Pleasant Weather This Week

A fairly calm and uneventful week is in store weatherwise. Temperatures this week will be much warmer than the values experienced late last week.

NAM temperature forecast for Tuesday, February 14, 2012

NAM temperature forecast for the Prairies – valid Tuesday, February 14, 2012

Weather conditions for the first half of this week will be pleasant. High temperatures on Monday and Tuesday will generally remain in the -2 to -6C range in Southern Manitoba with light wind speeds. However, those parts of Southern Manitoba that are mostly snow-free will see temperatures around zero during this time period. The stronger February sun will heat these darker, snow-free areas more easily than white snow-covered regions resulting in warmer temperatures. Regions west of the Red River Valley and south of the Trans-Canada highway are the most snow-free parts of Manitoba and therefore will be noticeably warmer than areas around Winnipeg (that have more snow cover). By Wednesday it appears that all of Southern Manitoba will get up close to the 0C mark as a light southerly flow brings in some warmer air.

The weather outlook for the end of the week is just a little bit cooler from that of the beginning of the week. Temperatures on Thursday and Friday look to remain around or slightly warmer than seasonal values (normal high is -9C). Models are currently showing the potential for some light flurries on Friday into Saturday, but other than that it doesn’t look like we have any chance of seeing snow this week.

Seven weeks of winter down, six to go.

Elsewhere in Weather News

Super Cyclone Giovanna Threatens Madagascar

A very strong and dangerous cyclone named Giovanna has spun up in the western Indian Ocean this past Friday.

Super cyclone Giovanna marches towards Madagascar on IR satellite, with its predicted path. (Source: Sat24)

Super cyclone Giovanna marches towards Madagascar on IR satellite, with its predicted path. (Source: Sat24)

Giovanna is currently categorized as a category three cyclone (wind speeds of 160km/h) and is forecast to sustain its wind speed as it barrels towards the island of Madagascar. The combination of low wind shear and relatively warm waters of the western Indian Ocean is what has contributed to the rapid intensification of Giovanna. With the storm moving slowly towards the southwest, the landfall will take place on the Tuesday the 13th, bringing with it: torrential rains, a high storm surge and powerful sustained winds. The areas taking a direct hit with this cyclone will have a shot at receiving 250mm of rainfall (more than half of what Winnipeg receives yearly), wave heights reaching 24 feet and not to mention the sustained winds of about 190km/h. The storm may slowly intensify into category four, depending on whether or not an eyewall replacement cycle will occur before Giovanna makes landfall. This occurs when the cyclone’s thunderstorm bands circling around the eyewall use up some of the eyewall’s moisture and subsequently collapses the eyewall. Eventually, a new eyewall forms, strengthening the cyclone further and the cycle restarts.

Models predicting rain rates of above 45mm/h (red) as Giovanna approaches Madagascar (Source: Sat24)

Models predicting rain rates of above 45mm/h (red) as Giovanna approaches Madagascar (Source: Sat24)

The remnants of Giovanna will emerge into the Mozambique Channel and may re-intensify into a tropical storm. Unfortunately, this would bring heavy rains to Mozambique again, as they have already been plagued with torrential rains this season, more notably cyclone Funso (blogged about three weeks ago) which dumped significant rainfall on Mozambique’s coast not too long ago.

Elsewhere in Weather News has been provided by Matt

Weather Education: Low Pressure Systems

Low pressure systems are the “engine” that drive weather. They are what interact with warm and cold fronts, and they function to convert and dissipate the energy stored within the fronts. These systems have a “traditional” or “classical” progression that can be observed.

Frontal Wave

A simple schematic of a developing frontal wave. The blue line represents a cold front and the red line represents a warm front.

The first sign of the development of a low pressure system is the appearance of a frontal wave. This is a slight bend in an area of high thermal contrast. This is the beginnings of the warm and cold fronts.

Surface Low Appearance

A representation of a maturing surface low. The frontal wave has tightened into a near 90° angle at the intersection point of the warm and cold front, a sign that an occlusion will soon form.

