Wednesday, December 7, 2011

More About the Fog

Yesterday's post describes an encounter on Monday with dense fog at Bridgeport (KBDR) and the resulting diversion to Waterbury-Oxford (KOXC). It includes the observation that neither the arrival of the fog, nor its persistence, were effectively incorporated in the Terminal Area Forecasts (TAF's) issued for KBDR.

Today I had an exchange on Google+ on the subject with Scott Dennstaedt. Scott is a former National Weather Service (NWS) research meteorologist and the guiding genius behind AvWxWorkshops.com, an excellent source of weather wisdom. He suggested that a comparatively obscure NWS product, the Area Forecast Discussion (AFD), might have helped me to "crack the code" when I was trying to sort out the effects of the fog using the TAF's.

The AFD's are produced by each Forecast Office (Bridgeport is covered by the office at Upton, NY on Long Island) and each AFD includes an Aviation section. The current AFD for the Upton office (OKX) can be seen HERE. Scott was kind enough to send me the Aviation portions of the AFD's that covered the time of interest on Monday.

As I noted yesterday, at 5:00 AM local time Monday KBDR was already fogged in. The TAF extant at that time was predicting that the fog would clear by 9:00 AM (14Z) while the AFD fretted about fog "over much of the area":

 
350 AM EST MON DEC 5 2011
.AVIATION /09Z MONDAY THROUGH FRIDAY/...
MAIN CONCERN THIS MORNING WILL BE FOG/LOW STRATUS OVER MUCH OF
THE AREA. NYC TERMINALS GENERALLY RANGE FROM IFR TO VFR...
WITH EWR VSBYS AS LOW AS VLIFR. OUTSIDE OF THE NYC
TERMINALS...CONDITIONS VARY WITH VSBYS AS LOW AS 1/4 MILE TO
OVER 3 MILES. WITH FOG CONDITIONS CHANGING...
IT IS DIFFICULT TO FORECAST JUST HOW MUCH CONDITIONS IMPROVE
TO WORSEN HOUR TO HOUR...THEREFORE AMENDMENTS ARE LIKELY
AND SHOULD BE EXPECTED THROUGH 12Z.
The forecaster pointed out that the behavior of the fog was difficult to forecast, which I knew well from experience. Still, the TAF came down on the side of the fog being gone by 14Z.

14Z came and went, with no sign of the fog dissipating. An amended TAF valid at 14Z extended the period where fog was considered likely to 15Z, projecting significant improvement thereafter. The Discussion in support of that TAF was still optimistic about improving conditions, but a new AFD issued just before 15Z – shortly after N631S and I had taken off and flown north – incorporated new pessimism:

 
956 AM EST MON DEC 5 2011
.AVIATION /15Z MONDAY THROUGH FRIDAY/...
A COLD FRONT OVER THE ERN GREAT LAKES WILL TRACK SLOWLY THRU
THE AREA TNGT. RIBBON OF IFR ACROSS THE METRO THIS MRNG.
THERE IS EVIDENCE OF SOME UPSTREAM CLEARING...AND WILL
CONTINUE TO EXPECT VFR THIS AFTN. EXACT TIMING OF THIS
CONVERSION REMAINS LOW CONFIDENCE...AND AMENDMENTS HAVE
SLOWED DOWN IMPROVEMENT.
So the AFD's offered through 14Z would not have kept me from departing, while those from 15Z onward might have inspired second thoughts.

As recounted yesterday, N631S and I arrived at KBDR about 1715Z, missed on the ILS Runway 6 approach (when the weather was 1/2 mile and 200 feet), held for a while and diverted to KOXC. We landed there about 1745Z. By that time the 18Z TAF was out and it called for low IFR conditions (a mile and 300 feet) through until 10 AM Tuesday, except for brief periods of slightly better conditions during the afternoon. The supporting AFD had this to add:

 
1252 PM EST MON DEC 5 2011
.AVIATION /18Z MONDAY THROUGH SATURDAY/...
A COLD FRONT OVER THE ERN GREAT LAKES WILL TRACK SLOWLY
TOWARDS THE AREA TNGT. AREA REMAINS SOCKED IN WITH LLVL
MOISTURE THRU THE TAF PERIOD. TAFS INDICATE MAINLY IFR...
HOWEVER SOME BREAKS TO VFR ARE EXPECTED THIS AFTN. THIS
HAS BEEN HANDLED WITH A TEMPO. CIGS LOWER AGAIN
TNGT. SOME -RA POSSIBLE TUE AS THE FRONT GETS CLOSER.
CIGS/VSBY FCST THRU 00Z REMAINS LOW CONFIDENCE...IN
PART DUE TO MID CLOUD COVER OBSCURING LOW CLOUDS ON STLT.
This discussion certainly casts a shadow over the TAF. In fact, the ceiling at KBDR went up to 400 feet for about 1/2 hour mid-afternoon (long enough for N631S and I to make an unsuccessful attempt to sneak into Bridgeport) and promptly went back down to 200 feet, there to stay overnight.

The Area Forecast Discussions are a useful tool, giving insight into the forecaster's thoughts and level of confidence. But having reviewed them, I don't think that they would have changed any of my decisions on Monday. The forecaster's multiple references to "low confidence" in aspects of the TAF's do reinforce the notion that the behavior of fog is hard to predict.

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