN ARCTIC
REGIONS.
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ARCTIC OUTBREAK
THIS LIGHTER FLOW ALLOWS AIR TO SIT ALMOST IN THE
SAME LOCATION FOR DAYS ON END, SO IT GETS
PROGRESSIVELY COLDER. WHEN THE JETSTREAM TAKES
ON A HIGH AMPLITUDE FLOW, THE ARCTIC AIR IS
DISLODGED, AND IT CHARGES SOUTHWARD INTO THE
UNITED STATES.
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ARCTIC OUTBREAK
THE FRIGID AIR HAS BEEN CONSTANTLY MANUFACTURED
OVER THE FROZEN SNOW FIELDS OF CENTRAL AND
NORTHERN CANADA. LARGE DIPS IN THE JET STREAM
HAVE ALLOWED LARGE CHUNKS OF THIS FRIGID AIR TO
BREAK AWAY AND MOVE DOWN INTO THE UNITED STATES.
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ARCTIC OUTBREAK
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ARCTIC OUTBREAK
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ARCTIC OUTBREAK
ON AVERAGE, JANUARY IS THE COLDEST MONTH OF THE
YEAR. A REASON IS THAT ARCTIC AIR MASSES REACH
FULL MATURITY OVER THE FROZEN SNOW FIELDS OF
CENTRAL AND NORTHERN CANADA. ANY BIG DIPS IN THE
JET STREAM WILL TRANSPORT LARGE BODIES OF THE
ARCTIC AIR INTO THE UNITED STATES.
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NORTHEAST BLIZZARD
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NORTHEAST BLIZZARD
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NORTHEAST BLIZZARD
THE BLIZZARD OF 1993 ROARED UP THE EASTERN
SEABOARD ON MARCH THIRTEENTH. THE STORM WAS
IMMENSE. AT ONE TIME DURING THE MORNING,
MODERATE TO HEAVY SNOW FELL FROM ALABAMA ALL THE
WAY TO MAINE. THE GRAPHIC SHOWS THIS AND ALSO
THE SIZE OF THE SNOW SHIELD OF A TYPICAL WINTER
STORM FOR COMPARISON.
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NORTHEAST BLIZZARD
In March of 1888 a great blizzard dealt a cruel
blow to the northeastern states. Total snowfalls
were crippling, exceeding 50 inches in some
locations. The storm generated winds near
hurricane force, which produced drifts over two
stories high. The huge amount of snow resulted
when the storm slowed and took and entire day to
complete a loop to the south of New England. To
this day, the blizzard of 1888 remains the
benchmark by which all other storms are measured.
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OCCURRENCE OF SEVERE THUNDERSTORMS ACROSS THE
SOUTHERN STATES SHOWS A MARKED INCREASE DURING
THE MONTH OF FEBRUARY. THIS IS A RESULT OF BIG
CLASHES BETWEEN COLD AIR COMING DOWN FROM THE
NORTH AND MUCH WARMER AIR BUILDING ACROSS THE
DEEP SOUTH.
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THE FREQUENCY OF SEVERE THUNDERSTORMS THROUGH THE
SOUTHERN STATES INCREASES DRAMATICALLY DURING
LATE FEBRUARY AND MARCH. CLASHES BETWEEN COLD AIR
TO THE NORTH AND BUILDING WARMTH TO THE SOUTH
PLUS STRONG JET STREAM WINDS PROVIDE ALL THE
ENERGY NEEDED FOR DESTRUCTIVE THUNDERSTORMS.
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FEBRUARY CAN BE A VERY SNOWY MONTH, ESPECIALLY
ACROSS THE EASTERN PART OF THE COUNTRY. THE
REASON IS THAT TEMPERATURE CONTRASTS GROW AS IT
REMAINS COLD TO THE NORTH WHILE SIGNIFICANT
WARMING IS TAKING PLACE THROUGH THE SOUTH. ALSO,
AS THE AIR AT SOUTHERN LATITUDES WARMS, IT CAN
HOLD MORE WATER VAPOR. STORMS THEREFORE HAVE
ADDITIONAL MOISTURE TO WORK WITH IN FEBRUARY,
WHICH SOMETIMES LEADS TO EXCESSIVE SNOWFALL.
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FLOODING ALONG RIVERS CAN OCCUR IN THE WINTER
EVEN WITHOUT HEAVY RAIN. KNOWN AS "ICE JAMS",
BLOCKS OF ICE COLLIDE AND PILE ON EACH OTHER,
FORMING A DAM IN THE RIVER. THIS OCCURS DURING A
WARM SPELL WHEN A FROZEN RIVER BEGINS TO THAW.
THESE JAMS USUALLY DO NOT LAST VERY LONG, BUT CAN
CAUSE A SERIOUS BACKUP OF WATER ON THE RIVER,
THUS FLOODING NEARBY LOWLANDS.
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THE USUAL SUSPECTS
Take A Break ... we'll be right back with ...

