Anatomy of a Sneaky Land Breeze – Charleston Coast 6/12/13

~—Weak front pressing offshore with High pressure set up in the NE GOM.

                         -—Recipe for zonal W/WSW flow.

—~Weak Lee side trough developed Tuesday and models suggest will redevelop and strengthen into Wednesday.

                         -—Generally, conducive to increased SW flow.

850mb Analysis Reveals:

~Decent W/WNW flow just aloft late Tuesday but fading into Wednesday AM.

~Also note inland trough looks more pronounced Wednesday AM even @ 850mb level.

So thus far just judging by surface and upper air analysis my guess is you are thinking that the AM period will show with mild SW or may W wind that goes SW about as soon as the sun comes up.  So now that we have an idea of what we think looks likely let s look at some guidance.

3 consequetive runs of NAM 12km show SW flow along the coast with light and variable flow just inland.

The 00z run of 15km WRF shows light SW/WSW flow during predawn hours,with trough easy to pick out of to the W.  More or less inline with the 12z and 18z runs.

 

I was not able to hunt down archived images of the higher resolution WRF 5km and NAM 5km, but CMC GFS and NAM 5km all suggested more or less similar SW/WSW flow between 6-12kts during the predawn and morning hours.  The lone 00z dissenter was the WRF 5km, which suggested WNW flow around 10kts for the morning hours.  So if you are like me you looked at the surface analysis, trends, upper air and model guidance, a say to yourself well looks like that SW flow is gonna stay with us for tomorrow (which it does later). But lets looks at what really happens.

 

So what had happened??? Maybe we missed some clues….

 

00z observations at KCHS (airport inland)

T~78F      Td~74F     Rh~87F

Early morning visible satellite and upper air sounding reveals clear (drier) air has slipped into the coastal region overnight. Which suggests dry 700-850mb air was able to work its way to the surface in the form of a light W/WNW wind. But what was the mechanism??? Its early AM so no convective mixing, no real turbulent or frictional mixing???

I would contend:

~Evening convection was able to generally stabilize/cool airmass earlier than otherwise would have occurred overnight.

               – This also led to a less defined inland trough after the air mass was turned over, reducing the SW gradient.

~Lee trough was over strengthened by models leading to steady SW/WSW flow outputs.

               -WRF 5km only model that got it right, possibly b/c of its Advanced Convective Parameterization Scheme.

~Cooling by evening rain gives a land breeze circulation a head start.

~Dry WNW flow at 850mb just waiting to hop in to the party, which possibly made flow more persistent till midday-ish.

Would I catch it if i was on duty??? I wasnt on duty but wanted badly to go offshore and kill something, and had been watching the weather like a hawk.  Cancelled the trip at 1AM because I could not convince myself that it would be nice enough to go in the 20ft boat.  Woke up at 9:30AM to see these delightful buoy reports. DOH!!!!

Capers Nearshore 8NM NE Charleston Harbor

#YY  MM DD hh mm WDIR WSPD  
                 degT m/s

2013 06 12 12 00 290  3.0

Edisto Buoy 40NM SE Charleston Harbor

#YY  MM DD hh mm WDIR WSPD GST  WVHT   DPD   
                 degT m/s  m/s     m   sec

2013 06 12 14 00 270  3.0  4.0    1.1  10