Heavy snow showers with northeast flow over RAF Lakenheath located in East Anglia, United Kingdom can be challenging to forecast. East Anglia is a region of England that is situated on the southeast side of the island. RAF Lakenheath is located about 80 miles northeast of London. Major cities in the area are Norwich which is located 36 miles northeast of Lakenheath and Cambridge which is located 22 miles southwest of Lakenheath. Precipitation Climatology Background—East Anglia Despite the maritime location, Lakenheath only receives 22 inches of precipitation per year. Each month averages about 1.5 to 2 inches precipitation per year with no significant peaks throughout the year and is usually very light. Frozen precipitation, except hail, occurs from October through early June, but is most common in January and February. On average, there are eight days per year with snowfall greater than or equal to 0.1 inch with just four days greater than 1.5 inches. Snowfall usually occurs with northwest through northeast winds (RAF Lakenheath Forecast Reference Material, 2001). Forecasting Difficulties One major forecasting problem here at RAF Lakenheath is forecasting heavy snow showers with northeast flow. The atmospheric set up that produces this event is when there is a strong high pressure system sitting just northwest of Ireland during the winter months of November through March. This will bring north to northeast winds over East Anglia. The snow showers are seen approaching the station from the north or northeast. The ceiling will usually decrease to around 500 to 1000 feet and the visibility usually drops to around 3200 meters. The snow showers usually last for 10 to 20 minutes and will drop from a trace to 1 inch per snow shower. Keeping a close eye on radar helps weather personnel forecast the timing of these snow showers and the total snowfall amount. Each individual cell shows up very well and is very simple to track by extrapolating. This forecast problem does not occur frequently. It typically happens three or four times a year. The Icelandic Low semi-permanent low pressure system usually sets up to the northwest during the winter which makes it very difficult for high pressure to set up in that location. The Icelandic Low will continually spin out low pressure systems that will pass through the region, keeping the weather very unsettled. Every other day a frontal system will move through the area bringing some sort of light precipitation. One consequence of this forecast problem is major impact to local travel throughout East Anglia. The area is not accustomed to having snow on the ground, so the slightest accumulation of snow makes traveling the roads very dangerous. This is not primarily because of the road conditions, but because people in the area do not have experience when it comes to driving in snow. They do not understand the need for slowing down or canceling travel plans if there is snow on the ground. Also, the quality of the secondary roads in the country is very poor. They are narrow, have no shoulders and are very rough and uneven in places. Because Lakenheath is approximately 52 degrees north latitude, the sun is very low in the sky during the winter months. This decreased amount of sunlight further contributes to travel difficulties. An accurate snow forecast is essential for the safety of everyone. Snow is difficult to forecast in this area because it requires specific weather conditions for it to occur. The winds must be from the north to northeast with a long enough fetch, the surface temperature must be within a few degrees of freezing, and the water temperatures in the North Sea must be cold enough to support snow. Forecasters should look at the synoptic scale and then the mesoscale to ensure the proper set-up. The 850mb level is very useful to determine temperature advection. The -5C isotherm at 850mb is a good indicator that the precipitation will fall as snow. As colder temperatures move into the area, precipitation will change from rain to mixed precipitation and then into snow. Being in a maritime location has a significant influence on the weather all year around, but is most significant in the winter months. After a period of cold weather with temperatures near or below 0C and warm air advection over the cold surface, the rain that falls will change into snow depending on the thickness of the cold dome that is in place. In this area, the 540 thickness line is not the primary indicator of snow; it is the 522-528 thickness lines. The 540 thickness line will only bring rain to the area due to the maritime environment. Research Opinions and Recommendations Area forecasters use a few rules of thumb based on their experience with weather conditions in the area. For example, significant snowfall is mainly associated with east-northeast flow, drifting can be enhanced when the temperatures are below -1C. Another principle is to use the 522-528 thickness lines to separate areas of forecasted solid from liquid precipitation. A wet bulb zero height of less than 2,000 feet will indicate snow, while above that value would indicate rain. One last thing to consider would be the 850mb wet bulb potential temperature. If it is below 2C there is a high probability of snow (RAF Lakenheath Forecast Reference Material, 2001). Some of these experimental principles can be used in discovering a method to deal with this forecasting problem. A solid knowledge of the regime pattern that will bring snow showers with northeast flow is essential. If the strong high pressure is not in the correct place it will keep the area from getting the exact north through northeast wind flow that is needed for snow. A wind flow coming too much from the east will bring in an excess of dry air from the continent which will also eliminate the chance for snow. A wind flow too much from the northwest will not bring in the needed moisture from the North Sea. A good overall understanding of the local topography and local effects also helps forecasters predict this event. Lakenheath sits in a bowl with higher elevation on every side except the north through east-northeast. All other wind flow will create a slight down slope warming effect which will dry the air out. One last thing that would help forecast for this event would be to have a good understanding of the local rules of thumb. Merely knowing that northeast winds can bring snow showers is not sufficient knowledge of weather patterns in the area. Forecasters must understand certain thickness values, wet bulb potential temperatures, and wet bulb zero heights are needed and will certainly increase your accuracy in forecast northeast flow heavy snow showers. In conclusion, forecasting for northeast flow heavy snow showers can be very difficult to do but is very important to be accurate. A precise forecast directly impacts transportation and the safety of the public in this area where snow is not the norm. Proper knowledge of the regime, local topography, and rules of thumb will greatly improve the accuracy of the forecast and lead to a much safer commute for the public. Reference RAF Lakenheath Forecast Reference Material (2001), 21 OWS Technical Services. |