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.

Saturday, 2 August 2014

Offshore Cutoff Low to Enhance Thunderstorm Threat Over Weekend

Offshore Cutoff Low to Enhance Thunderstorm Threat Over Weekend

Saturday Evening Update

It was an active afternoon across the sierra today as moisture and weak lift from the broad offshore cutoff low – with a few severe thunderstorms and lots of heavy rain. These diurnal mountain storms died about 2 – 3 hours after sunset, however a rather strong vort-max over southern California is pushing northward a band of increased lift and better moisture throughout the mid/upper-levels of the atmosphere. Instability is a bit harder to come by through the night, however as the band works it’s way northward showers and perhaps some thunderstorms are expected to develop from north to south, perhaps along a band, or segments of showers/storms.

Vorticity-max is readily notable over southern California via water vapor imagery, 500mb heights, and perhaps most of all 500mb relative vorticity. Lift and forcing from this as it rotates northward along the eastern fringes of the larger scale closed upper low off the California coast, which by the way has been sitting there for about the last 1.5 – 2 weeks, meandering around.

Water vapor imagery combined with 500mb heights from 10:55pm Saturday.

500mb heights + relative vorticity from 10pm RUC analysis. Can you see the vort-max?

Basically all models agree to a certain extend that some elevated showers/possibly thunderstorms will develop through the night tonight through Sunday, but it’s hard to determine the exact potential in the lower elevations. The higher elevations, including the western slope of the sierra, sierra crest, portions of the foothills, and northern mountains have a rather decent chance (60 – 80%), but once you get into the valley some issues include lack of decent instability, pockets of dry air low-levels of the atmosphere, and patchy CIN/capping… but in general the potential is there and any potential is worth noting.

Something I noted in the initial blog, and that the NWS in Sacramento also mentioned in their evening AFD is that behind and ahead of the morning band of convection surface heating will occur allowing for moderate to strong destabilization once again over the mountains. Ahead of the morning band/vort-max, it seems there lies a chance of strong to severe storms, where upper-level winds, lift, and moisture will be maximized, and this threat area may be north of Truckee to the northern mountains. South of the moisture plume surging northward ahead of the vort-max, enough moisture will remain aloft to support thunderstorms, still possibly strong to severe. Both behind and ahead of the band, I wouldn’t even be surprised to see daytime convection in the Sacramento valley if sufficient moistening occurs overnight and Sunday morning from the vort-max’s associated moisture plume.

1.3 – 1.6″ precipitable water values combined with 500 – 1000 j/kg CAPE over the mountains will support thunderstorms capable of hail, very heavy rain, and strong winds again, along with of course deadly lightning. CAPE/instability in the valley will be more sparse, but any pockets that models aren’t picking up could certainly support the development of diurnal valley convection.

Stay tuned, if things continue to look interesting for Sunday afternoon I’ll post another update to this post come Sunday morning.

A couple of offshore upper-level lows that have sat off the California coast for many days now will begin an eastward push this weekend, enhancing larger-scale lift and aiding instability across northern California. In addition, the southeast flow ahead of the upper-low will transport a good amount of monsoonal/sub-tropical moisture northward, not to say we don’t already have quite a bit over the state, which has supported the last several days of mountain thunderstorms.

Water vapor imagery from 8:45pm Friday, with upper-lows/cutoff lows pointed out.

A stronger shortwave embedded in the southeastern side of the cutoff low will rotate northward from Baja California, Arizona, and southern California through the day Saturday, supporting a more widespread increase in thunderstorm development as it slides north. This shortwave will also enhance mid and upper-level winds, including a pulse of 30 – 50kt 500mb winds rotating northward through the day Saturday. This band of noticeable mid-level winds will nose perhaps as far north as the Sacramento area before getting shunted eastward as the upper low continues it’s eastward track. While upper-level winds up to Sacramento may only make it up to 20kts, any amount can aid in the sustainment of convective activity… usually we have little to no upper-level winds to work with during summertime convective setups.

4km NAM’s forecast 500mb winds and heights, valid for 1am Sunday.

It appears lift/moisture/instability will be maximized in the mountains, where thunderstorm initiation will of course begin Saturday by late morning or early afternoon after a fair amount of surface heating. Later in the day, the southeasterly to perhaps east-southeasterly flow will tend to try and push storms to drift northwest to to west-northwest, into the lower elevations, including the foothills and possibly parts of the San Joaquin and Sacramento valley by evening and overnight. Once diurnal convection dies out, that shortwave in southern California will have nosed it into central California by mid to late-evening, and may provide enough support for nocturnal convection to continue overnight and into Sunday with little to no problem. During the day, mountain storms will have 1000 – 2000 j/kg of SB/MUCAPE to work with, which could make for another day of decent hail producing storms, as well as lots of heavy heavy rain directly under precipitation cores given a more than fair amount of available moisture.

4km NAM’s forecast MLCAPE for early afternoon Saturday. Good amount of instability over the sierra, so expect another round of daytime storms across the higher elevations.

500mb heights and relative vorticity, valid for 2am Sunday.

By the overnight hours, low-level instability will fade, however elevated instability will linger and perhaps even strengthen in addition to increased moistening of mid-levels as the shortwave works northward. Appears convection could develop in the southern sierra and San Joaquin valley late Saturday night in association with the shortwave, and slide northward into north-central California (Stockton to the I-80 corridor in my opinion) through Sunday morning. That, or storms could fire up ahead of the shortwave throughout the night, and fade off behind a final band associated with the shortwave/upper low as it exists eastward Sunday into Monday. These are both a couple of ideas/solutions brought to us via models and past experience with summer nocturnal thunderstorm events brought on by cutoff lows. Models have failed us quite a bit over the last few weeks, though, regarding low elevation precipitation/convection/thunderstorms… but this appears to be a bit more promising given a very notable shortwave and surge of moisture, with definite chances of showers and thunderstorms in the mountains and a general chance in the lower elevations, from Saturday morning through Sunday afternoon. As mentioned, the best chance for stuff (showers or thunderstorms/convection) in the valley will be overnight Saturday into Sunday morning, when lift, moisture, and instability is maximized.

Given 1.4 – 1.6″ precipitable water values and surface dewpoints in the lower elevations in the 50s, moisture in general will be decent and supportive of heavy rain under precipitation cores. However, some soundings across northern California indicate dry layers between 800 and 600mb, so gusty winds and initially dry storms are possible. Even with decent precipitation under cores, lightning can certainly strike outside rain/hail cores and spark fires. Fire starts combined with gusty outflow winds from nearby thunderstorms could create hazardous fire weather conditions, keep in mind.

Storms could continue in the mountains Monday, but after that the future becomes a bit more uncertain.

Portland Radar. NEXRAD Radar.

Weather On The One, Current National Temperatures Radar

Northern Weather