[Generally] Active Week Ahead

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It’s been a few weeks since our last bout of unsettled weather, with temperatures well above normal across California over the duration of this dry spell. This dry spell’s ridge will be nudged east through the day Monday as an offshore longwave trough begins pushing ashore by late in the day.

Increasing southwest flow in the mid and upper-levels through the day will likely support some mountain showers by the afternoon, however most of the region should remain dry through the day with temperatures climbing into the 70s in the inland lower elevations. By the overnight hours, a vort-lobe off the northern California/Oregon coast will push inland, at the same time pushing a cold front through from late evening through around 3 – 6am in the south. The front itself will be strong dynamic wise with a strong jet streak wrapped around the base of the trough with a broad brush of moderate-strength low/mid-level lift, but moisture will be limited quite a bit. Precipitable water values <.7″ will limit the amount of precipitation overall, but given increasing colder temperatures aloft, instability may promote some convective development overnight. Limited low-level instability will likely limit thunderstorm development, though.

0z GFS's forecast 300mb wind and 500mb heights valid for Tuesday afternoon, indicating the base of the trough pushing into the great basin and the nose of a jet associated with another system beginning to enter northern California.

0z GFS’s forecast 300mb wind and 500mb heights valid for Tuesday afternoon, indicating the base of the trough pushing into the great basin and the nose of a jet associated with another system beginning to enter northern California.

By Tuesday, the base of the trough moves directly overhead and takes on a more negatively-tilted phase. -28 to -30c 500mb temperatures atop surface temperatures climbing into the 60s will promote steep low and mid-level lapse rates, however peaking during the morning and early afternoon based on current model timing. This is due to the base of the trough shifting east of northern California through the day, shunting cooler upper-level temps to the east along with moisture and favorable S to SW flow. Nonetheless, there will be enough low-level moisture and surface heating to promote SBCAPEs in the 200 – 500j/kg range. One downside to the base of the trough is the zone of weaker winds rotating cyclonically around the base, limiting shear. If timing of the system slows down a bit, shear would be more favorable for rotating storms, but at the moment more multicellular/pulse-like convection appears likely. I’ll monitor future model trends for change, though surely.

April 20th

Precipitation amounts will be very limited from this system – with .10 – .5″ in the mountains, and a trace to a tenth in the valley. Along the north coast, a tenth to half an inch is also possible… similar in the northern mountains. Much higher amounts are/will be possible with thunderstorms and stronger convection as always.

Ridging returns for Wednesday and Thursday, however another, stronger trough arrives later Thursday into Friday. This system will be colder and pack more moisture, bringing some widespread precipitation with the system’s cold front Thursday morning. Behind the front, -30 to -32c 500mb temps slide overhead Tuesday late morning and afternoon as this second trough also begins to go into a more negative phase, but this time it does so just offshore, putting northern California in a more favorable position for lift and the back edge of the jet for some upper-level support and at least somewhat better shear. Forecast soundings from the Sacramento area on north indicate 200 – 400 j/kg SBCAPE values with decently clockwise curved hodographs favoring right-moving storms… possibly supercellular if clearing were to occur behind the cold front for the afternoon hours to boost low-level instability. The timing and track of this system will be key to thunderstorm and convective severity – and will be closely monitored through the week by me and I’m sure quite a few others.

500mb relative vorticity and heights valid Friday afternoon, as the trough's base begins pushing inland.

500mb relative vorticity and heights valid Friday afternoon, as the trough’s base begins pushing inland.

Precipitation wise, it appears around a quarter to half an inch of ran will fall across the lower elevations, with half an inch to an inch in the mountains and along the north coast. Locally higher amounts will be possible in areas impacted by convection. Moist westerly flow behind the system may continue precipitation chances into Saturday morning, but by Sunday ridging returns.

Stronger ridging next week may bring widespread upper 80s to low 90s by early to the middle part of the week if current long(er)-range indications are correct, which means I’ll (hopefully) be removing & relocating snakes by the bucket full!

If needed, I’ll post an update Monday evening regarding Tuesday’s convective threat. If it still looks relatively minimal severe wise, I’ll skip it. Expect an update on the late-week storm Wednesday or Thursday, and a final convective update Thursday evening. Stay tuned!

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