Today, scattered convection was evident across the Pacific Northwest, especially around Washington state. The surface analysis from this morning revealed a low-pressure system off of the coast of British Columbia, bringing WNW flow from the Pacific onshore. This provided a source of moisture for the area to help provide a base for convective activity as surface level temperature and dewpoints were close together signaling locally high relative humidity, but what was going to provide the lifting mechanism necessary for initiation of convective activity? Looking to the upper levels (specifically @250mb), there is evidence of a trough feature over the Pacific Northwest and in particular, the right side of the trough is situated over the area with a noticeable jet streak present based on the analysis. While not a particularly strong or amplified trough feature, this implies upper level ageostrophic divergence and mass evacuation which will induce upward vertical motions filtering down to the lower levels. The cyclone and subsequent onshore flow is evident as you go down to the 500mb and 700mb levels as well. The 700mb analysis gives you an idea of low-level moisture advection which was present with the onshore flow.
00z 11/19 and 12z 11/19 250mb upper air maps (AOS-UW-Madison) respectively
Isobars (black contours), Temperature (red dashed contours), Wind Speed (wind barbs), Stronger Jet Stream winds (fill pattern)
Now that the synoptic stage is set, we’re going to look at some sounding data to analyze the state of the atmosphere near the surface and see if there are more signs of convective initiation. Today’s 12z sounding (on the left) from KUIL (Quillayute,WA) was observed and from this we can see a very saturated low-level profile at least up to 800mb meaning there is definitely precipitation occurring over the area. Modest lapse rates (5.5-7.5 C/km) were shown with this sounding, implying modest parcel buoyancy to aid in the overall lift and upward vertical motions to induce convective activity. While this sounding depicts rain as the form of precipitation, there can be a changeover to snow depending on where you are in the region. The sounding on the right is from KOTX (Spokane, WA) also taken today at 12z depicts a similar sounding in terms of saturation and likely precipitation, however the temperature profile is close to the freezing temperature and there was in fact snow falling in the area. One reason for this is the effect of orographic lifting over the Cascade Mountains that forces air up from the surface into higher elevations where it condenses and in this case precipitates snow. The other thing that suggests snowfall is the wet-bulb temperature (thin blue line in between temp and dwpt profiles). Wet-Bulb temperature is the temperature a parcel of air will be at when it is cooled to saturation. When the wet-bulb temperature is at freezing or lower, this can imply snowfall even when the actual temperature is above freezing.
The image below shows radar imagery from this morning over Washington state, and you’ll notice that in addition to the onshore precipitation there seems to be an absence of precipitation to the east of the onshore flow but then all of sudden starts up again around and just east of Seattle. This area where the precipitation has started up again is known as the Puget Sound Convergence Zone (PSCZ).
The PSCZ is an area in which WNW winds converge and initiate lift and convection after diverging around the Olympic Mountains to the west. The abundance of moist WNW flow due to the low-pressure system talked about earlier have converged and were able to induce upward vertical motions, working in tandem with the upper level ageostrophic divergence from the upper trough, producing shows and thunderstorms near and east of Seattle this morning.