HMT Forecast made 1930Z Tu 1 Jan 08
6.7 micron satellite imagery shows that a continuous band of upper tropospheric moisture now extends from the equatorial Pacific se of Guam to s of HI to near 20N/145W. The low-latitude upper potential vorticity anomaly that has been plunging sewd across the Dateline toward HI is becoming deformed and elongated ENE-WSW and is reinforcing the subtropical jet near the north edge of the above-noted band of upper-tropospheric moisture. Farther N, the eastward moving upper trough upon which our hopes rest for a major IOP Th - Sunday 3-6 Jan is near 145W and showing the anticipated indications of amplification. The leading edge of the tropical upper-tropospheric moisture band is becoming entrained into this trof's upper wind field. SSM/I imagery shows this moist band has a lower tropospheric component, with integrated water vapor of up to 5 cm indicated s of HI. At this time, the low-level winds in the Pacific E of the Dateline are primarily easterly S of about 25N. This is still far enough south to permit entraining of some of the deeper Pacific moisture into this upcoming event.
The major trough in the westerlies noted yesterday as being over the Japan - Kamchatka area is slowly shearing out to the e and ene, with the nose of a 170kt jet and associated s/w trough on its se flank just now crossing the Dateline. This upper-tropospheric long-wave feature will be feeding short-wave troughs eastward that are expected to traverse the central Pacific ridge into the currently evolving eastern Pacific long-wave trough position. The first of these s/w trofs is a little east of the Dateline at present and is indicated to be the first part of the 3 part event for Th-Sunday. Meanwhile, a deep low associated with the upper trough near 145W is moving into the Gulf of Alaska. The occluded front with this low is near the BC coast and approaching the Northwest coast. With the collapse of the Pacific High during the past 24h, the surface winds over CA, OR and WA are offshore, bringing dry air into the southerly flow offshore, ahead of the approaching front. Surface dew point at KBLU is near -15C at present, although foggy conditions prevail in the Valley. A weak upper trough in the subtropical branch of the upper flow is approaching northern Baja, bringing some upper moisture toward srn CA, but no pcpn.
This morning's progs differ from earlier ones in details, but not in their main theme of a major event in 3 parts, in regard to Th - Sunday. Overall, relative to earlier forecasts, the second part of the event on Friday is given more emphasis, the onset of pcpn Th is delayed by maybe 6h from what was indicated yesterday, and there remains some difference of opinion on the third part of the event, both timing and intensity. The first part of the event, as noted above, is now just e of the Dateline, and is forecast to approach the Northwest coast Th evening. Close on its heels is part 2, manifesting itself in the progs as a deep surface low (965mb) approaching the Northwest coast late Friday. I think this will bring a frontal passage through the ARB Friday evening with probably the heaviest pcpn, and a drop in the melting layer to below KBLU by Saturday morning. The melting layer should start out below KBLU on Th 3 Jan and then rise fairly rapidly so that KBLU will be in the melting layer (or perhaps slightly below it briefly) through at least a signficant portion of Friday's heavier pcpn. The melting layer will remain below KBLU for the third part of the event late Saturday/Sunday. The prospects for strong surface winds, particularly Friday, remains, with a possiblilty of gusts to 50kts at KSAC and KSCK.
The matter of a break in the pcpn between parts 2 and 3 is still uncertain. This feature is not yet apparent as a distinct entity in the data I have examined, and so we are at the mercy of the progs. Even in the GFS ensembles (which often tend to underestimate uncertainty), there is considerable disagreement on the timing and even the existence of this feature. Today's GFS indicates less of a break than was suggested yesterday. I am confident that pcpn will be less intense on Saturday than F, but whether it will be sufficiently light to permit plowing at KBLU is uncertain. It may be Th 3 Jan before we can say with confidence whether or not there will be a major break in pcpn on Saturday.
Monday and Tuesday, 7-8 Jan appear likely to have strong w-nw winds at ridgetop, and some instability showers, but significant (> 1"/24h) pcpn seems unlikely. For late next week the potential for IOP-candidate situations decreases to less than climatology.