Marine Weather Forecasting

The United States Power Squadrons® Webinar/Seminar Guide

2011 Copyright USPS® Acknowledgements

The Marine Environment Committee gratefully acknowledges NOAA’s continuing support of the United States Power Squadrons® weather education mission and its contributions to this Webinar/Seminar Guide including: • The Mariners Weather Log article – Mariner’s Guide to the 500 – Millibar Chart – co-authored by Joe Sienkiewicz (NOAA/NWS) and Lee Chesneau (Lee Chesneau’s Marine Weather), and • Images from NOAA/NWS’s weather web pages, including the images used on the cover. The committee appreciates very much the contributions of Stf/C Robert F. Anderson, AP, an Assistant Chair of the Committee, who originated the idea for this Seminar/Webinar and did much of the work in putting it together. We also gratefully thank the Chairperson, R/C S. Patricia Greer, SN, and members, of the Publications Support Committee who reviewed and formatted this Guide for publication. R/C Ronald Kessel, SN Marine Environment Committee, Chairman

The following are registered trademarks and service marks of the UNITED STATES POWER SQUADRONS® AMERICA’S BOATING COURSE PARTNER IN COMMAND AMERICA’S BOATING CLUB SEA VESTER DESIGN (Cartoon caricature) BOAT SMART and Design THE ENSIGN BOATING IS FUN --- WE’LL SHOW YOU HOW THE ENSIGN (Stylized) CHART SMART THE UNITED STATES POWER SQUADRONS and Design ENSIGN FLAG DESIGN UNITED STATES POWER SQUADRONS ENSIGN FLAG DESIGN IN A SHIP’S WHEEL USPS JET SMART USPS UNIVERSITY PADDLE SMART WATER SMART FROM THE START

THE UNITED STATES POWER SQUADRONS® also owns the USPS AQUA TRADE DRESS OF ITS SEVERAL PUBLICATIONS (Left side of front covers, including text and graphics)

One or more of these trademarks or service marks may be found in this publication.

Copyright 2011, United States Power Squadrons All rights reserved Printed in the United States of America

United States Power Squadrons 1504 Blue Ridge Road P.O. Box 30423 Raleigh, NC 27622

Voice: (919) 821-0281 1-888-FOR-USPS (367-8777) Fax: (919) 836-0813 www.usps.org

This publication is designed to provide general information on boating safety, piloting, and navigation. It was prepared by the United States Power Squadrons and its members, and is sold with the understanding that professional nautical services are not being provided. Do not use this publication as a substitute for original sources of nautical information, including government publications, rules, regulations, laws, and manufacturers’ recommendations, where appropriate. The subject matter of this publication is likely to become outdated over time, and may have changed since our last publication date. Consequently, you must research original sources of authority to update this material and ensure accuracy when dealing with specific “on the water” conditions or boating problems.

Marine Weather Forecasting

Webinar/Seminar Guide

“ I must go down to the seas again, to the lonely sea and the sky…” John Masefield

These familiar words of John Masefield capture some of the wonder and fear that the sky and the sea can evoke. It is no surprise that ever since mariners set out to sea they all shared the same awe and fear. The sails and courses they set, their livelihoods, and even their lives were weather dependent. The effect of weather on the sea always was for them, and still is for us, a vital concern. The Marine Weather Forecasting webinar deals with these same ages‐old concerns, but in a modern way taking full advantage of today’s meteorological advances and presenting a seminar via the Internet as a webinar.

Marine Weather Forecasting has been designed with coastal and offshore cruisers in mind, but it will also be of interest to all boaters and those generally fascinated by the weather. Seminar topics include:  Review of Surface Maps and Winds  Upper‐Air Winds and Charts  The 500 mb Chart  GRIB Forecasts  NWS Marine Weather Forecasts.

The 500 mb chart is a major important tool in forecasting the weather for those in coastal/offshore areas as well as those on the high seas. It is no accident that understanding and interpreting this upper air chart is a major focus of the webinar.

In addition to this Webinar/Seminar Guide participants will be able to download in pdf format the classic article co‐authored by Joe Sienkiewicz and Lee Chesneau: Mariner’s Guide to the 500‐ Millibar Chart. Webinar participants reviewing the intricacies of the 500 mb chart are encouraged to read this article. The illustrations will be easier to see and the more detailed explanations accompanying them will be particularly helpful.

