|Heavy Snow with Northeast Flow|
over Lakenheath, England
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).
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.
RAF Lakenheath Forecast Reference Material (2001), 21 OWS Technical Services.