Basics of weather interpretation

Basics of weather interpretation Safety at Sea Seminar, April 2 nd 2016 Dr. Gina Henderson Oceanography Dept., USNA ghenders@usna.edu Image source: http://earthobservatory.nasa.gov/naturalhazards/view.php?id=80399, accessed 03/03/13

The goal of this session is to: Understand main features on a surface weather chart Cloud interpretation as it relates to sailing conditions Severe weather on the Bay & en route to Bermuda Introduce upper atmospheric chart @500 mb specifically Image source: www.opc.ncep.noaa.gov

Weather products from NOAA Ocean Prediction Center

Let us identify some features on this Surface Analysis map: Image source: www.opc.ncep.noaa.gov

Let us identify some features on this Surface Analysis map: Surface map ISSUED à time analysis was released VALID à Time conditions apply UTC/GMT/Zulu time Image source: www.opc.ncep.noaa.gov

Let us identify some features on this Surface Analysis map: H vs. L? H vs. L relative to what? Mean sea level pressure à 1013 millibars Pressure gradients? Fronts? Image source: www.opc.ncep.noaa.gov

Pressure gradients: At the surface winds cross the isobars toward low pressure and away from high pressure

Surface High pressure center Winds diverge at center Clockwise (anticyclonic flow) Surface Low pressure center Winds converge at center Counter-clockwise (cyclonic flow)

Wind 101 Wind à horizontal movement of air Naming wind o From source direction

Wind 101 Measuring wind speed o Knots o MPH o Beaufort Scale. force 1-12

Pressure Gradient Increased gradient à increased wind speed

Let us identify some features on this Surface Analysis map: Where are the strongest winds on this chart? What direction are they flowing? Image source: www.opc.ncep.noaa.gov

Let us identify some features on this Surface Analysis map: Interpreting a station model Image source: www.opc.ncep.noaa.gov

Interpreting a station model

Interpreting a station model: What information are we given? Surface pressure (upper right) Pressure tendency (not shown here) Wave height (lower right) Temperature (upper left) Dew point temp (lower left) Wind speed & direction Sky conditions (middle) Sea surface temp (lower middle)

Interpreting a station model: What information are we given? 10.9 109 Pressure range is 960 mb to 1060 mb. You must decide if a 9 or 10 should go in front of the pressure.

Interpreting a station model: What information are we given? 10.9 In this case 1010.9 mb 109 Pressure range is 960 mb to 1060 mb. You must decide if a 9 or 10 should go in front of the pressure.

Interpreting a station model: What information are we given? Surface pressure (upper right) Pressure tendency (not shown here) Wave height (lower right) Temperature (upper left) Dew point temp (lower left) Wind speed & direction Sky conditions (middle) Sea surface temp (lower middle)

What can clouds tell us about approaching weather systems?

Sky conditions: Cloud types & identification Named based on: Height Shape

Cirrus = thin and wispy

Stratus = flat clouds in layers

Cumulus = puffy clouds in heaps

Nimbostratus = rain (grey in color)

Cumulonimbus = thunderstorm

4 main types of thunderstorms, depending on how organized the system is... 1. Airmass à away from frontal boundary driven by intense surface heating, rarely produces hail/tornadoes 2. Squall line à a line of storms, often along a frontal boundary Associated with heavy precipitation, winds and potentially hail/tornadoes

4 main types of thunderstorms, depending on how organized the system is... 3. Multi-cell à storms occur in clusters Most common in spring/summer 4. Supercellà most intense and severe Supercell thunderstorm updrafts ALWAYS rotate

Thunderstorm clouds -- green tinge may indicate presence of hail

Thunderstorm clouds -- Hook echo, Tulsa 03/30/16

Squall Line Long line of thunderstorms o individual cells are so close together the heavy precipitation forms a long continuous line Typically form along an advancing cold front o Sometimes associated with a cold front aloft Can be hundreds of miles long Most commonly associated with strong straight-line winds o Can produce hail and/or tornadoes, too Called squall because of the abrupt wind changes

Squall Lines Squall line thunderstorms

Squall Lines Squall line thunderstorms L

Squall line approaching Memphis, TN. Note the heaviest precip is along the leading (eastern) edge of the line, with moderate but still continuous rainfall occurring 100+ km behind (to the west) of the line

Shelf cloud: found along the leading edge of the gust front

Squall Lines

Let us identify some features on this Surface Analysis map: Pressure gradients Fronts? A front is a transition zone between two air masses of different temperature and moisture content (they have different densities) Image source: www.opc.ncep.noaa.gov

Air Masses have uniform temperature & moisture characteristics Figure 5.21

Let us identify some features on this Surface Analysis map: Fronts, 4 main types 1. A stationary front 2. A cold front 3. A warm front 4. An occluded front Image source: www.opc.ncep.noaa.gov

Mid latitude cyclones = clash of two different air mass types!

Image source: http://www.hpc.ncep.noaa.gov/sfc/usdwmsfcbw.pdf, accessed 04/01/14 Let us compare the surface observations ahead, and behind the front

Stationary Front Cold air. X Warm air Cloud types depend on the atmospheric stability Precipitation intensity depends on the situation

Cold Front marks the front of a cold air mass How is it represented on a map? What type of clouds and weather is it associated with?

Warm Front marks the front of a warm air mass How is it represented on a map? What type of clouds and weather is it associated with?

Occluded Front Clouds and precipitation depend on the atmospheric stability

The following criteria are used to locate fronts: 1. Sharp temperature changes 2. Sharp dew point changes 3. Wind shift lines 4. Pressure changes (pressure tendency ) 5. The presence of clouds and precipitation

cp mp The following criteria are used to locate fronts: ct mt 1. Sharp temperature changes 2. Sharp dew point changes 3. Wind shift lines 4. Pressure changes (pressure tendency ) 5. The presence of clouds and precipitation

Interpreting a station model: What information are we given? Surface pressure (upper right) Pressure tendency (not shown here) Wave height (lower right) Temperature (upper left) Dew point temp (lower left) Wind speed & direction Sky conditions (middle) Sea surface temp (lower middle) Water holding capacity of atmosphere.. Amount of water in atmosphere. Dew point depression = T - TD

Image source: http://www.hpc.ncep.noaa.gov/sfc/usdwmsfcbw.pdf, accessed 04/01/14

How do we forecast where surface systems will move? Look UP!

Let us identify some features on this Surface Analysis map: Where will our surface systems move to? Will they intensify or weaken? Image source: www.opc.ncep.noaa.gov

500mb Chart: forecast -- Gives us the height where we reach 500mb of pressure

500mb Chart vs. Surface chart

500mb Chart: forecast -- Also called the steering level of the atmosphere

Let us track where our surface Low and surface High go... L H

24-hour forecasted surface map

Weather products from NOAA Ocean Prediction Center

24-hour wind & wave product

24-hour wind & wave product Wave height (ft) = total height from crest to trough

Hurricanes Atlantic hurricane season runs from June 1st to November 30th

The goal of this session is to: ü Understand main features on a surface weather chart ü Cloud interpretation as it relates to sailing conditions ü Severe weather on the Bay & en route to Bermuda Dr. Gina Henderson Oceanography Dept., USNA ghenders@usna.edu ü Introduce upper atmospheric chart @500 mb specifically Image source: www.opc.ncep.noaa.gov