• WINTER STORMS AND SYNOPTIC PATTERNS
• December through March

WINTER WEATHER
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Type A
Heavy/widespread snow producer
Axis of 850mb jet
L
L
850mb low tracks south of VA Brings max. moisture
inflow to region.
500mb low tracks near VA
500 MB
850 MB
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Type B
Heavy/widespread snow producer
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L
Axis of 850mb jet
500mb low remains well north of area but
maintains negative tilt
850mb low redevelops near Outer Banks.
850 MB
500 MB
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Type C
L
850 MB
L
850 MB
Changes over from snow to rain. Similar to type
B except that redevelopment of low occurs near SE
VA, allowing 0 degree C isotherm to move north of
VA.
Axis of 850mb jet
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Type D
500 MB
850 MB
Axis of 850mb jet
High-zonal flow with a weak short wave
moving rapidly eastward.
Low-level WAA
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MAJOR STORM TRACKS
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ALBERTA CLIPPER

• LOW PRESSURE FORMS EAST OF MOUNTAINS IN ALBERTA,
  CANADA, MOVES RAPIDLY SOUTHEASTWARD ACROSS
  NORTHERN GREAT PLAINS
• LACKS TAP IN MOISTURE FROM PACIFIC, ATLANTIC OR
  GOMEX, THEREFOR PRODUCES FLURRIES OR LIGHT SNOW
• RE-ENFORCEMENT OF COLD AIR POLAR OR ARCTIC,
  PRODUCES SINGLE DIGIT/BELOW ZERO TEMPS OVER
  PLAINS TO NEW ENGLAND AND 10-20S OVER THE
  MID-ATLANTIC STATES
• 2 POSSIBLE TRACKS
• ACROSS THE GREAT LAKES (LAKE EFFECT SNOW)
• DIPS FURTHER SOUTH, MOVES ACROSS OHIO VALLEY,
  EAST ACROSS MID ATLANTIC EXITING OFF VA/NC
  (POSSIBLE HATTERAS LOW OR NOREASTER)

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STORMS THAT TRACK FROM THE LOWER MISSISSIPPI
VALLEY TOWARD THE OHIO VALLEY OR GREAT LAKES
REGIONS OFTEN HAND OFF ENERGY TO A NEW STORM
ALONG THE EAST COAST. THE STORM OVER THE INTERIOR
LOSES ITS FUEL SUPPLY AS COLD AIR TO THE WEST AND
COLD AIR TO THE EAST PINCH MILDER AIR OUT OF THE
PICTURE. TO THE EAST, HOWEVER, THERE IS PLENTY OF
MILD AIR OUT OVER THE ATLANTIC OCEAN. THE
TEMPERATURE GRADIENT THAT SETSUP BETWEEN THE
OCEAN AIR AND THE COLD AIR OVER THE INTERIOR
PROVIDES THE FUEL FOR THE DEVELOPING SECONDARY
STORM.
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HATTERAS LOWS FORM OFF THE MID-ATLANTIC COAST
FOUR WAYS ON AN OLD STATIONARY BOUNDARY A WEAK,
NON FRONTAL LOW OR TROF FROM EASTERN GOMEX OR THE
SOUTH EAST OR FROM FROM AN ALBERTA CLIPPER. THE
BOUNDARY CREATED FROM WARM MOIST AIR FROM
ATLANTIC WATERS, COLD AIR DROPS SOUTH AS STORM
INTENSIFIES. CAN PRODUCE HEAVY RAIN/SNOW IN
ADVANCE OF FRONT, COASTAL FLOODING, HEAVY SURF
AND GALE FORCE WINDS BATTER THE EASTERN SEABOARD
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HATTERAS LOWS
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HATTERAS LOWS
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HATTERAS LOWS
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HATTERAS LOWS
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EXPLOSIVE CYCLOGENESIS
EXTRA-TROPICAL LOW UNDERGOES EXTREME RAPID
DEVELOPMENT AND INTENSIFICATION METEOROLOGICAL
BOMB - 24mb DEEPENING/24HOURS (1 BERGERON) -
WEAK 1.0 BERGERON - MODERATE 1.3-1.8
BERGERON - STRONG gt1.8 BERGERON
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EXPLOSIVE CYCLOGENESIS

