Overview

After the very dry winter, March was wet…April was wetter…and May was the wettest! Throughout Oregon, precipitation was well above normal. In fact, not a single NOAA Cooperative station (Table 3) reported below-normal precipitation for the month, while some exceeded 400% of normal.

Table 1 is a summary of monthly averages and totals at selected stations throughout the state. Table 2 lists daily temperatures and precipitation for most of the locations listed in Table 1. In Table 3, monthly and seasonal precipitation totals throughout the state are listed.

Basin Summary

Here is a summary of precipitation, water supply, and snow pack as of the end of the month, by river basin:

	Precipitation			Stream Flow		SWSI
BASIN	(1)	(2)	(3)	(4)	(5)	(6)
OWYHEE	226	121	116	265	88	-1.1
MALHEUR	300	118	89	200	47	-1.3
GRAND RONDE, POWDER, BURNT	200	101	81	100	76	-2.4
UMATILLA, WALLA WALLA, WILLOW	196	75	72	59	56	-2.8
UPPER JOHN DAY	209	102	83	110	58	-1.8
UPPER DESCHUTES, CROOKED	269	92	71	41	42	-1.8
LOWER DESCHUTES, HOOD RIVER	222	77	65	62	52	-3.1
WILLAMETTE	186	71	69	107	54	-2.3
ROGUE, UMPQUA	271	88	81	143	58	-1.2
KLAMATH	123	67	79	108	63	-1.9
LAKE COUNTY, GOOSE LAKE	310	136	105	151	78	-1.0
HARNEY	291	120	97	186	98	-0.8
NORTH COAST	161	74	61	114	65	-0.8
SOUTH COAST	204	85	n.a	191	72	-2.2

n.a. Not available
(1) Percent of normal May precipitation, from NOAA Cooperative sites
(2) Percent of normal seasonal precipitation (since Oct. 1), from NOAA Cooperative sites
(3) Percent of normal seasonal precipitation, from Natural Resources Conservation Service (NRCS) SNOTEL sites
(4) Percent of normal May stream flow, from U.S. Geological Survey (USGS)
(5) Percent of normal seasonal stream flow (since Oct. 1), from USGS
(6) Surface Water Supply Index, from NRCS (-4 = very dry, 0 = normal, +4 = very wet)

Forecasts

The Climate Prediction Center’s (CPC) forecasts for June-August appear below. Temperatures for Oregon (and all of the West) are likely to be near normal, except along the coast, where above-normal temperatures are more likely. Precipitation probabilities suggest an increased chance of above-normal rainfall.

The above forecast seems reasonable to us. Oregon Climate Service predicts normal temperatures and above-normal precipitation for the next three months. June should be wetter than average, but July and August are predicted to be warm (not excessively hot) with average precipitation - and since these are the two wettest months of the year, that means not much rain! One thing that we have noticed: when spring rains linger into early summer, the fall is likely to be unseasonably mild and dry well into October.

El Niño update
The Australian Bureau of Meteorology (BOM) suggests that the chances of an El Niño event this year have decreased, and are now only slightly above average. Their June 1 update states:

Sea surface temperatures remain warmer than average in most parts of the tropical Pacific. During the past fortnight there was further cooling in the NINO2 and NINO3 regions, although these indices remain positive. Small areas of cool anomalies have appeared in the far-eastern Pacific.

- Subsurface data for May showed that the spread of warmer than average water into the eastern Pacific has been further eroded by the propagation of negative anomalies into the eastern region.

- The Trade Winds have generally been a little weaker than average during the past two weeks. Westerly wind anomalies have persisted in the western to central Pacific.

- Ten of twelve computer models predict neutral eastern Pacific conditions in October 2005, with two models predicting warm conditions.

NOAA’s Climate Prediction Center reported on June 12:

“ Sea surface temperature (SST) anomalies decreased by more than 2°C in the eastern equatorial Pacific during May. By the end of the month, negative equatorial SST anomalies were observed in most areas between 120°W and the South American coast. The decrease in SST anomalies in the eastern equatorial Pacific during May was reflected by a decrease in the SST anomalies in the Niño 3 and Niño 1+2 regions and by a decrease in the upper-ocean heat content in the eastern half of the equatorial Pacific. A majority of the statistical and coupled model forecasts indicate that ENSO-neutral conditions will prevail during the northern summer (June-August) and fall (September-November). The spread in the forecasts indicates increasing uncertainty during the last half of 2005.”

“Thunderstorms” - or just convection?
Q. I have noticed that all of the weather forecasters on TV and in print, including you, refer to thunderstorms rather than lightning storms. Since thunder can occur only after there has been a lightning discharge, wouldn’t it be more accurate to call them lightning storms? Bert Cleary, Corvallis

A. Technically, a thunderstorm is “a storm that produces lightning, and thus thunder.” Our area gets relatively few, for reasons explained below. In fact, we often hear of thunderstorms in the forecast, yet seldom hear thunder. Is that because the forecasts are really bad?

No, it’s because there’s another definition. A thunderstorm is caused by rising air. This can result when air moves over mountains and rises; that’s why summer afternoons frequently see thunderstorms over the Cascades. Air can also rise following warming from the ground (on a sunny day, for example). This is known as “convection.”

