Major Pattern Change for North America and Arctic Next Week.

A major weather pattern shift will be occur next week for North America into the Arctic as the jet stream…which already has been largely higher in amplitude and experiencing some blocking with little eastward progression of long-waves in the upper-atmosphere, will becoming extremely amplified (north-south) next week bringing very warm air up into Alaska, Yukon and the Arctic Ocean and a modified Arctic air mass from Nunavut and the Northwest Territories of Canada into the central US. Let’s take a look at things.

The current pattern dominating North America has been strong ridge of high pressure over the Western US or Eastern Pacific with a prominent trough over the eastern US with some fluctuation in the wave pattern east or west, but not much significant change, except in the center of the country which has seen more significant swings between these two states. The east, including even the Southeast saw significant snow. The west has seen abnormal warmth with record fires in California. Currently the ridge of upper-atmosphere ridge is forecast by US and European models to build to an extremely high amplitude the end of next week north over portions of Alaska and Yukon and into the margins of the Arctic Ocean. This as a very intense trough is forced south over the US.

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European model forecast for the wave pattern of the mid-level atmosphere valid 6 pm CST 12/23.

This extreme amplification will drive an Arctic surface air high pressure system out of the Northwest Territories with very cold air this week, with this air mass advancing into the US beginning Thursday into this weekend. Meanwhile stormier conditions will moving from the Bering Sea into the Chukchi Sea driving up temperatures in the far north. And California with all the fires? Remains abnormally warm and dry.

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Temperatures the afternoon of Christmas Eve (European Model forecast).
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Greatest signal for low to no precipitation the next 10 days is south-central to southern CA into much of AZ and NV.

The Arctic:

As I spoke about in a previous post, the Arctic is having its second warmest year on record and lowest annual sea ice volume on record as climate change continues to abnormally warm the Arctic. The highly amplified wave pattern is much a product of the current weak La Nina pattern. However, the intensity of the amplification and resulting amplified warming of the Arctic is also a function of the long-term global warming regime dominating the polar region and causing record warmth and reductions in sea ice. I noticed this amplified wave pattern will have interesting impacts on the Arctic weather pattern and possibly the tenuous sea ice beginning next week.

Right now, a prominent surface high pressure region…associated with the Beaufort Gyre…is over the Arctic Ocean north of Alaska and eastern Siberia. By the middle of next week, this gyre will weaken as strong low pressure systems approach the Arctic from both the Bering Sea and the far North Atlantic.

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Prominent high pressure of the Beaufort Gyre over the sea ice of the Arctic Ocean.
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European Model depiction of low pressure system advancing into the Arctic Ocean from the Bering Sea on Christmas Eve. This may be the strongest in a series of lows (2-3) beginning late week. Stormy conditions will also impact areas near Svalbard (islands just east of northeast Greenland) late-week and weekend.

The Gyre is vulnerable because of the areas of open water and tenuous sea ice which remains over the Chukchi Sea…record low extent for this time of year. The ice being cold creates the surface high pressure system and clockwise circulation. But last year, this gyre collapsed because of slow sea ice growth allowing for storms with warm, moist air to move into the Arctic and further slowed sea ice growth. It appears this may be forecast to happen again during the tail end of this month.

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European Model forecast surface temperatures showing well above normal temps shifting northward late week into Christmas Eve over the Arctic Ocean north Svalbard and the Chukchi Sea. While exact values will change, general pattern appears likely.

Depending on the strength of the low pressure systems, not only will the tenuous sea ice in the Arctic…widespread areas 1.5 meters or less in thickness (less than a meter in the Chukchi Sea)…deal with more warm air temperatures limiting sea ice growth, but also wave action which may destroy the ice, particularly from the Pacific side as cyclones are expected to move across the Arctic from the Pacific. We’ll see how much impact those storms have and how intense they are. If the upper-level wave pattern is as amplified as forecast by models 5-8 days out (no reason to think otherwise as he reach the point of good reliability for the upper-atmosphere), it’s a good set up for strong low pressure systems to develop in both the North Pacific and North Atlantic. And with the highly amplified blocking high over the Eastern Pacific, storms will be forced to track into Alaska and into the Chukchi and Beaufort Seas and deep Arctic Ocean.

–Meteorologist Nick Humphrey

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La Nina Pattern Begins in the Pacific Ocean

A weak La Nina atmosphere-ocean pattern has fully developed in the Pacific Ocean. This phenomenon is part of the cool phase of the El Nino Southern Oscillation (ENSO). It is characterized by abnormally low surface pressure in the Western side of the Pacific Basin and abnormally high pressure on the Eastern side. This causes an enhancement of the easterly trade winds, causing significant upwelling of cold water along the equatorial coast of South America, with a build up of very warm water in the Western Pacific.

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Schematic of La Nina oceanic ocean-atmosphere pattern in the Pacific and expected jet stream behavior and temperature/precipitation impacts in the US/Canada during a La Nina winter.

ENSO patterns, as shown above can cause noticeable changes in the seasonal weather patterns over North America, particularly during the winter months. The jet stream can become more amplified, leading to a Pacific jet producing cooler and wetter than normal conditions over the Pacific Northwest, extending into the northern tier states. Meanwhile, the “Sun Belt” of the US can see abnormally warm, dry conditions.

The caveat of all this is is the strength of the La Nina versus the degree of influence other atmospheric patterns have on the seasonal climate variability. Other patterns include the North Atlantic Oscillation (NAO-surface pressure variability between the semi-permanent Icelandic Low and Azores High), Arctic Oscillation (AO-pressure anomalies between Arctic and mid-latitudes, closely related to NAO), and the Madden-Julian Oscillation (which can speed the development and enhance the effects of a El Nino or La Nina phase), among others on various timescales.