This guide consists of slide images and complete notes that cover all the webinar subjects. There may be some minor differences from the slides the presenter selects for the actual webinar presentation, but they will not be major ones. This Webinar/Seminar Guide with its images and notes will serve as a review and will be a useful reference for years to come.

Marine Weather Forecasting 1

Slide 1 Welcome to Marine Weather Forecasting This seminar is presented by: • The United States Power Squadrons® (USPS) • USPS is a non‐profit educational organization made up of 450 individual squadrons throughout the US and the Caribbean. • This seminar on Marine Weather Forecasting is only one of many USPS courses and seminars designed for the recreational boater. • Other subjects include seamanship, piloting, navigation, engine maintenance and marine electronics. • The National Oceanic and Atmospheric Administration (NOAA) and its National Weather Service (NWS) • The National Weather Service provides weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas. • Its products are used by both government entities and the private sector. • The NWS accomplishes this mission through a collection of national and regional centers, and 122 local weather forecast offices. • The NWS collects meteorological data from all over the globe. Slide 2 Acknowledgements • The United States Power Squadrons® gratefully acknowledges NOAA’s continuing support of the USPS weather education mission and its contributions to this webinar: • The Mariners Weather Log article – Mariner’s Guide to the 500 – Millibar Chart – co‐authored by • Joe Sienkiewicz (NOAA/NWS) • Lee Chesneau (Lee Chesneau’s Marine Weather) • Content from NOAA/NWS’s weather web pages • The webinar’s NWS Instructor

Slide 3 Seminar Instructor • Joseph Sienkiewicz Chief ‐ Applications Branch Science and Operations Officer Ocean Prediction Center NOAA National Weather Service

USPS University Series 2 Marine Weather Forecasting

Slide 4 Seminar Materials • Mariner’s Guide to the 500 – Millibar Chart article– co‐ authored by Joe Sienkiewicz (NOAA/NWS) and Lee Chesneau (Lee Chesneau’s Marine Weather) appearing in the December 2008 issue of the Mariners Weather Log (Vol. 52 No.3). • The article contains a more detailed explanation of the 500 mb chart and the slides relating to it. • It is available to the public at no cost and can be downloaded at: www.vos.noaa.gov/MWL/dec_08/milibar_chart.shtml • Where the slide notes contain references to figures, the references are to the illustrations in the Mariner’s Guide to the 500 – Millibar Chart.

• Marine Weather Forecasting Seminar Guide prepared for this webinar. • The guide contains thumbnail images of the slides in this presentation together with notes for each slide. • The images, however, are smaller than the same 500 mb chart images in the article. • The guide is available only to webinar participants at no extra cost and can be downloaded at www.usps.org/eddept/webinars/wx/wx_forecasting_guide.pdf

Slide 5 Seminar Focus • Coastal and Offshore Marine Forecasting • Topics: • Review of Surface Maps and Winds • Upper‐Air Winds and Charts • The 500 mb Chart • GRIB Forecasts • NWS Marine Weather Forecasts

Slide 6 Seminar Topic

Review of Surface Maps and Winds

USPS University Series Marine Weather Forecasting 3

Slide 7 Surface Weather Maps Surface Weather Maps show: • High – an area of comparatively high pressure with winds spiraling out • Low – an area of comparatively low pressure with winds spiraling in • Trough – an elongated region of comparatively low pressure • Ridge – an elongated area of comparatively high pressure • Front – the boundary between two air masses of different temperatures • Isobar – a line of equal pressure drawn on a weather map Slide 8 Surface Weather Map Symbols • Pressure Regions • Highs are designated with a blue H. • Lows are designated with a red L. • Troughs are designated with a tan dashed line.

• There are four main types of fronts (boundaries) between two air masses. • Warm: advancing surface front with warm air mass behind it depicted by red domes. • Cold: advancing surface front with cold air mass behind it depicted by blue triangles. • Stationary : a surface front that is not moving depicted by alternating red domes and blue triangles on opposite sides. • Occluded: a front where warm and cold air masses meet above the surface depicted by domes and triangles on the same side usually colored purple. • Dry Line: a front that separates warm dry and warm wet air masses depicted by gold domes on the same side. Dry lines occur over the plains. Slide 9 Isobars • Isobars (line of equal pressure) are normally drawn at 4 mb intervals with 1000 mb being the base value. • The isobars are drawn on a “best fit” basis and smoothed to make the map more readable. • The patterns of isobars are useful in forecasting weather and identifying weather systems. • This weather map clearly shows different weather systems: • Low pressure systems where the weather is usually foul; and • High pressure systems where the weather is usually fair.

