Weather West: Monsoonal Moisture to Bring Widespread Thunderstorms

The following is an excerpt from a recent article on the Weather West Blog. 

By Daniel Swain of Weather West

Continued heat, with record vegetation flammability across interior NorCal

I’ll keep this section brief, as there is much to discuss regarding the upcoming pattern itself. But suffice it to say that the interior of NorCal has continued to bake for most of July, despite the slightly lessened pace of record-breaking high temperatures. Some areas across the NorCal interior are continuing to experience their hottest summer on record to date, and this is in the midst of an extreme to record breaking drought in the same region. As a result, soil and vegetation dryness are now approaching or have already reached record levels–not just for the calendar date, but for any time of year. This has yielded vegetation flammability that is similarly off-the-charts. And that’s reflected by the extraordinary and extreme fire behavior being almost continuously observed on large fires like the Dixie Fire, currently burning in (primarily) Plumas County at the moment, even in the absence of extraordinary ambient fire weather conditions. This record vegetation dryness and drought, especially in northern and central CA, is the context through which the upcoming probable lighting event should be viewed from a fire weather perspective.

Deep surge of monsoonal moisture & instability across *all* of CA

I continue to be fairly impressed by the extent and depth of projected moisture and instability across a good portion of California at various points over the next few days. This will be a true monsoonal surge–with distinct easterly flow over SoCal, and southeasterly flow across NorCal–complete with multiple easterly waves that will provide some mid-level lift and focal points for convective activity. Additionally, a weak low pressure system off the central CA coast will offer some mid-upper diffluence over the regions through mid-week–further enhancing the mid-level dynamics that are sometimes missing during monsoonal surge events.

This looks to be a classic, relatively high-end monsoonal pattern across CA. There will be at least a slight chance of some thunderstorm activity across every corner of the state during this event, but the likelihood will vary significantly from place to place and the character of the storms themselves will vary much between NorCal and SoCal (see below).

This week’s event will be a fairly impressive and prolonged event one that will affect most of CA.

At the moment, the first wave (due in tonight) appears to be the weakest and least significant of the week. Some isolated SoCal thunderstorms are possible, but this one could potentially fizzle out. But that’s much less likely to happen with subsequent waves beginning Monday and continuing through at least Wednesday. During that period, subtle mid-level features (vorticity maxima as well as broader easterly waves) will contribute additional bursts of instability and weak mid-level lift–which will likely be more than enough to trigger occasional high-based thunderstorms across the region at times. Timing these subtle features is still almost impossible this far out, so I won’t try to do so. But there will likely be multiple peaks and lulls during this fairly extended monsoonal event, and SoCal will likely see activity before NorCal.

Wet(ter) storms south, dry(er) storms north; fire risk highest NorCal

This event appears poised to bring quite a moist airmass to California, with precipitable water in the 98-99th percentile for summer. That means there will be plenty of moisture to work with, by July standards, to generate convection. But most of this moisture will be in middle and upper levels of the atmosphere, especially as one moves progressively farther north within California. What does that mean in practical terms?

Well, in SoCal, most thunderstorms will be accompanied by rain, perhaps even heavy rain. Isolated dry lightning strikes will be possible outside of rain cores, but in general I don’t expect widespread dry lightning across SoCal during this event. Instead, I would expect to see pretty good coverage of downpour-producing storms, which could still spark local fires but could also produce localized flooding (mainly mtns and deserts) due to heavy downpours. I would also expect very muggy and uncomfortable conditions across SoCal during this event, as the marine layer becomes suppressed and surface dewpoints become very elevated.

Very favorable synoptic setup this week for widespread monsoonal convection: an offshore low, plus inland ridge.

In central and northern California (west of the Sierra Nevada crest), the story is more complicated. Here, there will be a much drier sub-cloud layer–so despite the large amount of overall moisture in the atmosphere, and the relatively high likelihood of convective activity, wetting rainfall is pretty unlikely. Brief showers are still likely with thunderstorms that develop across NorCal, but due to a combination of that dry sub-cloud layer and relatively fast motion of cells, dry lightning is much more likely up north. Rainfall ranging from only a trace to maybe a few hundreths is most likely from these thunderstorms. This is bad news from a fire weather perspective, because (as noted above) vegetation is at record dry levels and the lightning ignition efficiency will likely be quite high. The brief showers that may occur up in NorCal with this probable lightning event will still likely allow for a high likelihood of new fire ignitions, since there just won’t be enough water to extinguish any new starts.

Read the rest of the weather report here.