• PRIMARILY A MARITIME, COLD SEASONEVENT MAY
  DEVELOP FROM FALL THROUGH SPRING WITH PEAK
  FREQUENCY JANUARY THROUGH FEBRUARY
• USUALLY FOUND WITHIN OR POLEWARD OF THE MAIN
  WESTERLIES BELT, WITHING OR AHEAD OF PLANETARY
  SCALE TROFS.
• LOOK FOR NEGATIVE OR NEUTRAL TILT, USUALLY FOUND
  APPROXIMATELY 400NM DOWNSTREAM FROM AN AGGRESSIVE
  500MB TROF
• REGION 1/4 OF THE WAVELENGTH DOWNSTREAM FROM THE
  TROF TO RIDGE IS FAVORED FOR DEVELOPMENT
• TYPICALLY ASSOCIATED WITH 140KT JET MAXIMA
• IN STUDY (Sept 76-May 79), GREATEST DEEPENING WAS
  3.4 BERGERON (81.6mb/24hr)
• ALTHOUGH PACIFIC BOMB FREQUENCY WAS 1.5 THAT OF
  ATLANTIC, ATLANTIC HAD TWICE AS MANY EXTREME
  EVENTS(gt/2 Bergeron)

Source Monthly Weather Review article
"Synoptic-Dynamic Climatology of the Bomb"
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FREQUENCY OF EXPLOSIVE CYCLOGENESIS IN THE
NORTH ATLANTIC
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NOREASTER

• OCCURS DURING LATE FALL, WINTER AND EARLY
  SPRING (OCTOBER THROUGH APRIL), A
  WINTERPHENOMENON THAT ARE MOST COMMON IN LATE
  JANUARY AND FEBRUARY.
• CALLED NOREASTERS BECAUSE THEY TYPICALY
  PRODUCE STRONG NORTHEAST WINDS OVER THE EAST AS
  THEY MOVE NORTH ALONG THE ATLANTIC COAST.
• TRUE NOREASTERSDEVELOP MORE RAPIDLY AND GROW
  WITH GREATER INTENSITY OVER WATER, BUT THEY CAN
  GENERATE OVER LAND OR IN THE GOMEX.

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NOREASTER

• CLOSELY RELATED TO THE DEVELOPMENT OF LOW ZONAL
  FLOW AND BLOCKING PATTERNS
• FAVORED DEVELOPMENT AREA IS JUST OFFSHORE CAPE
  HATTERAS (HATTERAS LOW), IMMEDIATELY NORTH OF THE
  STRONGEST SST GRADIENT ASSOCIATED WITH THE WARM
  GULF STREAM.
• BAROCLINIC INSTABILITY AND THE SELF DEVELOPMENT
  PROCESS ALONE CANNOT EXPLAIN THE EXPLOSIVE
  CYCLOGENIS A NOREASTER CAN EXHIBIT.
• AS cP AIR MOVES OFF THE US OVER A WARM CURRENT,
  IT MODIFIES RAPIDLY FROM BELOW. OCEAN
  SURFACE/SENSIBLE HEAT IS TRANSFERRED FROM THE
  WATER INTO THE AUR AS IT WARMS.
• AS STORM RAPIDLY INTENSIFIES, WARM AIR FLOWS
  INLAND FROM OVER RELATIVELY WARM ATLANTIC OCEAN
  WATER.
• COLD AIR CONTINES TO MOVE SOUTH OVER THE EAST
  COAST.
• CAN PRODUCE HEAVY RAIN, SNOW, SLEET, FREEZING
  RAIN, AND SEVERE COASTAL FLOODING TO THE ATLANTIC
  COASTAL REGION.

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NOREASTER
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NOREASTER
THE EXACT TRACK OF THE STORMS CENTER DETERMINES
THE DIVIDING LINE BETWEEN RAIN AND SNOW. IF THE
STORM MOVES OVER THE COAST OR INLAND JUST EAST
OF THE APPALACHIAN MOUNTAINS, IT WILL USUALLY
PUSH ENOUGH WARM AIR INLAND TO BRING RAIN TO THE
COASTAL PLAIN, WITH SNOW CONFINED TO THE
MOUNTAINS AND POINTS WEST. IF THE STORM MOVES
FURTHER EAST OVER THE ATLANTIC OCEAN, HEAVY SNOW
CAN FALL ALONG THE COASTAL PLAIN
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NORTHWALL EFFECT
7
ExtremeWinds Seas
ColdAirFlow
AT 21oF
53oF Cold Water

Land
SourceSurface Warfare article"Up Against the
Wall"

• TYPICALLY A COLD SEASON EVENT WITH HIGH WINDS AND
  SEAS IN THE VICINITY OF THE GULF STREAM
• WARM OCEAN CURRENT, FLOWS NORTHEAST AT
  APPROXIMATELY 1.2 TO 2.5 KNOTS
• CUMULONIMBUS BUILDUP
• INCREASED SFC WINDS(15-25 KTS), GUSTINESS,
  TURBULENCE
• SEAS 10 - 15 FT HIGHER IN EXTREME CASES

Land
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NORTHWALL EFFECT
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6
NORTHWALL EFFECT
PERPENDICULAR WIND FLOW

