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HMT forecast discussion for Thursday 3 January 2008

The first part of our long-awaited 3-part event has begun. This morning rain has begun on the coast, with the first main band now extending inland but still west of the ARB. South winds gusting over 40 knots in buoy reports near the coast from SFO northwards. Dry air pervades the area at low levels, as air involved in part 1 of this event is mainly recirculated air from the North American interior. This should keep precip in the ARB today on the light side. However, this will change overnight, when the remnants of this first band combine with the frontal band to produce a period of heavy precipitation. The start of this period in the NAM is near 0300 UTC, with a little over an inch forecast for KBLU by 1200 UTC/Fri and close to an inch at KSAC. The first of the surface cyclone centers forecast by the progs is now off the OR - WA coast as about a 980mb center and moving nwd. Its frontal band is not too far off the coast of Nrn CA at present and will move inland later today and tonight, shifting the low level winds offshore more into the sw - w. There will be no real break between this event and part 2, as the next deep low center heads eastward and deepens, then turns more northward and heads for Vancouver Island. Part 2 promises to be a major storm over northern and central CA with strong winds and heavy pcpn, as has been indicated by the progs for several days now. Winds and melting-level evolution looks still consistent with yesterday's blog and telcon discussions. Heaviest pcpn is forecast for the daytime hours tomorrow into tomorrow evening. The NAM picks up the precip rates considerably by just after 1200 UTC/Fri, with a forecast of .75-1.00 inches per 3h thru 0000 UTC/Sat, then a burst of even heavier precip as the main wave pushes across the ARB, with ~1.7"/3h ending at 0300 UTC/Sat, then rates falling to "only" 0.3-0.5"/3h through 1200 UTC/Sat, by which time the total precip is ~6.3" at KBLU, but a max of near 8" just to the west, ranging down to 3.3" at KSAC. Cold advection orographic precip then continues for the next 24h at a pretty good clip, 0.25 to 0.5 or more inches per 3h period, so that by 1200 UTC/Sun/6 Jan the NAM has total precip values for the event of near 9" at KBLU, ~11" for a max value just to the west of KBLU, and 4" at KSAC. This appears to be the 3rd part of the system, which is a somewhat ill-defined wave moving out of the wnw around the big Alaska upper low that would then pass across the ARB near 1200 UTC/Sunday, after which the precip in the NAM then tapers off to light activity through the day on Sunday. These amounts are not far off the HPC forecasts: 5.2" near KBLU for the 24-h ending 0000 UTC/Sat, another 3-4" (max of 5+ is to the south in their forecast) for the next 24-h ending 0000 UTC/Sun, then more precip thereafter. Here is a comparison to some other models. The 1200 UTC GFS has similar evolution through Saturday to the NAM, but is not as tied to the topography, and so keeps its max amounts near the elevation of KBLU, and then shifts things south a bit more than by Friday night. Initial precip is somewhat less for part 1; by 1200 UTC/Fri ~0.7" at KBLU with 0.55" at KSAC. But precip cranks up on Friday after 1200 UTC, with ~4" at KBLU in the 12-h period ending 0000 UTC/Sat, then precip tapering off but continuing at 0.25-0.33"/6h through 1200 UTC Sun. Totals in the GFS then by 1200 UTC Sun/6 Jan are near 7" at KBLU and 3.8" at KSAC, so similar numbers to the NAM at lower elevations but less higher up, which could simply be a reflection of the better resolution in the NAM leading to greater orographically forced precip. This idea is consistent with the output from the hi-res 5 km NCEP window runs, with the 0600 UTC WRF/NAM having total precip by 0600 UTC/Sat of up to 8" in the ARB, and closer to 9" for the same time frame in the WRF/ARW. One difference that is present between the GFS and NAM is the precip does not diminish in the GFS at 1200 UTC/Sunday, but instead actually increases just ahead of the shortwave passage around 1800 UTC/Sunday, with a 6-h burst of ~0.8" ending at 1800 UTC/Sun before precip then diminishes. Some precip amounts for comparison from the ECMWF show less overall than the GFS but with some differences. For part 1, the EC is very similar to the NAM in total precip by 1200 UTC/Friday. Not quite as robust for the big show on Friday, with an additional total of ~3" in the ARB for the 24-h ending 1200 UTC/Sat, but a little heavier for the period later Sat into Sunday, with the precip then diminsihing by ~1800 UTC on Sunday in the ECMWF.

As for wind and snow levels, consensus on the telcon is for the snow level to initially be rather high, ~7000 ft or so, and remain above the KBLU elevation (5282 ft) thru the daytime hours on Friday when the very heavy precip is expected. Winds will be very strong, with 700 mb SW winds forecast to increase to a peak of 85 kts by 1800 UTC Fri (when 700 mb temps are predicted to be in the -2 to -3C range), then the 700 mb front passes around 0000 UTC in the ARB and temps drop, to -10C by 0600 UTC at KBLU with winds shifting more to WSW and decreasing to ~50 kts. The winds continue to decrease into the 30-40 kt range on Saturday with cold 700 mb temps around -10 to -12C. An increase in wind occurs as we near 1200 UTC/Sunday with the approach of the final wave in this series, with its passage then shifting the flow at 700 mb more to the west on Sunday, consistent with the decrease then in precip. Even though the heaviest precip falls as rain at KBLU on Friday, telecon consensus was for several feet of snow possible by the time all was said and done (the 9th or so) at KBLU, and good snowfall beginning Friday night and continuing into early Sunday.

Although precip tapers off in the models sometime on Sunday, there is another separate system that rapidly moves across the Pacific and reaches the ARB not long after Sunday. This is a fast-mover but potentially wet again, as it appears to have tapped into the long stretch of moisture that still would be in place across the Pacific. Precip with this one picks up in the ARB very late Sunday night (~0600 UTC/Mon) through Monday night, with another 1.5-2" in the GFS by 1200 UTC Tue/8 Jan. There are variations in the model solutions for this event, particularly with when the potentially heavier precip starts up. So it is quite possible that there could be a break in the heavier precip from ~1800 UTC/Sunday until sometime on Monday, the way it looks right now.

But all is not finished even then, as yet another wave in the series, this one likely to be even more moist than whatever happens on Monday, approaches the coast with the flow becoming more wsw and again tapping into some deep moisture. The result in the latest GFS is another burst of heavy precip, beginning right as the other one ends ~1200 UTC Tue, and continuing through ~1800 UTC Wed, with an additional 4-6" forecast for the KBLU vicinity in the GFS. Ridging follows this system and then builds along the West Coast through the weekend (12-13 Jan) with drier conditions in the GFS, finally. This is consistent with the ensembles and other model forecasts. In fact, the timing of these events from today through Wed is pretty close in the GFS to the latest ECMWF 1200 UTC run, with the EC being just a little quicker to end the final event (by ~1200 UTC/Wed). The 1200 UTC Canadian Global model is also relatively close to the GFS and ECMWF solutions, although one difference is it has more precip later Sunday night than some of the other models.

Looking much farther ahead, the 1200 UTC GFS ensembles support a drying period beginning after the Tue/Wed event and continuing not only through the weekend of 12-13 Jan, but through much of the following week as well, with a ridge along the West Coast.

ed szoke NOAA/CIRA/GSD