By WeatherFlow Forecaster

This blog is meant to highlight a scenario where a early AM land breeze was not forecasted by the majority of model guidance nor by the human forecaster. We will examine the synoptic and mesoscale features for the area, during the period leading up to the land breeze event in order to determine if there were enough clues to tip us off even with incorrect model guidance.



Synoptic Regional Analysis Reveals:
~—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.
NAM12z NAM06z NAM00z

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

00z 15km WRF @ 09z
00z 15km WRF @ 09z

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.

Fort Sumter Range Light 6/12
Fort Sumter Range Light 6/12

Type Value Time #
Wind HI 13 kts 4:25 PM 288
Wind LO 1 kts 4:25 AM 288
Wind Gust HI 15 kts 4:10 PM 288
Wind Gust LO 2 kts 4:25 AM 288
Air Temp HI 87 °F 12:10 PM 288
Air Temp LO 77 °F 6:20 AM 288
Pressure HI 1019 MB 7:45 AM 288
Pressure LO 1012 MB 6:20 PM 288
Water Temp HI 0
Water Temp LO 0
Wave Height HI 0
Wave Height LO 0
Wave Period HI 0
Wave Period LO 0
Total observation count 288


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


Screen Shot 2013-06-26 at 2.50.52 PM

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
                 degT m/s  m/s     m   sec
2013 06 12 14 00 270  3.0  4.0    1.1  10