Extratropical cyclones a forecaster s perspective

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1 Extratropical cyclones a forecaster s perspective Meteorol. Appl. 4, (1997) Martin V Young, National Meteorological Centre, Meteorological Office, London Road, Bracknell, Berkshire RG12 2SZ, UK Despite considerable improvements in numerical models in recent years, weather events associated with extratropical cyclones continue to present forecasters with a challenge. For most cyclogenesis events model forecasts display, at the least, subtle errors in track and intensity of the system. Occasionally, errors can be significant a depression can deepen suddenly and unexpectedly, with major implications for wind strength as well as other weather elements. Many of the weather events within extratropical cyclones are essentially mesoscale and, on occasions, a single cyclone can be associated with several simultaneous severe events such as gales, heavy rain, snow and thunderstorms. This paper describes how the forecaster can add value to numerical model products by using information from a wide variety of sources such as satellite and radar imagery, as well as by monitoring hourly surface observations. Examples are shown in which the forecaster has to make adjustments to model predictions, reconcile conflicting model guidance, and add the all-important mesoscale detail. 1. Introduction In recent decades, numerical models have evolved to become one of the most important tools for weather forecasting. Despite continuing advances in numerical weather prediction, the forecaster retains a vital role in adding value to numerical model guidance, and conveying this information to customers. In this paper, specific examples will be presented which illustrate how the forecaster can add value to numerical guidance (concentrating on the period up to 24 hours ahead), using information from a variety of sources. 2. Synoptic scale evolution 2.1. Track of a depression Numerical models are usually good at predicting the pressure field and wind distribution and it is becoming increasingly difficult for the forecaster to improve upon these aspects. However, it remains a fact that for many depressions whose evolution can be followed into data-rich areas such as the UK there are, at the least, subtle errors in the prediction of track/intensity. On a few occasions errors can be much more significant, with major implications for weather over populated areas, and it is important that these can be identified in advance. The Meteosat image for 0600 UTC on 16 February 1995 in Figure 1 shows the cloud signature associated with a developing depression to the west of the British Isles. Features worthy of note are the northward extension of layered cloud to about 53 o N exhibiting a convex poleward boundary, and a cloud-free slot penetrating into the upstream end of the cloudmass near 49 o N. Interpretation of the imagery and latest observations pointed to the existence of a deepening depression near 49 o N, 19 o W. This confirmed the forecaster s view that the depression was centred somewhat further north and also deeper than indicated in the latest available UK Limited Area Model forecast for this time. Such errors in a model s analysis or in the very early stages of the forecast are often transmitted into later stages of the forecast. The model predicted that the depression would be a relatively shallow feature over southwest England in 12 hours time (Figure 2(a)). In view of the latest evidence, the forecast track of the depression was adjusted to be further north from southwest Wales to the Wash (Figure 2(b)) with greater deepening. This manual adjustment of almost two degrees of latitude comprised at least a degree to cater for the model s initial positional error plus a further slight adjustment to allow for the tendency for deepening depressions to be steered towards the left (owing to buckling of the upper flow in their vicinity). This adjusted track implied that gales would be likely to affect much of southern Britain later that day, rather than just English Channel coasts as predicted by the model. The actual track of the low and surface analysis for 1800 UTC on 16 February (Figure 2(c)) shows that the forecaster had indeed added considerable value to the model products Conflicting model guidance Conflicting guidance between different models or even successive runs of the same model is a problem which 293

M V Young Figure 1. Meteosat infra-red image 0600 UTC on 16 February 1995. The location of the deepening surface depression is superimposed. often confronts the forecaster.

