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August 2014
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Wx: Another Round of Instability

Posted: August 11th, 2014

 

A cut-off low pressure system will drift through the region this week and produce scattered showers and thunderstorms across the lowlands and mountains. Most rainfall should remain spotty and very light, however some forecast models are indicating a good chance of heavier showers late Tuesday and into Wednesday. The system will exit the area by Friday and more normal, slightly cooler conditions are expected for the weekend.

 

In this morning’s National Weather Service Area Forecast Discussion, the chance of heavier rainfall is addressed in terms of a deformation band, a zone in which the atmosphere is stretched and unsettled…

 

TODAY SHOULD BE THE HOTTEST DAY OF THE RECENT WARM SPELL … THE AIR WILL BECOME UNSTABLE AND AS SOUTHERLY FLOW ALOFT CONTINUES TONIGHT AND TUESDAY THE CHANCE OF SHOWERS AND TSTMS WILL INCREASE. LOW LEVEL ONSHORE FLOW WILL DEVELOP TONIGHT AND STRENGTHEN TUESDAY…FOR A COOLING TREND. AN UPPER LOW WILL MOVE INLAND OVER OREGON TUESDAY NIGHT AND WEDNESDAY. A DEFORMATION BAND COULD SET UP AND GIVE HEAVIER MORE ORGANIZED SHOWERS TO WRN WA TUESDAY NIGHT OR WEDNESDAY. CONTINUED LOW LEVEL ONSHORE FLOW AND A GENERAL COOLING OF THE OVERALL AIR MASS WILL RESULT IN HIGHS GETTING BACK TO NEAR AVERAGE. THE UW MM5NAM SHOWS SCATTERED SHOWERS GIVING WAY TO A GOOD SOAKING RAIN OVER SW WA LATER TUE NIGHT WITH PRECIP OVER A GOOD PORTION OF WRN WA DURING THE DAY WED. THE GFS DOES NOT SEEM TO HAVE ORGANIZED PRECIP BUT DOES LOOK SHOWERY TUE AND WED.

 

Below is the “soaking” as depicted by the latest 4km UW-WRF model run. Note, this single deterministic model is forecasting up to an inch in a 3-hour period. Not likely, or at least highly uncertain, but certainly worth watching…

 

2014-08-11 4km wrf

 

This system is at least the fourth of its type to affect our region this summer. The most notable provided around an inch of rainfall and broke daily records on July 23rd. The other disturbances were significant only for their isolated lightning and made-for-Instagram sunsets.

 

This week’s cut-off low is unique in that it is preceded by a thermally-induced trough, a classic Pacific Northwest heat wave scenario in which dry easterly winds descend the Cascades and blow offshore. The regime is very unstable, hence the forecasted thunderstorms above, and as a result the National Weather Service has issued a Red Flag Warning, it’s first in a while…

 

THUNDERSTORMS ARE EXPECTED TO DEVELOP FIRST ALONG THE CASCADE CREST THIS AFTERNOON … ERRATIC OUTFLOW WINDS WITH GUSTS UP TO 30 MPH WILL BE POSSIBLE … LIGHTNING IN DRY FUELS CAN CAUSE MULTIPLE FIRE STARTS. GIVEN THE RECENT STRETCH OF HOT AND DRY CONDITIONS…ANY FIRE THAT DEVELOP COULD SPREAD RAPIDLY.

A RED FLAG WARNING MEANS THAT CRITICAL FIRE WEATHER CONDITIONS ARE EITHER OCCURRING NOW…OR WILL SHORTLY. A COMBINATION OF STRONG WINDS…LOW RELATIVE HUMIDITY…AND WARM TEMPERATURES CAN CONTRIBUTE TO EXTREME FIRE BEHAVIOR.

 

By next weekend, we should be back to normal, however with ongoing injection of tropical cyclones into the Pacific jet stream, our weather could remain quite interesting.