It’s late April, but we’re already getting a typical Los Angeles basin weather trend that we call “May Gray” and “June Gloom” (since that’s usually when it is at its worst). When we are experiencing this, we get low clouds and visibility that may or may not “burn off” later in the day. This means a lot of delayed or cancelled flights for private pilots and students and can be bad enough to ground even IFR pilots.

I woke up this morning to drizzle. This means a cloud layer that may be 3000 feet thick. I check PIREPs and see base and top reports that confirm that:

SMO UA /OV SMO270005/TM 1538/FL037/TP BE58/SK OVC008-TOP037
POC UA /OV POM/TM 1525/FL043/TP UNKN/SK TOP043

Thicker clouds usually translate to a later burn off. This is reinforced by the TAFs, with Burbank forecast to improve from very low overcast to low broken then to higher scattered clouds finally in the late afternoon and Ontario showing broken clouds all day

KBUR 231522Z 2315/2412 00000KT 2SM BR OVC006
FM231700 15007KT 6SM HZ BKN015
FM232200 18010KT 6SM HZ SCT020
FM240400 16006KT P6SM OVC015

KONT 231124Z 2312/2418 VRB05KT 5SM BR OVC012
TEMPO 2312/2315 2SM -DZ BR OVC009
FM231700 VRB05KT P6SM BKN020
FM231900 26012KT P6SM BKN025
FM240400 VRB05KT P6SM BKN025
FM240800 VRB04KT 3SM BR OVC010
FM241600 VRB05KT 5SM BR SCT025

Current conditions at El Monte would preclude any sort of VFR flight and would be low enough that IFR pilots might not be able to get back into El Monte until conditions improve:

KEMT 231447Z 19005KT 2SM BR SCT000 OVC009 A3002 RMK BR SCT000

Using the TAF for BUR and ONT, we might see improvement by around 1000 local. One thing that pilots will quickly learn is that the timing of burn off can be very hard to forecast and that it can be off by hours. When burn off comes, it can be very quick. Overall, during these periods, planning for afternoon flights is safer, otherwise, expect to do some weather watching and going when conditions improve enough.

Want some scientific backing on why this happens?

UPDATE:
Now we can look back and see how the forecast fared. Answer? Not too well. EMT wasn’t usable VFR until almost 1400 local. Even then, it might be ok for pattern work, but not really for going anywhere:

KEMT 232147Z VRB05KT 5SM BR BKN020 OVC040 A3000
KEMT 232047Z 20005KT 5SM BR BKN018 OVC040 A3004
KEMT 231950Z 20005KT 5SM BR BKN012 OVC040 A3003
KEMT 231848Z 20005KT 3SM BR SCT000 BKN012 OVC040 A3003 RMK BR SCT000
KEMT 231749Z 00000KT 3SM BR SCT000 BKN009 OVC040 A3004 RMK BR SCT000
KEMT 231647Z 21005KT 1 1/2SM BR SCT000 OVC008 A3001 RMK BR SCT000
KEMT 231447Z 19005KT 2SM BR SCT000 OVC009 A3002 RMK BR SCT000