• Air less modified
• Extreme gustiness
• Effects felt further
• seaward Often occurs with
• rapidly forming,
• intense lows
• (explosive
• cyclogenesis)

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NORTHWALL EFFECT
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5
NORTHWALL EFFECT
PARALLEL WIND FLOW
Wind fetch parallel but opposite Gulf
Stream Narrow zone with steeper waves
Cold air warmer than when perpendicular
3
75o
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COLD AIR DAMMING
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COLD AIR DAMMING

• COLD SURFACE HIGHPASSES NORTH OF THE MID ATLANTIC
  STATES AND NEW ENGLAND (AT LEAST 1020MB,
  PREFERABLY gt/ 1024MB)
• COLD AIR SUPPLIED BY THE SURFACE HIGH IS
  CHANNELED SOUTHWARD ALONG THE EAST SLOPES OF THE
  APPALACHIANS.
• RIDGE OF HIGH PRESSURE DEVELOPS BETWEEN THE
  MOUNTAINS AND THE OCEAN. THIS HELPS KEEPNORTHERLY
  COMPONENT TO THE LOW LEVEL WINDS OVER LAND.
• EASTERLY OR NORTHEASTERLY LOW LEVEL WINDS OVER
  THE LAND TIGHTEN THE THERMAL GRADIENTNEAR THE
  COAST.
• WHEN ARCTIC AIR IS PRESENT, THE MODELS OFTEN HAVE
  A HARD TIME HOLDING ONTO COLD ENOUGH LOW
  TEMPERATURES WHEN THE SURFACE HIGH IS STILL OVER
  THE NEW ENGLAND OR GREAT LAKES REGION.

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COLD AIR DAMMING
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COLD AIR DAMMING

• LOW LEVEL AIR MASS IS TRAPPED TOPOGRAPHICALLY
• INFLUENCES THE DYNAMICS OF OVERLYING AIRMASS
• AFFECTS MOISTURE, TEMPERATURE, AND STABILITY
• MOST EVENTS IN DECEMBER AND MARCH

L
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COLD AIR DAMMING
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COLD AIR DAMMING
COLD AIR DAMMING
UPPER LEVEL FEATURES

• Upper Level Features

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COLD AIR DAMMING
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COLD AIR DAMMING
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OVERRUNNING
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OVERRUNNING
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OVERRUNNING
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OVERRUNNING
BIG WINTER STORMS OCCUR WHEN A VAST QUANTITY OF
MOISTURE IS LIFTED UP AND OVER A LARGE EXPANSE OF
COLD AIR. THE JET STREAM PLAYS A VERY IMPORTANT
ROLE IN CREATING THIS LIFT.
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OVERRUNNING
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THE WHOLE NINE INCHES
Take A Break ... we'll be right back with ...

• WINTER PRECIPITATION AND FORECAST CONSIDERATIONS
• December through March

WINTER WEATHER
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ACCUMULATION
WINTER PRECIPITATION
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WINTER PRECIPITATION
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WINTER PRECIPITATION
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WINTER PRECIPITATION
RAIN / SNOW CRITERIA

• Surface Air Temp lt/ 34F gt/ 40F
• SST lt/ 38F gt/ 41F
• Freezing Level lt/ 800ft gt/ 1200ft
• 850mb Critical Temp lt/ -3C gt/ 0C
• 1000-850mb Thickness lt/ 1310m gt/1320m
• 1000-700mb Thickness lt/ 2804m gt/2865m
• 1000-500mb Thickness lt/ 5370m gt/5410m
• Wind Direction NW thru NE

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WINTER PRECIPITATION
SYNOPTIC CRITERIA

• Polar High over NE/Mid Atlantic States
• High or Ridge over NE U.S.
• Low or Trof over along SE U.S.

500mb CRITERIA

• L/W trof extends from central Great Lakes /St.
  Lawrence to GOMEX
• S/W trof extends SW from Newfoundland
• Mid Atlantic High ridges into Greenland
• Weak ridge off east coast oriented north/south

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WINTER PRECIPITATION
FORECASTING GUIDELINES

• Western edge of moderate to heavy snow shield
  ends at the 700-mb trough line in 90 of cases
  east of 100 longitude, regardless of what the
  model says.
• Light snow ends at the 500mb trough axis.
• Moderate snow usually needs a 14 Vort max. Heavy
  snow are usually associated with a 20 vort max.
• Average 500mb temp w/3 degrees latitude of the
  Vort max24 to 37C
• Moderate/heavy snow is in an area of 7 degrees
  latitude downstream of the Vort max AND within 3
  degrees to the Left of the Vort max.
• Moderate/heavy snow to fall is under closed 500
  low and never further east than the inflection
  point where the 500 flow turns from cyclonic to
  anti-cyclonic .
• Moderate/heavy snow usually occurs 90 miles to
  the left of the 850mb low (if the cold air mass
  is arctic then the distance is 50 miles) and
  120-140 miles north of the surface low (east of
  100 longitude).
• In the Southeastern U.S. -below 35N latitude and
  east of the Mississippi river- the snow band to
  surface low distance is more like 200 miles.

