AXAU01 APRF 311925 IDW27600 TROPICAL CYCLONE TECHNICAL BULLETIN: AUSTRALIA - WESTERN REGION Issued by AUSTRALIAN BUREAU OF METEOROLOGY TROPICAL CYCLONE WARNING CENTRE at: 1925 UTC 31/03/2023 Name: Severe Tropical Cyclone Herman Identifier: 21U Data At: 1800 UTC Latitude: 18.2S Longitude: 106.9E Location Accuracy: within 15nm (30 km) Movement Towards: south southeast (155 deg) Speed of Movement: 4 knots (8 km/h) Maximum 10-Minute Wind: 110 knots (205 km/h) Maximum 3-Second Wind Gust: 155 knots (285 km/h) Central Pressure: 936 hPa Radius of 34-knot winds NE quadrant: 30 nm (55 km) Radius of 34-knot winds SE quadrant: 40 nm (75 km) Radius of 34-knot winds SW quadrant: 50 nm (95 km) Radius of 34-knot winds NW quadrant: 35 nm (65 km) Radius of 48-knot winds NE quadrant: 25 nm (45 km) Radius of 48-knot winds SE quadrant: 25 nm (45 km) Radius of 48-knot winds SW quadrant: 25 nm (45 km) Radius of 48-knot winds NW quadrant: 25 nm (45 km) Radius of 64-knot winds: 15 nm (30 km) Radius of Maximum Winds: 5 nm (10 km) Dvorak Intensity Code: T6.0/6.5/D1.5/24HRS STT:W0.5/06HRS Pressure of outermost isobar: 1008 hPa Radius of outermost closed isobar: 150 nm (280 km) FORECAST DATA Date/Time : Location : Loc. Accuracy: Max Wind : Central Pressure (UTC) : degrees : nm (km): knots(km/h): hPa +06: 01/0000: 18.6S 107.0E: 035 (065): 100 (185): 936 +12: 01/0600: 19.0S 107.2E: 050 (090): 090 (165): 950 +18: 01/1200: 19.4S 107.2E: 060 (110): 070 (130): 968 +24: 01/1800: 19.7S 107.2E: 070 (130): 050 (095): 984 +36: 02/0600: 20.1S 106.8E: 065 (120): 040 (075): 995 +48: 02/1800: 20.3S 106.1E: 085 (160): 035 (065): 998 +60: 03/0600: 20.4S 105.3E: 105 (195): 030 (055): 1002 +72: 03/1800: 20.1S 104.2E: 120 (225): 030 (055): 1003 +96: 04/1800: 19.4S 101.5E: 160 (300): 030 (055): 1004 +120: 05/1800: 18.9S 98.1E: 230 (425): 025 (045): 1008 REMARKS: Severe Tropical Cyclone Herman showing initial signs of weakening but remains a very small and intense category 5 system. While Herman maintains a distinct eye surrounded by very cold cloud tops, recent imagery shows a less defined eye heralding the initial weakening that has been expected. Intensity: 110 kn based upon Dvorak holding CI=6.5 higher than FT=6.0 for initial weakening which is based upon an eye pattern: DT=6.0 White surround (6.0) and LG/W (+0.0) eye adjustment; consistent with adjusted MET on a +1.5/24h trend but short term weakening of 0.5. The analysis is consistent with objective ADT FT/CI=6.0/6.6. Other objective guidance broadly consistent (all 1min mean): SATCON 117kn; ADT (CI=6.7) 132kn; and AiDT=128kn . Note SAR at 1127UTC indicated peak winds of 94kn but this is likely to be ~20kn too low. Structure: SAR and ASCAT B and C passes show gales extending only 30-50nm from centre and radius of maximum winds of just 6nm (from SAR). Motion: Herman is currently being steered to the southeast by the combined influences of the ridge to the east and the mid-level trough but the influence of the trough is expected to weaken as it slips further away. The environment is becoming more hostile to the circulation while shear remains NNE at 15kn with ongoing poleward upper level outflow associated with an upper trough away to the southeast, the peripheral dry air may just be starting to impact the circulation. SSTs are 27.5C. Herman is highly vulnerable to the dry air being ingested into the circulation as it expected to wrap around from the north and hence weakening is expected to be rapid during the weekend. Deterministic and ensemble global model guidance has been significantly underestimating the strength of the system, not surprising given its small size. This has affected the track with some guidance taking it on a more westerly track sooner than what is expected under the influence of a ridge to the south. The most recent deterministic guidance is showing a more likely scenario of continued albeit slower south southeast motion for the next 24 hours before the turn to the west starts to eventuate. Nevertheless Herman is still highly likely to remain well west of the Australian mainland and have no direct impact upon the coast. Copyright Commonwealth of Australia == The next bulletin for this system will be issued by: 01/0130 UTC.