USPS University Series 4 Marine Weather Forecasting

Slide 10 Surface Analysis (Fig. 1) • This surface analysis shows the features of a typical surface weather map: • highs; • lows; • troughs; • fronts (warm, cold, stationary, and occluded); and • isobars.

• It adds • red and blue Xs – 24 hr forecast positions of lows and highs • ship observations; and • a satellite overlay. • Note the clouds associated with the large frontal system extending S‐SW from the Aleutians.

Slide 11 Wind Speed and Direction • There are three determinants of wind speed and direction. • The Pressure Gradient Force causes the air to move. • The greater the pressure difference and the closer the isobars – the faster the wind. • Surface Friction slows the wind. • The Coriolis Effect causes the wind to curve • to the right in the Northern Hemisphere • to the left in the Southern Hemisphere.

• The Coriolis Effect is 0 at the Equator, increases with latitude, and is greatest nearest the poles. • The Coriolis Effect also increases with wind speed. • For this reason Friction weakens the Coriolis Effect. • Technically speaking, the vector sum of all of three determines the wind’s speed and direction.

Slide 12 Surface Wind Flow

• Winds blow from high pressure to low pressure due to the Pressure Gradient Force. • Winds turn to the right in the Northern Hemisphere due to the Coriolis Effect.

USPS University Series Marine Weather Forecasting 5

• The winds cross isobars at an angle because • Surface friction slows the wind’s speed. • Slowing the wind weakens Coriolis Effect since it depends on speed, but Pressure Gradient Force stays the same. • Result ‐ Pressure Gradient Force, now stronger than Coriolis Effect, pushes the wind toward the low center or away from the high center.

Slide 13 Northern Hemisphere Surface Winds: Highs and Lows • Winds spiral out of Highs in a clockwise direction. • Winds spiral into Lows in a counter‐clockwise direction. • The winds cross the isobars at an angle. • The closer the isobars the stronger the wind. • Lows usually have higher winds than highs.

Slide 14 Surface Cross‐isobar Flow • Since surface friction is greater over land than over water, the wind crosses isobars at a greater angle over land than over water. • about 30 degrees over land • about 15 degrees over open water • The angle is greater over land because surface friction is greater – slowing the wind and reducing the Coriolis Effect.

Slide 15 Seminar Topic

Upper‐Air Winds and Charts

USPS University Series 6 Marine Weather Forecasting

Slide 16 Comparison Surface/Upper Air Winds • Surface Winds • Vary in direction • But SW‐W‐NW dominates • Have more rotary patterns • Swirl into Lows • Swirl out of Highs

• Upper‐Air Winds • More consistent west to east flow • Smoother streams with gentler curves • Sweep around Highs and Lows – the absence of friction means the Coriolis Effect balances the Pressure Gradient Force. • Wind speeds are higher because PGF increases with altitude

Slide 17 Surface Map – 18 Feb 2011 • This slide shows a surface weather map for 0700 on 18 February 2011. • The isobars indicate the many different directions of the wind. • Note the closed circular Low in the Northwest and the much stronger one north of the Great lakes

Slide 18 Mid‐Level Chart – 18 Feb 2011 • At the same time on the same date, this slide shows wind flow at a middle altitude level (18000 feet) ‐ 500 millibar pressure level. • Note the two closed Lows on the surface map are no longer closed on the mid‐level chart. • The flow is prevailing westerly and influenced less by rotary convective air‐flows. • The winds are smoother streams with gentler curves.

USPS University Series Marine Weather Forecasting 7

Slide 19 Constant Pressure Surface Aloft • The slide shows a “Constant Pressure Surface Aloft”. • For any given pressure level (e.g. 500 mb) the imaginary surface representing that level will be higher when there is warm air below it (a ridge) and lower when there is cooler air below it (a trough). • A constant pressure surface can be represented on a chart using contour lines.

Slide 20 Upper‐Level Chart • Resembling the ordinary weather map in some respects, these upper‐air charts show wind direction and speed, temperature and water vapor content at fixed pressure levels (e.g., 500 mb, 200 mb). • The different heights of a constant pressure surface are depicted as contour lines (lines of equal altitude) just like a topological map. • In this slide the contour numbers give the height in meters – the altitude ranges around 18,000 feet (the 500 mb level).