2 M V Young Figure 1. Meteosat infra-red image 0600 UTC on 16 February The location of the deepening surface depression is superimposed. often confronts the forecaster. On occasions this can be ascribed to something as simple as model resolution in other cases the reasons are much more complex. Figure 3 shows 24-hour forecasts from the UK Limited Area Model (LAM) and Global Model (GM), valid at 0000 UTC on 18 April The LAM with a horizontal resolution of about 45 km shows a sub-992-mb depression just west of Ireland (Figure 3(a)), whereas the GM (resolution about 90 km) produced only a weak trough (Figure 3(b)). The pressure difference between the two models exceeded 12 mb in that region. Upper-air charts (not shown) indicated that the depression was developing ahead of a confluent trough. In this pattern there is a fine balance between whether a depression will run rapidly east as a non-developer beneath strong zonal flow, or be engaged by the upper trough, thereby concentrating ascending motion into a compact area leading to significant cyclogenesis (Bader et al., 1995). In this case the LAM was followed, partly because of a better initial analysis than the GM and greater resolution. The verifying analysis for 0000 UTC on 18 April (Figure 4) shows that significant cyclogenesis did indeed occur, giving gales over Northern Ireland and Scotland. 294 On the face of it, the decision to follow the LAM solution was an easy one, since one major purpose of a high resolution model is to capture the strong circulation around developing depressions. However, care must be taken since occasionally, high resolution models can over-deepen depressions. This emphasises the importance of treating each case on its merits and continually monitoring latest imagery and observations, as well as understanding the characteristics of the forecast model. 3. Mesoscale evolution 3.1. Rapid intensification Whatever the quality of the broadscale guidance from models, one major responsibility of the forecaster is to add mesoscale detail, and it is in the nowcasting period (up to 12 hours ahead) where there is the greatest opportunity. However, in the case of extratropical cyclones, it is the potential for sudden intensification that creates major forecasting problems. In such cases, changes happen quickly and can be difficult to keep abreast of, as the following example illustrates.

3 Extratropical cyclones - a forecaster s perspective (a) (c) (b) Figure 2. (a) 18-hour LAM forecast of sea-level pressure, valid 1800 UTC on 16 February (b) Actual and forecast tracks of the depression centre. Cross-dashed line is the model s forecast track, dot-dashed line is the manually adjusted forecast track, whilst the continuous line is the actual track of the depression. (c) Subjective surface analysis for 1800 UTC on 16 February Figure 5 shows LAM 12-hour forecasts of (a) MSLP and rainfall and (b) 10 m wind for 0000 UTC on 19 May A large area of heavy rain was predicted around the northwest flank of the low. Issued forecasts (based on this guidance) additionally emphasised the likelihood of rain turning to snow over high ground in southwest England owing to unseasonably low freezing levels north of the depression, with the coldness accentuated by the strengthening northerly wind. The LAM predicted winds of little over 20 kn around the depression whilst the 12-hour Mesoscale Model forecast (not shown) produced only marginally stronger winds. By 1800 UTC on 18 May (Figure 6) evidence from surface data (and satellite imagery) confirmed the forecaster s view that the forecast winds around the depression looked far too weak in both models, and gale warnings were issued for the English Channel. During the next few hours marked intensification of the pressure gradient took place to the rear of the depression as it continued deepening, giving winds of severe gale-force in gusts by 0000 UTC on 19 May (Figure 7), necessitating upgrading of the gale warnings already in force. In this situation the forecaster faced the dilemma of whether the system would deepen further, with the danger of having to update the forecasts on the hoof. A useful clue in this event was that shortly afterwards, satellite imagery (not shown) indicated that a hook of upper cloud was already wrapping round onto the forward side of the depression, indicating a vertically deep circulation, and therefore that the surface low had reached its maximum intensity. 295

4 M V Young (a) (b) Figure 4. Subjective surface analysis for 0000 UTC on 18 April crucial in order to remain one step ahead of developments A multitude of severe events The following example illustrates from the perspective of a local forecaster the complexity of the nowcasting problem during a typical cyclogenesis. Several severe weather events can occur simultaneously presenting considerable challenges to the forecaster. Figure hour forecasts of MSLP and precipitation valid at 0000 UTC on 18 April 1996 from (a) LAM and (b) GM. In both (a) and (b) circles represent areas of dynamic rain, and angular symbols areas of convective rain. Despite very successful forecasts of the precipitation forward of this depression (including the occurrence of widespread snow over high ground of southwest England) as well as its track, the somewhat unexpected sting in the tail in the form of severe gales is by no means unusual in such systems. This constitutes a serious forecasting problem especially at a time of year when many weather-sensitive craft may be crossing the English Channel. Numerical models usually underestimate the intensity of the circulation near the centre of developing depressions, even at very short ranges (it is interesting that even the 1800 UTC run of the models on 18 May 1996 failed to capture the intensity of the cyclogenesis). Monitoring of hourly surface observations and imagery is therefore 296 Figure 8 shows the precipitation pattern associated with a complex depression and its rapidly occluding frontal system moving into the British Isles. Mesoscale Model products (Figure 9) suggested that the depression and its extensive precipitation would move east across England as a complex feature, with the rain turning to snow over high ground of northern England and southern Scotland. From the viewpoint of a forecaster at Newcastle, successive radar images confirmed the idea that the frontal precipitation would move steadily east across northeast England. Available surface observations showed exclusively rain at this time. In view of wetbulb freezing levels of around 3000 ft at Boulmer (just north of Newcastle) it seemed reasonable to assume that significant snow would penetrate down to 1000 ft above sea-level. Therefore, warnings were issued for snow leading to difficult driving conditions over higher routes in northern England. The sequence of surface observations at Newcastle Weather Centre (Figure 10) showed rain beginning as expected by 0200 UTC. By 0400 UTC the rain had become heavy then very quickly turned to snow. At this stage, the tricky decision had to be taken regarding how long

