Itâ€™s been a weird old summer so far with the active jet stream we had for most of the winter not really packing up for the summer (it had a bit of a break during April) and its continued to hit us with a steady precession of lows. Unfortunately the jet stream has mostly been aligned with a dip over the eastern US seaboard and another dip over the UK.
This could be described as a bloody nuisance as its meant that lows have raced across the Atlantic and then stopped right on top of the UK bringing bad weather, blown-out surf and general depression all round.
So lets have a look at predictions for the hurricane season 2007:
NOAA (Posted 22 May 2007)
13 to 17 - Named storms
Dr. Gray's Tropical Storm Forecast (Posted 31 May 2007. Next update 1 Aug)
17 â€“ Named storms
UKMET / ECMWF (Posted 19 June 2007)
7 to 13 â€“ Named storms
So not much consensus there then. Anyone know of any other prediction out there? Mystic Meg perhaps?
Dr Grayâ€™s â€˜the daddyâ€™ of hurricane season predication so his 1 Aug update may well be illuminating. Incidentally thereâ€™s quite an amusing and informative recorded presentation of Dr Grayâ€™s here about TS activity and global warming.
http://ams.confex.com/ams/27Hurricanes/ ... 107533.htm
Here are some of the factors that effect tropical storm activity and have thus far kept the tropical Atlantic quiet:
Sea Surface Temperatures (SSTs)
The warmer the ocean gets the higher the probability is of Tropical Storms (TS) forming and the greater maximum theoretical intensity they can reach.
Current SST anomaly charts show slightly above average temps over most of the western tropical Atlantic whereas in the east near the Cape Verde islands is looking a little cool. Overall sea temps will continue to increase for a fair while yet.
https://www.fnmoc.navy.mil/products/NCO ... nomaly.gif
A worrying feature is that the Caribbean and Gulf of Mexico (GOM) have high anomalyâ€™s and can support very intense hurricanes.(see http://wxmaps.org/pix/hurpot.html
). Overall I get the feeling that should these SST trends persist we might see some real nasty hurricanes in the Caribbean and GOM that do a lot of damage but not quite the intensity of Fish (a Fish is a Hurricane that does not make landfall) we might hope for heading our way.
Wind shear is a measure of the difference between winds at various levels of the atmosphere. TS need low wind shear to grow as high wind shear tears them apart. July has seen quite a lot of wind shear in the tropical Atlantic but August often sees this shear relax. You can see some lovely colourful wind shear charts here:
http://www.wunderground.com/modelmaps/m ... &domain=TA
You will need to add the wind shear chart in using the menu. I been looking at this over the past few days and it does seem that there is more of the red (less than 8 m/s) shear predicated as we head into August.
Saharan Air Layer (SAL)
Dust blowing about in the atmosphere coming from the Sahara can inhibit TS formation and growth. Thereâ€™s been a lot of it about recently which is another reason for the quiet season so far.
http://cimss.ssec.wisc.edu/tropic/real- ... java5.html
Not found any way of predicting the SAL activity so itâ€™s a real wildcard factor that could really reduce the number of TSâ€™s we see.
El NiÃ±o Southern Oscillation (ENSO)
From what I can tell the main thing here is that El Nino suppresses TS formation because it increases wind shear over the tropical Atlantic. Neural or La Nina conditions cause less shear and are thus more conducive to TS formation/growth.
The current situation is that we have roughly neutral conditions. Things may go a little La Nina later on in the season.
These are the main factors I know about. There are others such MJO and sea level pressure but itâ€™s all a bit chicken and egg and I donâ€™t really understand it.
I will probably update this thread if I spot any interesting developments.