• These contour lines should not be confused with isobars on surface maps (lines of equal pressure, all at the same altitude). • Constant Pressure (rather than fixed height) surfaces are used because it makes the physics and math easier for meteorologists. • The height contour lines are particularly useful because upper‐air winds flow parallel to them. • This slide and the previous one show the relationship between a Constant Pressure Surface and its Upper‐Air Chart.

Slide 21 Cross Section View: Constant Pressure Surfaces (Fig. 3) • White contours – constant pressure surfaces • Warm to cool colors – air temperature • Purple contours – air temperatures in degrees C • Red dashed line – trough in height field • Blue dash/dot line – temperature trough (coldest axis) • Note the surface low over New England • Note also the westward tilt with height of the trough axis – a sign of a deepening storm system • Different Constant Pressure Surfaces Charts have different uses.

USPS University Series 8 Marine Weather Forecasting

Slide 22 Bird’s Eye View –500 mb Surface (Fig. 2) • Same date and time as previous slide (Cross Section View: Constant Pressure Surfaces) • White – 6000 m fixed height surface • Green shaded surface – 500 mb surface • Yellow lines – surface isobars • Note how the 500 mb surface slopes down going north (colder air) • Note also the low pressure trough on the 500 mb surface supporting the New England surface low.

Slide 23 Jet Stream • This slide shows some fragmented Jet Stream winds at the 500 mb level, but the more continuous core is much higher. • To locate the Jet Stream core: • a 300 mb chart (about 29,000 ft.) is used in the winter when the jet stream is lower; and • a 200 mb chart (about 39,000 ft.) is used in the summer when the jet stream is higher.

Slide 24 Different mb Charts • 300 mb and 200 mb Charts: • These charts represent constant pressure surfaces at altitudes that range around 29,000 ft (5.5 miles) and 39,000 ft (7.4 miles) respectively. • The charts are particularly useful in locating the polar jet stream. • The 300 mb chart is mainly used in the winter when the jet stream is lower and the 200 mb chart is used in the summer when the jet stream is higher.

USPS University Series Marine Weather Forecasting 9

• 500 mb Charts: • These charts represent constant pressure surfaces at altitudes that range around 18,000 ft (3.4 miles). • They are powerful surface weather prediction tools • The charts show upper‐level steering winds, the vorticity (curving/spinning flow) associated with rising and sinking air, and the trough and ridge patterns responsible for the development and displacement of surface weather systems. • 850 mb and 700 mb Charts: • These charts represent constant pressure surfaces at altitudes that range around 5,000 ft (1 mile) and 10,000 ft (2 miles) respectively. • They are particularly useful for showing thermal (warm and cold) and moisture advections. • The 700 mb chart is used in the mountainous west part of the country where surface elevations are high. The 700 mb wind flow indicated by the contour lines steer air mass or single cell thunderstorms.

Slide 25 Seminar Topic

The 500 mb Chart

Slide 26 500 mb Chart Basics

• 500 mb surface • average. height 5600 m ‐ 18,000 ft. • range – 4700 m near poles to 6000 m near equator. • Contour Lines • Higher the height – higher the pressure • Lower the height – lower the pressure o Add a 0 to get altitude in meters • Closer the contours – faster the wind

• Winds • flow mostly parallel to the contour lines unlike surface winds that cross isobars on surface weather maps. o wind barbs show direction and speed • unlike near the surface, friction does not slow the wind reducing the Coriolis Effect • patterns used to predict surface weather

USPS University Series 10 Marine Weather Forecasting

Slide 27 500 mb Analysis (Fig. 4)

• 500 mb showing the North Pacific. • Ls and Hs are minima and maxima heights. • Purple Xs are present surface low centers and red Xs are 24 hr forecast positions. These are the same positions shown on the Surface Analysis slide. • Brown dash lines are shortwave trough axes. • Blue zigzag line is ridge axis.

• Note the 5640 mb “storm track” (thicker yellow contour). • Simple rule of thumb: the tighter the height contours, the higher the wind speed, the stronger the temperature difference below 500 mb. • The tight contours indicate the bottom of the jet stream (not its higher faster core). • The highest wind speeds (tightest contours) indicate a jet streak.

Slide 28 North Atlantic mb Analysis (Fig. 5)

• 500 mb analysis for the North Atlantic. • Upper level charts vary somewhat in their manner of presentation. In this version: • Wind speeds are color coded. • The 564 “storm track” is thick red line. The “564” line is called the 564 decameter height contour.