5 Extratropical cyclones - a forecaster s perspective (a) Figure 6. Surface analysis and observations at 1800 UTC on 18 May Maximum hourly gusts (kn) are preceded by letter G. (b) Figure 7. Surface analysis and observations at 0000 UTC on 19 May Symbols as in Figure 6. Figure hour forecasts of (a) MSLP and precipitation (symbols as in Figure 3) and (b) 10 m wind from the LAM valid 0000 UTC on 19 May the snow would last, and how deep any accumulations would become. In view of the likelihood of there being several hours further of heavy precipitation, a flash message of severe weather was immediately issued, warning of very hazardous driving conditions with falls of 4 6 cm likely in any location and cm on high ground. The sequence shows that several hours of moderate snow occurred (causing severe traffic disruption), which only reverted to rain when the precipitation became light. Clearly the conditions for snow near sea-level were extremely marginal, and depended on the correct combination of several factors. It is often observed that marked lowering of the freezing level can occur due to cooling by melting snow during heavy precipitation ahead and north of a deepening depression a process rarely captured properly even by mesoscale models. 297

6 M V Young Figure 8. Radar derived rainfall distribution for 0000 UTC on 12 February 1996 with fronts and depression centres superimposed. N and B show location of Newcastle and Boulmer respectively. Rainfall intensities are as follows: hatching < 4mm h 1, stippling > 4mm h 1. Figure 10. Sequence of hourly surface observations from Newcastle (labelled N in Figure 8), 12 February Station site is 52 m above sea-level. Time (UTC) is shown below each observation. Figure 11. Surface analysis and observations for 0600 UTC on 12 February Dashed lines are isallobars in mb/3h. Figure hour Mesoscale Model forecast valid for 0600 UTC on 12 February The area of snow is highlighted by hatching. Other symbols are as in Figure 3. It could be argued in this instance that the forecaster was merely reacting to events, rather than forecasting in the true sense. However, even when a severe event has already started, customers will need to know how long the severe conditions are likely to last. Although more general warnings of snow can be issued much further in advance (and indeed were given in this instance by guidance from the National Meteorological Centre, Bracknell the previous day, including the possibility of sleet or snow at low levels), in marginal circumstances 298 specific severe weather warnings can often only be issued once confidence is high, which sometimes entails waiting until the event is literally starting. Further south, other problems were confronting forecasters. Figure 11 shows the surface chart for 0600 UTC on 12 February and Figure 12 the corresponding satellite image. A pocket of large pressure falls was evident just west of Wales associated with a commashaped cloud cluster (C in Figure 12) which had become detached from the cloud band of the occlusion. This was taken as evidence for formation of a small new depression which looked set to cross southern England. The idea of a separate centre forming south of the main circulation was not conveyed by the

Extratropical cyclones - a forecaster s perspective Figure 12. Meteosat infra-red image for 0600 UTC on 12 February 1996. C is the comma-shaped cloud cluster referred to in the text. Figure 14.

Surface analysis and observations for 1200 UTC on 12 February 1996 Figure 13. 12-hour LAM forecast of 10 m winds valid for 1200 UTC on 12 February 1996.

However, the Mesoscale Model (Figure 14) did hint at such a development, as indicated by the opening out of the isobars over East Anglia and the minor trough over the Thames estuary.

Winds, having eased considerably following passage of the active frontal system, would increase to the south of the new low.

