By WeatherFlow Forecaster

Early Spring Fog/Marine Layering along the SC coast

Tim Kent


(sorry in advance for strange layout of text captions)

First Visible Image of the Day

Screen Shot 2014-03-23 at 11.40.15 AM

Low Level fog clearly visible along the coast below some upper level cloud streaks

Synoptic Setup for the day

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Zonal flow (Westerlys) with High pressure over the NE GOM and a slow moving front approaching the coast.  Most models predicting fairly light zonal or SW flow.

Models on track with Observations

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Models do a pretty good job with the wind field and the NWS was all over the potential for a widespread fog event in the Saturday pm discussion.  With very light winds showing at 5AM and multiple observations confirming I took it as a given that Fog was fairly widespread while still in the dark.

Surface Fog and Sea Fog

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Fog has burned off over land by lunch time and westerly flow has advected much of the Sea fog into the open Atlantic.  Early morning would have revealed a near continuous fog bank from land to sea (seen in the 1st image)

Moist Low Level Air over Cold Surfaces

-Dewpoints in the 50s to low 60s overnight were easily met by moderate cooling inland overnight.

-At the same time light 850mb W/SW winds allow solid Sea Fog to develop over coastal locales.
-Inland surface fog burned off once the Sun rose and good compressional heating from zonal downslope flow began.
How the Winds were effected for the day

-NWS forecasted for fairly solid prefrontal SW/WSW flow in the low to mid teens despite model consensus depicting considerably lighter flow.

-We know that fog and particularly Sea fog will decouple the atmosphere and not allow stronger warm winds aloft reach the Surface.

-So when the sun rose and we were able to confirm the solid fog presence the more conservative model solutions are easy to side with

Dewpoints and Wind direction play a critical role

-Dry air with Dewpoints less than surface water temp not favored for fog development
-Wind directions that allow for longest residence time of air over Cold shelf and coastal waters are most efficient fog producers
             –However in this scenario we saw light offshore/sideshore flow overnight which still produced widespread fog and allowed Sea Fog to settle nearshore early on before advecting offshore.
This was a interesting scenario because:
-Two separate but continuous fog banks early in the day.
-Also, serves as a early season reminder of how that cool coastal water can kill warm moist flow at the surface.
Moral of the story:
–Keep an eye on dewpoints and wind direction during Warm Air Advection as these greatly determine strength of winds at the surface.