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WINTER PRECIPITATION
AT TEMPERATURES AROUND THE FREEZING MARK,
SNOWFLAKES CONTAIN MORE WATER, ARE HEAVIER AND
THUS FORM A COMPACT MASS AFTER REACHING THE
GROUND.
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WINTER PRECIPITATION
WHEN THE AIR IS QUITE COLD THE INDIVIDUAL SNOW
CRYSTALS MAINTAIN THEIR SHAPE AND LITERALLY STAND
ON ONE ANOTHER AFTER SETTLING TO EARTH. SO, GIVEN
THE SAME AMOUNT OF WATER VAPOR IN THE ATMOSPHERE
THE AMOUNT OF SNOW THAT FALLS WILL BE GREATER
WHEN IT'S VERY COLD THAN WHEN THE TEMPERATURE IS
AROUND FREEZING.
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WINTER PRECIPITATION
EXTENSIVE SNOW COVER IS SOMETIMES JUST ENOUGH TO
ALLOW A STORM TO BRING ALL SNOW INSTEAD HANGING
TO RAIN. THE REASON IS THE COOLING PROPERTIES OF
SNOW. FRESH SNOW COVER REFLECTS 70 TO 90 PERCENT
OF INCOMING SOLAR RADIATION INTO OUTER SPACE. AS
A RESULT, THE AIR ABOVE THE SNOW PACK STAYS
COLDER AND, IN MARGINAL SITUATIONS, THE ODDS
SHIFT IN FAVOR OF SNOW.
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WINTER PRECIPITATION
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WINTER PRECIPITATION
WET SNOWFALL COMMONLY OCCURS DURING THE MONTH OF
MARCH. THIS HAPPENS WHEN THE LOWEST THOUSAND FEET
OR SO OF THE ATMOSPHERE IS CLOSE TO THE FREEZING
MARK. SNOWFLAKES FALLING INTO THIS ENVIRONMENT
PARTIALLY MELT AND STICK TOGETHER, FORMING LARGER
POORLY DEFINED FLAKES. AT THE ROUND, THE FALLEN
SNOW COMPACTS AND FORMS A DENSE MASS. THE SLIGHT
AMOUNT OF (LIQUID) WATER PRESENT MAKES THE SNOW
STICKY. THIS IS THE PERFECT TYPE OF SNOW FOR
SNOWMEN AND SNOWBALLS. IT IS A CHORE TO SHOVEL
THOUGH. THE COMPACTED WET SNOW ON THE GROUND CAN
WEIGH OVER 15 POUNDS PER SHOVEL FULL.
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WINTER PRECIPITATION

• BLIZZARD -WINDS gt/ 35MPH
• -SNOW AND BLOWING SNOW
• -VISIBILITY lt1/4 MILE FOR 3HOURS
• BLOWING SNOW -WIND DRIVEN SNOW
• -REDUCESVISIBILITY
• -MAY BE SNOW THAT HAS ALREADY FALLEN
  DOWN, PICKED UP BY WINDS
• SNOW SQALLS -BRIEF INTENSE SNOW SHOWERS
• -ACCOMPANIED BY STRONG GUSTY WINDS
• -MAY HAVE SIGNIFICANT ACCUMULATION
• SNOW SHOWERS -FALLING AT VARYING INTERVALS
• -BRIEF PERIODS OF DURATION
• -SOME ACCUMULATION POSSIBLE
• SNOW FLURRIES -LIGHT SNOW FALLING
• -SHORT DURATION
• -LITTLE TO NO ACCUMULATION

