Southerly Low Level Jets Hampered by Marine Layering

Forecast Blog by WeatherFlow meteorologist Shea Gibson 11/6/13

The last 2 weeks of October 2013 showed a few blasts of cold air spilling down into the SE Region as Northerly events unfolded in sequence, with resulting Sea Surface Temps falling to upper 60’s to NC down to northern SC, and low 70’s from Myrtle Beach and southwards into GA. November 1, 2013, a strong cold front approached the SE Region from the west. Central pressure was occluding and situated over Quebec at ~976mb @10Z EST with a healthy convergent frontal boundary draped all the way down the country and over into the western Gulf.  This would typically show strong low level jetting along pre-frontal build, as GFS (with WW3/WW3 0.5), NAM12k and NAM5k models were forecasting upper teens to mid 20’s along the SE Coast, and higher northwards.  Boundary layer decoupling kept winds from fully mixing as air became more stable at the cooler water surface. CMC 0.6 downplayed this one by a few knots and seemed to get closer to what actually occurred, but it is uncertain that they specifically picked up on the possibility of marine layering to occur all along the SE Region. Will have to watch future trends to prove this observation.

This illustration shows predicted outlook as front approached:

And the 300mb stream:

Areas north of Chesapeake Bay showed most consistent model forecast speeds. Areas from Chaesapeake, Virginia and southward is where the decoupling occurred.

It appears that short wave radiation was trapped near the surface under a thick blanket of stratocumulus clouds with shallow cumulus underlying from inland. Most cloud cover turned into debris and dissipated as it reached out over the Atlantic – this did provide breaks and very small gaps of sunshine along the immediate coast from time to time; however – it further diminished speeds locally the warmer it became.  Wind speeds did better with heavier cloud cover and even better with additional lower cumulus entry overhead. Air temps reached upper 70’s but never broke 80 without sunshine. Dewpoints were forecast to stay in the upper 60’s in Charleston, SC, but that number peaked in the low 70’s during the afternoon, keeping relative humidity levels a little high – not quite enough for a marine visibility statement to be issued. After looking at webcams up and down the outer coast, this seemed to occur from Jekyll Island, GA all the way into northern NC. Sounds and Bays excluded of course as inland wind speeds generally reached their potentials.

You can see the thin layer over the water here at Breach Inlet, SC.  No precipitation was present at the time.

Here is the forecast for Georgia and what occurred – both areas dumped just at midday and tried to build back up, showing UP’s and DOWN’s – with Jekyll holding just at or below and Tybee struggling to hold elevations through the higher period.

This is Jekyll Island, south Georgia:

And South Tybee Island, northern Georgia:

Here is the forecast for SC and what occurred – with UP’s and DOWN’s  struggling to hold elevations through the higher period.

Here is the Isle of Palms pier:

And Ft. Sumter Rear Range Light (Charleston Harbor) – which showed some promise for consistency then unbuckled and became inconsistent in peaks and valleys. Overall- this area got the speeds right for a total average – good call on the harbor Tim!

Here is the forecast for SENC and what occurred – also struggled with UP’s and DOWN’s  through the higher period.

This is Oak Island, which bets represents the Cape Fear area (central SENC)

This is from Fort Macon, next to Cape Lookout (northern SENC) – fell well below values.

This is from Cape Lookout, (northern SENC) – which started out well and gradually fell. Picked back up as sun went down and boundary drew in close.

Sandbridge, VA seemed be the area where things changed overall with just some inconsistency…with areas north of the Chesapeake seeing surface winds escaping any decoupling issues and ramping up just at forecast models.

Sandbridge:

 Cape Charles (Chesapeake) showed peaking then gradual downward trend with one push as boundary passed over:

Ocean City South Beach, NJ – finally showing consistency all the way into NYC  – you can clearly see exactly when boundary cleared the coast around 12Z:

Wrap-up: Overall – this was a tough call for us as we predicted this possibility within the Extended Forecast from a few days out. Mentions of it did hold value as flow was altered to show lower numbers throughout the region. Archiving of this data will be useful in predicting future occurrences with approaching strong cold front boundaries when SST’s drop below certain temps and pre-frontal warm air tries to mix down – as that is key in catching these events with seasonal climate shifting, whether it be summer–>fall or winter–>spring.

Future prediction: As air temps and water temps cool down in unison into the winter, we should see Southerly wind events begin to match forecast model predictions ahead of fronts provided that warm air doesn’t elevate too far above water temps. Adversely, advection layering could occur where air temps are significantly lower than SST’s.

Cheers,

Shea Gibson

WeatherFlow Forecast Team

SE Region/East Coast