Case Profile

The case is presented in four parts:

Case Profile
Introduction to the case setting with a synoptic overview.

Case Challenge
A series of forecasting questions accompanied by a rich set of data and background content to challenge forecasting skills.

Case Summary
An experts overview of the case as it unfolded.

Resources
Bibliography and links to more information on ocean effect snow events.

Eta 500hPa Height

Current Time: 15UTC 13 January 1999

It is currently 15UTC (10AM local time on the East coast) on 13 January 1999. You are on shift in the Boston forecast office and are familiarizing yourself with the current synoptic conditions over eastern North America.

500hPa analysis valid 12UTC 13 January 1999 This chart shows a sharp upper trough over central Québec. It extends southwestward into Ontario. Comparison with previous 500hPa analyses (not shown) indicates that the trough has been moving very slowly. The trough is positively-tilted with strong upper-level winds on its east side over the northeastern U.S. and southeastern Canada. To the west of the trough is a northwesterly flow over much of Ontario and Manitoba, downstream of an upper ridge over the northern Canadian prairie provinces.

Eta 500hPa Height Analysis (m) valid 12UTC 13 Jan 99

• No Trough Axis
• Show Trough Axis

Eta 250hPa Isotachs

Current Time: 15UTC 13 January 1999

The 250hPa isotach chart shows the core of a strong jet streak, oriented southwest-northeast, extending from the lower Great Lakes to the northern Gulf of the St. Lawrence.

Comparison with previous 250hPa analyses (not shown) indicates that the streak has been moving eastward fairly rapidly.

Eta 250hPa Isotach Analysis valid 12UTC 13 Jan 99

• No Jet Streak
• Show Jet Streak

Eta MSLP and 1000-500 Thickness

Current Time: 15UTC 13 January 1999

MSLP (hPa)

1000-500hPa Thickness (m)

Show Anticyclone

Show Trough Axis

The 12UTC MSL pressure analysis shows a moderately strong anticyclone centered over northwestern Ontario.

Comparison with previous MSLP analyses (not shown) indicates that the high has been strengthening and moving eastward.

A trough is present farther south, with an axis from southern Illinois to Nova Scotia. This is a frontal trough, as shown by the 1000-500 thicknesses. The air mass over Canada is extremely cold, with thicknesses as low as 4740 m over north-central Québec.

Use the check box buttons at the top of the image to reveal or hide fields, and use the check boxes below to reveal the features of interest.

GOES-8 10.7-µm IR

Current Time: 15UTC 13 January 1999

The 10.7-micrometer IR image shows mostly clear conditions over much of Ontario and Quebec, with very cold ground temperatures there.

Strong, upper-level west-southwesterly flow is clearly evident over New England, with upper-level clouds moving from as far west as Missouri to off the New England coast. By 13 and 14 UTC the deepest clouds with the coldest tops seem to be dissipating over northern Pennsylvania and New York.

Radar Composite

Current Time: 15UTC 13 January 1999

The composite radar reflectivity image shows a long band of light-to-moderate snowfall from Missouri to the Canadian Maritimes. The snow is generally moving eastward with the upper flow.

Case Challenge

Overview

From the perspective of a forecaster in the Boston Forecast Office, respond to this series of forecast challenge questions. The case begins at 10AM local time (15UTC) on 13 January 1999.

The first few questions in the Challenge examine this event on the synoptic scale. Subsequent questions will add mesoscale considerations to the mix.

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question

Current Time: 06UTC 14 January 1999

Data: Soundings | Radar

15 hours have passed. It is now just after 06UTC on 14 January, and eastern Massachusetts, including the Boston area, is experiencing light snow with periods of locally heavy snow due to bands of ocean effect snow moving onshore from the east. As was forecast earlier, conditions favourable for ocean effect snow have indeed come together: very cold air with a steep lapse rate in the low levels along with winds that are approximately parallel to the general New England coast. You are interested in the depth of the ocean effect snow bands at this time.

From the radar image at 0607UTC and/or from available forecast sounding data, what is the approximate depth of the ocean effect snow bands over coastal Massachusetts? Depending on how you estimate the heights, you may need the sine of 0.5°, which is 0.0087.
(Choose the best answer)

View the relevant Supporting Topics below to help you with your answer.

a) 200 - 600 m

b) 600 - 1200 m

c) 1200 - 1800 m

d) 1800 - 2400 m

The correct answer is c)

Overview

The best choice is the range of 1200-1800m, or roughly 4000-5000 ft. Use of radar or use of soundings are the two possible approaches for estimating cloud heights in this situation.

