Unsettled Pattern to Arise This Weekend Into Next Week

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It’s been a long while since I’ve posted something on here, and the reason is probably quite obvious: there’s been very little to post about weather wise. To put it simply, it’s been pretty boring, though we have had some weak systems here and there. Now when I use the term “boring” describing the pattern much of this year, I mean that in terms of active weather. As of now, the California snow pack is at an all-time low. From the southern sierra north, the snow pack is at 5% of average. Today’s (April 1st) snow survey turned up nothing but bare ground, which isn’t too surprising given the lack of recent snow. D4 (exceptional) drought has increased to over 40% of the state once again, and has steadily intensified over the last few months given the pattern after December has been quite horrific, plague with the super ridge (not an official term). The ridge has not only kept us extraordinarily dry, but also fairly warmer than average, too. The very abnormally dry and warm pattern we’ve been stuck in has kept weather pretty boring, but has also got snake season going earlier than normal, keeping me busy while weather has been quite.

Drought-3-panel

Pattern Change

This may change, at least for a short period of time, beginning this weekend. It currently appears that our ridge will get kicked out eastward into the central U.S. over the next few days as a trough digs southward out of the Gulf of Alaska. The first system embedded in this trough as a whole looks like it’ll slide into the west coast later Saturday, with showers perhaps beginning in the northern sierra and mountains through the day. By Sunday, the trough and upper-low off the Pacific Northwest coast begin to push ashore, bringing inland more widespread precipitation and more instability as the colder air associated with the base of the trough slides overhead. There’s some positioning differences between models, and these positioning discrepancies make it a bit tough to forecast how widespread showers could be, and how much precipitation we could pick up in general. An in-between/blend of models would generate around a tenth or two for much of the coast and east side of the valley from Sacramento northward, with perhaps up to a half inch of liquid precipitation in parts of the western slope of the sierra, foothills, and northern mountains. Much of the precipitation is expected after sunrise Sunday, which could make for an interesting day of Easter egg hunts.

4-1

Snow levels look like they could drop down to around 4000ft through the day Sunday, with perhaps 4 – 8″ of snow up around 5000ft, enough to potentially cause some travel issues in the sierra/mountains Sunday into Monday. These numbers as well as rainfall amounts could certainly fluctuate over the coming days as models (hopefully) come into better agreement on positioning.

It’s That Time of Year

The positioning differences among models makes thunderstorm forecasting tricky given the difference in positioning affects how cold air air aloft would be (colder = more instability, warmer = less instability, generally). The GFS has the core of the upper-low closer, thus 500mb temps around -30c, while the ECMWF has the upper-low further north, and has 500mb temps closer to -20c. That makes for about a 10c difference in models, which affects instability a good bit. The GFS depicts surface-based CAPE values between 300 and 600 j/kg from Sacramento to Redding, while the ECMWF depicts minimal instability in the valley. Directional shear becomes pretty decent through the afternoon in the valley as winds become more backed out of the south/southeast locally beneath the westerly mid/upper-level jet, though speeds themselves currently appear to remain modest, though potentially effective in the right environment. This is something I’ll continue to monitor over the days ahead to see in what way models trend. One thing to remember is that we are in spring now, and it doesn’t take a whole lot of cold air, moisture, and lift to get storms going as it would in the colder months given the sun is up a while longer.

12z GFS's forecast MLCAPE, valid at 2pm Sunday.

12z GFS’s forecast MLCAPE, valid at 2pm Sunday.

On Monday, a new system may drop into the trough, strengthen, and impact the west coast by Monday night on into Tuesday. There’s a bit of model agreement that this one could be a bit wetter, too, with a somewhat better moisture tap and more well held-together low. Models are still a bit sketchy at this point this far out, but there is definitely a chance of another system early next week, with more rain, mountain snow, and potentially a chance of thunderstorms (again). I’ll hold off on going too far into detail for now given how models can be 5+ days in advance. I had initially planned to write more on this system (even prepared a couple of maps, but since I realized we were still several days out, I figure, or rather have a feeling models will alter their current solutions over the coming days).

I’ll get another post out Friday or Saturday depending how models trend (if they’re close to the same I’ll do a day-before final post, but if they come into decent agreement by Friday I’ll post then).

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