Cutoff Low, Thunderstorms, Fire Weather Threat

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9:15pm Saturday: I’ll have an updated post out Sunday regarding our thunderstorm threat.

Friday’s post:

After a week or so of below normal temperatures, and a week of near or slightly above normal temperatures, we’ll continue the trend of near normal temps through the weekend. Near 100 degree temperatures are expected Saturday, and by Sunday, potentially widespread 100s are possible. 12z NAM indicates valley temps Sunday between 100 and 106 degrees… with the GFS maxing out around 105 and locally 107.

Over the weekend while temperatures warm, a offshore cutoff low will strengthen with a 300mb wind max looping around the low of around 70kts. Sunday, the low works its way eastward slightly, positioning itself about 150 miles off the central coast. At this time, it appears moisture begins to feed into the systems flow pumping over northern California from the south/southeast as a ridge in the southwest also pumps some monsoonal moisture northward, meeting the cutoff lows flow at the California coast.

Graphic indicating regions with thunderstorm potential over the coming days.

Graphic indicating regions with thunderstorm potential over the coming days.

Beginning Sunday, expecting widespread mountain convection to blow up during the afternoon. Sufficient lift may exist into the overnight hours to allow for some nocturnal storms continuing into the early morning hours of Monday as well.

Convection is expected to continue developing daily over the mountains through at least Thursday, with some nocturnal storms possible some nights depending on the placement of the cutoff low.

There is a fairly wide spread among models regarding how far west convection may make it (as there will be a hint of a southeast flow rolling over the sierra) due to there being a fairly steep low level temperature inversion hovering around 900mb. However, above this inversion lies some fairly steep lapse rates equating to some instability. Generally, any day between Sunday an Thursday, the valley could see some action.

Right now it is a bit early to call anything (also do to the fact that it’s a cutoff low, which models struggle with), but given that there are at least a couple of models indicating the chance of valley convective activity, it is certainly possible, of course depending where the low actually places itself an how strong it gets before it weakens and pushes eastward.

Between Sunday and Thursday, instability in the form of MUCAPE is expected to generally top out around 500 – 1500j/kg each afternoon… with LIs between 0 and -3c. Though not extraordinary, these instability progs more than support elevated convection. Some periods of increased shear (15 – 25kts of effective shear) may also introduce the threat of more organized convective clusters over the several day period.

Thunderstorms over this event timeframe will be capable of hail, possibly severe in some cases given this low will have a -10 to -12c 500mb cold core aloft some fairly warm surface temperatures. Also expecting some storms to drive down some strong winds (via microbursts, outflow, or downdrafts), especially since the cutoff low will have a 70kt wind max rotating around.

12z GFS 300mb winds and heights valid for Tuesday morning, though the low looks about the same Monday - Wednesday..

12z GFS 300mb winds and heights valid for Tuesday morning, though the low looks about the same Monday – Wednesday.

This low is expected to tap into precipitable water values in excess of 1.2 to 1.3″, along with spiking dewpoints into the 60s locally. Once convection wets the lower atmosphere over regions the travel over, rain could become heavy. Despite the high probability of rain occurring directly under storms, the large amount of lightning expected could become worrisome as not all strikes occur in precipitation cores. Since brush is dry across much of the state, fire starts will be easy to create, and could become fanned by thunderstorm winds blowing out of collapsing storms.

Apart from the fire weather threat, a blend of models including the ECMWF, GFS, GEM, and GEFS indicate QPFs of .25 – .75″ across much of the sierra, northern mountains, and southern cascades of Oregon. Amounts between 1 – 2″ also seem plausible given the length of the event… possibly locally higher amounts given this is a full blown convective event, and models can’t simply pinpoint exact areas thunderstorms could slam daily.

12z GFS 300mb heights and 700mb vertical velocity indicating the cutoff low moving inland Wednesday into Wednesday night, possibly bringing a peak to the convective activity.

12z GFS 300mb heights and 700mb vertical velocity indicating the cutoff low moving inland Wednesday into Wednesday night, possibly bringing a peak to the convective activity.

Things should dry out by the weekend as the cutoff low exits the coast an moisture shifts eastward into the desert southwest.

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