Eastern Pacific Hurricane Watch - Reading a Surface Chart
Since the season to pay close attention to Baja hurricane threats is not yet upon us and knowing how to read a Surface Chart for yourself can be handy at anytime of year here, this edition of the Eastern Pacific Hurricane Watch Report will begin a three part look at how to read a Surface Chart. This isn't intended to replace your doctorate studies on meteorology, but will give you a much clearer understanding of Surface Chart interpretation.
If you are just here to see what is in store for the next few weeks and a strategic look at the possibilities of tropical cyclone formation in the Eastern Pacific click here.
Reading a Surface Chart - Part 1
What is a Surface Chart? ( Click here to see the current Eastern Pacific Surface Chart) It is a map of weather conditions and forecasts for the next period. According to Answers.com a Surface Chart is as follows:
"An analyzed synoptic chart of surface weather observations; essentially, a surface chart shows the distribution of sea-level pressure (therefore, the positions of highs, lows, ridges, and troughs) and the location and nature of fronts and air masses, plus the symbols of occurring weather phenomena, analysis of pressure tendency (isobars), and indications of the movement of pressure systems and fronts. Also known as sea-level chart; sea-level-pressure chart; surface map. "
The most noticeable features on a Surface Chart are the High and Low pressure systems. Indicated by large letters, and when in color Highs are blue and Lows are red. In the northern hemisphere High pressure systems orbit clockwise, Low pressure systems orbit counter-clockwise. Movement of a Low pressure system is indicated by an arrow indicating direction of movement and a 
, located at the forecast position of the system at the end of the forecast period. Movement of a High is also indicated with an arrow of direction of movement and a 
. The direction of movement arrow is sometimes omitted if space doesn't allow. If the system is not moving "STATIONARY" is usually printed next to the system. The current barometric pressure reading for the system is indicated in millibars located usually above the symbol and underlined, (i.e. 1006). The future barometric pressure can be indicated by an underlined two digit number, near the future location of the system (i.e. 04 ) If the system is new, rapidly strengthening or this can be indicated with the written words such as "DISSIPATING, NEW or RAPIDLY STRENGTHENING" Hurricane strength conditions are indicated with "XX".
The reason our Eastern Pacific Hurricane usually move off into the Pacific is this counter-clockwise rotation of 
these extreme Low pressures. Like a rolling tire, the rotating action drives them to the southwest. Other environmental factors push them more to the north, and as the northern hemisphere cools the jet stream pushes them back toward the northeast and sometimes into Baja after late July.
Isobars are the long lines forming unusual shapes on a surface chart. and are directly associated with Highs and Lows and relative barometric pressures. indicated in solid or dotted lines, brown on color charts, these are rough demarcations of barometric pressure, taken from reporting stations. The closer together the lines, the greater the wind in that area. Wind does not blow parallel to these lines, because of the earth's rotation, but slightly off axis to a point of lower pressure. In the case of intense winds "DEVELOPING GALE" or "GALE" will be written on the chart
Fronts are a very visible and important feature too, although not usually for hurricane weather. (although Tropical Waves are a form of a front) In a color chart cold fronts are indicated in blue, Warm fronts in red, stationary fronts in red and blue and occluded fronts in purple. Fronts are a line, indicating position of the front. Warm Fronts have semi-circles, Cold Fronts triangles and stationary fronts have both. The side of the line on which the added symbols exist indicate the direction of flow from the front. Stationary fronts have both semi-circles and triangle on opposing sides of the line, indicating that the front isn't moving. Occluded front is where cold air has overtaken a warm front, using both triangles and semi-circles in purple, indicating the movement of the front.
Station reporting symbols are perhaps the most complicated and information packed little items on a surface chart. If you know the 'code' you can tell wind direction, wind speed, temp, dew point, sky and precipitation condition of each provided plot. In our next Hurricane Watch Report we will take a look at that tiny little type and symbols within a symbol and other surface chart features.
