2006-02-26 Wx discussion and Forecast
Major features at 12Z this morning were the upper trough, axis from near 50N/138W
to 30N/146W, and strong surface low near 37N/132W and tracking NNE.
However, this doesn't tell the whole story. There are lots of details. The trof offshore has at least 2 main pieces. A short-wave feature near 35N/130W at 15Z this morning was clearly associated with the main surface low off Nrn CA. Of significance is a large area of open cellular convection with cloud-top temperatures of -40C or so, between 30 and 40N, 142-149W at 15Z this morning. This has begun to turn cyclonically after heading mostly south earlier, and is currently on an ESE to E trajectory. It is also elongating and there is emerging a vorticity center estimated to be near 31N/145W at 2000Z. Beginning about 15Z there began formation of Ci to the E of this feature, and that has now expanded considerably, sgstv that there is appreciable dynamics (i.e., synoptic-scale upward motion) with this, separate from the main surface low and upper wave farther NE mentioned above. When I made a strenuous effort to do so earlier, about 16Z, I couldn't detect any separate cyclonic flow center at the surface with available data (tracking of low cloud tags, ships, buoys, etc.), but there
is clearly cyclonic shear in the sfc flow to E of the large patch of deep, open-cellular convection, probably concentrated along the surface front.
Closer to HMT, there is evidently good surface confluence just offshore Nrn and central
CA as the occluded/cold front approaches the coast. It should be noted that at the moment there is dry air over all CA, with dew points in the 40s at the sfc (35 at KACV!), and mostly offshore component to the surface flow along the coast also. However, GOES PW from RAMSDIS showed very strong precipitable water gradient just offshore and along the coast at 16Z. (This area has since become obscured by high clouds.) So, the picture emerges of a narrow wedge of high PW alg and just ahead of the cold front approaching the coast, with associated pcpn, and a wind shift behind the front into the WSW and decrease in wind speed.
At 16Z the pcpn offshore appeared to be in 2 batches: one from W of the Bay Area S
to Piedras Blancas, and the other from Pt. Arena to the CA-OR border. The southern of these has since moved NNE into the central coast and Delta areas, roughly with the 700mb flow. This morning the 12Z raobs had the freezing level near 700mb, but the wet bulb zero near 850, reflecting a very dry layer from near the surface to near 700mb. This sgsts to me that the snow level in the HMT may be closer to 5,000 ft with this IOP. There is also the likelyhood of significant evaporation initially that may delay the onset of significant surface pcpn. In the Valley, the surface dew points are highest in the rainy areas (e.g. the Delta), indicating this evaporation is already having a notable effect.
I believe the NAM may be 3-h or so slow with the onset of the pcpn over HMT, which I think should be underway by 03Z this evening. However, synoptic forcing of upward motion appears to not reach its peak till tomorrow.
Of phps more interest is what the NAM is doing with the overall sitn. It shows sfc wave formation offshore (near 30N/130W) tonight, and a new deepening surface low moving toward Nrn CA. The GFS is basically similar, but slower and slightly farther SE with the track of this wave. I am inclined to slightly favor the GFS in its handling of this feature, based on experience. This development will retard the eastward movement of the front now off the coast, setting up what looks to be a 24-30h period of pcpn over the HMT, with heavy pcpn ending before 12Z Tu as this new surface low moves rapidly ashore OR or Nrn CA and weakens. Frontal passage by GFS reckoning is anticipated to be about 08Z Tu (midnight LT), but I think this can at this point be considered only +/- 3h or more. Heaviest pcpn appears likely between 20Z Monday and frontal passage. Following the front snow levels will drop to 3500 to 4000' as low level flow continues with an upslope component and much colder air moves in aloft. However, synoptic forcing is likely to steadily diminish after frontal passage and be nil by 18Z Tuesday - 00Z Wednesday.
Then, there is a break of 36h or so before the next, colder system begins to affect the HMT. I expect this beginning will be late Wednesday, based on GFS indications.