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HMT discussion for Saturday 24-March-07

Summary: Plume is still present and currently directed into the Pacific Northwest where flood watches and warnings prevail in Washington. Main change appears to be a slower southward movement to the plume, so that it is not really over the HMT area until later Sunday. Operational models still not showing precip with this except hinting at maybe scattered late Sunday activity, but some get decent precip close by 12z Monday. Maybe a later start to the soundes? Otherwise overall system still looks good with strong dynamics and at least brief entrainment of tropical moisture ahead of trough passage monday night.

Discussion: Our system is still on track with a well-defined plume present in the water vapor and SSMI imagery, the later showing up to 35 mm or so of total PW, which agrees with the analysis on the 12z NAM and GFS models. There continues to be some run to run variation on the overall total precipitation but overall things still look good for the main event with the strong dynamics passing in good position for the HMT area and the heaviest period probably from 18z/Mon through 03 to 06z/Tue, which is a good consensus time for trough passage near 700 mb. Decent cold advection precip thereafter until about 00z/Wed. Totals from the 12z GFS are 1.5-1.75 in a solid area centered on the HMT area, 1 to up to 1.5 to the south a tad on the 12z NAM. The 00z GFS had more, up to 2.5 centered on SAC, and the other 00z models all looked favorable and moist. HPC in their discussion felt the 00z GFS was too moist, and also were unimpressed with any tropical connection. Perhaps they are thinking more of a direct Hawaii type connection and not such a stretched out one like we now have. The HPC has a total of 1.75 for the event. Finally, the 09z SREF run has similar timing to the GFS and other models, with mean precip totalling 1-1.25 and some members over 2 inches.

The tricky part remains whether the moisture increasing as the tropical plume sinks south over the region will produce precip. Pretty much nothing but some very light activity suggested by the models until perhaps 12z/Mon, with the GFS getting closer by this point and then having a decent max, separate from the main event, just to the nw of the HMT for the 12z-18z/Monday period. The NCAR/RAP weather site has a nice total PW presentation from the GFS and NAM and this indicates the plume is slower to sink south than yesterday, but does show some increase in PW by 00z/Mon then the plume sinking over the area. Our local models at this point only go out to 00z/Monday, but by then there are echoes and a band of accumulated precip about 70 miles to the nw of BLU, all with max values under 0.50 inches, and also lighter activity into SFO. The WRF-NAM at 3 km out to 72 h (12/Tue) has total precip values of around an inch, which seems low, and some maxima up to 1.5. Precip in this model does develop Sunday afternoon but to the nw of the HMT, moving into the higher terrain of the HMT area by 12z/Mon, then increasing after 18z through Monday evening. The 700 mb flow becomes more southerly in the cross-section for this run on Monday and increases to at least 40 kts, consistent with the GFS and NAM, then trough passage 06-09z Tue. So overall it looks like the plume may be slower to set up but still does so, with the advent of precip holding off in the models despite its presence. Thus we still have a potential null case perhpas, or maybe there will be more precip beginning earlier. The main issue it seems would be whether to delay the start of the sondes 6h or so.

Looking ahead, overall the ridge actually starts to build with anything for Thu-Sat becoming much less likely at this point, and uncertainty beyond that but no system of note.

ed szoke NOAA/GSD


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