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Two Denver Blizzards - 09.27.06

This year Doc Weather is on the lookout for blizzard patterns in the High Plains. Find out why in this article.

The classic climatic pattern for the emergence of a blizzard in Denver involves the sudden dropping of a cold air mass from British Columbia southward into the Basin and Range area between Denver and California. (arrow) The upstream stimulus for the cold air to drop from a high-latitude is often a high-pressure area situated in the central Gulf of Alaska that pushes the jet stream up into Alaska. (H) There, the cold air forms the cold front that eventually drops south along the West Coast. A low- pressure area usually accompanies the cold front (L). As the low moves into southerly latitudes, its counter clockwise circulation picks up moisture from the Gulf of Mexico, as the low turns eastward at the bottom of its southward swing through the inter-mountain west. The cold low- pressure area draws warm moist air into the leading edge of the circulation. The warm moist air streams up from Texas and into Colorado especially in the Denver area (black dot) dumping wet snow mixed with rain that can accumulate rapidly in the mixing conditions of the cold front meeting the warm monsoon. Snow forms easily and falls in abundance bringing remarkable accumulations in a short time. These types of storms often make a dismal water year a success instead of a disaster. It would therefore be good to know in advance if there was an increased potential for these types of storms to form in a given water year.

In standard climatology the Denver blizzard pattern is most often linked to El Nino years where the dominance of a high latitude high- pressure area over the Gulf of Alaska splits the Pacific jet stream into two currents (figure 2). Warmth from the El Nino in the eastern Pacific supports the formation of strong high pressure in the Gulf of Alaska. When this high latitude high pressure area forms in the Gulf of Alaska, the storm jet divides into two currents. One rides up and over the block into Alaska and the other drops south through southern California and the desert southwest. The split storm jet recombines over the southern part of the Colorado Plateau. The counterclockwise circulation of the storm draws the moisture from the Gulf of Mexico and delivers it to the High Plains. The record snowfall of the blizzard is the result. The big question is why in some years does this pattern unfold and then for the longest time there is a drought pattern in the High Plains? If the blizzards only occurred during El Nino years then it would be a simple thing to predict that when an El Nino event was underway a blizzard season would be probable. The double catch here is that the pattern for Denver blizzards does not only occur during El Nino years and if we could predict El Nino years exactly then a lot of things in climate study would fall into place.

To try to get some perspective on this double catch this article will cover two major Denver blizzards, one in 2001 and one in 2003, and link them to the jet curves during those years rather than just the El Nino connection.

Figure 3 shows the jet curves for the second week of April 2001. In this situation the solar and lunar eclipse points generating the jet curves are due south of the West Coast. This southerly placement of the eclipse points in April 2001 projected the set of four 22° jet curves (blue) into a bracketing pattern straddling the West Coast. Since the eclipse points were due south of the West Coast there was a set of 22°jet curves to the west of the eclipse points in the center of the Gulf of Alaska and another set to the east over Central America. This placement created a symmetrical pattern in the four jet curves with regards to their position along the coast.

Starting on the 8th the approach of Mars towards the eclipse points coincided with a strong surge of high- pressure on the two sets of 22° jet curves. (blue high pressure areas) This was coincident with high- pressure areas intensifying in the central Gulf of Alaska and over west Texas. Over the ocean, high- pressure surged out of the middle of the eastern Pacific and up into the center of the Gulf of Alaska following the path of the two jet curves (blue arrow) . Over the continent, high-pressure pushed up over the High Plains also following the other set of jet curves (green arrow). In patterns like this these kinds of symmetries can often be observed. They make for interesting climatic scenarios. In between these two surging high-pressure areas, a weak low- pressure cell started to drift south along the West Coast (red arrow). Cold air accompanied the low as it drifted across California into the Great Basin on the 10th.

On the 11th Mars moved in arc. This created low- pressure values on the 22° jet curves from the solar point (red curves). This move also created moderate high pressure values on the lunar 22° jet curve (blue curves). On that day the low that was transiting Idaho consolidated and dropped south towards the newly shifted low- pressure jet curve situated along the coast of Baja California. At the same time, the high over the continent was still supported to high pressure along the lunar jet curve in the vicinity of Brownsville, Texas. The block forced the trough to the south and allowed the southward moving low to pick up moisture from the Gulf of Mexico as it dropped into the southwest. The low slowed and turned east into the Colorado plateau and deliver the goods to Denver. Although there was a weak warming trend in the eastern Pacific at this time in this sequence it was not really a strong or significant El Nino that instigated the blizzard. The trough that was transiting to the north in the area of Idaho suddenly turned south as Mars shifted the values on the jet curves. It tracked towards the jet curve over Baja California that had turned towards low pressure. It can often be seen that transiting fronts will seek the curve that has just made a shift from one value to another value. In this case this appears to have been influential on the formation of the blizzard.

In figure 5, the eclipse lines of the blizzard of March 2003 are depicted. That year there was a strong la Nina cooling event taking place during the time of the blizzard, so the standard El Nino support for a southerly placement of the Pacific storm jet was not available. On the15th of March a strong low-pressure area formed off of the southern coast of Alaska. A continental high in the Pacific Northwest blocked this low from transiting through the north (blue high over PNW). The only strongly transiting planet at this time was Mars. It had passed the eclipse points in late January and was now crossing the coastline on the West Coast. In the system used by Doc Weather a West Coast transit like this happens at 15° Sagittarius. When a planet crosses the coastline a strong turbulence is often the result since the planet that is crossing the coast is often moving at a maximum southerly declination. The constellation that straddles the West Coast is Sagittarius. This is the most southerly of the constellations and a planet moving though it often is at an extreme southerly declination. The passage of a planet at an extreme southerly declination is often accompanied by the southerly drooping of the storm jet. This low declination affect occurs about 85% of the time. In 2007 Mars transits the West Coast on the last day of January. The last week of January would be a good time for watching the High Plains for storms.

In 2003 the storm jet dropped to the south on the West Coast as Mars transited 13° of Sagittarius. At that longitude Mars also stood at a low pressure value to the solar eclipse point. The 45° jet curve from the solar eclipse point was over Denver (red curve). At this time a low-pressure area dug to the south on the West Coast (red arrow) moving around the southern flank of the block over the PNW. The storm barreled into Los Angles and then drifted turned eastward over the southwest. As the storm approached Denver it came into the vicinity of the low pressure jet curve from the solar point. The storm exploded in this area. This was a major blizzard for Denver and it is significant that there was no El Nino influence for this one.