7 Extratropical cyclones - a forecaster s perspective Figure 12. Meteosat infra-red image for 0600 UTC on 12 February C is the comma-shaped cloud cluster referred to in the text. Figure hour Mesoscale model forecast valid for 1200 UTC on 12 February Symbols as in Figure 3. C marks the forecast precipitation area associated with cloud cluster C on Figure 12. Figure 15. Surface analysis and observations for 1200 UTC on 12 February 1996 Figure hour LAM forecast of 10 m winds valid for 1200 UTC on 12 February hour LAM forecast, either in the surface wind field (Figure 13) or the pressure field (not shown). However, the Mesoscale Model (Figure 14) did hint at such a development, as indicated by the opening out of the isobars over East Anglia and the minor trough over the Thames estuary. From the viewpoint of a local forecaster responsible for about 30 airfields in southeast England, there were several major implications regarding warnings. Winds, having eased considerably following passage of the active frontal system, would increase to the south of the new low. Hence strong wind warnings (gusts > 25 kn) would need consideration for the extreme south. Several sferic (lightning) reports were received near the convective cluster, which also necessitated consideration of thunderstorm warnings. Radar imagery showed a significant area of rain associated with the convective cluster. Although partially supported by convective heating from the sea, model products (Figure 14) suggested that this cluster C would retain some of its integrity as it moved east across southern England due to dynamical forcing. In view of the convective nature, warnings of thunderstorms accompanied by isolated gusts of kn were issued for a number of airfields. 299

8 M V Young With wet-bulb freezing levels falling below 2000 ft, the rain would be likely to turn to sleet/snow, especially on high ground therefore, warnings of snow for various airfields had to be issued. Detailed forecasting would be complicated by the inherent hit/miss problem with convective situations, as well as the need to give guidance regarding possible accumulations of snow. The analysis for 1200 UTC (Figure 15) shows the new depression, by this time over East Anglia where two locations were reporting sleet or snow. Many other places had reported snow showers, whilst a cluster of thunderstorms moved southeast across central southern England, with gusts as high as 50 kn at 1000 UTC on the Isle of Wight. In addition, as pressure over southern England continued to fall, a region of locally storm force winds propagated across southwest England with a gust of 72 kn at St Mawgan. Although LAM winds (Figure 13) gave a strong signal for gales, the continued strengthening had been identified and a flash message of severe weather with winds up to 75 mph was issued at 1030 UTC for Devon and Cornwall. Ironically, as gradients became slack in the southeast, with clearing skies, the airfields forecaster then had to consider frost warnings in his region for the evening. However, even this was uncertain because cloudy conditions, with further sleet or snow, could swing quickly south in the so-called backwash as the complex depression moved east over the North Sea. With several severe events occurring simultaneously over different parts of the UK, this example demonstrates the importance of having dedicated forecasters concentrating on developments over regional areas, to minimise the risk of significant developments being overlooked. The recently developed Met. Office NIMROD model (Golding, 1998), designed specifically for nowcasting, offers considerable hope for tackling such problems. By assimilating latest observations, satellite and radar imagery and mesoscale model diagnostics in an hourly update cycle, along with improved physics, NIMROD provides the forecaster with useful additional detail regarding a variety of parameters such as precipitation type. An additional general consideration pointed out by McCallum & Norris (1990) is that the latest model guidance is not always the best it is essential to examine a succession of model runs and identify trends, although these can, on occasions, be misleading. 4. Conclusion This paper has demonstrated a range of problems which confront the forecaster in the prediction of extratropical cyclogenesis. Many of the problems are essentially mesoscale, for which model guidance is far from perfect owing to constraints imposed by resolution and the difficulty of representing the physical/dynamical processes given the available data. The forecaster is very much dependent upon satellite and radar imagery as well as hourly observations in order to keep ahead of events. Using such information sources it has been shown that the forecaster can often add considerable value to numerical products; even though cyclogenesis itself can, on occasions, present some unpleasant surprises. Acknowledgements The author wishes to thank D. A. Mansfield and E. McCallum for helpful comments on the first draft of this paper. G. Holpin obtained the satellite images and the raw radar data was extracted by S. Higginson. References Bader, M. J., Forbes, G. S., Grant, J. R., Lilley, R. B. E. & Waters, A. J. (1995). Images in weather forecasting. Cambridge, Cambridge University Press. Golding, B. W. (1998). NIMROD A system for generating automated very short-range forecasts. Meterol. Appl. (accepted for publication). McCallum, E. & Norris, W. J. T. (1990). The storms of January and February Meteorol. Mag., 119:

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