As the wave tightens, and the cold front becomes more perpendicular to the warm front, the low pressure center appears on the inflection point of the warm and cold fronts (where the two fronts attach to each other). The low will then move with the mean flow aloft, following the troughs of upper-level waves (more on that later). As the low moves, the fronts move along with it. And naturally, the weather associated with those fronts moves along as well.

Mature Low

Analysis of a mature low. Convection is visible along the cold front over the Eastern United States. The warm front extending from Southern Quebec southeastwards into the Atlantic Ocean has a large cloud shield ahead of it. A trowel is visible arcing backwards around the low from Southern Quebec through Central Ontairo and bad around the Low in Minnesota into Michigan.

A satellite image depicting a mature low pressure system. The cold front is represented by the blue line, the warm front by the red line, and the trowel by the blue/red half-arrows. The center of the low circulation is marked by the large L.

One characteristic of cold fronts is that they are often faster moving that warm fronts. This can result in the warm front moving “along” the warm front and lifting the warm air up. This is called an occlusion process. The warm air aloft then is pulled towards the low and around it, rising in height. This warm air aloft is called an occlusion, or more frequently today, a trowel.

A mature low will have 4 distinct areas and kinds of precipitation: warm front precipitation, cold front precipitation, occlusion/trowel precipitation, and “wrap around” precipitation. Wrap-around precipitation is the weather that occurs in extremely close proximity.

Low Dissipation

Eventually, the warm and cold fronts pull themselves off the low pressure system. It can be likened that the “gas” for a low pressure system is the temperature contrast present in the fronts. When the fronts leave the low, warm air wraps around it and soon there is no more sharp temperature contrasts. When this happens, the low will “fill in” and dissipate.

Why Is It Called A Low Pressure System?

A cross-section schematic of the air flow through a low pressure system.

When a low pressure system begins to form, air is pulled in towards the center of it. We have discovered that, more or less, air in the atmosphere doesn’t like to compress. So instead of compressing as all this air meets in one place, it pushes air upwards. This creates a circulation where air moves in towards the low at the surface, rises some height, then flows out and away from the low. This results in low pressure near the surface, where the air is rising, and higher pressure somewhere above, where the air is moving outwards. Thus, a low pressure system is called such because the surface pressure is actually lower than the areas around it. As it “dies,” the surface pressure will return to the normal pressure around the low.

I should mention that this is an extremely brief overview of low pressure systems. If you would like to learn more, there are entire books written on the subject, and to this day it is still an area of active research.

A Mild Day Before The Deep Freeze

Southern Manitoba will see a beautiful afternoon today with plenty of sunshine and highs approaching 0°C in southwest sections of the Red River Valley. This afternoon/evening would be a great time to get out and enjoy the warm weather; a cold front sweeping southwards is going to bring a drastic change on Thursday.

500mb Wind Prognosis

+42 Hour 500mb Winds & Height forecast valid 12Z 09 February, 2012 from the GEM-REG Model

A strengthening upper ridge over the western portion of the continent will organize itself and amplify significantly today and tomorrow. By Thursday morning, it will stretch from Nevada northwards all the way to the Arctic Sea shoreline of the Yukon Territory. As a response, long-wave troughing will be enhanced over the central portions of the continent. Practically, this means that large-scale troughing will be induced in the arctic vortex and rotate around it.

In winter time, long-wave troughs are often associated with cold temperatures and incoming surface ridges of high pressure.

As this long-wave trough rotates from the Kivalliq region of Nunavut through Southern Manitoba, arctic air will be allowed to spill southwards. With this particular upper trough and associated cold front, there appears to be a significant push of cold air: temperatures on Thursday will only recover a degree or two from our overnight low of around -12°C, and temperatures will plummet on Thursday night. Currently, it looks like most areas in the Red River Valley will see overnight lows between -28°C and -32°C on Thursday night, with the colder temperatures east of the Red River and the warmer temperatures west of the Red River courtesy some light downslope winds over the escarpment. Like the previous cold front, very little precipitation is expected with it’s passage.

