Main(An abridged version of Weir, 1982)
The purpose of this technique is to provide tropical cyclone forecasters with a real-time synoptic prediction technique for determining where and/or if a northward-moving tropical cyclone will undergo a significant increase in speed of movement as it approaches the domain of the mid-latitude westerlies.
This technique assists in the identification of specific flow patterns at the upper-tropospheric (mean 200 hPa) level which have been associated with the acceleration of previous tropical cyclones. It further provides guidance in identifying northward-moving tropical cyclones which do not experience significant accelerations.
Application of the technique requires use of the most-recent 200-hPa data and the 200-hPa (numerical weather prediction model) prognostic series that covers the next 24- to 48-hour period. The essence of the technique is identification of the domain of the mid-latitude westerlies. The forecaster must carefully review all 200 hPa wind data prior to conducting the TAPT evaluation; raw data, i.e., rawinsonde/pibal winds, AIREPS and cloud motion winds, are preferred over gridded analysis winds. If sufficient raw data exists the technique will provide:
(1). a YES/NO decision for a significant increase in the speed of movement,
(2). a "best" location (latitude) for the initiation of the acceleration process,
(3). speed of movement guidelines including duration and upper-limits, and
(4). insight on the probable path of the TC .
It should be noted that it is imperative that the forecaster closely evaluate the applicability of the TAPT conclusions each time the technique is attempted. The validity of the results is highly dependent on: the 200-hPa chart containing a sufficient amount of data to depict the southern extremity of the mid-latitude westerlies; the wind data present on the 200-hPa chart being representative of a deeply penetrating layer of mid-latitude westerlies, i.e., 200-700 hPa stratum; and the TC maintaining a northward movement and being in position to be affected by the mid-latitude westerlies.
The following environmental factors have also been observed to affect the subsequent acceleration of the TC and may be at variance with the TAPT forecast:
A. A strong northerly or easterly low-level flow that will impede TC movement. Normally, when this effect occurs early in the predicted acceleration period, the TC will drift northward for one to two degrees latitude before acceleration commences. If it occurs later in the acceleration period, it often marks the end of the acceleration process. Thus, a check on low-level steering is necessary prior to applying this technique.
B. A poorly-defined or very small mid- or upper-level circulation center which does not (directly) link the TC 's low-level center to the mid-latitude westerlies. In such cases, the low-level steering should be more representative of the TC 's movement.
C. A well-defined tropical upper-tropospheric trough (TUTT) lying between the TC and the mid-latitude westerlies. Interaction with the TUTT will often slow a TC and alter its upper-level circulation pattern, causing it to respond to steering influences at lower levels.
1. Instructions
Step 1. Locate the position of the TC at the valid-time of the 200 hPa chart. Annotate this position with an "X". From 15 to 20 degrees of longitude west of the TC , locate the southern extremity of the mid-latitude westerlies by sketching the 30 kt isotach eastward to the longitude of the TC . Be careful not to drop the isotach into the low originating from the tropical latitudes (Fig. B-1).
Step 2. Place the overlay (Fig. B-0) on the 200-hPa chart with the "large-scale" TAPT diagram's typhoon symbol above the chart's "X" symbol. Orient the overlay to true north and maintain this orientation throughout the evaluation. Use the "small-scale" diagram with the numerical prognostic charts.
Step 3. With the TAPT overlay in place, average the wind speeds present in Window A. If the average wind speed is less than 30 kt, continue the evaluation by moving the TAPT overlay northwestward while maintaining the chart's "X" symbol under the overlay's baseline.
Step 4. STOP the evaluation when the average wind speed is 30 kt or greater in Window A (Go to step 5), or when the "X" symbol is located beyond the "arrow" at the end of the baseline. If the evaluation has been stopped at the end of the baseline and if a prolonged northward track is still likely, climatology may be used to estimate the latitude where acceleration may begin.
Step 5. If the evaluation has been stopped with 30 kt or greater in Window A, then the following instructions will identify the potential for acceleration. TO OBTAIN THE "BEST LATITUDE" FOR SIGNIFICANT ACCELERATION TO BEGIN, THE PATTERN OF THE UPPER-TROPOSPHERIC, MID-LATITUDE WESTERLIES MUST BE IDENTIFIED NEXT.
Step 6. If the overlay's typhoon symbol is within two degrees of the chart's "X" position, then the current analysis chart and the 24-hour prognostic chart should be used to determine the upper-tropospheric wind pattern that will prevail during acceleration. Evaluate any differences between the wind pattern shown in the analysis and the prognostic chart before deciding on the future upper-tropospheric wind pattern.
