Monday, June 25, 2018

Unusually Strong Cold Front (with Lightning) Hits the Northwest

Summer cold fronts are generally unimpressive around here, often bringing cooler marine air accompanied by a transition to increased onshore flow.   But last night we had a stronger than normal summer cold front that brought lightning and burst of heavy rain.

The radar image around 6 PM Sunday was impressive, with the yellow colors indicating heavy precipitation.


By midnight, the heavy precipitation reached Puget Sound and as many folks can attest, there was some serious lightning with the frontal band.


In fact, the regional lightning detection network observed hundreds of lightning strikes during this frontal passage (the figure below shows the strikes for the 24-h period ending 1 AM Monday).


If you want to get a real feel for the lightning, here is a video looking north from the northern Kitsap Peninsula from Greg Johnsons Skunk Bay Weather facility.  Pretty exciting at Skunk Bay.  No missile shots or UFOs!


The infrared satellite image at 8 PM Sunday was impressive for June, with a strong frontal band making landfall on the WA coast.



The precipitation totals from this band were modest...precipitation was heavy but it moved through quickly.  Particularly, heavy precipitation fell over the north Cascades (half inch), but lots of locations observed a tenth or two of rain.


On top of the atmospheric science building at the UW, the frontal passage was very obvious around
07-08 Z (GMT)--around midnight to 1 AM Monday.  About .15 inch of rain (bottom panel), an increase of wind (top panel), and an interesting, sudden jump down of pressure (fourth panel).


So why did we have so much action last night?   The front was fairly strong but a key was elevated instability (known as elevated CAPE, Convective Available Potential Energy, in the weather business).  The upward motion with the front released the instability, resulting in numerous thunderstorms.

The existence of the upper level instability was indicated in the cloud late Sunday afternoon with the existence of lots of altocumulus castelanus clouds.



Our high-resolution forecast models had a good idea about what would happen, as illustrated by the one-hour precipitation ending at 9 PM Sunday (04 UTC) and 1 AM Monday (below)



This week should be cool with plenty of low clouds in the morning and early afternoons.  Typical weather for this time of the year.  But don't worry....the latest model runs suggest a big improvement by early July.

6 comments:

Ian Reed said...

For as good as the weather models are, it always fascinated me when stuff like this happens seemingly by surprise.

Did no one think a sharp front like this after a warm/hot day would create the instability needed for lightning? It seemed to surprise the NWS, as they scrambled to put out an AFD update as it happened, but there was no mention leading up to landfall about any thunder possibility that I saw.

Is this a case of parameters being borderline and nature just surprising us? Or did geography play a role in aiding strength? Definitely curious.

lhsouthern said...

It really rocked here in chehalis last night. Thunder so intense it shook the house for a minute.

John K. said...

Question about the "lightening strikes" - isn't a "strike" technically when lightening reaches a solid object like the ground, or perhaps an aircraft? Is that what gets recorded, or is most of the lightening that gets recorded actually between clouds, ie, not lightening that is actually striking anything?

Ian Reed said...

I believe only cloud to ground lightning gets recorded by lightning detection equipment.

JWG said...

Saturday night, viewed from the tricities, looking west, there was a long slanted horizontal cloud dropping ice crystals out the bottom that were drifting southward(as it appeared to us).
Monday morning till 1 pm there were abundant clouds dropping out ice crystals.

jay

Foo said...

This is where a stronger NWS emphasis on probabilities, as Cliff has discussed, would come in handy. The ingredients for thunderstorms here usually add up to borderline situations - but the models obviously are on the right track to some extent. Seems like the NWS preference is to play it safe by not mentioning this in its forecasts, and as a result they almost always end up looking like they were caught unaware.

On the other hand, there are situations where thunderstorms here really do seem to defy reliable prediction. If i recall correctly, that happened back in Aug. 2015 when some rather potent storms developed right over the city and dropped a ton of rain - and the forecasts didn't have a clue. Maybe chalk that up to the generally lower reliability of forecast models here due to a lack of data to initialize them correctly?