HMT forecast discussion for Friday 25 January 2008 (entered 2030z/Friday)
A complex situation for the next 72 h with many uncertainties, as discussed in the telcon. Bottom line is that IOP6 was called based on the potential for a significant period of precipitation across the ARB beginning late Saturday or Sunday morning and continuing into Monday, with the chance that the snow level could rise to above Blue Canyon on Sunday with increasing southerly to sw flow.
The upper low that has persisted all week off the central California coast has strenghthened and retrograded this morning as a new shortwave has dug southward on the east side of the upper level ridge centered near 155W. The current upper low sits west of SFO near 37N/130W with a slight drift to the west. The result is a new round of heavy precipitation that has moved into Northern California this morning and is pushing towards the ARB, but stalling JUST to the west of SAC at this time and may not make much further eastward progess over the next 24-36h. This band of heavy precip is relatively narrow, with the heaviest precip at 20z over SFO where there is even some lightning, then extending offshore to the wsw. The upper low is forecast to elongate off the coast over the next 24-48 h and then move inland ahead of the next wave which is at this time forecast to progress across Northern California late Sunday into Monday. The combination of these systems could bring some pretty heavy precipitation to the ARB very late Saturday into Monday; certainly IOP-worthy amounts. Following these 2 systems the pattern is forecast to change to a much more progressive one with a series of storms beginning ~Tue/29 Jan and conintuing into next weekend. Each system appears to tap into some tropical moisture, but also is relatively fast-moving. The details are below, but I think it should be noted that there are some significant uncertainties, in my view, in how this system will evolve, with the possibility that a heavy precip period could occur late Saturday through early Monday.
There are several questions worth considering that were discussed in the conference call. First, up to this point, the upper low that has been hanging off the California coast this week has not had any real tap into an extensive moisture plume. There is a chance that could change over the weekend, as there is currently a surge of high PW air moving northward towards the Southern California coast. This does not factor into the precip for the next 36h, but even so, the latest 12z NAM produces nearly 6" of rain north of SFO and up to 5" of accumulated precip just north of the ARB, but with a sharp gradient and thus only 1.05" at SAC and 1.25 at BLU for the 36-h ending 00z/Sun. The latest trend in what is actually occurring suggests this may be an over-forecast for this period, with the ARB staying JUST east of the heavy precip. The current snow level is below BLU, with KBLU this morning at 32 in S- and a light ese wind. The snow level is forecast to remain below BLU by WFO SAC through the weekend, but as was noted in the telcon there is the possibility it could rise on Sunday. As the upper low drops farther south and elongates the flow becomes more southerly and increases in strength. In the NAM 12z analysis at KBLU 850 mb flow is se at 10 kts with a temp of -2C, at 700 mb -11C with a ssw wind at 35 kts. By 00z/Fri (this evening) the 700 mb flow is forecast to increast to ssw at 55 kts, with a temp rising to ~-7C, at 850 mb the values are +2C and ssw-s at 20 kts. Then hovering between 0 to +2 for the next 24 h at KBLU with s-sse 850 mb winds of 20 kts (but much stronger to the west, up to s at 50-60 kts at 850 mb at KSAC), and at 700 mb -7 to -8C at KBLU with ssw winds 50-55 kts. The wild card comes into the Sunday period. The aforementioned moisture plume now far to the south is working northward on Saturday with the increasing southerly flow, but is forecast to stay just east and south of the ARB on Sunday as the upper low is predicted by the NAM to weaken rapidly and push inland as an open wave, passing the ARB about 18z on Sunday as the next wave approached from the north but takes an inland track. If this upper low is slower to move inland then i would think the surge of warmer and more moist air would move farther norhtward on Sunday with the potential for some really heavy precip totals in the ARB. This whole pattern has been hard for the models to predict certainly in the long range. Some of the SREF members from the 09z run are in fact slower moving the trough inland though they all eventually do....something to discuss would be how confident we all are that this will occur. The SREF mean still has 3-4" total for the ARB area by 00z/Tue with up to 7" on the coast. Through the same period the 12z NAM has ~1.5 at KSAC and ~4.5 at KBLU (up to 7-8" to the nw and along the coast). Note that the last ~24h of the system are quite chilly with dropping snow levels as the wave passes from the wnw, assuming it takes this track. An alternative on the dry side, as brought up by Dave Reynolds during the telcon, is that the ARB never gets into a good moisture plume, with the upper low passing too far south (the old "south of 40N rule"). Indeed, it is a risk that the main moisture could never make it far enough to the north and the main precip would be with the second shortwave, so the system also remains cold and all snow at KBLU. On the other end of the spectrum, though, is the possibility of a heavy event, with amounts ending up per the HPC ~7" range in portions of the area by the end of Monday.
The 00z ECMWF model was in general agreement with the evolution of the systems described above, but has some distinct differences in the precip forecast. By 00z/Sun, for example, the accumulated precip max of just over 5" is found SOUTH of SFO, with 3.3" at SAC and 2.9 at BLU. The weakening upper low passes near SFO at 12z/Sun, with the next wave from the nw quickly following this and passing the ARB by or before 12z/Monday, with precip ending. Total accumulation by late Monday in the 00z/ECMWF run then is ~4.5" at both SAC and BLU. Timing in the latest 12z ECMWF is similar to what is noted above, but it should be mentioned that it looks like the EC has too much precip in the first 36 h too far east, compared to the latest trend and the other models. The 12z GFS is best described as slower in moving everything inland, and keeps the heavy precip just west of the ARB until later in the weekend. As a comparison then, by 00z/Sun in the GFS the accumulated precip is 0.6" at SAC and 0.8" at BLU, then the precip increases as the weakened upper low passes to the south followed quickly by the next wave, with only cold advection precip after 15-18z on Monday. Final totals by 00z/Tue are 1.4" at SAC and 3.1" at BLU. The max of 4-5" is on the coast from just north to south of SFO, and to the north of the ARB. So at this point none of the models are stalling the upper low off the coast, though the GFS is slower, allowing for some of the plume to make it into the ARB on Sunday just ahead of the next wave.
Following this system, as noted earlier, are some faster-moving waves approaching from the west-northwest. In the latest GFS the first of these is ~21z/Tue through ~21z/Wed, with about 1" of precip at BLU, and likely snow as it looks fairly cold. A weaker wave on Thu-Fri, then maybe something more significant on the weekend (2-3 Feb). The weekend trough looks to be a slower one again, but it may dig farther inland and not be a big precip producer along the coast. At the moment, it is not clear that any of these will be worthy of an IOP, but it is of course too early to tell.
ed szoke NOAA/CIRA/GSD