Case Study: 29 - 30 January 1998

• I. Introduction and Short Description of the Case
• Introduction
• The situation over the Arctic Sea before the development
• II. Analysis for 29 January 1998 06.00 UTC - Early Wave Stage
• 06.00 UTC - Satellite Overview
• 06.00 UTC - Frontal, Wave and Jet Streak diagnosis
• 06.00 UTC - Cross sections
• III. Analysis for 29 January 1998 12.00 UTC - Developing Wave Stage
• 12.00 UTC - Satellite Overview
• 12.00 UTC - Frontal and Wave diagnosis
• 12.00 UTC - Cross sections
• IV. Analysis for 29 January 1998 18.00 UTC - 30 January 1998 06.00 UTC - Rapid Development
• Rapid Development - Introduction
• Rapid Development - Satellite Overview
• Diagnosis of surface and Upper Level Lows
• Diagnosis of frontal parameters
• PVA and jet streak diagnosis
• Rapid Development - Cross sections
• Diagnosis of precipitation patterns with help of relative streams and NORDRAD radar
• V. Overview of Period 30 January 06.00 UTC - 18.00 UTC - Further Development of the Occlusion and ECMWF - HIRLAM Model Comparison
• Further development of the Occlusion
• Additional invesstigation: Comparison of HIRLAM fields vs. ECMWF fields of isotachs and PVA at 300 hPa

Introduction

Scandinavian countries frequently experience passages of intensive low pressure systems originating over the Arctic areas. These depressions are challenging both forecasters and numerical models. This is because the depressions deepen in data-sparse regions. Depressions can also bring intensive weather with them, such as heavy snowfall and strong winds. The structure and development of these arctic depressions can be somewhat different from the classical mid-latitude cyclones.

This case study illustrates rapid the development of a low pressure system in a cold air stream over Scandinavia on 29 - 30 January 1998. The development can briefly be described as follows:

• A fresh northerly air stream is blowing on the Arctic Sea. An Arctic Front separating the polar air mass from extremely cold arctic air extends from east of Greenland towards northern Norway. A minor low pressure system forms on this front (north of the Lofoten Islands) early on 29 January, within an area of enhanced convection.
• A Wave forms along the Arctic Front.
• Associated with an upper trough moving south from the Arctic, the low pressure begins to deepen. The deepening is at first only gradual, but becomes more rapid later.
• As the low pressure deepens, it moves further south-east and occludes.
• Some moderate or heavy snowfall is observed especially during the Occlusion process, in particular on the south-east side of the low pressure centre. The most intensive weather is observed, however, during the late stages of development at the rear side of the system with extremely strong winds and heavy snow showers.