Step 7. If the overlay's typhoon symbol is greater than two degrees from the chart's "X" position, then refer to the numerical prognosis charts with valid-times closest to the forecast arrival of the TC at the latitude shown under the typhoon symbol to determine the upper-tropospheric wind pattern that should prevail during the acceleration.
Step 8. From Figures B-2, B-3, B-4, B-5, and B-6 that follow, identify the upper-tropospheric wind pattern that best suits the pattern determined in Step 7 or 8. Follow the instructions given for the selected figure. When completed, review the Discussion Section before applying the technique (TAPT) to the forecast.
2. PATTERN RECOGNITION
2.1 South-southwesterlies (180-200 degrees) (Fig. B-2)
SOUTH-SOUTHWESTERLIES from 180 to 200 degrees: Generally a very favorable pattern for acceleration. Pattern usually develops from a WEST-SOUTHWESTERLY. Common pattern from early summer to early autumn.
SIGNIFICANT ACCELERATION: will commence when wind speeds average 40 knots or greater in Window A. Move the TAPT overlay toward the northwest (with the "X" symbol remaining under the baseline) to meet the 40-knot criterion. Refer to Table B-1 to determine the recommended acceleration rate.
DURATION & UPPER LIMITS: With this pattern, acceleration is usually very sudden and a fairly rapid extratropical transition normally follows. Thus, most of the acceleration occurs within 24 to 36 hours and may reach speeds above 30 knots (often ahead of those predicted in the acceleration tables).
TRACK: Normally the track will be toward the north-northeast or the north-northwest moving from 10 to 20 degrees left of the upper-level wind pattern.
CAUTION: This pattern often sets-up within 12 hours of acceleration, aided by interaction of the TC with the westerlies, a ridge building process often occurs east of the upper-level trough.
2.2 West-southwesterlies(225 to 255 degrees) (Fig. B-3)
WEST-SOUTHWESTERLIES from 225 to 255 degrees: Generally the most favorable pattern for a sustained acceleration. A common pattern from late September to early November, also seen from late April to early June.
SIGNIFICANT ACCELERATION will commence when wind speeds average 40 knots or greater in Window A. Move the TAPT overlay toward the northwest (with the "X" symbol remaining under the baseline) to meet the 40-knot criterion. Refer to Table B-1 to determine the recommended acceleration rate.
DURATION & UPPER LIMITS: If this pattern is maintained throughout the acceleration process, the acceleration will normally be sustained for over 30 hours and may well exceed 30 knots before extratropical transition.
TRACK: Given a persistent pattern, the track will recurve toward the northeast and will move from 10 degrees (initially) to 25 degrees (in later stages) left of the upper-level wind pattern.
CAUTION: This pattern can quickly change to a SOUTH-SOUTHWESTERLY which will noticeably affect both the duration and track.
2.3 Westerlies (Fig. B-4)
WESTERLIES from 260 to 285 degrees: Generally a favorable pattern for acceleration. Common pattern in high zonal situations, especially in the spring, late autumn and winter months.
SIGNIFICANT ACCELERATION will commence when wind speeds average 30 knots or greater in Window A. Refer to TABLE B-1 to determine the recommended acceleration rate.
DURATION & UPPER LIMITS: Normally this is a very stable upper-level pattern and the effects of the westerlies on a TC will usually weaken the system due to increasing vertical wind shear. Acceleration will peak within 18 to 30 hours with speeds reaching the 20- to 30- knot range.
TRACK: A fairly sharp recurvature track toward the east-northeast moving from 10 to 15 degrees left of the upper-level wind pattern.
CAUTION: In the late fall, winter and early spring, this pattern may be present in the upper-levels while a strong northeast monsoonal flow is dominating the low-levels. In such cases the TC will often draw toward an upper trough then turn toward the west-southwest with the low-level flow.
2.4 West-northwesterlies (Fig. B-5)
WEST-NORTHWESTERLIES from 290 to 325 degrees: Generally an unfavorable pattern for significant acceleration. Common pattern in transition periods, especially in May and November.
SIGNIFICANT ACCELERATION should not occur with this pattern. However, some acceleration may occur before the shearing process weakens the TC . When wind speeds average 30 knots or greater in Window A the shearing and limited acceleration process should commence. Refer to TABLE B-1 to determine the recommended acceleration rate.
DURATION & UPPER LIMITS: Acceleration will be limited to 12 to 24 hours with the maximum speed generally less than 20 knots.
TRACK: If the TC recurves, the track will be toward the northeast and moving 30 to 50 degrees left of the upper-level wind pattern. If a strong northeast monsoon flow is present in the lower levels, the TC will normally track west-southwestward with the low-level flow instead.
