Sunday, October 3, 2010

Another Rainy Monday

Looks like another rainy Monday morning in store. At left is a clip from the 24-hour forecast map from the NCEP site, valid at 12Z. As I plan to depart KVKX (in the DC area) about that time headed for KBDR (Bridgeport, CT), I anticipate a damp flight. From the position of the low I also expect headwinds due to the cyclonic flow.

For departure conditions, lets look at the Terminal Area Forecasts (TAF's) for Washington National (KDCA) and Andrews AFB (KADW).

KDCA 031739Z 0318/0418 04012KT P6SM BKN028 BKN040 OVC060 
     FM032000 02010G17KT P6SM -RA OVC025 
     FM040100 02011G18KT 6SM -RA OVC020 WS020/04035KT 
     FM040500 36015G21KT 5SM -RA BR BKN012 OVC020 
     FM040900 35013G20KT 4SM -RADZ BR OVC008 
     FM041400 33012G20KT 5SM -RA BR OVC009
KADW 0317/0417 03010G15KT 9999 BKN020 OVC030 QNH2997INS 
     TEMPO 0317/0320 SCT020 OVC030 
     BECMG 0405/0406 36012KT 4800 -RA BKN010 OVC020 QNH3000INS 
     BECMG 0412/0413 35012KT 9999 NSW BKN020 OVC030 QNH2998INS
The pertinent part of the DCA forecast (the line starting "FM040900", meaning "From 09Z on the 4th") predicts winds from the north at 13 knots gusting to 20, four miles visibility in light drizzly rain and mist and an 800 foot overcast, continuing through 14Z.

The Andrews forecast predicts conditions becoming, between 12Z and 13Z, wind of 12 knots from the north, good visibility with "no significant weather" (NSW) and a broken ceiling at 2,000 feet, overcast above at 3,000. This is one where I hope the Zoomies have it right!

In either case, the forecast conditions are good enough to depart. How about the arrival a couple of hours later?

KBDR 031720Z 0318/0418 04014G20KT P6SM SCT040 
     FM031900 04014G22KT P6SM BKN035 BKN100 
     FM040500 05020G25KT 6SM -RA SCT012 OVC020 
     FM041300 05020G26KT 5SM -RA OVC012

The Bridgeport TAF predicts northeast wind of 20 knots gusting to 26, with five miles visibility in light rain, overcast at 1,200 feet, from 13Z. That's a sporty wind but it's nicely aligned with Runway 6. The ILS approach will be on offer and the high ceiling ought to make for a straightforward arrival.

With rain forecast at both ends of the flight there's a good chance of getting wet en route. It's October now, so the vital question becomes, "Where will the freezing level be?"

The image at left is clipped from the ADDS - Winds/Temps page. It shows where the isotherms (especially the zero degree curve) will be at 12Z tomorrow. This one is for the 725 millibar level - about 9,000 feet. If I file for 7,000 feet I'll be comfortably below the freezing level all the way along my route.

One other item to note. The nice folks at FltPlan.Com are telling me to expect an average tailwind component of +2 knots at 7,000 feet but an average headwind component of -17 knots at 5,000 feet. That is a lot of wind shear! The predicted time for the flight is 2:02 at 7,000 and 2:20 at 5,000. Of course it isn't that simple.

Potomac Approach will want me at 7,000 feet above the KBWI arrivals and New York Approach will, for their own mysterious reasons, want me at 5,000 feet over the top of KJFK. So, I'll file for 7,000 (secure in the knowledge that the temperature will be warm enough to keep water in the liquid phase) and try to convince ATC to let me stay up there as long as possible to avoid the headwind at 5,000.

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