We will show below that the meteorological situation was favourable for an appearence of a geometer/geometeorite. For more suitable work, below are a couple maps of the region.

And more detailed:

Below are hourly maps of some weather data of July 15, including a cloudiness ( most of the data was taken from the University of Washington www-site ). The event took place a few minutes after 7 a.m. GMT (=Z), July 15, 2003.
Maps below show general meteorological situation over USA on that day at:
1 Z
2 Z
3 Z
4 Z
5 Z
6 Z
7 Z
8 Z
9 Z
11 Z
14 Z
17 Z
20 Z
23 Z

An appearence of cloud's cover to the south of the area at about 2 Z is seen, which almost disappeared at about 9 Z.

The situation with the cloudiness with more details is seen on pictures taken by weather radars. In Seattle at 7.02 Z:

at 7.31 Z:

and at 8.59 Z:

And in Portland, Oregon at 4.59 Z:

at 5.58 Z:

at 7.02 Z:

at 7.31 Z:

at 8.29 Z:

at 9.33 Z:

and at 10.02 Z:

GOES-10 satellite infrared (band 4) pictures also show the cloudiness cover on July 15 at:
2 Z
3 Z
4 Z
5 Z
6 Z (or here)
7 Z
8 Z
9 Z
10 Z
12 Z
Also from the pictures it looks like some brighter (i.e. cooler, i.e. higher) clouds appeared later in the region, but I am not an expert in this.
Anyway, there was a lot of water vapor in the region as seen from the "water-vapor" band picture of GOES-10 taken at 8 Z:

Pictures (smoothed and averaged over time period of 6-12 Z) below present more info on water vapor (relative humidity) at pressure levels:
1000 millibar (near ground)
850 millibar (about 1.5 km altitude)
700 millibar (about 3 km)
500 millibar (about 5.5 km)
300 millibar (about 9 km)
An increase of relative humidity in the region is seen at altitudes above about a couple kms.

Possibly the increase is related with the jet-stream, as seen in the map below:
A profiler in the Seattle area illustrates this:

More detailed (but still smoothed and averaged over time period of 6-12 Z ) pictures show the "from-the-south-to-the-north" component of a wind at pressure levels:
1000 millibar (near ground)
850 millibar (about 1.5 km altitude)
700 millibar (about 3 km altitude)
1000 millibar (about 5.5 km)
1000 millibar (about 9 km)
100 millibar (about 16 km)
A strong increase in the region of the "from-the-south-to-the-north" component of the wind above the ground is well-seen.

And now about "from-the-west-to-the-east" component of the wind at pressure levels:
1000 millibar (near ground)
850 millibar (about 1.5 km altitude)
700 millibar (about 3 km altitude)
500 millibar (about 5.5 km)
300 millibar (about 9 km)
100 millibar (about 16 km)
A strong increase in the region of the "from-the-west-to-the-east" component of the wind above the ground is well-seen too.

Here you can read meteorological data of that period of time of the following meteorological stations from the area of the event: KGRF (47 deg 04' N; 122 deg 34' W); KPWT (47 deg 30' N; 122 deg 45' W); KHQM (46 deg 58' N; 123 deg 56' W); KOLM (46 deg 58' N; 122 deg 54' W); KSHN (47 deg 14' N; 123 deg 08' W).
The meteo-stations data shows that in the area, periods of "clear-sky", and "cloudy" swiftlty changed each other.