Background:

In the lower troposphere, waves are often observed to develop on both warm and cold fronts, though warm front waves are less frequently observed. They can however be equally, if not more important, in terms of impact on the low level weather conditions for the operational meteorologist. The most conducive environment for a frontal wave to develop is in frontogenetic confluent flow. In the case of warm front waves these are often found on the forward side of a confluent upper ridge. A shallow low develops along the warm front and is steered on the warm side of the jet. The low associated with the warm front wave seldom develops appreciably, and just ripples along the warm front until they finally fill as they enter an anticyclonic region downstream towards the right exit of the jet stream ahead of the surface front. These waves travel on the warm side of the jet, approximately 100-250 miles west of the main jet axis (see Figure 1, right, for an example from 18 November 2017), which itself may be moving slowly E or NE, with a speed rarely exceeding 20 knots.

Rules for forecasting:

Formation of the wave is favoured when the warm front is slow moving in the right entrance area of a 300 hPa jet on the east side of a 300 hPa confluent ridge. Sawyer (HWF) found that they occur when the configuration of the 1000–500 hPa thickness pattern was a confluent ridge with a jet core speed >60 knots. Direction of the movement of a warm front wave when formed is close to the 300 hPa flow, since they remain shallow features, with a maximum diversion of 15° towards the cold air. Their speed of movement is approximately ⅓ of the jet core, or ½ of the speed of 300 hPa flow over the wave.

Characteristics of warm front wave:

Associated with the wave is an area of enhanced precipitation, usually over a relatively small area, or 100-200 km in length, parallel to the front, and centred somewhat ahead (in the cold air) of the wave tip. The average direction of movement is 060-170° with an average of 110°. The average speed of movement is 33 kn, (this resulting in an often rapid and unexpected deterioration in the colder air ahead of the warm front wave) with typical progression speeds ranging between 20 and 55 kn. Generally these waves move along the front without significant deepening occurring, with central pressure generally between 990-1028 hPa but most likely between 1010 and 1020 hPa. There is infrequent formation of a closed isobar, generally 1 in every that 5 that have been observed show a closed isobar pattern.

Saturday 18 November 2017 Case Study:

In this particular case study the warm front wave is forecast to move approximately ESE (110 deg) at 40 knots. This is approximately equal to ½ the speed of the 300 hPa flow over the wave (80 knots) and ⅓ of the jet core speed (130 knots) which fits the above rules well. Only a very shallow depression is expected to form with no closed isobar for much of its lifetime, and an MSLP no lower than around 1023 hPa. It is expected to track across central southern England through Saturday and the UKPP output models this well as a narrow band of much poorer conditions with areas of low stratus (200-400 FT AGL) and outbreaks of rain and drizzle. The deterioration just ahead of the wave is likely to be rapid, but the main forecasting challenge (as ever) is the exact track as subtle changes in this can and will have a dramatic impact on the forecast for Saturday in these areas!

• METAR EGVN 180950Z 25004KT 9999 FEW016 SCT025 BKN050 07/05 Q1022 BLU BECMG BKN024 WHT=
• SPECI EGVN 181008Z 25005KT 9999 BKN006 07/06 Q1022 YLO1 TEMPO BKN008 GRN=
• METAR EGVN 181050Z 27007KT 9999 -DZ OVC005 08/07 Q1022 YLO1 TEMPO 7000 BKN004 YLO2=
• SPECI EGVN 181103Z 25007KT 5000 -RADZ OVC004 08/07 Q1022 YLO2 TEMPO 3000 RADZ YLO2=
• METAR EGVN 181150Z 25005KT 4000 -RADZ OVC003 08/08 Q1021 YLO2 TEMPO 3000 YLO2=
• METAR EGVN 181250Z 26005KT 3000 DZ BKN004 09/09 Q1021 YLO2 BECMG 7000 -DZ BKN006 YLO1=
• METAR EGVN 181350Z 28005KT 7000 -DZ SCT005 BKN020 10/10 Q1020 YLO1 BECMG 9999 NSW BKN030 BLU=