• Surface low centers and their fronts (particularly cold fronts moving south across latitude lines) move at 1/3 to 1/2 the 500 mb wind speeds. • Surface wind speeds (especially in west to southwest quadrant ‐ cold air) is about 50% of 500 mb wind speeds.

USPS University Series Marine Weather Forecasting 11

Slide 29 The 5640 m Contour • The 5640 line (or 564 line when the reference is to decameters) has special significance. • The line marks the area with the greatest mixing of cold dry air and warm moist air – an area of great low pressure development. • Known as the “storm track line” it is often shown as a thicker contour. • Surface storm tracks usually lie 300 to 600 nm north of and track parallel to the 5640 contour.

• The worst weather conditions are usually poleward of the 5640 contour (north in N.H.) • In winter 5640 is southern extension of Force 7 (28‐33 kts) westerlies. • In summer 5640 is southern extension of Force 6 (22‐27 kts—occasionally Force 7) westerlies. • In mid‐latitude storm systems the strongest winds and seas are usually in the cold air (the western quadrants).

Slide 30 500 mb Patterns • The wind flow patterns shown on the 500 mb chart are tools used to forecast surface weather conditions. The main patterns are: • Long Waves • Short Waves • Short Wave Troughs and Surface Lows • Zonal Flow • Meridional Flow • Blocking Ridge • Cut‐off Lows

Slide 31 Long Waves (Fig. 6) • Long waves (wavelength 2000 to 3000 nm) with northward and southward amplitudes circle the globe in the mid‐latitudes. • They are easier to see on Polar charts than Mercator projections. • The waves appear to stand still, move slowly to the east for a period of time, and then breakdown or retrograde (move westward). • A specific long wave pattern tends to last 10 days or more without significant fluctuation. • Long waves determine the overall weather or storm track (e.g., drought, storminess, abnormal temperatures).

USPS University Series 12 Marine Weather Forecasting

Slide 32 Short wave and Surface Low – 1 (Fig.7) Short waves • Short waves range in size from 500 to 1500 nm and have a life cycle of less than a week. • Use the 5400 m contour to locate the short wave’s mid‐point. • Short waves tend to move rapidly from west to east passing through the longer waves. • They tend to flatten or enhance as they do so. • When short waves and long waves are in phase they enhance one another; when out of phase they flatten. • Sometimes short waves move north or south; they “cut‐off” from the main contours that then shift north.

Atlantic Storm • The sequence of 500 mb and corresponding surface images cover a 24 hour period. They depict a developing Atlantic low pressure system in the late winter of 2008. • The images show the 12 hourly development of a short wave trough (brown dashed line) and surface low as they move across the United States and cross into the western Atlantic propelled by strong west to east or “zonal flow”. • The superimposed satellite cloud images show how the storm system is organizing into the classic comma shape. • Also shown is the vertical cross section of the trough axis between the surface low and the 500 mb trough (red line is exaggerated 500 mb surface and dashed red line is vertical trough axis). • For the development of a mid‐latitude low pressure system, it is necessary for the short wave trough axis in the vertical to tilt back towards cold air (lower heights) with altitude.

Slide 33 Short Wave and Surface Low – 2 (Fig. 8) Atlantic Storm (continued) • These images depict the same storm system for the next 36 hours. • The 500 mb trough over the western Atlantic has amplified significantly as the upstream ridge has built up rapidly. • The longitude distance between the western Atlantic short wave and the upstream ridge has shortened. • The short wave trough no longer appears elongated and has a more curved appearance with the heights falling 60 m as the trough has deepened. • The surface low responded by dropping to 972 mb (20 mb in 12 hours). • The 500 mb circulation closes off—an indication the storm will slow.

