Noteworthy April weather not unusual

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The normally staid Walla Walla Valley experienced a couple of wild weather days this past week. April kicked up its heels with a springtime smorgasbord that featured a severe thunderstorm on Thursday evening and heavy rain early Sunday morning, followed by a strong wind event during the day.

Meteorologically, April is a transition month and is no stranger to noteworthy weather occurrences anywhere in the United States. With winter weather patterns yielding to more summerlike trends, it is no wonder that some of the region’s most violent weather takes place during this month.

Cold fronts continuing to plunge obstinately southward from Canada into the nation’s midsection and those coming ashore in the Pacific Northwest, bearing frigid air from their spawning grounds in the Gulf of Alaska, clash with warm, moist air originating in the Gulf of Mexico or the subtropical Pacific. The result is a recipe that is guaranteed to occasionally be as explosive as a high school chemistry experiment gone awry, with potentially fatal consequences for those who find themselves on the receiving end.

A warm front passed through the Valley on Thursday afternoon, on its way from central Oregon to northeastern Washington, leaving a warm, moist air mass in its wake. A ripple of low pressure in the flow moving from west to east that evening provided just enough lift to destabilize the atmosphere. This touched off a thunderstorm just to the west of Pendleton that quickly blossomed into a monster of a storm that assumed severe proportions as it rumbled northeast at the astonishing speed of 55 miles per hour.

Soon thereafter, the National Weather Service office in Pendleton issued a severe thunderstorm warning for the local area, confirming what anyone who was looking to the southwestern sky already knew: something very unusual was afoot.

The sky in that direction had assumed a purple/black look accented by the appearance of a cloud that appeared part flying saucer in “Independence Day” and part “Wizard of Oz” tornado scene in which Zeke (Bert Lahr, aka the Cowardly Lion) runs around screaming “It’s a twister! It’s a twister!” — just as your somewhat crazed weatherperson did on Thursday evening — which was approaching at an alarming clip. This eye-popping formation is known as a “shelf cloud,” and is the result of cold downdrafts from the thunderstorm cell fanning out above the ground ahead of the storm and lifting and condensing the warm, moist air they encounter. It is generally a reliable harbinger of a severe weather event.

The rain arrived shortly thereafter, lashed by winds gusting to 52 miles per hour at the airport along with a couple of vivid flashes of lightning — a fabulous weather spectacle, and one that was, quite frankly, unforeseen. Although, to be fair, the Pendleton weather office had mentioned a slight chance of a thundershower for northeastern Oregon in its earlier forecast discussions.

Early Sunday morning a fairly strong low-pressure system brought periods of moderate to heavy rain to the Valley, bringing the three-day total to nearly two-thirds of an inch. This outburst was welcomed heartily by many who are involved in agriculture in our area — particularly a certain Walla Walla Community College vineyard manager who was very concerned that he would need to begin irrigating his vines soon in view of the abnormally dry start we have gotten off to here in 2013. That decision can now be postponed for a while, pending future developments.

Following the passage of the low pressure to the east, gusty winds developed as a result of a steep pressure differential between the departing low and high pressure to the west pushing a cold front through the state. As with water flowing down a hill, the steeper the gradient the faster the flow, and so winds gusting in excess of 50 miles per hour Sunday were emblematic of the sharp pressure differences between the outgoing low and the incoming high.

In the short term, we can expect some slight warming Wednesday (and a chance of showers) with fair weather Thursday and Friday, followed by gradual cooling, along with an increasing chance of showers as we head into the weekend.

In the longer term, there does appear to be a fair chance of some unusually cold air settling over the Valley next week, with lows possibly dipping below freezing from next Monday through Wednesday. Remember where you heard it first, vineyard people.

A lifelong fan of both the weather and the Baltimore Orioles, Jeff Popick is an instructor at the Center for Enology and Viticulture at Walla Walla Community College and manages the school’s teaching vineyard. Send your questions and comments to him at jeffrey.popick@wwcc.edu.

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