Storm Day! Part 2

Greetings from rain-soaked Bodega Head!

We have now picked up over an inch and a half of rain since this morning at 7 am. We have had periods of light rain, heavy rain, big ole fat rain, and rain that even seemed to come up from underneath. The short range, high-res MM5 forecast suggested that we would get 1.5 inches for all of wednesday:

DRI-MM5-24hrAccum_1200UTC0226-1200UTC0227

24 hr accumulated precipitation forecast by 4 km MM5 valid at 4 am UTC on Feb 26, 2014. Credit: DRI-CEFA.

We reached Atmospheric River conditions here at the BML HMT station just after midnight and have been under an AR ever since. The surface met time-series over the last 48 hours or so looks like:

HMT-BBYsfc_0100UTC-022714

Sfc met variables collected at the 10m tower at BML for the 48 hours ending at 5pm PST on Feb. 26, 2014. Remeber that the time ordinate increases to the right! Credit: NOAA-HMT.

My favorite panels are the lowermost 2. Winds have been southerly or south-southeasterly all day. This has been the case throughout deep layer overhead:

HMT-BBY449UFlx_0100UTC-022714

Time-vertical profile of low-layer winds, freezing level, and upslope water vapor flux. Credit: NOAA-HMT.

Since the 449 MHz wind profiler is alive and kicking again, I couldnt resist posting a plot of the near-surface winds. For the most part the wind profiles themselves have been uniform and southerly in the lowest 5 km. The approaching storm is a very deep system, and since we are on the east flank, we will continue to get southerly winds until the first cold front passes us.

The radar senses virtual temperature (via density) using RASS. The black dots in the later stages of the picture correspond to the freezing level above BBY, which is currently about 1.9 km high. Notice that after 2 pm, the winds in the lowest 2 km have veered. As a result, the freezing level has raised slightly. Veering winds correspond to warm-air advection (WAA), so this low-level warming should be expected.

We did not have a prolonged period of westerly winds as this storm approached, and therefore did not see a coastal jet. Interestingly, even though we did not, and we have not had terribly stable stratification, Cazadero has recieved less rain this event than BBY. Upslope water vapor flux corresponds to the low-layer integrated component of the wind which points from BBY to CZC multiplied by the water vapor mixing ratio. In the lower panel of the plot above, this value has been increasing along with wind speed. It is expected that as upslope wv flux increases, orographic enhancement will cause more rain to fall at CZC.

CZC has thus-far recieved near 1.4 inches. Often, this rain has formed in a deep layer extending above the freezing level:

HMT-CZCSNR_0100UTC-022714

Timeseries of S-Band radar signal-to-noise ratio above CZC. Credit: NOAA-HMT.

The freezing level can be diagnosed from this plot by the location of the “bright band” of echos near 1.9 km.

Even though we have struck out on it so far, we may yet get a short lived coastal jet. The first front is expected to come ashore at BML tonight near 10 pm. This is depicted in the MM5 forecast below:

DRI-MM5-925hPaUT_1200UTC0226-0600UTC0227

925 hPa winds (barbs and isotachs) from 4 km MM5 valid at 0600 UTC Feb 26, 2014. Credit: DRI-CEFA.

After the front, we expect a short (until 1 pm, PST Thursday) period of westerlies, during which we may have the right conditions for a jet.

Winds will turn southerly again Thursday into Friday, until another front reaches us in advance of Friday’s storm system.

At this point, to call today’s and Friday’s system separate is probably improper. The upper level potential vorticity maximum associated with Fridays storm is so rapidly deepening, that it will engulf the PV max associated with today’s storm soon:

See that here Credit: U Hawaii

At the lower levels, this rapid upper level deepening is going to intensify a small, but intense extratropical cyclone which will hit California early Saturday morning:

See the fun cyclogenesis Credit: U Hawaii

Just for fun, I thought I would post what the high-resolution forecast of this event looks like at 72 hours simulation time.

WCoast_TC

10 m wind and SLP from 4 km MM5 valid at 12 UTC on Mar 1, 2014. Credit: DRI-CEFA.

Is that a west coast TC?

Other curiosities from today:

In this post, I mentioned that we encountered low level dust this morning, and that this system was relatively very dust laden. We also found Petaluma Gap flow for most of the morning. This can be seen in the surface station map from near 10 am PST:

MWest-Obs-USLP_1800UTC0226

Surface station temperature and sea-level pressure from stations in N. California. Credit: Meso-West, U Utah.

High pressure over the CV and East Bay, compared to relative low pressure in the North Bay and coastal Sonoma, are causing flow through the Petaluma Gap to our site in this picture.

As a result, we measured very elevated particle counts, NO, NOx and Ozone for much of today. It’s a strange result, but sometimes when it is raining the air is more polluted at Bodega Bay than when it is sunny and high pressure is in control.

This is especially true in light of the data we have recently worked up for periods of quiet conditions from early in the trip. We now have an idea of just how clean the air can be here when flow is onshore. A post reviewing that information will appear here tomorrow.

-Andy.

which is going to accelerate and intensify Friday’s system, so that it will come right on the heels of today’s rain.

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