USPS University Series Marine Weather Forecasting 13

Atlantic Storm Summary (this and previous slide) • The 500 mb ridge to the west of the short wave trough rose in height and amplitude quickly. • The short wave trough began to descend by 60 m toward the surface with further deepening to a minimum of 5220 m. • The slope of the trough axis from the surface low to the 500 mb short wave trough became more vertical as the vortex grew to the 500 mb level and the cyclone slowed. • This low pressure system in 60 hours moved from the Louisiana Arkansas border to south of Newfoundland and intensified rapidly into a very intense ocean storm with hurricane force winds. • The rapid travel speed was a direct result of the 500 mb flow being west to east or “zonal”. General Comments • Think of an eddy in a stream as it deepens into the water and begins to slow. • The atmosphere is a fluid and lows are eddies embedded in the main fluid stream. • Note that vessels transiting east or west bound in transoceanic crossings in such a “zonal flow pattern” will avoid the heaviest winds and seas located to the north if they stay south of the bold 5640 contour line. Slide 34 Short Wave Winds (Fig. 9) • Red shading on 500 mb charts highlights winds of 50 kts or greater. • Enclosed white contour shading shows areas of winds 100 kts or greater inside. • Higher speed wind barbs indicate flow is spreading out indicating vertical venting. • The wind speed is fragmented into jet streaks associated with each short wave trough. • Diffluent (spreading out) wind flows indicate vertical venting. • The deepening trough in the horizontal plane corresponds to the deepening trough in the vertical plane. • The tightening contours indicate higher wind speeds.

Slide 35 Life Cycle of Short Wave Trough (Fig. 7, 8, 9) • Life Cycle of a short wave trough has three stages. • Developing Phase: • 500 mb short wave lags behind surface low and amplifies with time. • Tilt vertical trough axis decreases as surface low deepens. • Closing‐off Phase: • Height contours encircle the newly formed mb low • Circulation has become closed from surface to 500 mb. • Surface low slows forward motion. • Mature Phase: • 500 mb low is vertically stacked above surface low. • Surface low deepest with maximum wind speeds • Then surface low will begin to fill and weaken.

USPS University Series 14 Marine Weather Forecasting

Slide 36 Zonal Flow (Fig. 10) • A zonal flow pattern means contour lines are arranged west to east. • Westerly wind flow at 500 mb extends from central North America across the Atlantic to western Europe with an 85 kts maxima over the mid‐Atlantic southeast of Newfoundland and again south of Iceland north of the Azores. • Surface fronts embedded in these areas may move as rapidly as 40 to 45 kts. • Zonal flow patterns tend to be unstable and short lived. They often break down into a more amplified pattern fairly rapidly. • When a transition from a zonal to a more amplified or “meridional flow” pattern is taking place, a strong surface low of storm or hurricane force will usually develop.

Slide 37 Meridional Flow Pattern (Fig. 11) • In a meridional flow pattern, the contours have more amplitude (north‐south orientation) than in a zonal flow pattern. In other words, the height contours cross more latitude lines than longitude lines. • Meridional flow patterns tend to move cold air south and warm air north in a highly visible aspect when comparing to a zonal flow pattern. • Surface lows and 500 mb short waves will move more toward the north or south along a meridian than in a zonal pattern.

• In this example a surface low existed near 42°N 150°W to the east of the 50 kts wind barb. That low was moving due north at 20 kts. • The entire meridional flow pattern may move from west to east over a period of days.

Slide 38 Blocking Ridge (Fig. 12) • A high amplitude ridge that blocks the west to east progression of the upper level “Westerlies” is aptly named a “blocking ridge”. • The large 5810 m High at 50°N 30°W is blocking the west to east progression of weather systems. • Large open longitude space between height contours of the same value that cover up to sometimes 30° of longitude is a characteristic of a block. There is very little air flow penetrating the area.

USPS University Series Marine Weather Forecasting 15

• Blocks may last 10 days or more. • Generally short waves will be steered northward over the blocking ridge. • Closed lows that do dig southeastward of a block tend to be fairly strong. (e.g., surface wind of 45 – 50 kts.) • The rule of thumb “up to fifty (50) percent of the 500 mb wind speed can translate to the surface” applies.

Slide 39 Cut‐off Lows (Fig. 13) • If a 500 mb meridional flow pattern becomes amplified enough, an upper‐level low may form in the southern boundary of the upper‐level “westerlies” and become “cut off”. • This slide depicts near zonal flow extending from eastern Canada, to near 30 degrees West longitude over the Atlantic, with two cut‐off lows to the south. The first is over the southern United States. The second cut off is near 30 degrees N, and 41 degrees W longitude.

• Cut‐off lows may be accompanied on the surface by strong winds perhaps with showers and thunderstorms. • They occur most in the spring and fall when the upper level westerlies migrate north and south, respectively. • Cut‐off lows tend to remain stationary and persist for several days – sometimes up to two weeks. • They either gradually weaken or are picked up by the upper‐level westerly flow when the pattern changes.

