A First Look at the Big Day Forecast

I am in the middle of a 13-day work binge. Pretty sure that's the longest stretch of consecutive work days for me at the station. It will all be balanced out when I take 9 days off starting Wednesday of next week. For those who may not be aware, I am getting married on September 3rd. So the time off will all be related to that.

There have been a few people asking if I am nervous for the ceremony. I'm not really nervous (at least not yet) for the vows. Rather, I am more nervous about the weather cooperating as we are having an outdoor ceremony. As always, best to have a "Plan B" which we do. That involves cramming about 130 guests into an outdoor tent and doing the ceremony/reception all in one place. Luckily, climatology says that the likelihood of a "rainout" is very slim. The average temperature in Portland on September 3rd is 79 degrees. Our wedding will be in Oregon City which typically runs a degree or two warmer than Portland. The last 5 years have seen an average of about 82 degrees with nothing but sunshine.

It appears our hottest stretch of weather this year just wrapped up. It's going to be tough to see more triple digits the later into summer we get. We've entered a stretch of below-average temperatures with a lot more cloud cover now as we begin the second half of August. I'm wondering how long it will last and will it impact the wedding. Portland hasn't seen any measurable precipitation in 31 days but that's not even half of our longest stretch, which was 66 days set in 1967. The GFS model run is finally showing the first glimpse of what we might expect for September 3rd.

  

Here is the 18z (11 A.M.) snapshot on the 3rd. This is the 500mb chart, giving us an idea where systems are and might be headed. The wind barbs show a strong "on-shore" flow, air will be coming off the ocean, thus giving us a cooler air mass.A westerly flow in late summer typically means that we start off cloudy as the marine air surges inland. This is all behind a system that comes onshore a day or two before the wedding that may produce some rain. Models that predict cloud cover do not go out this far, so using the map above I would say that we are looking at a somewhat cloudy start to the 3rd. Does that mean it will rain?

Above is the graphical precipitation output from the GFS model for the 3rd. It leaves us high and dry around 12 noon on Monday. Numerical numbers also back the maps.

FCST Hour	Valid Time	Max Temp	Min Temp	Td	10m Wind mph	850mb Wind mph	Total Precip(")	Conv. Precip(")	500-1000 THKNS	500mb Height	850mb Temp °C	500mb Temp °C	MSLP mb	Total Cloud Cover
192	Wed 08/29 06Z	61 °	58 °	58 °	NNW 4	SW 4	0.00	0.00	564	578	13 °	-12 °	1016	0 %
204	Wed 08/29 18Z	59 °	56 °	57 °	SSW 11	SSW 16	0.00	0.00	555	566	8 °	-16 °	1013	3 %
216	Thu 08/30 06Z	60 °	57 °	56 °	SSW 13	SW 20	0.01	0.00	552	564	6 °	-19 °	1014	10 %
228	Thu 08/30 18Z	58 °	57 °	42 °	W 7	W 11	0.06	0.04	551	566	5 °	-17 °	1018	44 %
240	Fri 08/31 06Z	58 °	57 °	33 °	NW 7	NW 9	0.01	0.01	553	571	5 °	-15 °	1022	15 %
252	Fri 08/31 18Z	59 °	57 °	34 °	N 7	N 11	0.01	0.00	555	576	7 °	-13 °	1024	69 %
264	Sat 09/01 06Z	59 °	56 °	47 °	N 18	NNE 9	0.00	0.00	559	579	9 °	-13 °	1023	15 %
276	Sat 09/01 18Z	58 °	55 °	54 °	N 18	NNE 11	0.00	0.00	560	578	11 °	-13 °	1021	2 %
288	Sun 09/02 06Z	60 °	56 °	57 °	NNW 20	N 7	0.00	0.00	561	577	12 °	-13 °	1019	0 %
300	Sun 09/02 18Z	59 °	55 °	56 °	N 16	NW 11	0.00	0.00	558	575	9 °	-14 °	1021	12 %
312	Mon 09/03 06Z	59 °	55 °	54 °	N 16	NNW 11	0.00	0.00	557	575	8 °	-15 °	1021	12 %
324	Mon 09/03 18Z	57 °	55 °	45 °	N 11	N 11	0.00	0.00	557	575	8 °	-13 °	1022	25 %
336	Tue 09/04 06Z	59 °	55 °	39 °	N 20	N 11	0.00	0.00	559	576	10 °	-13 °	1019	26 %
348	Tue 09/04 18Z	58 °	55 °	39 °	N 18	N 11	0.00	0.00	558	574	9 °	-14 °	1019	2 %
360	Wed 09/05 06Z	59 °	55 °	51 °	NNW 20	N 9	0.00	0.00	558	574	10 °	-14 °	1019	2 %
372	Wed 09/05 18Z	58 °	55 °	55 °	NNW 16	NNW 11	0.00	0.00	556	574	9 °	-15 °	1021	5 %
384	Thu 09/06 06Z	59 °	55 °	56 °	N 16	N 11	0.00	0.00	559	576	10 °	-14 °	1020	8 %

I have highlighted the precipitation (yellow) and mid-level air temperature (red) columns above. This model showing zero rain. That's good. It is also showing an 850mb air temperature of 8 degrees Celsius. We use the temperature at 850mb (that's about 5,000 ft) to help translate a surface temperature. On average, 850mb temperature of 8 degrees Celsius translates to a surface temperature of about 71 degrees Fahrenheit at the surface. That is a bit cool considering the average high for that day is some where around 79 degrees F. You may notice the temperature output for that day is 57 degrees F! This model is notoriously bad at temperature forecasting but has strengths in other areas.


These charts give us a good idea of what ALL model runs are doing with temperatures. Each model runs several different parameters and these charts compile all the 850mb temperature outputs and compares them against average (green line). The red line is the average of ALL those runs, and the blue line is the operational (published) model run.
 The first model is the ECMWF (European model). The red arrow indicates the 3rd. Not surprised to see such a wide variety of temperatures this many days out. However, the average of all the models looks to be pretty close to climatological average. One model goes as high as 22 degrees C, which would translate to 85-90 degrees F at the surface! The Euro also goes as low as 4 degrees C which would mean

 Let's look at the GFS, my personal favorite model. September 3rd is indicated by the black arrow this time. GFS is thinking that things will be a bit cooler, indicated by all model averages (red line). Still plenty of variation and even more time to go for things to change. But this chart kind of supports what we saw on the numerical output with cooler than average temperatures on the 3rd.

Mind you, we are still some 320+ hours away from the wedding and weather is ALWAYS changing. But if I had to make a forecast guess for the 3rd right now, I'm saying things will be in the mid to upper 70's with partly cloudy conditions by the afternoon. I'll be here to update the forecast as I see things change! Only 13 days away now!