Rain in the Pacific Northwest

December 8th, 2015 |
GOES-15 Band 3 Water Vapor (6.5 µm) imagery for 6-8 December 2015 [click to play animation]

GOES-15 Band 3 Water Vapor (6.5 µm) imagery for 6-8 December 2015 [click to play animation]

Persistent southerly to westerly flow over the Pacific Northwest has allowed a series of disturbances to produce rain each day so far this month in Seattle, including a record rainfall on December 7th (a day that was particularly dark, given the extensive clouds and the low sun angle: Link). (Update: Seattle also had record daily rainfall on 8 December.) The GOES-15 Water Vapor animation, above, from 0000 UTC on 6 December through 2100 UTC on 8 December (available here as an mp4 file) shows that series of impulses on the south side of a strong circulation in the Gulf of Alaska (Surface Analysis), and later scenes suggest a jet extending to the southwest. MIMIC Total Precipitable Water, below, for the 72 hours ending at 1700 UTC on 8 December, shows the signature of an atmospheric river moving moisture towards the Pacific Northwest coast, auguring rain. The broad southerly and southwesterly flow has meant above-normal temperatures as well (Seattle recorded a daily record high maximum — 60 F — on December 8).

MIMIC Total Precipitable Water for the 72 hours ending 1700 UTC on 8 December 2015 [click to enlarge]

MIMIC Total Precipitable Water for the 72 hours ending 1700 UTC on 8 December 2015 [click to enlarge]

The Aqua Satellite, carrying a MODIS (MODerate resolution Imaging Spectroradiometer) instrument, overflew the Pacific Northwest at about 2045 UTC on 8 December. The MODIS instrument senses radiation at 1.38 µm, a water vapor channel that is particularly sensitive to Cirrus Clouds (GOES-R will also detect radiation at this wavelength). The toggle below, between the 1.38 µm near-infrared and the 0.64 µm visible, shows that the storm in the Gulf of Alaska and the jet moving in from the southwest are prolific cirrus producers! Cirrus is present almost everywhere. The cirrus channel on MODIS is dark (that is, it is not detecting upper-level clouds that are inferred to be cirrus) only over the central Plains, over the Pacific Ocean west of California/Oregon and over parts of north-central Oregon and western Washington.

Aqua MODIS Cirrus Channel (1.38 µm) and Visible Channel (0.64 µm) imagery, 2045 UTC on 8 December 2015 [click to enlarge]

Aqua MODIS Cirrus Channel (1.38 µm) and Visible Channel (0.64 µm) imagery, 2045 UTC on 8 December 2015 [click to enlarge]

One Response to “Rain in the Pacific Northwest”

  1. This event has produced some large precipitation amounts at our OLYMPEX (olympex.atmos.washington.edu) gauge sites. One site at ~2000′ elevation received 150 mm (6+”) in 12 hrs and is at 165 mm and still going. Other sites in higher terrain receiving similar amounts. Gauges in the Chehalis river basin are at 50 – 100 mm (and still going) and along the coast 35 – 45 mm. Major flooding expected at multiple sites. With multiple waves along the large band of moisture, this event could produce event totals in the 300 mm range at the high terrain sites.

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