Two rain watches for Hawke’s Bay

MetService has issued two weather watches for Hawke’s Bay through Saturday and the Regional Council is monitoring the situation. They are for possible severe weather Friday and Saturday.

The possible Friday afternoon rain locations are still quite variable. The three scenarios show heavy rain in Wairoa, or Hawke’s Bay around Napier, or Central Hawke’s Bay and the southern coast.

The best option is to plan for any of these scenarios.

What MetService issued:

Severe Weather Watch

Issued by MetService at 8:41 am Wednesday 22-Feb-2023

• Heavy rain from Otago to Hawkes Bay
• A frontal system over the South Island is moving northwards to reach the central North Island tomorrow (Thursday), bringing rain with heavy falls.
• Rain should ease over the South Island later today, but this Watch now includes Wairarapa and Hawke's Bay. Hawke's Bay, in particular, could see an extended period of rainfall, and this Watch is likely to be upgraded to a Warning.
• People are strongly urged keep up to date with the latest forecasts, in case changes are made, or further areas are added.

Heavy Rain Watch

• Area: Hawke's Bay
• Valid: 48 hours from 12:00 pm Thursday to 12:00 pm Saturday
• Forecast: Periods of heavy rain. Rainfall amounts may approach warning criteria.

The charts show about 100 mm over 3 hours in CHB and southern HB on Friday around 7 p.m.

Most other time periods show scattered showers.

River level summaries

These are based on several scenarios – impacts from the Friday rain are still quite variable.  A more accurate river forecast will take place on late on Wednesday or early Thursday.

Wairoa – Forecast rain is not significant for this large catchment, however, showers may cause smaller rivers to rise.  Very unlikely to be significant rise in major rivers.  Levels expected to stay below alert levels.

Esk – forecast up to 80 mm rain over 24 hours in upper catchment.  Will cause small rise in river, very unlikely to be at berm level.  Caution for areas where river has cut new channel.  Minor surface flooding possible from rainfall.  Caution needed around silt deposits becoming saturated.

Mangaone – Rissington to Dartmoor Road – up to 70 mm rain over 24 hours.  Likely to see Mangaone rise about 0.5 m by 12 noon Thursday.  If rain is persistent, may rise another 0.5m by early hours Friday Feb. 24.  Caution needed around bridge rebuilds.   If additional rain occurs all day Friday, expect river rises of an additional 1m in the afternoon to evening on Friday.

Tutaekuri – Dartmoor, Moteo, Puketapu – up to 70 mm rain over 24 hours.  Likely to see Tutaekuri rise about 0.5m to 1m by early hours Friday Feb. 24.  This is still within the berm levels, i.e. water is expected to stay in main channel, however, caution is needed if there has been a new channel formed between the main channel, through the berm and to any location of stop bank breach.  Mapping of these locations is continuing.  This water level rise is unlikely to cause any significant depth of water over any floodplain area if it were to escape the main river channel.  If additional rain occurs all day Friday, expect river rises of an additional 0.5 m to 1m in the afternoon to evening on Friday.

Ngaruroro - up to 70 mm rain over 24 hours – Likely to see Ngaruroro at Fernhill rise about 1m by around 7am Friday Feb. 24.  Present estimate is for water to remain in main channel, with a small chance to rise to berm level.   Water is unlikely to cause any significant depth of water over any floodplain area if it were to escape the main river channel.

At the Ngaruroro River mouth, including Awatoto, the rise in water level is likely to be less than 1m.  If additional rain occurs all day Friday, expect river rises of an additional 1m in the afternoon to evening on Friday.  High tide at Napier is around 10am, low at 4pm.

Upper Tukituki – Scattered showers on Wed. to Thursday, with additional rainfall of up to 100 mm over 3 hours on Friday at 19:00.  The Thursday rain will cause minor river level rises, unlikely to be significant, likely to remain below alert level.  Friday rain is likely to cause short term spikes in minor rivers, and then a minor rise in the major rivers.  Caution needed around stop bank breach areas if the river has carved a new channel to the breach location.

27 February 2023

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