Image: Estimating Cloud Height with Radar

Use of Radar

This depth of the snow bands can be estimated from the PPI (plan-position indicator) reflectivity scan at Taunton (BOX), which shows ocean effect bands extending out to about 100 km east of the radar on the 0.5° scan at 0557UTC (the 100-km range ring is available in the data set for this question). Beyond that distance the beam is more or less above the bands.

Simple geometry shows that at 100 km, the centre of the beam at 0.5° elevation is above the tangent to the earth through the radar station by some 873 m. This number is surely an overestimate, due to beam spreading, since the center of the beam could pass somewhat above the tops of the snow bands while the lower part of the spread-out beam could still provide some radar returns from them.

Another factor that must be accounted for is the earth's curvature: the correction for the earth's curvature at 100-km range is that the earth's surface lies under the tangent by about 785 m. The sum of the two, 1658 m, overestimates to some degree the depth of the snow band clouds, following the comments made above about beam spreading.

One final factor to consider in this case is beam refraction: the increase of temperature with height in the inversion will cause the beam to refract downward, but this will be offset by the upward refraction of the beam due to the increase of humidity with height. The net refractive effect may be small in this case.

Use of Soundings

The operational forecaster might use forecast or observed soundings to get a rough estimate of the height of the clouds producing the ocean effect snow bands. The 3-hour Meso-Eta forecast sounding for BOS valid 06UTC shows the strong inversion above 925 hPa, and we can assume that a similar inversion exists over the Atlantic off Massachusetts.

The ocean effect clouds are convective clouds with their origins over the waters of the Atlantic. In such a cold air mass, an air parcel just above the water surface is buoyant and will rise (recall that the SST is observed to be 4°C at a buoy just off the Massachusetts coast). Under various assumptions about the temperature and dew point of such a parcel, it is clear that it would rise to at least the level of the inversion in the vicinity of 900 to 850 hPa. This would put the tops somewhere in the 1150-1500m range (see the 850 and 900hPa height forecasts for these numbers). This rough estimate is not too different from that obtained through the use of the radar reflectivity data.

Please make a selection.

Question 7

Question 8

Question 9

Case Summary

Overview

Between the evening of 13 January and early morning of 15 January 1999, eastern Massachusetts experienced regions of heavy snowfall. An area of 25-41 cm of snowfall accumulated south of Boston (BOS). The maximum amount of 41 cm was recorded in South Weymouth (NZW).

Ocean effect snowfall began over eastern Massachusetts during the evening hours of 13 January 1999 and continued overnight and through most of the day on the 14th. During the late afternoon of the 14th, an upstream system brought synoptic scale precipitation to the area. Most areas, especially north and west of Boston, received a few more hours of snowfall before it changed to rain. The first observation of light rain took place in Cape Cod at around 06UTC on 15 January. Subsequent heavy rain was observed at many locations through 15 January, with temperatures soaring to 7-13°C (45-55°F). Rain amounts of up to 50 mm were observed.

It was not possible to separate the ocean effect snow measurements from the synoptic snowfall measurements, so the observed amounts in this chart present the sum of both types of snowfall. It is estimated that not more than 5-10 cm of the snow was synoptically produced (Waldstreicher, 2002). Thus, some locations likely received up to 30 cm of ocean effect snow from this event.

The Case Summary begins with a synoptic view beginning approximately 36 hours prior to the time that most of the precipitation changed to rain, and narrows in on the mesoscale. Details of the low-level ocean effect snow with accompanying banding that contributed so much to the heavy snowfall amounts will be highlighted.

Note: This Case Summary provides a brief overview of the event. The Case Challenge provides a more in-depth analysis.

A Cold Front Moves Through

At 12UTC on 13 January, a high pressure center was located over Ontario, while very cold air covered Ontario and Québec. A frontal trough was found from southern Illinois to Nova Scotia with some snow associated with the front. As the high built and moved eastward, the front was pushed southward from the New England area.

Use the time and field buttons to view Eta MSLP, 1000-500hPa thickness, 2m temperatures, and 10m winds at 12, 18, and 00UTC on 13 and 14 January.

