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HMT forecast discussion for Saturday 23 February 2008 (entered 2020z)

Wrapped up surface low analyzed at ~974 mb (528 dm at 500 mb) in the 12z/NAM and mb in the 12z/GFS is sitting west of KSFO this morning with initial bands of precip already on the coast west of the ARB. Not a strong elongated plume into this system but a tap into the plume that does extend well w-wsw into the Pacific. The intensity of the storm will make the most of whatever tap it is making, certainly though not a classic-looking elongated atmospheric river into the ARB, as there is a very strong system to the west near 160W (the one that built the upstream ridge for our current storm) that is also tapping into the stream of moisture in the Pacific. The measurements of IPW this morning do show a rapid increase as the system approaches, with Bodega Bay increasing in the last ~6h from 0.25" to now close to 0.8" from the GPS site there. The upward trend has just begun at the Big Bend site. The 12z OAK sounding still has relatively light southerly flow with a freezing level near 850 mb. The model analyses indicate however that much stronger winds were just off the coast at 12z this morning and the flow is forecast to rapidly increase out of the sw today. This is in fact being seen by the various ESRL profilers, with the Bodega Bay profiler now (19z) showing 50+ kt flow from near 2 km and higher out of the south (more sw by 3.5 km MSL). Sloughhouse winds have also dramatically increased in the last few hours. The NAM predicts 700 mb winds near KBLU approaching 50 kts by 00z/Sun then peaking around 65 kts overnight. Winds continue out of the sw tomorrow and will probably remain quite strong (~50 kt flow at 700 mb for most of the day) then gradually weaken very late as the concentrated upper low weakens rapidly after 12z/Sun, to an open wave at 700/500 mb by 00z/Mon. The winds shift at 700 mb to more westerly after 00z/Mon with the trough passage at 700 mb; maybe a few hours earlier with a shift to more westerly flow at 500 mb. This scenario is forecast in good agreement with both the NAM and the GFS from this morning. WFO SAC indicated strongest winds at KBLU likely to be from about 06z/Sun into Sunday afternoon.

In terms of the timing of the precip, still pretty much as discussed yesterday. The larger scale models indicate precip beginning by 21z in the ARB. Our local models have the first main band into the ARB at 21z this afternoon, then heavy precip continuing through 00z/Mon, then gradually becoming more terrain forced and slowly decreasing, with most over by 06z and totally done by 12z/Mon. This is similar to the 12z NAM and GFS, which have heavy precip through ~06z/Mon, then over by 12z/Mon. Totals from the NAM are max amounts in the ARB near 4", with 2-2.5" in the GFS. A large area in the 4-5" range from our 3 km individual runs with embedded spots above 5". Our ensemble mean totals from the 06z runs gives ~3.5" or so max in the ARB for the event. Discussion during the telcon noted that right now the first band moving into the higher terrain of the ARB is hitting some drier air and widespread virga is occurring but no precip yet (as of 20z). So maybe more like 22z or so for the start of precip at KBLU.

The precip type forecast continues to be tricky, with the current temperature at KBLU at 32F at 20z. However, as the winds increase out of the sw a slug of warmer air is forecast to push northward and it looks likely that the freezing level will rise to at least 800 mb/~6000 ft or so by later this afternoon and remain above KBLU into the evening. There was considerable discussion on this aspect of the storm. WFO SAC noted that with some clearing last night there was more cooling and drying in the higher terrain, and the stronger winds and warming aloft are still a bit off to the west, so actually with wet-bulb cooling as the atmosphere initially saturates the precip should start as snow at KBLU this afternoon. Then it will probably (tho not a certainty) change over to rain for at least a few hours before further dynamic cooling brings it back to snow, probably near or after 06z. This is also seen in the NAM and GFS, with gradual cooling as the upper low approaches but and it still looks like the precip at KBLU would go over to snow around 06z/Sun. However, this is also somewhat uncertain; our local model keeps the freezing level close to 800 mb in the vicinity of KBLU through the entire day on Sunday, having only gradually come down from reaching closer to 750 mb later today. It seems unlikely that it would stay warm enough for all rain past 06z or 12z/Sun, but the snow conceivably could be so wet it might slip off the radar (or hopelessly stick to it driven by the strong winds?). WFO SAC also noted on the BUFR soundings (from the NAM) that there was a little bit of warming aloft actually tomorrow just ahead of the Pacific cold front, so there was some talk of maybe precip at times going back to rain, tho this seemed unlikely in the view of WFO RNO. Our latest 18z runs from our local models show a freezing level right near or even above KBLU into Sunday, but precip type as frozen down to just below KBLU. This is certainly a very tricky part to the forecast. We decided at the telcon to revisit this with another call at 430pm between John and I, and then later John and Dave would talk. At issue is what the radar crew would do in terms of leaving the site as the precip goes to snow and radar operations become marginal.

A brief look at the longer range...this week still should be dry with the next wave riding north into the Pacific NW; this storm is the strong one now near 160W which has a great tap of Pacific moisture, but weakens dramatically over the next few days. However, some models (EC, Canadian, NOGAPS) split this system and move a very weak closed low towards the nrn/ctl CA coast ~Thu/28 Feb, probably not enough to be an event but may bear watching. It still looks like the next potential system of note could be late Friday into the weekend, although the latest 12z GFS is not favorable being fast-moving and a bit north. Other models and some ensemble members are farther south with this system, however, and the GFS PW look from the U of Hawaii does show at least some Pacific connection intact as it approaches CA. Beyond that into the first week of March the deterministic 12z GFS run keeps a ridge along the West Coast, but other models/ensemble members remain interesting with an active Pacific and progressive systems that do reach the coast, so quite a bit of uncertainty in early March at this time.

ed szoke NOAA/CIRA/GSD