Today's surface charts are available from the US National Weather Service. A world wide detail chart is located here and a Eastern Pacific Basin Surface Chart is located here.
Looking at Eastern Pacific Tropical Cyclones in the two weeks ahead...
June 8, 2010 - The Eastern Pacific has had more than a week of limited tropical wave activity. Following the passage of Tropical Storm Agatha which brought driving rain, flash flooding and killed around 150 people in southern Mexico, Guatemala and Honduras at the end of May there hasn't been any threat of tropical cyclone formation. Now, as we look at the new 10 days or so we can see a series of 5 tropical waves headed across the Atlantic toward our basin at 3-5 day intervals. Since at least 4 of these appear unlikely to form a tropical cyclone in the Atlantic, that energy is probably going to make it to our basin.
One advantage we have this early in the season is the positioning of the ITCZ. Right now the Inter Tropical Convergence Zone is moving across significant portions of land in the northern reaches of South America. Moving over this hot, but diverse terrain, the tropical waves lose more of their 'punch' than those that travel over warm waters.
More below...
Sea Surface temperatures give us a strategic look at where storms might form. When taking a look at this weeks Eastern Pacific Sea Surface Temperatures There is little Net change in the temperatures in the off the southern coast of Mexico, where storms at this stage in the season form. For the next several weeks tropical waves will excite moisture over the warm waters of this region. Historically speaking, nearly all of the storms that form from now until July make their way off into the Pacific. (See Baja Hurricane History) Much of the +28° to +30°C waters just mulled around. The most significant change to sea surface temps seen is the warming of the Sea of Cortez. The all important +26° thermo cline is indicated in red. Tropical cyclones are unlikely to persist in waters cooler than 26°C, so the threat to Baja this early in the season is VERY remote. But that red line does extend up into the Sea and by mid to late July that waters surrounding Baja begin to be warm enough to support cyclonic activity. But there are other factors involved too.
Over the past 6 years of this Hurricane Watch Report I have studied weekly the surface temperature anomalies. It has been a rare occurrence to see almost all of the Eastern Pacific hurricane basin at nearly 'normal' temperatures. The green on the Surface Temperature anomaly graphic shown right indicates water at normal temperatures for this time of the year. There is a slight elevation south of Baja, but less than +1°C. There are two separate areas, one along the coast near Manzanillo and the other well west of the Socorro Islands that are just more than -1°C below normal. We have had winds every night for the past week here in La Paz. Along with unseasonably cool conditions during the day (we haven't broached 100°F yet this season) the Sea remains cool and about normal for this time of year. Last year at this time . arts of the Sea of Cortez are JUST above normal
I didn't start the season so well, but not so terribly either. Last edition I predicted that we would make it through June without a tropical cyclone in the Eastern Pacific. I was wrong by a mere matter of hours. Tropical Storm Agatha developed from TD1E, moved ashore and was gone in less than 32hrs. The short life and little warning provided by the storm may have been partly responsible for the 150 deaths in Agatha's wake.
We do have a parade of tropical waves and statistically we should have 4 named storms form and 2 are likely to be hurricanes. I'm going to go with the numbers and say before our next Hurricane Watch Report we will have 1 tropical storm and one hurricane. Both will form several hundred miles west of Guatemala and move west into the Pacific without threatening land. Since the waters are cooler than they have been in recent years I'm gonna go for the extra credit points and say the second storm to form will emerge as the hurricane.
But what do I know...
Our Hurricane Watch Reports will be issued every two weeks until mid August, when our Tropical Cyclone threat begins to heat up. So look for this report again in the third week of June. Until then, fair winds....
Tomas Zyber
Our Eastern Pacific Hurricane Watch is an editorial/entertainment analysis of data from the National Hurricane Center, NASA and NOAA and is based on information provided by the same, but is an amateur endeavor. For actual storm information readers should refer to notices and warnings posted by the National Hurricane Center. or visit the Mexican Nation Metrological website for more information.
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