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WINTER PRECIPITATION
93
WINTER PRECIPITATION
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WINTER PRECIPITATION
ICE STORMS CAN BE SLEET, FREEZING RAIN, OR A
COMBINATION OF BOTH, BT WHATS THE DIFFERENCE? IN
BOTH CASES, SNOWFLAKES THAT FORM HIGH IN THE
CLOUDS FALL THROUGH A MILD LAYER AND MELT.
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WINTER PRECIPITATION
UPON ENTERING A SHALLOW LAYER OF COLD AIR AT THE
SURFACE, THE RAINDROPS REFREEZE.IF THE RAIN
DROPS FREEZE ON THE WAYDOWN, ITS CALLED SLEET.
IF THE RAIN FREEZES UPON IMPACT WITH THE GROUND,
ITS CALLED FREEZING RAIN.
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WINTER PRECIPITATION
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WINTER PRECIPITATION
FREEZING RAIN ON BARE WALKWAYS AND HIGHWAYS IS
VERY TREACHEROUS. IF THERE IS ALREADY SNOW AND
SLUSH ON THE GROUND THE PROBLEM IS LESS SERIOUS.
THE REASON IS THAT THE FALLING RAIN IS ABSORBED
BY THE SNOW AND IS NOT ABLE TO FORM A SHEET OF
GLAZED ICE. IN EITHER CASE, THE ICE WILL
ACCUMULATE ON TREES AND POWERLINES, WHICH CAN BE
QUITE DESTRUCTIVE.
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WINTER PRECIPITATION
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WINTER PRECIPITATION
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WINTER PRECIPITATION
ITS NOT JUST A RAIN EVENT, BUT WHETHER YOU WANT
TO CALL IT VIRGA OR SNIRGA IN THE WINTER IS
REALLY UP TO YOU.
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WINTER PRECIPITATION
102
WINTER PRECIPITATION
DURING THE WINTER, FLOODING BECOMES A CONCERN
WHEN RAIN IS IN THE FORECAST AND THERE IS A
SUBSTANTIAL AMOUNT OF SNOW ON THE GROUND. THE
SERIOUSNESS OF THE FLOODING IS DIRECTLY RELATED
TO THE TEMPERATURE AND HOW MUCH WIND THERE WILL
BE. A MILD WIND THAT BOOSTS TEMPERATURES TO 50
DEGREES OR HIGHER WILL MELT SNOW VERY FAST. THAT
COMBINED WITH AN INCH OR TWO OF RAIN HAS CREATED
SOME OF THE WORST FLOODS. HOWEVER, IF THE AIR
STAYS CHILLY, THE RATE OF SNOWMELT IS MUCH SLOWER
AND THE SNOW IS ABLE TO ABSORB A GOOD PORTION OF
THE RAIN THAT IS FALLING. IN THIS CASE, FLOODING
IS MUCH LESS SERIOUS.
103
WINTER PRECIPITATION
SNOW MELTS THE QUICKEST WHEN THERE IS A MILD WIND
AND THE RELATIVE HUMIDITY IS HIGH. THE WIND KEEPS
VENTING MILD AIR ACROSS THE SNOW SURFACE WHICH
KEEPS THE SNOW CONSTANTLY EXPOSED TO TEMPERATURES
WELL ABOVE FREEZING. WHEN THE HUMIDITY IS HIGH,
THE SNOW SURFACE IS NOT COOLED BY EVAPORATION, SO
THE MELTING PROCESS SPEEDS UP EVEN MORE.
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WINTER PRECIPITATION
IN LIGHT TO MODERATE SNOWFALL WITH TEMPERATURES
IN THE 26- TO 32-DEGREE RANGE, ROADS OFTEN STAY
WET BY DAY DURING MARCH. THE REASON IS THE
HIGHER SUN ANGLE. THIS ALLOWS ENOUGH SOLAR
RADIATION TO PENETRATE THE CLOUDS, WHICH WARMS
THE PAVEMENT SUFFICIENTLY TO MELT THE FALLING
SNOW.
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BAY EFFECT SNOW
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BAY EFFECT SNOW
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BAY EFFECT SNOW
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LAKE EFFECT SNOW
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VISIBILITY DEGRADATION
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VISIBILITY DEGRADATION
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VISIBILITY DEGRADATION

• KEEP IN MIND THE IMPACT OF WINTER PRECIP IN
  VICINITY OF COASTAL AREAS
• WHITE OUT FROM HEAVY SNOWFALL
• BLOWING SPRAY
• FOG/ICE FOG

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ICE ACCRETION

• AIR TEMPERATURE
• WATER TEMPERATURE
• WIND VELOCITY

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ICE ACCRETION

• STABILITY/MANEUVERABILITY
• EQUIPMENT EFFECTIVENESS
• SAFETY ON DECK
• MAY PROHIBIT SOME OPERATIONS AT SEA

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EXTREME TEMPERATURES

• WIND CHILL
• FROSTBITE
• HYPOTHERMIA
• FROZEN TOPSIDE EQUIPMENT

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WINTER WARNINGS

• NORTHWALL ADVISORY -FAVORABLE CONDITIONS FOR
  NORTHWALL SETUP AND FORECASTED
  WINDS/SEAS/DURATION
• SMALL CRAFT WARNING -SUSTAINED WINDS 18-33 KTS
• SNOW WARNING CONDITIONS -ACCUMULATION gt/1INCH
  IN lt/6HRS (WITHIN 50 MILES OF NORFOLK)
• FREEZING PRECIP WARNING -ANY ACCUMULATION OF
  FREEZING PRECIP THAT COULD HINDER
  DRIVING CONDITIONS
• STORM SURGE CONDITIONS -/ 3 FT ABOVE NORMAL
  TIDE
• TSTORM WATCH ARE EXPECTED W/I 25NM OF NORFOLK
  WITHING 8 HOURS
• TSTORM WARNING ARE OCCURING OR FORECAST TO
  OCCUR W/I 25NM OFNORFOLK IN 1 HOUR
• SPECIAL WEATHER ADVISORIES
• EXTREME COLD,
• POTENTIAL FOR RAIN/FREEZING PRECIP/SNOW AND
  ACCUMULATION AMOUNTS
• OTSR

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WEATHERMAN
Take A Break ... we'll be right back with ...

