In most places in the mid-latitudes of the northern hemisphere, the temperature rises more during March than during any other month.
In addition, temperatures in the north rise more rapidly than in the south. The difference in average maximum temperature between Chicago and Dallas is 26 degrees at the end of January; 23 at end of February; 20 at the end of March; and 14 at the end of April.
A weakening of the temperature gradient (change of temperature with distance) is correlated with a weakening of the jet stream.
Currently, the powerful jet stream associated with Weather Event Yeti is holding fast in the eastern US. But long-range forecasts suggest a change is coming by mid-March. Not only will the jet stream weaken, bringing an end to Extreme Weather Event Yeti — the pattern of waves might reverse.
The US, with its ongoing battle against Yeti, is the focus of this week’s weather, but there’s news from the Pacific Ocean and Asia too. Let’s go Around The World.
The Long And The Short Of Waves In The Jet Stream
The long-wave pattern in the jet stream tends to have four or five dips around the globe. These dips (troughs) move slowly, if at all.
As the long waves hold their positions, short waves ripple through. An especially strong short wave can reconfigure the jet stream. Depending on where the short wave amplifies, the longer wave may prograde (move eastward, with the flow) or retrograde (move westward, against the flow)
Cutting Off Can Force A Major Change In The Long-Wave Pattern
Sometimes a low or high pressure system will become so magnified that it cuts itself off from the main flow. When this happens, the entire long-wave structure of the atmosphere can reverse.
When a low cuts off, a ridge can form, bypassing the low and the entire pattern flip-flops. Similarly, when a high pressure cuts off, the jet stream may form a branch at a lower latitude and become a trough where the ridge was.
Some forecasts suggest that such a reversal of the jet stream pattern will occur over the US during March.
El Niño Eggplant Goes On: NOAA About To Recognize It
Decoded Science named El Niño Eggplant a couple of months ago. NOAA’s arcane definition requires five consecutive 3-month overlapping periods of at least a half degree departure from normal in sea surface temperature in the central Pacific.
February marked the end of the fourth such consecutive three-month periods, and unless something unexpected happens in March, NOAA will declare an El Niño at the end of the month.
There isn’t any difference in effect between an official El Niño and El Niño-like conditions. NOAA needs to update its criteria for declaring an El Niño.
El Niños are correlated with a jet stream flow from near Hawaii into the southwest US, often referred to as the Pineapple Express. This flow brings rain to California.
Since the waters near Alaska are still anomalously warm, and this tends to create dry conditions in California, there has been a tug-of-war going on. November and early February were rainy on the California coast and January was dry.
Water temperatures in the tropical Pacific are notoriously hard to predict. The latest forecast calls for the weak El Niño to peak in late spring. Conditions are expected to return to neutral in the late summer and fall.
El Niños are also correlated with subdued tropical storm activity in the Atlantic, so the strength and duration of El Niño Eggplant is of critical importance to next summer’s Atlantic hurricane forecast.
The weak El Niño conditions that began last summer appeared to depress overall tropical activity in the Atlantic (all systems reaching tropical storm intensity), though the numbers of hurricanes and major hurricanes were average.
Raining Cats And Dogs In Texas
On Saturday, there was a deluge in Dallas. In nearby Little Bits, a cat appeared in the backyard of Decoded Science’s CEO. The cat probably didn’t fall from the sky, but everyone is familiar with the saying “It’s raining cats and dogs.” Not so familiar is the origin of the phrase.
There are several explanations for the saying, but the most appealing is that it comes from a Pennsylvania Dutch adage that “It’s raining to bring in the cats and bring out the ducks.” Like a phrase that changes during a game of ‘telephone,’ the saying morphed into “raining cats and dogs” as it was repeated.
Another Facet Of Yeti: Fog
As warm air overspreads the cold or frozen landscapes that Yeti visited last week, fog can form. The warm air flowing from the Gulf of Mexico is laden with moisture. As it passes over the colder surfaces, the air is cooled below the dew point. Since cold air cannot hold as much moisture as warm, some of the moisture must condense into tiny water droplets: fog.
Raleigh, Savannah, and Houston had visibilities under a mile Monday morning. This morning, Houston and New Orleans are reporting visibilities under one-quarter mile.
As the warm air works its way north and passes over the snow cover, fog could become very widespread today and tomorrow.
Avalanches In Afghanistan
As if the country didn’t have enough problems already, avalanches, the worst in thirty years, hit a mountainous area north of Kabul, Afghanistan last week. Heavy snow appears to have precipitated the fifty or more avalanches that have killed at lest 200 people.
Avalanches occur preferentially in the spring, when the old snow pack has hardened into a layer of compacted ice and snow. Any disturbance can shatter the sheet and the resulting granular frozen particles cascade down a hill. The disturbance is most often an additional heavy snowfall, as was the case here.
Avalanches are related to mudslides in that they depend on the angle of repose (the angle a pile of the stuff would make with the ground if allowed to settle) of the particles of soil or ice involved. Fresh snow has a higher angle of repose than ice granules (as dry ground has a higher angle of repose than water and soil mixed), so when the compacted snow sheet is broken into small ice particles, the whole volume of ice and snow rolls down the hill.
The Coast is Clear; No Tropical Activity
It should be the height of the tropical cyclone season in the southern hemisphere, and the western North Pacific Ocean can produce tropical storms in any month. But currently there is no tropical activity- not even a suspicious area — anywhere on earth. Nobody is complaining about this.
What’s Up With Spring?
You know it has to come, although in Boston, where four inches of snow will break the all-time winter record, that may not be so obvious. What evidence of spring is there in your neighborhood? Let us know with a comment.
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