• 1200UTC
• 1800UTC
• 0000UTC 14 Jan

1200UTC

MSLP (hPa)

1000-500hPa Thickness (m)

2m Temperature (°C)

10m Winds (kts)

1800UTC

MSLP (hPa)

1000-500hPa Thickness (m)

2m Temperature (°C)

10m Winds (kts)

0000UTC 14 Jan

MSLP (hPa)

1000-500hPa Thickness (m)

2m Temperature (°C)

10m Winds (kts)

Six hours later, at 18UTC, the cold front was approaching Cape Cod, coastal Connecticut, and Rhode Island. Very cold air moved across northern and central New England.

By 00UTC the cold front was well south of New England as cold air continued to move over the region. Note how the flow shifted to a northerly-to-northeasterly direction along the New England coast.

Cold Air Advection

The 00UTC 14 January sounding at Chatham, Massachusetts (CHH) shows that a low-level cold layer extended to about 925 hPa. Note the backing winds with height, indicating cold advection. Temperatures near 850 hPa at this time were in the range of 0 to 2°C. Also note that the sounding below 700 hPa is saturated, or nearly so.

Use the radio buttons to view each station's sounding.

• Location of buoy and ship SST observations
• CHH Sounding
• GYX Sounding

At the same time, 00UTC on 14 January, the sounding at Gray, Maine (GYX) shows a much deeper layer of cold air. The 850hPa temperature is -13°C. Buoy data indicated that ocean water temperature was approximately 4°C. The ocean-to-850hPa temperature difference, (delta-T), of 17°C indicates considerable instability, which could lead to ocean effect snow development given the appropriate wind scenario. The subsidence inversion, indicated by both increasing temperatures and abrupt drying, is based slightly above 850 hPa.

For more information on the relationship of the lapse rate over bodies of water and the implications for lake/ocean effect snow, please view the accompanying Supporting Topics.

Enhancement of the NNE Flow across Eastern New England

By 06UTC on 14 January, the surface high over southwest Quebec had strengthened to a central pressure of 1052 hPa, further enhancing the cold, northeasterly low-level flow across eastern New England and its coastal waters.

MSLP (hPa)

1000-500hPa Thickness (m)

2m Temperature (°C)

10m Winds (kts)

Ocean Effect Set Up

At this point, conditions for ocean effect for the Boston area are evident. The Boston (BOS) sounding for 06UTC on 14 January shows the following important features:

• A very cold and moist boundary layer
• A strong inversion at around 925 hPa
• The sounding is very moist—nearly saturated—all the way up through 600 hPa
• North-northeasterly flow in the low levels with west winds above the inversion
• The ocean-to-850hPa temperature difference, delta-T, is only 11°C, smaller than the commonly used 13°C threshold (which roughly corresponds to a dry adiabatic lapse rate from the sea surface up to the 850hPa level) indicating potential for lake or ocean effect snow. However, the 850 level is well above the base of the inversion, where temperatures are as low as -17°C. This results in an extremely unstable 20°C temperature differential between the water surface and the base of the inversion.

View the accompanying Supporting Topics on lake/ocean effect snow for more information.

Ocean Effect Snow with Banding

The 4-km resolution (pixel size) 10.7-µm IR loop follows the same general pattern in the vicinity of New England for the 22-hour period 03UTC on 14 January through 00UTC on 15 January. This pattern has the middle- and upper-level clouds invading the Massachusetts area from the west. Much of the region over the course of this time period experienced areas of light snow and, later on, rain of varying intensity.

The Taunton (BOX) radar loop for the same 22-hour period follows in more detail the evolution of the precipitation. Ocean effect bands of heavy snow began to appear in the Boston area at around 03UTC, in addition to the more widespread light snow related to the westerly upper flow. Ocean effect bands of heavier snow to the east and south of the Boston area were approximately parallel to the prevailing low-level winds.

These bands moved slowly from east to west. The westward motion of the ocean effect bands over the Atlantic is also nicely shown in the 2-km resolution visible satellite loop in the period 1332 to 2032UTC. Though this loop only covers 7 hours, the ocean effect bands persisted for close to 16 hours and continued to produce heavy snow in some areas through part of the afternoon.