• WEATHER WHYS
• December through March

WINTER WEATHER
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ANSWERS TO THE QUESTIONS YOU WERE AFRAID TO ASK
WEATHER NERD
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WEATHER NERD
Outdoor Nerd
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THE WIND DOES NOT LOWER THE TEMPERATURE BUT IT
MAKES IT FEEL MUCH COLDER. THE REASON IS THAT AS
THE WIND GROWS STRONGER, MORE AND MORE WARMTH IS
REMOVED FROM YOUR BODY. OF COURSE, IF THE AIR
TEMPERATURE DOES DROP AS WELL, THE WIND CHILL
VALUES BECOME EVEN LOWER.
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WEATHER NERD
Outdoor Nerd
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HALO COMMONLY OCCURS WHEN MOIST AIR IS LIFTED UP
AND OVER A DOME OF COLDER AIR. THE MOIST AIR
COOLS AND CONDENSES, FORMING AN EXPANDING DECK
OF CLOUDS. ON THE LEAD FLANK OF THE OVERRUNNING
MOISTURE, THE CLOUDS ARE VERY HIGH AND ARE
COMPOSED OF ICE CRYSTALS. SUNLIGHT COMING THROUGH
THE VEIL OF ICE IS REFRACTED IN SUCH A WAY AS TO
CAUSE A BRIGHT RING, WHICH WE REFER TO AS A HALO.
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WEATHER NERD
Outdoor Nerd
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THE WIND MAKES THE AIR FEEL COLDER BECAUSE IT
REMOVES HEAT FROM YOUR BODY. THE STRONGER THE
WIND, THE FASTER BODY WARMTH IS REMOVED AND THE
COLDER YOU FEEL. NOTE THAT THE WIND DOES NOT
LOWER THE ACTUAL AIR TEMPERATURE.
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WEATHER NERD
Outdoor Nerd
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THE WIND CHILL TEMPERATURE CAN BE SIGNIFICANTLY
LOWER BETWEEN BUILDINGS. IN THE OPEN, THE WIND
MIGHT BE BLOWING AT 20 MPH AND THE WIND CHILL
MIGHT BE AROUND ZERO. WHEN FORCED THROUGH THE
RELATIVELY NARROW PASSAGE WAYS BETWEEN BUILDINGS
THE AIR ACCELERATES. THIS INCREASE IN WIND SPEED
LOWERS THE WIND CHILL TEMPERATURE BY SEVERAL
DEGREES.
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WEATHER NERD
Outdoor Nerd
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SUNBURN DURING THE WINTER IS CALLED A SNOWBURN.
ONE WOULD NOT THINK THAT THE SUN'S RAYS ARE
STRONG ENOUGH DURING THE WINTER TO CAUSE A BURN
BUT THEY ARE, ESPECIALLY WHEN THERE IS SNOW
COVER. ON A BRIGHT FEBRUARY OR MARCH DAY WITH
SNOW ON THE GROUND, A DOUBLE DOSE OF SOLAR
RADIATION IMPACTS YOUR SKIN THE DIRECT ENERGY
FROM THE SUN AND THE ENERGY BEING REFLECTED OFF
THE SNOW.
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WEATHER NERD
Outdoor Nerd
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DURING EXTREMELY COLD WEATHER, MITTENS ARE BETTER
THAN GLOVES. ASSUMING BOTH ARE MADE OF THE SAME
BASIC MATERIAL, A WELL-CONSTRUCTED MITTEN WILL
KEEP OUT THE COLD AND RETAIN BODY WARMTH MUCH
BETTER THAN FINGERS OF THE GLOVE. THIS IS
BECAUSE THE INDIVIDUAL FINGERS ARE EXPOSED TO THE
COLD ON ALL SIDES AND LOSE HEAT MUCH FASTER.
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WEATHER NERD
Outdoor Nerd
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MELATONIN, A SLEEP-RELATED HORMONE, IS PRODUCED
AT GREATER LEVELS IN THE DARK. THE WINTER IS
INHERENTLY A DARKER TIME OF YEAR DUE TO THE LOW
SUN ANGLE AND LONG NIGHTS. SO, THE BODY NATURALLY
PRODUCES A GREATER AMOUNT OF MELATONIN DURING
WINTER WHICH IS BELIEVED TO BRING ON MILD
SYMPTOMS OF DEPRESSION OFTEN REFERRED TO AS THE
WINTER BLUES.
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WEATHER NERD
Outdoor Nerd
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WEATHER NERD
Nerd Driving
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WEATHER NERD
Nerd Driving