Slide 40 The Tropics (Fig. 14) • 500 mb charts are best suited for mid and high latitude applications where surface weather systems movement and intensity respond to evolving 500 mb wind flows. • But, the same use can be made in the tropics when amplified patterns dig south enough. • The slide depicts a 500 mb analysis and surface analysis for the eastern North Pacific from 1200 UTC 9 February 2002 showing a deep 500 mb trough and associated cold front southeast of the Hawaiian Islands.

USPS University Series 16 Marine Weather Forecasting

Slide 41 Tropical Storm Steering Winds (Fig.15) • 500 mb height, winds and satellite image from 1200 UTC 6 SEP 2008. • Tropical Storm “Hanna” can be seen over North Carolina • Hurricane “Ike” is east of the Bahamas. • 24 and 48 hour forecast positions for Hanna and 24 through 72 hour forecast positions for Ike are shown by the symbols of red Xs with circles. • Hanna is being steered to the east between the 5820 and 5880 m contour lines moving at about 1/2 the 500 mb wind speeds. • Ike is being steered to the west by the 5970 m high and blue jagged ridge axis.

Slide 42 Seminar Topic

GRIB Forecasts

Slide 43 GRIB Basics • GRIB Forecasts are computer generated forecasts without the benefit of interpretation or modification by meteorologists. • The forecasts are based on two different computer models run 4X a day. • Wwave3 (also called WW3, Wave Watch # ) • generates wind (at sea level) and wave forecasts. • wave forecasts cover height, direction and period. • covers only ocean waters. • forecasts produced in 3 hour intervals out to 7 days.

• GFS (Global Forecasting System) • The GFS Model has a native resolution of 0.5 degrees. • Forecasts surface winds (direction and velocity), pressure, temperature, humidity, and precipitation. • Forecasts 500 mb contours and winds. • Forecast is a 7 day forecast with a 3 hour time step.

USPS University Series Marine Weather Forecasting 17

Slide 44 GRIB File Selection • GRIB files can be obtained from GribUS at www.grib.us • Select the weather area you want. Select the number of days forecast and the interval. • Select the resolution and parameters, wind, rain and pressure. • Download the GRIB file.

Slide 45 GRIB File – 1 • This is the weather data selected and shown on the GRIBUS viewer. • The time shown on the tool bar is Universal, UTC, time. The cursor location will give you the live data for that location. • You can select the data layers you want to show in the display layer box. • You may also chose to animate the weather data by selecting the animation tab.

Slide 46 GRIB File – 2 • You may zoom in or out. • Note the blue areas of precipitation.

Slide 47 GRIB Overlay on Coastal Explorer • The GRIB files for Coastal Explorer shown are downloaded from Rose Point’s server. • Moving the cursor will give you the weather forecast for the time shown at the cursor location. • With Coastal Explorer you can view graphical as well as text forecast.

USPS University Series 18 Marine Weather Forecasting

Slide 48 GRIB Overlay on Nobeltec • The GRIB file can be imported into Nobeltec navigation software and overlaid on the navigation chart as shown. • The green line is an isobar. • You may move the cursor to any location and read the weather conditions forecast for that location.

Slide 49 Seminar Topic

NWS Marine Weather Forecasts

Slide 50 NWS Marine Forecasts • NWS marine weather forecasts are based on • many models; and • the analyses of meteorologists (unlike the strictly computer generated GRIB forecasts). • Coastal and Great Lakes Marine Forecasts • for coastal, nearshore, and open lakes zones • issued by coastal NWS Weather Forecast Offices (WFOs)

• Offshore Forecasts • for areas between the Coastal Zones and the High Seas • issued by the Ocean Prediction Center (OPC) and certain NWS offices • High Seas • geared to needs of ocean‐going vessels • Issued by OPC, the Tropical Prediction Center (TPC) including its National Hurricane Center (NHC), and the Honolulu Weather Forecast Office.

USPS University Series Marine Weather Forecasting 19

Slide 51 NWS Coastal and Great Lakes Marine Zones • Marine zones are based upon area differences in weather and sea conditions. • The number of zones and their coverage areas are continually being modified as meteorological models and graphics change. • There are about 360 Coastal and Great Lake Marine Zones: • 228 Coastal Marine Zones • 88 Great Lakes Nearshore Marine Zones • 45 Open Lake Marine Zones • Marine forecasts emphasize wind speed and direction, visibility, and sea state.

Slide 52 NWS Offshore & High Seas Marine Zones • Marine zones are based upon area differences in weather and sea conditions. • The number of zones and their coverage areas are continually being modified as meteorological models and graphics change.