The normal daytime high for Winnipeg right now is -10°C. The normal overnight low is -20°C.

We’ll be under the influence of this bitterly cold air for a couple days before some warmer air advects into our region from the north again and we return to more seasonal values for Sunday and the start of the new week.

A Cooler Week Ahead

A cooler week is in store for us as we move into the last month of meteorological winter. Temperatures don’t, however, look like they’ll be too far from normal through most of the week.

Surface Pressure Map

Surface Pressure Map – valid 12Z Tuesday, February 6, 2012

With the passage of a cold front this morning, gusty northerly winds will usher colder air into Southern Manitoba. As a result temperatures will fall throughout the day today, with midday conditions in the mid-minus single digits. There will be a few scattered flurries this morning through Southern Manitoba as the cold front works its way through the area, however there will be no significant accumulations with them. Tonight, a strong area of high pressure will build into Southern Manitoba, with the central pressure expected to be as high as 1045mb. This will bring significantly colder temperatures than we’ve enjoyed over the past week to our area overnight, with lows expected to be near -20°C in Winnipeg and as low as -25 or -26°C in some areas in the Red River Valley. Tuesday won’t be an extremely cold day as a weak warm front will push into our area from the north, but it will still be colder than we have been used to lately with highs unlikely to exceed -10C. Conditions through Wednesday will be slightly warmer, with highs expected to rise back up into the minus single digits.

Friday Morning GEMGLB Forecast Temperatures

Forecast surface temperatures from the GEM-GLB model valid 06Z Friday, February 10th with analysis. The thick black line represents the cold front.

There is some uncertainty as to how the weather will unfold later this week. Models are hinting at the potential for another push of cold air on Thursday into Friday as another cold front sweeps through the province. Models currently differ on the exact strength of the cold push, however, as shown in the image above, some models push temperatures as low as -30°C back into Southern Manitoba. Which if it comes true, we can at least take solace that we don’t live in Northern Saskatchewan, where temperatures are forecast to be as low as -45°C!

Elsewhere in Weather News

Europe Battles Cold and Snow

This past week Europe was battered with what Canadians would call a “light snowfall” where many parts of the United-Kingdom, including London, received 10cm of snow this past weekend. The European infrastructure isn’t equipped to handle a snow storm of such magnitude which was affirmed by the fact that there were widespread airport cancellations and traffic was crawling to a standstill – some drivers were even stuck in their cars on the freeways overnight. The storm also prompted the UK Met offices to issue freezing rain warnings and cold weather alerts. The snowfall was due to a warm front pushing inland off the Atlantic Ocean and clashing with colder air that was present in the region prior, thanks to another low pressure system off to the east. The snow storm that has been wreaking havoc is forecast to lift and leave the area by Tuesday morning, leaving warmer temperatures behind and clearer conditions to come.

Surface Map for the UK

System’s analysis with the warm front arriving inland, clashing with the colder air to the east (occluded front). (Source: Met Office: UK)

Cold weather also affected other parts of Europe including Ukraine and Italy, with Ukraine being one of the hardest-hit of the Eastern European countries and experiencing tragedy. Kiev, Ukraine’s capital city, normally has temperatures averaging minus five Celsius for the month of February. During this cold snap however, temperatures dipped lower than minus thirty Celsius and many people have died – over 300 as of this posting. Temperatures for the next week are forecast to remain well below average, with highs only reaching the minus teens.

Snow in Rome

City of Rome, largest snowfall in two and a half decades. (Source: Dailymail)

In Rome, Italy, the city has received its largest snowfall in two and a half decades. The city was buried in ten to twenty centimetres, leaving the city paralyzed and people freezing in their houses, since homeowners are only allowed twelve hours of heating per day (to reduce emissions). As the low pressure system plans to stick around for a couple more days, Rome will experience well below average high temperatures of about five Celsius (average of thirteen Celsius).

Surface Map of Europe

Europe’s system analysis, Feb 5th, showing both lows impacting Europe. (Source: Intellicast)

Elsewhere in Weather News has been provided by Matt