HMT Forecast 2/11/07
This very good atmospheric river event is finally coming to a close. This morning's reports are steady rain in Blue Canyon at 36F, 34F at Truckee with moderate snow, 50's at lower elevations mostly cloudy in the ARB with rain still on the coastal areas. The current atmospheric river has moved south of CA and is now impinging on Baja, however lingering moisture remains over the ARB and central CA valley. Out in the Pacific a high pressure center at 33N 155W is now directing the moisture from 2 plume sources and that moisture is just now cresting the ridge. This might be the source of the potential precipitation that we have been watching for later this week.
Near term forecast:
Precipitation in both the GFS and NAM end about the same time, only today the NAM appears to stop the precipitation a bit sooner than yesterday's GFS. The NAM has 0.1 inches liquid equivalent every three hours up until 3utc 2/12 after which precipitation appears scattered. The GFS on the other hand has the event ending somewhere in the same timeframe but reporting 0.01 inches in the 6 h prior to 06utc 2/12. The GFS ensembles on the other hand show an end to the event after 00utc 2/12. So it appears that the precipitation should wind down for sure around 03utc. After this the area goes dry for an extended period that will be discussed in the outlook below.
500 hPa winds remain fairly constant out of the W to SW at 30-50 knots up to and beyond the end of the precipitation with no noted trof passage at upper levels until well past the event at 15 utc 2/13 (Tuesday). Surface winds also remain fairly constant out of the SW at 25 knots up until about 12 utc 2/12 when they back to the S and lighten to 10 knots. This also corresponds to the time when the freezing level stops dropping and might indicate the arrival of the cooler air behind a weak front. As opposed to earlier forecasts that showed a definite fropa, it now appears that this event just sort of runs out of gas.
Freezing level begins this morning at about 6600ft which drops to mountain top levels for the duration of the precipitation. Then about 09utc 2/12 it drops to 4600 ft associated with the arrival of cooler air behind the front. This is after the precipitation is indicated to have stopped.
By 2/14 00utc the moisture coming over the ridge center at 30N 145W shows a new high pressure center spinning up at a more favorable location at 35N 130W with moisture making the coast but not inland. By the 15th 18utc 1.8 cm TPW is sown over the ARB with the main plume never making it to shore due to a low at 45N 155W that develops and redirects the main moisture back out to the Pacific. Though the plume "skinnies out" it appears to progress east with time and might eventually make it to the ARB sometime near 2/17.
On the other hand the GFS ensembles show that the current system ends for sure by 00utc 2/12 with 100% chance of precip in the 12h before that time but zero afterward. After that point, the ensembles indicate that the there is virtually no chance of precipitation over the ARB until a possible 50% chance on the 19th at 12utc and a higher (70% chance) on the 20th at 12utc. After this, weak probabilities are indicated for the 23rd and 26th.
Based on these progs, I would forecast that there will be an extensive dry period after the current event ends today. I would call 03utc 2/12 the end of this event.
Dan Birkenheuer - ESRL/GSD