Depth of Ocean Effect Bands

The ocean effect bands were composed of convective clouds with their tops in the vicinity of the inversion. Through the use of radar data and compared to model sounding data, it was shown that the tops of the bands were in the range of 1200-1600 m (4000-5000 ft) above the surface. (For more on this, see the Discussion section in Question 6 of the Case Challenge.)

• GOES-8 2-km VIS, 10.7-µm, 3.9-µm at 16UTC 14 Jan 99
• Eta Forecast Sounding for Boston, valid 18UTC initialized 12UTC 14 Jan 99

GOES-8 2-km VIS, 10.7-µm, 3.9-µm at 16UTC 14 Jan 99

• VIS
• 3.9-µm
• 10.7-µm

Eta Forecast Sounding for Boston, valid 18UTC initialized 12UTC 14 Jan 99

The nature of these ocean effect bands was such that heavy snow was produced from bands that were much shallower than clouds in a traditional synoptic system for equivalent amounts of snow. In addition to the cloud-top measurements discussed in Question 6 of the Case Challenge, an estimate of the tops of the ocean effect bands can also be obtained from available IR imagery.

At around 16UTC on 14 January, a hole appeared in the mid-level cloud over the ocean off the New England coast. This hole can be clearly seen at 1602UTC on the visible, 10.7-µm IR, and 3.9-µm channels of the satellite loop. The ocean effect clouds can be seen through the hole near 42.5N, 69W. A close look at the IR temperature at that point gives a value of -11°C.

The Eta 06-hour forecast sounding at that point, valid 18UTC on 14 January, shows a temperature of -11°C at a level of about 875 hPa. This corresponds to a height of about 1370 m (4,500 ft), which falls in the range of values already calculated by the radar and IR imagery.

Ocean Effect Banding and Horizontal Roll Convection

The radar and satellite imagery both indicated a slow westward movement of the ocean effect snow bands. It is believed that such bands are an example of horizontal roll convection. In these bands, individual convective elements move with the mean low-level wind—in this case, from the northeast to the southwest. No clear explanation for the observed slow westward motion of such bands exists, however.

The observed hodograph at Chatham at 12UTC on 14 January shows northeasterly winds in the low-levels becoming northwesterly or westerly at higher levels above the inversion. The wind shear direction was from the southwest in this case. The main point to note from this hodograph is that the cold advection implied by the backing of the wind with height was acting to maintain the low-level cold air at that time, which in turn maintained the low level instability with respect to the lowest levels.

Heavy Snow and Cloud Microphysics

Much of the heavy snow in this case can partly be attributed to the microphysics of the setting. Though the bands were contained below the relatively shallow inversion layer and a little over 1 km in height, their temperature and water vapor content were primed for producing copious amounts of precipitation in the form of snow.

A look at the Boston sounding for this time period reveals that the temperature in and below the inversion layer included a zone of -12 to -18°C, the optimal temperature for crystal initiation and rapid growth of large dendrites. Also take note of the nearly saturated layer near 600 hPa in the same dendritic temperature range. Any crystal forming in this region fell through a relatively warmer and saturated layer between 850 and 650 hPa. Dendrites would not dissipate in these conditions, but instead would most likely grow through aggregation and subsequently fall into another dendritic growth zone under the inversion.

For more information on winter cloud microphysics, see the accompanying Supporting Topics.

Seeder/Feeder Effect

• Taunton MA (BOX)
  WSR-88D Radar 1857UTC
• 3.9-µm GOES-8 1915UTC

An overriding source of ice crystals in this case provided enhancement of the snowfall produced by the shallow ocean effect bands via a seeder/feeder process. As was seen in the previous soundings, a nearly saturated layer was present at about 600 hPa with a temperature around -15°C. Ice crystals and snow forming and falling from middle-layer clouds into lower-level cloud bands most likely acted as seeds for increased crystal production below the inversion and provided more dendrites for aggregation.

Radar during this time period confirms this by showing an increase in reflectivity in the banded areas as higher clouds move across from west to east.

Examination of 3.9-micrometer satellite imagery during the times of reflectivity enhancement confirmed that mid-level clouds likely contained primarily ice crystals. The darker areas with yellow highlights upstream from Boston show regions of lower reflectance for liquid. These are areas of clouds saturated for ice.

