Friday, 8 August 2014

Potentially Significant Thunderstorm Event Sunday/Monday Possible

Potentially Significant Thunderstorm Event Sunday/Monday Possible

A potentially significant thunderstorm/convective event looms across northern California (northward into southern Washington) as a tightly wound-up and rather fierce cutoff low positions itself off the northern California coast through the day Saturday, floating around offshore through Monday. This low will be origin/main factor to this event, and looking back at other similar events in the last decade or, potent/compact cutoff upper-level lows such as these, when combined with enough moisture in the mid/upper-levels, have been known to spawn very notable thunderstorm events across the state (take for example the dry lightning event of June 2008, and other cutoff low-induced thunderstorm events in September 2005, August and September of 1999, and July 2002, to name some memorable events).

Right now, the low is bout 400 miles off the OR/CA coast, and is working its way east-south-east at a steady pace, and should be about 100 – 200 miles off the San Francisco coast by Saturday afternoon. The low is expected to run into the dominant upper-level ridge over the Great Basin extending into the four-corners region, and essentially become quasi-stationary off the central/northern coast through Sunday and Monday.

Water vapor satellite imagery from 8pm Friday, with a cutoff low half-wrapped in dry and moist air off the CA/OR coast.

As it sits along the coast and spirals/wobbles around, due to the compression between the ridge to the lows east and a strong upper-low/trough in the Gulf of Alaska, it won’t have anywhere to go besides to sit along the coast and strengthen bit by bit until it can undercut the ridge to the east. Models have fluctuated and changed run-to-run, which is quite usual with cutoff lows, thus we’ll have to work with historical event occurrences and the general gist of model indications.

0z 12km NAM’s forecast 500mb relative vorticity and heights, valid at 11pm Sunday.

The NAM has been trending faster and faster with the timing and strength of this low run-to-run, which could make for a more impactful event when it comes to the strength/potency of the low. Overnight Sunday into Monday after strengthening the low will rotate back around itself to the northwest, as this occurs increased lift in general or a mid/upper-level front could slide through, which combined with decent mid/upper-level instability and moisture would support the development of more widespread convection, separate from afternoon storms across the mountains Saturday/Sunday. Convection would continue into Monday as the low continues rotating around offshore providing lift.

With precipitable waters around .8 – 1″ across northern California, moisture won’t be as significant as seen in previous events due to there not being a significant surge of monsoonal moisture northward, just a limited amount being fed into the mid/upper-level flow of the cutoff low. The cutoff low itself will also draw northeast dry(er) air from the central Pacific. This lack of deep available moisture combined with dry layers in the low/lower mid-levels, thus moderate to considerable rainfall evaporation is possible with thunderstorms, thus creating potential for scattered to widespread dry thunderstorms. This isn’t to say thunderstorms will be completely dry, though, as most will likely have some rain directly under their cores… it’s just the lighter precipitation around cores likely wouldn’t make it to the surface, and any lightning that occurs outside of the heaviest convective precipitation cores would probably be dry.

The low will slowly work its way inland Tuesday/Wednesday, continuing to bring scattered to widespread thunderstorms to the mountains. Tuesday the dry lightning threat in the mountains will still be considerable, but increasing moisture from the southeast should help out Wednesday to cut down on precipitation evaporation, though some would still occur and isolated to scattered dry thunderstorms could continue, just not as widespread as Sunday – Tuesday.

Because these cutoff lows are so tricky to forecast, and correlations between low elevation convection and these summertime cutoff lows is mixed, there is always potential for a bust in the lower elevations if something key doesn't set up right or set up at all. I’ll likely update this post Saturday and Sunday as the event closes in and impacts become more clear.

Portland Radar. NEXRAD Radar.

Weather On The One, Current National Temperatures Radar

Northern Weather