High Pressure Produces Both Nice and Dangerous Weather

Weather Talk

POSTED: Thursday, May 15, 2014 - 12:53pm

UPDATED: Thursday, May 15, 2014 - 2:03pm

High pressure is to thank for our Borderland sunny skies but it also produces record heat and wildfires on the West Coast

Most of us here in the Borderland would like to see an end to our “windy season”. In order for this to happen a ridge of high pressure has to build in strong over the Great Basin in the southwestern US. Now this weather feature will steer most of the “wind maker” systems to the north, but will also brings excessive heat and increases our fire danger. When this ridge of high pressure shuts off our winds, it sometimes helps produce the “Santa Ana Winds” which bring extreme heat and fire danger to southern California. In today’s “Weather Talk” I will talk about the pros and cons of the desert southwest’s Great Basin ridge of high pressure and the current dangerous weather conditions it is producing right now in southern California.

High pressure systems usually form where the air converges at the upper levels of the atmosphere. As this air converges, it forms an area of higher pressure and the air is forced to sink. The air that is sinking below the high spirals outward, clockwise in the Northern Hemisphere, counterclockwise south of the Equator. As the air sinks it warms. High pressure systems are steered or moved by upper-level winds much the same way low pressure systems are steered.

Very often you will hear me or another weather forecaster say that an area of high pressure will dominate the weather. This usually means the Borderland will experience sunny to mostly sunny days with little or no chance of rain. Air tends to sink near high-pressure centers, which inhibits precipitation and cloud formation. Air flows clockwise around a high-pressure system in the northern hemisphere. As a result, regions to the east of a high-pressure center often have northerly winds bringing in relatively cold air while regions to the west have southerly winds bringing in relatively warm air. Sometimes, high-pressure systems will stall or stay over a particular region for long periods of time and producing many days of sunny, calm weather with little or no precipitation.

The good effect for our region and the Borderland is that this ridge of high pressure that builds over the Great Basin in the southwest steers most all the wind making storm systems well to our north.

Sometimes this same high-pressure zone that is blocking cooler air from the north and causing extremely hot weather on the West Coast keeps in Arizona, New Mexico and Texas hot and dry as well. The summer high-pressure zone is what meteorologists call the “death ridge” as it not only blocks cool fronts but moisture as well. This in turn increases the wildfire danger.

This Great Basin ridge of high pressure produces regional weather phenomena called the “Santa Ana Winds" in southern California. The Santa Ana Winds are very dry offshore winds, meaning these winds are moving east to west. These winds blow into Southern California and Baja California usually during the late summer to late fall time of year, but can last from October until March after a build-up of air pressure in the Mojave Desert and the Great Basin. This air warms or heats up as it descends in elevation. This rapid warm up dries out the air. The combination of very warm to usually hot at, dry air and strong, gusty winds causes extreme fire danger and wildfires to spread quickly.

This Santa Ana wind weather scenario has been impacting southern California since Sunday producing record high temperatures and multiple wildfires.

Fire agencies in Southern California have been beefing up crews and pre-positioning equipment in wilderness areas because of Santa Ana winds and hot weather since late last week. The dry northeast “Santa Ana winds", along with dangerously low relative humidity and temperatures near or above 100º in some areas have been in place since Monday more possible record breaking heat again today, Thursday.

This dangerous combination of record heat, region wide drought and powerful Santa Ana winds has helped fuel several wildfires in California.

On Wednesday, fire crews battled a rapidly spreading 100-acre wildfire in Carlsbad, California, about 35 miles north of San Diego. The weather conditions at 1 P.M. in Carlsbad had wind gusts near 30 mph, a temperature of 100º, and only 3% humidity!

NBC News, Los Angeles reported the “Alert San Diego” emergency system called 11,600 homes, businesses and cell phones to instruct people to evacuate. The fire began at 10:40 a.m. P.T. and spread quickly. NBC 4 News also said there were at least five major fires burning in San Diego County, with multiple homes on fire in Carlsbad, California.

Downtown Los Angeles hit a record high of 99º Wednesday breaking the old record of 96º set way back in 1890! In Orange County, the temperature at John Wayne Airport hit 105º Wednesday. The average high temperature for this day is 72 º.

The National Weather Service has issued red-flag fire warnings, high-wind warnings and heat advisories from Ventura to San Diego counties everyday since Sunday. Heat Advisories and Red Flag Fire Danger Warnings are in effect again today.

The NWS also issued a high wind warning for the valleys and mountains Tuesday and Wednesday when there were sustained winds of 25 to 35 miles per hour and gusts of 55 to 65 mph. Some Isolated peak gusts of 75 to 85 mph were reported on the area mountain slopes.

The weather service is predicting afternoon highs in the western valleys today and from 95º to 103º again today.

So you can now see what kind of weather this ridge of high pressure can produce. For some areas of the southwest including the Borderland it gives sunny and warm to hot beautiful days. Sometimes when it sets up over the Great Basin, it can produce potentially dangerous record breaking heat and definitely dangerous and damaging winds. The extreme fire danger these northeast, dry and hot Santa Ana Winds produce and how quickly these winds can spread the fire. Our drought and strong winds also create a fire danger here in the Borderland so we need to take fire precaution here as well. The Santa Ana winds are thankfully forecast to weaken later today into the weekend as high pressure shifts our direction. Westerly winds and that ridge of high pressure will warm El Paso up to the mid to upper 90’s this weekend! We are edging closer to the hottest month of the year, June and we have the Great Basin ridge of high pressure to thank for that!

Chuck DeBroder, Chief Meteorologist
KTSM, NewsChannel 9, NBC, El Paso, TX

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