« HMT forecast discussion for Sunday 6 January 2008 | Main | HMT forecast discussion for Tuesday 8 January 2008 »

HMT forecast discussion for Monday 7 January 2008

KBLU METAR is back and last bits of snow from this long system are falling this morning with some light snow still at 19z and a temp of 28. According to the KBLU obs 0.44" of melted in the last 24 h.
Reports from the radar site indicate about 2 feet of snow fell from the event. A quick overview for this week. The two waves of interest are moving rapidly eastward across the Pacific as a strong jet of up to 150 kts+ continues, with the max from near 40N/140W back to 30N/160E. The first wave located near 45N/155W is a tight spiral at lower levels with upper level cloudiness well in advance of the upper level shortwave, while the second system looks stronger at this time aloft and is found near the Dateline at 40N. A nice plume of moisture remains in place north of Hawaii and extending back to the sw, and this is being tapped by the first system at this time, while the second wave is forecast to tap into the plume as well before it hits midweek. SSMI imagery indicates about 1.5" or so of PW in the plume now feeding into the lead wave. The main issue with both waves that will limit their potential for heavy precip is that they are fast moving for one, and that the second wave tends to hit the coast farther to the north, and weakens considerably from its current status. The latest 12z GFS is in good overall agreement with last night's ECMWF run on these two systems.

Some details: on the first system, timing still looks to have light precip beginning near 12z/Tue. The precip picks up by 18z, with the heaviest period about 18z/Tue to ~02z/Wed. The 00z ECMWF put out a 0.85" max just south of KBLU in the 12h ending 00z/Wed, then another 0.26" max in the next 12-h period. The latest 12z GFS is roughly similar but with the heavier precip shifted more to the nnw of KBLU. At KBLU, very light precip begins near 12z, by 18z the GFS has 0.17" total, then by 00z/Wed another 0.7", followed by 0.22" by 06z/Wed which is about the end of the precip from this system. The high resolution models run by NOAA have slightly different timing. The WRF/NMM has the same start time but only light precip until about 21z when it picks up dramatically, then continues until 03z and tapers off after 06z. Totals for this period then are 1.75-2" max. Only slightly less in the WRF/ARW. Both could be too much, as we have somewhat limited verification of these hi-res models, but clearly the max in these runs is tied to the terrain which is better resolved in the hi-res runs. The latest runs of our own 3 km models indicate similar timing to the NCEP hi-res runs; some echoes begin to appear in the ARB area at 13z, increasing in coverage down from the highest terrain by 17z, then really picking up by 21-22z as the main rainband with the shortwave moves inland, with ~0.7" by 00z/Wed (end of the current runs available from 12z). The 12z NAM has similar timing; precip beginning around 12z/Tue is quite light, with around a tenth of an inch or so by 18z, but then begins to increase, at KBLU 0.13" 18-21z, then 0.5" 21-00z (with up to 0.75" to the nnw), another 0.4" by 03z, then decreasing by 06z with only light showers thereafter. This adds up to ~1.5-1.75" for max amounts near or in the ARB area for the 12z/NAM run. The 12z Canadian Regional model run, which also has higher resolution, supports these higher totals. Latest HPC graphics indicate about an inch or so forecast for this first wave in the ARB region. It looks like this would be a worthy event, especially since the crew is already there.

NAM BUFR forecast soundings indicate good warm advection from ~800-650 mb tonight, then on Tuesday lowering as the wave approaches, with wsw flow increasing to around 50 kts in the 700-750 mb layer ~21z/Tue to ~03z/Wed. Upper level (500 mb) trough passage is around 04z/Wed at KBLU, with a rather ill-defined low level wind shift. The temperature profile never rises above the freezing point at KBLU, supporting the idea by WFO/SAC yesterday that the freezing level would remain lower than KBLU (lower than 5282 feet). At SAC the freezing level in the BUFR sounding forecast rises to near 6800 ft, so upslope cooling must be lowering the level as we head up towards KBLU in the model forecasts. The WFO/SAC forecast for KBLU has rain and 34 for tomorrow, so it looks close. A look at cross-sections from our 3 km runs indicates a rise in the freezing level for a time just above the BLU elevation tomorrow, then lowering below it as the precip increases. Consensus from the telcon was a freezing level near the radar site but probably staying below 5000 feet, so mainly snow at KBLU, but a close call.

The next system brings some light precip to the ARB by 06z/Thu, with the GFS then having 0.14" by 12z/Thu, then not much more, with the heaviest precip staying near the coast, so only about a quarter of an inch for the ARB area. The 12z NAM is quite weak with this second wave, having only very light precip by 12z/Thu, then only another ~0.33" or so shifted toward the higher terrain west of KBLU by the end of the run at 00z/Fri/11 Jan. Right now this looks like a very marginal event. If we can somehow delay a decision until the wave gets closer then we should know more tomorrow. The main potential for better rainfall would be a better tap into the moisture plume then the current model forecasts are showing. Possibly a little milder temperatures than the next system with this wave.

Looking ahead, still looks dry in the longer range forecasts beginning after the Thursday system, as an upper level ridge builds along the West Coast through midweek next week (Wed/16 Jan). Then the ridge
retrogrades a bit in most of the ensemble forecasts but things remain dry through the next week (ending 23 Jan). Pretty good agreement right now in the ensembles that this will be a prolonged dry period.

ed szoke NOAA/CIRA/GSD