John Pethick

Hi all

Just wondered what people thought of this mornings low level wind
forecasts in the Gippsland area on Mesolaps 050, mesolaps and laps at
18z.

Essentially in the Sale area the models had for the 2-3000ft ranges
speed forecast of 20 to 30 knots, stronger in Bass St.

The observed winds here at 18z were 1000ft 28020, 2000 26036, 3000 25040  4000 24036, 5000 24034kts.
(a later balloon flight here due to the inability of the Bureau to provide relief Technical Officer Observing staff and a 3 man roster is in force until 6 May)

Any comment on why the underforecast occurred, or is this just a local
phenomenon caused by funnelling under the inversion and horizontal
constriction through the Latrobe Valley?  perhaps there is a point out
east where ther may be a hydraulic jump or a breakdown point in the flow
where it becomes chaotic flow.

One of the probelms here with observed speeds of this magnitude is
whether the flow is laminar or chaotic and whether to forecast severe or
moderate turbulence.  The tail of the front/trough system emanating from
the low east of Tasmania seems to be the mechanism responsible for
tightening the gradient and I expect it to ease later.
Comments welcomed.

Regards

John Pethick
East Sale

John McBride

Hmm.... I have nothing clever to say offhand, other than the fact that
this morning's was an unusual kind of front for summer, with its east-west
orientation and its very weak surface structure (mainly just a slight
shift in wind direction).

Is the underforecast that you mention a regular feature or does it happen
often; and if often, is it in frontal situations mainly?  If the reason
was Latrobe-Valley funnelling it would occur in a range of low level
westerly situation, I would imagine.

Anyway, this is all from my hip, I haven't had a proper look at the
charts yet today.

cheers

John McB

John Pethick

Hi all again

The subsequent wind flight here at Sale has seen the low level winds
drop to 100ft 26022. 2000 25026, 3000 24026, 5000 25026, 7000 22524kts

Perhaps this feature is a result of wind trapped between the surface and
subsidence inversions, and is then spread through a deeper layer once
the surface temp rises.

Obvioulsly in conjunction with the pressure gradient weakening as the
trough moves away. Never the less the models did not forecast it at 18z as far as I could
see.

Regards

John Pethick

John Pethick

Hi John & all others

It is a regular feature in a lot of forced west/southerly flow synoptic
situations, and it usually evens out about 23z as per today.

It would be nice to have had a sonde at 18z to check the thermodynamic
aspects.

Milton Speer

FYI,
In NSW we issued a strong wind warning yesterday for 25/33 knots for our
Mid-north coast and southwards valid for today based on MesoLaps winds
(yesterday morning's run, 0.9974 level), but based on last night's run which we also
followed, that wind warning was reduced to the area south of Moruya. In
reality, there was no dome of strong winds observed just behind the front as often
occurs, especially at this time of year.

- the plot thickens!!

Milton