So what does it mean for our winter in the US? Well, as of now, the NWS Climate Prediction Center is generating winter temperature/precipitation forecasts accounting for the development of La Nina, with a strong latitudinal effect on temperature and precipitation. Below/above in the north and above/below in the south, respectively.

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In the meantime, long-range forecasts show the North Atlantic Oscillation becoming “negative” later in November (characterized by a south to north pressure gradient between the Azores high over Portugal and the Icelandic to the north). This pattern is favorable for an amplified upper-level jet stream wave pattern over North America and the North Atlantic and intrusions of cold air deep into the eastern half of the US. So in the shorter term colder than normal conditions may be possible for these areas this month (as has already occurred this week).

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When it comes to these “teleconnections”…the various cycles of variability within the annual climate regime of Earth…they can most definitely give us a head’s up on to what to expect in general. A canvas of how the weather may be behave over the course of days to weeks and months. But we must keep track of how these different cycles interact with each other and how they vary individually in terms of strength and mode. One curiosity is the strength and persistence of the La Nina. If it was fairly weak, it is more likely to be dominated by other teleconnections at times during the course of the winter, versus if it intensifies and produces more persistent effects on the upper-level air patterns.

Overall, the expected winter pattern is good news for drought-stricken areas in the northern tier such as Montana and the Dakotas. We will have to watch areas along the southern tier for potential further drought development. And as mentioned, November and at least early December could feature a more amplified jet stream so that even areas in the Southeast which may end up with an above average winter overall may see serious impacts from cold because of Arctic intrusions (something for citrus growers to watch out for in Florida, for example).

–Meteorologist Nick Humphrey

Category 1 Hurricane Franklin making landfall on the East Coast of Mexico

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Hurricane Franklin is making landfall on the East Coast of Mexico Wednesday night/early Thursday morning (~midnight CDT Thursday) with maximum sustained winds of 85 mph with gusts likely over 100 mph. Besides damaging wind gusts, very heavy rain – up to a foot or more – will be possible in the mountainous terrain once the system moves inland and weakens during the day Thursday. ┬áLife threatening flash flooding and mudslides will be the greatest threats to any populated mountain areas (storm surge will be the hazard for coastal areas in the hurricane warning area tonight).


NOAA Raises North Atlantic Tropical Activity Forecast

The National Oceanographic and Atmospheric Administration raised its confidence today that the North Atlantic Basin would have an “very active” season. They called for 14-19 named storms, 5-9 hurricanes, 2-5 major hurricanes. A normal season averages 11 named storms, 6 hurricanes, 2 major hurricanes.

The reasoning for this activity forecast include 1) No El-Nino in the Eastern Pacific which would otherwise produce unfavorable vertical wind shear and cooler sea surface temperatures in the North Atlantic 2) Above normal sea surface temperatures across the Caribbean, Gulf of Mexico and Main Development Region (open tropical Atlantic) 3) The continuation of the Atlantic Multidecadal Oscillation favoring above normal oceanic heat content.

So far, we are at 6 named storms, 1 hurricane, 0 major hurricanes (assuming no surprise intensification of Franklin prior to landfall).

The North Atlantic Tropics Looking to become Active Again This Weekend or Next Week

It’s August and with that it’s time for the North Atlantic to show its tropical cyclone “muscle”. Tropical waves become more numerous as mesoscale convective systems form over the tropics of West Africa and race off into the open Atlantic; their mid-level “vorticity” or spin the seed for possible further development. The National Hurricane Center in Miami has pegged one Thursday with an 80% chance of development between now and Tuesday (40% chance between now and Saturday).

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Shortwave infrared image (sometimes known as “nighttime visible”) taken at 8 pm EDT Thursday. This shows a cloud mass moving off West Africa associated with a new tropical wave south of the Cape Verde Islands. (NOAA)

Mid-range models suggest the system will develop possibly into a depression or tropical storm, moving generally westward toward the Lesser Antilles heading toward Tuesday. Much more on what will happen will depend on the system’s development. Mid-level dry air brought in from the Sahara Desert will be an issue for this system as it approaches the Lesser Antilles if it moves north of 15N. As far as upper-level winds, forecast shows a modestly favorable environment for development, but details will wait until down the road. Water temperatures in this part of the Atlantic – known as the Main Development Region (MDR) – are running up to 1.5 C (~3 F) above average with abundant warm sea surface temperatures above 26 C (79 F) west of 35 W.

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Current Sea Surface Temperatures over the tropical Atlantic Ocean. The orange-red and red shades indicate temps at and above 26 degrees C. (Wunderground.com)

Because of the strong semi-permanent “Bermuda High” expected to dominate the Central and Western Atlantic next week, this system will need to be watched by interests in the Western Atlantic Basin for potential impacts in case it does not curve northeastward out to sea because of the subtropical high pressure system to its north (assuming it develops).

Also of interest is a system in the Eastern Caribbean Sea. It is in a more hostile environment (shear and dry air main problems) and only has a 40% chance of development over the next 5 days.

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Shortwave IR image at 8:45 pm EDT showing generally disorganized thunderstorm activity associated with a tropical wave in the Eastern Caribbean. (NOAA)

The Atlantic has been running about a month ahead of schedule on named storms, but has been dead quiet on hurricanes. The 1966-2009 average for the first hurricane in the basin is coming up (August 10th), but given recent years of activity, the Accumulated Cyclone Energy (ACE) thru Aug. 2nd is running at its lowest level in the basin since 2009. But still 90% of the ACE on average occurs from here on out, so much can still happen, especially given the lack of one otherwise major hindering presence in El-Nino.

I’ll keep track of these disturbances in the coming days and have more for you if they develop into organized systems. Stay tuned!