HMT forecast discussion for Thursday 21 February 2008 (entered 2045z)
Relatively disorganized system still looking to come across the ARB in two stages, as discussed in yesterday's telcon. First part has entered the lower elevations as a weakening ~north-south band with precip at KBLU beginning as snow at 18z with a temperature of 34, now (20z) down to 32 with snow. A couple of other bands follow the first one, and the weakening of the first band is consistent with the latest 12z NAM, which has the main precip beginning about 18z today. A surface low sits off the far northern CA coast, with a trailing cold front southward that is also still off the coast at 20z, with evidence of a couple of weak shortwaves in the more westerly flow behind the front. There is an extensive moisture plume stretching back from this system to the west-southwest all the way back to the sw Pacific. Satellite PW values close to 1.5-2" in the eastern portion of the plume, much higher well west of the Dateline. The moisture plume is aligned near and just south of an upper level jet with max speeds near 150 kt stretching back to the Dateline. Another even stronger jet extends off Asia across Japan with the speed max south of a strong wave near 40N/160E. This is not the strong wave that will hit the West Coast this weekend, but instead the one that rapidly amplifies an upper level ridge near the Dateline, with a downstream amplification of a shortwave now near 40N/165W that is the next system. This shortwave intensifies remarkably from its current status to become an intense upper level small-scale low, down to 525 dm by 12z Sat near 35N/135W in the 00z/EC, which has now come on board with a very strong wave; in fact, the new ECMWF is even stronger, bottoming out the surface low at 968 mb at 12z Sat and the upper low at 524 dm, lower than the 12z GFS which has 977 mb at this time. This strong storm is forecast to track into northern CA later on Sunday and weaken rapidly. As it approaches strengthening sw flow ahead of the upper level wave should bring warming to the higher elevations of the ARB, with the threat of rain for at least the start of the storm at KBLU. More on that later, but first details on today/Friday's event.
In the latest NAM the increase in precip begins after 18z today, which is in fact happening, with precip continuing through ~21z/Fri (with a total for this period of ~1.5" at KBLU but only ~0.35" at KSAC) with a trough passage at 500 mb around 08z/Fri, then this is followed by another weaker shortwave that increases the precip late Friday for about a 6-h period decreasing around 06z/Sat (for an additional ~0.4" at KBLU and ~0.10" at KSAC), but with the precip falling with colder temperatures. Somewhat uncertain how much will occur with this second bit, may not be of great interest to the HMT, but can be addressed in tomorrow's telcon. Overall storm totals in the higher terrain of the ARB then by Saturday morning (precip ending by 09z/Sat) in the 12z NAM range from 2-2.5". It is currently (16z) right at 32F at KBLU and precip will be all snow with this storm, with temperatures holding steady today then falling tonight with the passage of the lead wave. In the 00z/ECMWF run total precip from this system is on the order of an inch at KBLU and 0.70" at KSAC, with generally similar timing to the NAM. The latest 12z/GFS predicts 1.1" at KBLU through 18z/Fri with 0.35" at KSAC, then another burst like in the NGM later Friday through 12z/Sat but with only ~0.25" more at both KBLU and KSAC. What I've seen of the local models is similar to the NAM, but with maybe a tad more max precip. The 09z SREF mean precip through the event is around 1.5", with some of the Eta members being more moist.