Precipitation Change

• Surface Observations
• CHH Sounding

Surface Observations

• 0000Z
• 1200Z

CHH Sounding

• 0000Z
• 1200Z

The surface high pressure center continued to move eastward on 14 January. During this time, the easterly component of the low-level flow over eastern New England gradually increased. At the same time, warm advection associated with the incoming synoptic-scale disturbance advanced northeastward toward New England.  

By around 20UTC, the low-level inversion had weakened to the point where the ocean effect snowbands dissipated. Synoptic-scale snow continued beyond this time over parts of New England, though. At 00UTC on 15 January, the observed sounding at CHH still shows some cold air in the low levels, and the surface chart shows that snow was falling at some stations in the Massachusetts area. Warm advection intensified overnight, the surface winds continued to veer, and the precipitation changed to rain. By 12UTC, heavy rain was falling at some locations in Massachusetts.

This is the end of the Case Summary.

Please take our User Survey

Resources

This case exercise is based on an article by Jeff Waldstreicher:

Waldstreicher, Jeff S., 2002: A foot of snow from a 3000-foot cloud: The Ocean-Effect Snowstorm of 14 January 1999. Bulletin of the American Meteorological Society: Vol. 83, No. 1, pp. 19-22.

Contributors

COMET Sponsors

• Air Force Weather Agency (AFWA)
• Meteorological Service of Canada (MSC)
• National Environmental Satellite Data and Information Service (NESDIS)
• Naval Meteorology and Oceanography Command (NMOC)
• National Oceanic and Atmospheric Administration (NOAA)
• National Polar-orbiting Operational Environmental Satellite System (NPOESS)
• National Weather Service (NWS)

Principal Science Advisors

• Peter Lewis — MSC
• Garry Toth — MSC
• Dr. Doug Wesley — UCAR/COMET

Project Lead/Instructional Design

• Bruce Muller — UCAR/COMET
• Patrick Parrish — UCAR/COMET

Multimedia Authoring

• Bruce Muller — UCAR/COMET
• Dan Riter — UCAR/COMET

Audio Editing/Production

• Seth Lamos — UCAR/COMET

Audio Narration

• Dan Riter — UCAR/COMET

Computer Graphics/Interface Design

• Heidi Godsil — UCAR/COMET

Illustration/Animation

• Steve Deyo — UCAR/COMET

Software Testing/Quality Assurance

• Michael Smith — UCAR/COMET

Copyright Administration

• Lorrie Fyffe — UCAR/COMET

Data Provided by

• MSC
• NWS — NOAA

COMET HTML Integration Team 2021

• Tim Alberta — Project Manager
• Dolores Kiessling — Project Lead
• Steve Deyo — Graphic Artist
• Ariana Kiessling — Web Developer
• Gary Pacheco — Lead Web Developer
• David Russi — Translations
• Tyler Winstead — Web Developer

COMET Staff, 2003

Director

• Dr. Timothy Spangler

Deputy Director

• Dr. Joe Lamos

Meteorologist Resources Group Head

• Dr. Greg Byrd

Business Manager/Supervisor of Administration

• Elizabeth Lessard

Administration

• Lorrie Fyffe
• Bonnie Slagel
• Penina Frager

Graphics/Media Production

• Steve Deyo
• Heidi Godsil
• Seth Lamos
• Dan Riter

Hardware/Software Support and Programming

• Tim Alberta (Supervisor)
• David Ellis (Student)
• James Hamm
• Karl Hanzel
• Ken Kim
• Mark Mulholland
• Max Parris (Student)
• Carl Whitehurst

Instructional Design

• Patrick Parrish (Supervisor)
• Dr. Alan Bol
• Dr. Vickie Johnson
• Bruce Muller
• Dr. Sherwood Wang

Meteorologists

• Dr. William Bua
• Patrick Dills
• Kevin Fuell
• Jonathan Heyl (Student)
• Dr. Stephen Jascourt
• Dolores Kiessling
• Heather McIntyre (Student)
• Wendy Schreiber-Abshire
• Dr. Doug Wesley

Software Testing/Quality Assurance

• Michael Smith (Coordinator)

National Weather Service COMET Branch

• Anthony Mostek (National Satellite Training Coordinator)
• Brian Motta
• Elizabeth Mulvihill Page
• Dr. Robert Rozumalski (SOO Science and Training Resource Coordinator [SOO/STRC])

Meteorological Service of Canada Visiting Meteorologists

• Peter Lewis
• Garry Toth