As air rises, it cools, and eventually condenses, forming clouds. If it continues to rise, precipitation will occur. If the air rises quickly enough, and high enough, the ingredients for lightning and thunder may be present: ice crystals and “supercooled water drops.” The latter are drops that remain liquid even at temperatures of 40 degrees below zero. The movement of the ice and liquid water inside the storm produces an electric field that can result in a lightning flash.

But in western Oregon, where I live, we seldom have the right ingredients, so thunder and lightning are rare. Nevertheless, when we get convection, and clouds, and maybe some rain, forecasters say we’re experiencing thunderstorms. Maybe that’s short for “not fully developed thunderstorms” or “unripe thunderstorms.”

In other parts of the country, they get VERY ripe. I was in a doozy last week. I went to Washington, D.C. for meetings. After a cool, wet spring (very similar to our own), summer arrived in DC last weekend. It was over 90 with high humidity. I felt like a wilted flower.

The warm, humid air moving into the eastern US from the Atlantic was simply providing fuel for the development of thunderstorms - real ones! A cold front moved out of the Midwest and into the east, and the combination of cool, dry air and warm moist air creates a violent reaction. Monday evening, powerful storms roared through. The thunder shook the restaurant I was in. Lightning flashed repeatedly. Heavy rains filled the gutters to overflowing. And this lasted for 5 hours.
Philadelphia got hit much worse. Winds were so strong they toppled trees, and hail caused significant damage.

Storms like these are a regular feature “back East,” and in the Midwest, because of the contrast in warm/cool and moist/dry air mentioned above. In the coastal Northwest, the cool offshore water causes our marine air to be much cooler and much less moisture-laden than areas east of the Rockies. Thus, fewer thunderstorms - and pretty wimpy ones at that. And sometimes they don’t even thunder!

Rainfall Intensities
Not long ago I compared consecutive years and commented on how we often get big changes in weather from one year to the next: “what a difference a year makes.” In the last several months we’ve seen dramatic changes from day to day, ranging from intense rain and mild temperatures to hot, dry summer-like weather with record high temperatures. What a difference a week makes!

Rather than discuss the hot weather (which is rather dull, albeit pleasant), I’d like to focus on the intense rain of a few weeks ago. It rained about as hard as it ever does in the mid-valley. Streets were temporarily flooded, creeks and streams were full, and unlucky folks caught outside got drenched. Add in a bit of thunder and lightning, and we had quite a show. And for this weather nut, it was a fun one indeed!

On the afternoon of May 18th the real fun occurred. It was a wet day (.89 inches in 24 hours, a new daily record for the date), but most of the rain came in the afternoon. Our official weather station, at the Hyslop Experiment Station on Highway 20 near Independence Highway, recorded .32 inches of rain in a one-hour period between 3:00 and 4:00 p.m. In Corvallis, amounts were similar, but most of that came in much less than an hour.

My colleague Matt Doggett has an automated weather station at his home in southwest Corvallis. Matt reported .30 inches of rain - in three minutes! That works out to 6 inches in one hour, if the event had lasted that long.

Fortunately for us, they never do in our area. While we don’t have long-term hourly data for Corvallis, there are nearby stations whose records go back pretty far. At Salem Airport, for example, hourly data are available back to 1948. The highest one-hour total ever reported at Salem was 1.24 inches during a short-lived but intense thunderstorm on June 10, 1950. The successive hourly totals for that otherwise dry day were:

4-5 pm : .02 inches 5-6 pm: 1.24 inches 6-7 pm: .28 inches 7-8 pm: .02 inches

The only other time that Salem received more than three-quarters of an inch in an hour was on July 18, 1983, when .92 inches fell.

No, if you want intense rain, don’t bother with western Oregon. Go east, young man! East of the Cascades is where you’ll find intense rains, even though much of the Eastside is considered arid or semi-arid.

How about this one? Birch Creek (south of Pilot Rock and north of Ukiah, out in the boonies) received 2.5 inches in 20 minutes on June 22, 1938. Or this? 3.5 inches in 50 minutes, near Mitchell, July 13, 1956. This created a major flash flood which destroyed 20 buildings as well as bridges and a section of a highway. Mitchell’s annual average rainfall is less than 12 inches, so this represents a sizable chuck of an average year.

Heppner, the scene of a disastrous flash flood in 1903 which killed 224 people, has had several other notable floods. One was caused by a May 25, 1971 thunderstorm that deposited 3 inches of rain in 20 minutes. Over a longer period (3 hours), John Day received 7 inches of rain on June 9, 1969.

Yes, here on the Westside we get a lot of rain, but it’s spread out over a large number of days. In eastern counties, it doesn’t rain often, but when it really rains, it pours!

Oregon Climate Service

George H. Taylor, Oregon State Climatologist

Wayne P. Gibson, Programmer/GIS mngr.

Eileen Kaspar, Research Assistant/Manager of Data Services

Melanie Mitchell, Undergraduate Assistant

Wolf Read, Undergraduate Assistant

Emily Gibson, Student Assistant

Cadee Hale, Publications Assistant

Sara Joos, Student Assistant

Sean Daly, Student Assistant

Kirk Borgerding, Undergraduate Assistant

Kelsey Kuykendall, Undergraduate Assistant

Oregon Climate Service, Strand 326,                             	     Phone: (541) 737-5705  
Oregon State University                                                      Fax: (541) 737-5710   
Corvallis, Oregon  97331                                                     E-mail: oregon@coas.orst.edu   
                                                                             Web: http://www.ocs.oregonstate.edu