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SALT LOWERS THE FREEZING POINT OF WATER AND
THEREFORE EFFECTIVELY TURNS SNOW AND ICE TO
LIQUID ON THE HIGHWAYS. HOWEVER THE
EFFECTIVENESS OF ROAD SALT IS GREATLY REDUCED
WHEN THE TEMPERATURE IS BELOW 20 DEGREES, AND IT
HAS VIRTUALLY NO EFFECT WHEN THE TEMPERATURE IS
BELOW 10 DEGREES.
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WEATHER NERD
Nerd Driving
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WEATHER NERD
Nerd Driving
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A SNOW FENCE IS USED AS A SHIELD AGAINST DRIFTING
SNOW. UPWIND FROM THE FENCE, STRONG WINDS CREATE
CONSIDERABLE BLOWING AND DRIFTING. THE SNOW
FENCE SLOWS THE WIND AND THE SNOW IS DEPOSITED
INTO A CONTROLLED DRIFT. SNOW FENCES ARE USUALLY
SET UP ALONG HIGHWAYS WHERE DRIFTING SNOW COULD
BE A SERIOUS TRAVEL HAZARD.
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WEATHER NERD
HOME AND GARDEN NERD
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WEATHER NERD
HOME AND GARDEN NERD
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SINCE SNOW CONTAINS A LOT OF AIR SPACE, IT IS A
GOOD INSULATOR. A BLANKET OF SNOW ON YOUR ROOF
HELPS TO INSULATE AGAINST THE COLD AND KEEP THE
INTERIOR OF A HOME SOMEWHAT WARMER THAN IF THE
ROOF WERE BARE.
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WEATHER NERD
HOME AND GARDEN NERD
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DURING A SNOWSTORM, SNOW MAY DRIFT SEVERAL FEET
HIGH ON THE WINDWARD SIDE OF YOUR HOME. THE
REASON IS THAT THE HOUSE ACTS AS A WIND BREAK,
WHICH ALLOWS THE WIND-DRIVEN SNOW TO BE DEPOSITED
AND PILED UP.
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WEATHER NERD
HOME AND GARDEN NERD
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THE SAME WINDOWS THAT LET IN THE SUN'S WARMTH BY
DAY CAN LET OUT A LOT OF COLD AIR AT NIGHT. ONE
REASON IS THAT GLASS IS A POOR INSULATOR AGAINST
THE COLD. ALSO, SMALL GAPS BETWEEN THE WINDOW
FRAME AND THE HOUSE STRUCTURE CAN ALLOW COLD AIR
TO SEEP IN.
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WEATHER NERD
HOME AND GARDEN NERD
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WEATHER NERD
HOME AND GARDEN NERD
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SINCE YOUR HOME IS NOT A SEALED ENVIRONMENT, THE
OUTSIDE AIR FINDS A WAY INSIDE. DURING EPISODES
OF COLD WEATHER, THIS CAN LEAD TO VERY DRY
CONDITIONS. IF THE OUTSIDE TEMPERATURE IS 20 AND
THE DEWPOINT IS 5, THE RELATIVE HUMIDITY IS 50.
WHEN THAT AIR IS BROUGHT INSIDE AND WARMED TO 70
DEGREES, THE RELATIVE HUMIDITY LOWERS TO 10.
THIS OCCURS BECAUSE WARM AIR HAS A MUCH GREATER
CAPACITY TO HOLD WATER VAPOR THAN COLD AIR.
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WEATHER NERD
HOME AND GARDEN NERD
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THE MOST COMMON PLACE FOR ICICLES TO FORM IS ON
THE RIM OF A BUILDING. HEAT ESCAPING FROM THE
INTERIOR COMBINED WITH SOLAR RADIATION CAUSES
SNOWMELT ON THE ROOFTOP EVEN THOUGH THE AIR
TEMPERATURE MAY BE WELL BELOW FREEZING. AS THE
WATER STARTS TO DRIP OFF THE ROOF EDGE IT
ENCOUNTERS THE COLD FREE AIR AND A PORTION OF
THE DROP FREEZES. AS THIS PROCESS CONTINUES, THE
ICICLE ELONGATES.
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WEATHER NERD
Outdoor Nerd
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REFFERENCES
QUESTIONS?

• NLMOC OPTASKS
• FORECASTERS HANDBOOK
• NATIONAL WEATHER SERVICE
• ACCU WEATHER
• SURFACE WARFARE ARTICLE UP AGAINST THE WALL
• MONTHLY WEATHER REVIEW ARTICLE CLIMATOLOGY OF
  THE BOMB
• MR. DIXONS EXPANSIVE AMOUNTS OF WEATHER KNOWLEDGE