>A Tail of Two Highs: Los Angeles to International falls: 73 verses 37

Written by on August 29, 2009 in Rest of Europe with 0 Comments

>Firefighters battle the Station fire
(Al Seib / Los Angeles Times / August 27, 2009)
U.S. Forest Service firefighters race to battle the Station fire as it jumps Angeles Crest Highway in the Angeles National Forest. The blaze spread Thursday as canyon winds whipped flames into dry brush.

Currently as of 2.15am Pacific Time and 4.15am Central Time, it’s 73 degrees in Los Angeles whilst up in International Falls, Minnesota it’s 37 degrees. That number is the same only flipped around! These two locations very much under opposite air masses right now, two difference high’s, two different origins. It’s hot, dry throughout Southern California and indeed the Southwest region as strong high pressure dominates. Fires are worsening simply because of the high heat, low humidity and at times shifty winds that blow, through and around canyons and mountains. This combo ALWAYS increases wildfire risk in this region of the world. It would not be summer in SOCAL without high heat, low humidity and indeed fires. That’s the truth. Why, because this region of the world happens to find itself under the perminent or semi-permanent sub tropical high pressure belt that circles the world. High pressure always dominates summertime weather in California and the Southwest bringing sunshine, high heat, low humidity and lack of rainfall from June through September and at times into October. Wild fires are part of the makeup of California climate as well as the interaction of weather and geography, with such complexity of geography comes complexity in wind flow, you can get oceanic wind flow associated with the Pacific High and wind flow from the Four Corners high, winds from the Four Corners high can rise heat and lower humdity across the Southland and coastal Southern California, dramatically increasingly wildfires risk as the donsloping effect of winds blowing from inland towards the coast lower humdity and compression rises the temperature, the three way combo of higher temps, lower humidity and wind, creates a tinderbox environment for California and a dry lightning bolt, or cigarette could be the trigger for disaster. Luckily, this season hasn’t been all that bad, well up until now. Temperatures until Thursday only topped 92 for the hottest of the summer in LA (not including the 101 in April) and other areas despite periods of intense heat out into the deserts, coastal and inland coastal areas of California have escaped the normally brutalising heat until now, thanks to stronger heights further east over the desert, and in case you’ve forgotten, it’s close to September, that’s not bad going for a region that gets hot, like Scotland get rain..

As for Minnesota, it’s been a cool year and I believe 20s are heading for the north woods of northern Minnesota this morning, why? Lower thickness values (depth of atmosphere betweebn surface and tropopause) is growing colder as this northern North America origin high (polar high) is growing colder by the day is dropping south where nights are getting longer and therefore a longer time frame for the thermometer to fall more under thr right conditions. As I said, completely opposite of southern California where yes they too are under high pressure but their thickness values are sky-high which is normal as some of California’s coastal heat waves have been worst during fall when interior areas start to see increased frost and cold air. Heights start to rise more towards the coast and inland desert areas cool with their continental location, allowing heat build more towards the coast of Calif. When I mention “higher thickness values”, I am talking about the depth of the atmosphere from surface to tropopause, in warmer originataing high pressure cells, (pac, four corners, bermuda-azores highs) the depth of depth of the atmosphere is higher between surface and tropopause therefore heating the air vertically through atmosphere means the warmer surface readings will be and the 850mb or 5,000 foot level temperature is looked at by forecasters to judge what surface readings could be, but considerations in cloud, sunshine, humidity and wind must be taken into account. Whilst across the Great Lakes thickness values are very low and Canadian origin high pressures at this late stage in summer tend to come from a region where fall has already arrived and this means, after chilly, perhaps even rainy, cool and cloudy days, these Canadian or Arctic High’s drop down , southeast and clear out skies, lower wind speeds and the dryness of the air allows all the heat at the low levels to escape, radiational cooling sets in and it’s not difficult to see lows get to around freezing or lower in the wooded areas away from urbanised heating. Through the next few days these high is going to drop deep into the Midwest and even into the Tennessee valley. Lows in Minneapolis and Chicago may drop off into the mid-40s and areas away from downtown, may see some patchy frost form as temps get into the mid and upper 30s… Tis the time of season to have winter teasin… It’s really a natural sign of the change in season and that though days can still get warm, sticky, humid, for example, the current 37 degrees at International Falls is likely heading to 32 or even upper 20s for lows by sunrise, but yesterday’s high got to 63, even under Canadian high pressure, the sun can still heat this air nicely to comfortable even uncomfortable levels during the daylight hours but quickly escape after sundown. Under clear skies, low humidy and light winds it now gets tougher to hold onto the daytime accumulated heat at the surface.

I have ran out of time to talk more but I shall try and find time during days off (tues-wed) to discuss this further as it’s very appropriate to talk about this when we are beginning to see the first signs of fall.

I’m off to work! Have a great day.

Thanks for reading.

-Mark

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