The more interesting and stronger storm continues to be the next one for Sat into Mon, an intense storm that will tap into the moisture plume and send strong sw flow into the ARB. The starting time for the main precip with this storm is late Saturday (~23z in the 12z/NAM and similar in the 00z/ECMWF, but maybe a bit earlier in the latest GFS) with heavy precip overnight Saturday through the day on Sunday. This is a little faster than the NAM but similar to the 00z/ECMWF run. Some precip totals for this storm include: for the 00z/ECMWF 2.2" at KBLU and 1.2" at KSAC (through the end of precip around 12z/Mon/25 Feb); in the 12z/NAM about 3" at KBLU and 0.7" at KSAC (this is through the end of the run at 00z/Mon, so more would be expected for the entire storm); in the latest GFS about 4.2" at KBLU and 1.5" at KSAC; about 2-3" over the higher terrain (more to the south of the ARB) in the 12z/Canadian Global run, with similar timing to the EC, and about 2-3" total in the 12z/NOGAPS, but with a start time a bit later, around 00z/Sun, and finally the 09z SREF mean precip is in the order of 2.5" (through 00z/Mon). So good agreement on a significant event for the ARB. Considerable discussion of course during the telcon with lots of great input from various forecasters. Generally felt that max amounts of up to 4" likely with this event, maybe more given the uncertainties in the amount of moisture in the plume off the coast and the exact track of the wave, plus how intense it may be when it hits the coast. In this regard, the models generally agree that a strong surface low will be approaching the northern Calfornia coast by 12z/Sun, with the GFS at 985 mb and the EC 984 mb. But the EC is about 175 miles further south that the GFS, and this trend will have to be watched. Rapid weakening is forecast over the next 12-h period, with the position differences increasing so that by 00z/Mon the GFS has a 1003 mb surface low in nw OR, while the ECMWF has a more elongated low with the lowest pressure of 1004 mb in northern CA. Both scenarios will bring very strong sw winds into the ARB on Saturday into Sunday, with WFO SAC about to hoist a High Wind Watch for the area.
In terms of the snow level, it looks like it will be a tricky call, at least at storm onset and in the initial phases of the event, in terms of rain vs. snow at the radar site (BLU). This was also extensively discussed in the telcon, with possibility raised of a 6-h or so initial period of rain as warm air worked northwards on Saturday. WFO SAC favored all snow for the event, but other opinions ranged to an initial snow level of around 6000 feet. Some differences on starting time for the precip, but likely to be Saturday afternoon, with the heaviest 12-h period 00z-12z/Sun. A look at the BUFR soundings for KBLU from the 12z NAM indicates increasing deep sw flow, reaching 50+ kts by Saturday afternoon, but temperatures still warming only to near freezing at the surface and cooler aloft, with the entire saturated profile remaining near or below freezing through 00z/Mon. No BUFR soundings available (to me) from the GFS, so examined the model interpolated soundings only that have lower resolution. These soundings for KBLU have more initial warming, with a freezing level near 760 mb at 00z/Sun, which then quickly lowers to near 800 mb by 06z and then to near the KBLU elevation by 12z/Sun, with the profile holding below freezing with only some cooling on Sunday. Trough passage near 00z/Mon in the GFS, but no real cooling, with warming aloft already by early Mon as the upper level ridge builds in. In the current forecast the SAC WFO has snow levels rising to the 4000-5000 ft level as the storm commences on Saturday, and then lowering to 3000-4000 ft on Sunday, along with potential high wind warning criteria winds with the event. Whether all snow or rain to snow, the snow would initially be heavy and wet, getting to somewhat higher ratios on Sunday but certainly not Colorado powder, but the combo of heavy wet snow and strong winds could create major radar issues at BLU by Saturday night, and will be further discussed in the telcon tomorrow. Dave Reynolds mentioned the 4 Jan IOP with its very strong winds and the radar problems that resulted in that event.
Longer range another strong trough approaches (the one west of the Dateline now, which was discussed earlier) but never really makes it into the ARB and instead ridging occurs through next week as this wave drives eventually more to the north into the Pacific Northwest. Other waves then lined up in the Pacific, and the ECMWF 12z run brings the first of these into the area by 1-2 Mar, while the GFS is weaker and farther to the north. Ensembles show quite a bit of uncertainty by ~days 8 through day 15 (~1-8 Mar). So relatively high confidence of a dry period after our big weekend storm for most of next week, then some possibilities as we get into early March.
ed szoke NOAA/CIRA/GSD