CAUTION: The more northwest the upper-level wind pattern, the more rapid will be the shearing process and the shorter the acceleration period, if any.
2.5 North-northwesterly (Fig. B-6)
NORTH-NORTHWESTERLY from 330 to 360 degrees: A very unfavorable pattern for acceleration. This pattern can develop from the WEST-NORTHWESTERLY pattern in low zonal situations.
ACCELERATION should not occur with this pattern. A northward-moving TC would encounter very rapid shearing (within 12 to 24 hours) and would quickly become non-tropical in nature. However, surface wind speed may remain fairly high for 24 to 36 hours after the shearing process has begun. When this process occurs at relatively low latitudes, 15N to 25N, possible regeneration as a significant TC may occur if upper-level wind conditions change fairly rapidly and the TC ceases its northward movement.
TRACK: Quasi-stationary or erratic until the shearing is completed, then the surviving low will track with the low-level steering.
3. DISCUSSION
To this point, the most-recent 200 hPa analysis and prognostic charts have been evaluated and an upper-level wind pattern has been identified which has provided a prediction of the best latitude where acceleration will begin, the duration and upper-limits of the acceleration, and the track of a typical TC most common in the upper-level wind pattern. Additionally, the rate of acceleration has been determined by evaluating upper- and lower-level winds near to the TC .
This technique (TAPT) is a combination of synoptic and statistical predictors which, in most cases, will provide a good approximation of TC movement into the mid-latitudes. There are, however, many other factors which the forecaster should consider before applying the results of this technique to the forecast. Some of the meteorological factors which might be at variance with the TAPT forecast are listed below.
A. A weak TC with little or no mid- or upper-level support cannot be expected to draw into the westerlies and accelerate similar to the strong typhoon.
B. A very compact or midget TC with a very tight upper-level circulation may not accelerate unless it is adjacent to a deep-tropospheric steering current.
C. A well-established low-level wind regime which is directed against the upper-level wind pattern will most often overcome the upper-level steering and move the TC southwestward. A good example is a strong northeast monsoon off Asia and Japan in the late fall through early spring.
D. Rapidly changing upper-level wind patterns that may suggest one forecast scenario, then another. Any forecast during this situation is difficult and, accordingly, the reliability of TAPT is highly dependent on the identification of the upper-level wind pattern when interaction with the westerlies begins.
E. Movement of the TC away from a predicted northward track. Obviously, if the TC fails to attain enough latitude in a presumed period of time, the expected interaction with the westerlies will be delayed or never occur.
F. Interaction with a tropical upper-tropospheric trough (TUTT). If a strong TUTT lies between the TC and the main belt of the westerlies, interaction with the westerlies may be denied. The TUTT may weaken the TC (vertical wind shear); the TUTT may also slow the TC as they interact.
G. Interaction with another TC or other warm-core cyclone may cause one or both cyclones to rotate about a common point at the expense of moving toward interaction with the mid-latitude westerlies as would be expected in a single cyclone situation.
H. A significant displacement of the low-level (surface, 850 or 700 hPa) westerlies northward of the upper-level westerlies. In such cases, the surface center must overcome the influence of the low-level steering, which is often in opposition to the upper-level steering, before the TC can accelerate. If the upper-level circulation does not shear away from the low-level center and if the TC continues the drift northward, then the acceleration will tend to be gradual until the effects of low-level steering have been overcome. Table B-4 can assist in blending in this gradual acceleration.
I. Nonrepresentative analyses and/or prognostic data might produce the wrong interpretation of the upper-level wind regime and affect the viability of a northward-moving track.
If acceleration appears to be occurring much earlier than predicted (more than one degree latitude south), double-check the data and the TAPT results. If the reason for acceleration is not known, only apply further acceleration if it is certain that the acceleration is in response to mid-latitude westerlies and not due to normal fix accuracies.
If acceleration does not occur as predicted (TC is located more than one degree north of predicted latitude) and cannot be explained by one or more of the items (1-9) above, phase in the acceleration to reach the predicted location by 30 hours. If the TC exhibits strong shearing of its upper-level circulation, then only a modest (12 to 24 hours) acceleration should be forecast if the low-level steering favors a continued northward track.
If the TAPT results appear to be questionable, i.e., too low a latitude, check the results against the climatological recurvature prediction for the month. If there is a great difference (more than five degrees), the forecaster will have to make a decision consistent with the data at hand.
If it appears that there are no limiting factors to the TAPT results, then the forecaster is encouraged to apply the technique, consistent with good forecast judgement.
***** End of APPENDIX B *****
(HTML Revision date: 4 March 1999)
Main
Chapter 4
Section 3
Appendix B