Offshore Zones • There are 30 Offshore Marine Zones between Coastal Marine Zones and High Sea Zones. • They extend seaward up to several hundred nautical miles. • Offshore forecasts & warnings: • include significant surface weather features • identify major weather systems (strength, movement etc.) • winds and seas forecasts High Sea Zones • Five zones in the North and South Pacific Oceans cover: north to the Bering Strait, west to 160E Lo; and south to 25S L (18.5S Lo east of 120W Lo). • Two zones in the North Atlantic Ocean cover: north to 67N L; east to 35W Lo; and south to 7N L. • High Seas forecast and warnings: • are designed to meet the needs of ocean going vessels • emphasize gale force or worse conditions.

USPS University Series 20 Marine Weather Forecasting

Slide 53 Ocean Prediction Center • OPC Forecasts cover • Offshore areas (New England, Mid‐Atlantic, West Coast) • The High Seas • OPC Forecasts include • Unified Analysis Northern Hemisphere • Pacific and Atlantic Analyses • Forecasts [24, 48, & 96 hr] • 500 mb Charts and Surface Charts • Winds and Waves

Slide 54 OPC North Pacific –Surface Chart • OPC North Pacific chart shows fronts, lows, highs and isobars. • Note that ship reports are shown.

Slide 55 OPC Surface Chart • Surface chart is a 24, 48 or 96 hour forecast showing fronts, highs, lows, wind velocities as well as isobars. • Predicted movement is shown with arrows projecting from the high or low. • The circle with an x shows the predicted 24 movement and location of a high and an x is the predicted 24 hour movement and location of the low.

Slide 56 OPC 500 mb Forecast • The 500 mb forecast chart shows the 564 line (decameters) as a bold line. • The 564 line is referred to as the storm track line. Highs and lows are shown. • Wind velocities are show at this pressure level. • Wind velocities at the surface will be 30 to 50 percent of the 500 mb level. • Weather conditions on the surface will usually be 300 to 600 nautical miles polar of the 564 line.

USPS University Series Marine Weather Forecasting 21

• Example, if the wind is 50 knots at the 500 mb level, the surface velocity would be 15 to 25 knots 300 – 600 nm polar to the 564 mb line. Slide 57 OPC Wind/Wave Forecast • Wind and wave forecast shown is for 24 hours. • Both wind velocity and direction are shown. • Wave height is shown with an area high in a square box.

Slide 58 OPC Wave/Period Direction • The wave period and direction chart is a forecast of the direction of the waves and the time between each crest. • The chart shown is a 48 forecast of the north pacific.

Slide 59 How to obtain NWS Marine Weather

• NOAA • NOAA Weather Radio (NWR) • also HF Voice and Telephone • Internet (web, FTP, email) • Dial‐A‐Buoy

USPS University Series 22 Marine Weather Forecasting

• USCG • VHF, MF, and HF Voice • Radiofax – charts and text • Text broadcasts (special receivers necessary): NAVTEX for offshore & limited coastal; SITOR for high seas and limited offshore – also known as Narrow Band Direct Printing (NBDP) •WWV and WWVH (time tick) • These radio stations operated by the National Institute of Standards and Technology are known mainly for their time ticks (time announcements). • They also broadcast high seas marine storm warnings. •Satellite based Providers • Inmarsat‐C SafetyNET—promulgates safety information including high seas weather and warnings. It is a part of the Global Maritime Distress Safety System (GMDSS) and there are no user fees. • Private subscription services such as Sirius and XM.

Slide 60 Additional Reference Materials The following reference materials collectively provide descriptions, explanations, sources, schedules and broadcast frequencies for various marine products including forecasts: • A Mariner’s Guide to Marine Weather Services • Coastal, Offshore and High Seas www.nws.noaa.gov/om/brochures/mw_coastal.pdf • Great Lakes www.nws.noaa.gov/om/brochures/mw_lakes.pdf • Ocean Prediction Center’s Radiofacsimile Charts User’s Guide www.opc.ncep.noaa.gov/UsersGuide/UGprint.html • Worldwide Marine Radiofacsimile Broadcast Schedules www.nws.noaa.gov/om/marine/rfax.pdf

Slide 61 Thanks! • Thanks for attending Marine Weather Forecasting. • The United States Power Squadrons and the National Weather Service hopes this seminar helps you find fair winds.

USPS University Series