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2006-02-28 WX Discussion and Forecast

Light echoes in the ARB this morning, 40 dBz tapering down to 20 over the higher terrain. There appear to be numerous echoes still on and off of the coast and these may still portend some continued precip through out the day over the ARB as they progress inland. The models indicate that the event should begin winding down with probably only lingering showers past 06h utc Wednesday.

This morning HMT run shows precipitation occurring up until 06h 3/1 utc after which it falls off rapidly, so this high resolution run also corresponds well to the NAM and GFS.

The forecast question of the day is really the onset of the next event and its forecast duration.

The NAM has the approaching precip band with the progressing closed upper level low moving inland by 06h 3/2 (Thurs). The first precipitation in the ARB from both NAM and GFS appears to begin about 09h 3/2 utc. Of the two, the NAM is slightly stronger with precipitation out to 12h 3/2 with a lull for a few time periods and picks up at the end with an additional 0.2 inches in the final 3 hours. As far as the NAM extends, it shows a total QPF of 1.58 inches between 09h 3/2 and 06h 3/3 utc.

The GFS begins the precipitation about the same time, likewise with a strong surge of moisture. Both models tend to show bands of precipitation that have been entrained in circulation around the low pressure system. As each of these bands moves over the ARB the precipitation appears to rise and fall accordingly. There is an initial strong precip event at 12h 3/2/06 utc much like the NAM followed by a tapering of precip until 9-12h 3/3 when there is another surge. This is then followed by a complete lull until 03-06h 3/4/06 utc afterwhich we see 0.84 inches in a 6 hour period. The event continues until about 09h 3/4 utc (Sunday). It is followed quickly by another system. Total QPF for the GFS for this system through Sunday is 2.82 inches with 1.14 inches falling in the same time frame as the NAM forecast termination (which is comparable).

The wind field is not as strong as this current system with winds staying mostly south at 20-30 kts and an upper level wind shift passing about 12h 3/3 utc possibly associated with the decrease (lull) in precipitation noted in the model. However, this feature has little affect at the surface being evident only above 600 hPa. After this upper wave passes it appears that precip at the surface resumes until the moisture in the system moves through. The GFS never runs far enough out to show a good surface wind shift associated with fropa.

The freezing level with this next system appears to start the episode at 850 hPa just above mountain top level and then drops to mountain top level for the duration of the event. It is definitely not as cold a system as was forecast a week ago, nor does it look as interesting to me as the event that is currently taking place.

Dan Birkenheuer - ESRL/GSD