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November 30, 2005

2005-11-30 Daily Wx Forecast Discussion (2000 UTC)

Some fog is being reported along the windward slopes of the Sierras and in the Central Valley this morning, while IR imagery shows high/middle clouds starting to stream across the N half of CA with the onset of warm advection aloft ahead of the next approaching storm system. In fact, the KDAX, KBHX and KMAX 88-Ds are already showing some echoes moving eastward… probably associated mostly with virga for the time being. The Cazadero vertically pointing S-band radar is showing clouds moving in between 4-7.5 km MSL. Meanwhile, the latest SSM/I IWV imagery reveals a developing potent atmospheric river (core values of 3.0-4.3 cm) associated with the approaching southern-branch wave extending eastward to beyond 135W. This feature should impact CA tonight and tomorrow.

Overall, the various models are still on track for the landfalling storm to impact the ARB tonight through tomorrow…. although today’s model runs seem a little faster with both the onset of precip and its cessation. On the front end, the 12Z NAM is showing precip beginning in the ARB near or slightly before 00Z this evening, i.e., before dark. The vigorous mid-trop vortex approaching the Pacific NW tonight and Thursday will phase with the southern branch wave (this is seen nicely in the 12Z NAM with the consolidating northern/southern branch jet streaks over CA on Thursday). The northern branch wave is still progged to yield substantial large-scale dynamics while the southern branch system will provide the tropical moisture. The heaviest precip will likely occur between 12Z Thursday and 00Z Friday with the strongest jet dynamics, warm advection, onshore flow, and incoming tropical moisture transport within the atmospheric river, and the weakest stratification. During this period, there will likely be strong gusty surface winds >30-40kts. The newest models now show cold fropa across the ARB as early as 06-09Z Friday (i.e., before or near local midnight), with postfrontal showers persisting until perhaps 12-15Z Friday. We are still on track for a 3-5” precip event in the ARB, with perhaps 2/3 to ¾ of that falling between 12Z Thurs to 00Z Fri. Snow levels at the onset of precip will likely be in the 6-7kft range (or greater) this evening, rise to ~8kft later tonight into tomorrow, then fall dramatically with cold fropa tomorrow night… perhaps to as low as ~4-5kft by Friday morning. A trailing vort max Saturday may generate some light precip in the area, but most of that wave energy should remain N of the region.

Medium-range and longer models are now more consistent building midtrop heights offshore of the West Coast later this weekend and well into next week. The current thinking is that some wave energy may sag far enough S early/mid next week to bring some scattered light precip to the ARB, but once again, it is likely that most of the precip will remain to our N.

Paul Neiman

November 29, 2005

2005-11-29 Daily Wx Forecast Discussion (2130 UTC)

Post-cold-frontal showery precip at the back edge of a classic atmospheric river continues to form in-situ in the ARB, but with snow levels much higher now than they were yesterday at this time (>5.5kft vs. ~4kft). Precip amounts in the area have ranged from ~2-3” thusfar, about as expected. Precip should end this evening, and conditions should remain mostly dry later tonight through tomorrow afternoon in the shortwave ridge situated between this exiting disturbance and a more potent storm developing in the eastern Pacific.

A vigorous mid-trop vortex approaching the Pacific NW late tomorrow and Thursday will phase with a transitory disturbance approaching the CA coast in the southern branch. The northern branch wave will provide substantial large-scale dynamics while the southern branch system will entrain significant tropical moisture northward (as a potent atmospheric river) to the coast ahead of a slowly advancing polar cold front tied to the northern system. The southern system will tap into a rich moisture supply over the eastern Pacific with integrated water vapor (IWV) anomalies that are >2.5 standard deviations greater than climatology (NOAA/HPC). Deep warm advection on Wednesday afternoon will likely result in thickening clouds, but precip will likely not begin until after about 04-06 UTC 1 Dec. Once the precip commences Wednesday night, significant liquid equivalent is expected in the ARB through Thursday into Friday (~3-5”) due to very favorable orographics (700mb southwesterly - barrier-normal - winds exceeding 50kts) coupled with tropical moisture (IWV >3 cm) and large-scale dynamics. Moist-neutral stratification is forecast ahead of the cold front over the Sierras, thus allowing for further orographic precip enhancement. As the cold front makes landfall Thursday night or early Friday morning, precip may be further enhanced. Snow levels will start out at ~5.5-6.0kft Wednesday night, rise to at least 7kft on Thursday within the atmospheric river, then crash to as low as ~4-5kft by Friday evening in the maritime polar air stream. Following fropa, cold-core conditions aloft coupled with continued moist onshore flow may result in an extended period of showery conditions on Friday into Friday night.

The longer-range outlook for this weekend into next week is still uncertain. The suite of medium-range models are now hinting at low-amplitude ridging developing offshore of CA during this period. However, previous model runs have shown significant wave energy impacting northern CA. Bottom line: wait and see.

Paul Neiman

November 28, 2005

2005-11-28 Daily Forecast Discussion (2300UTC)

Hi All,

This is a short, test weather forecast discussion for the Sierra’s American River Basin (ARB) area that is hopefully readable on the HMT06 weblog. Daily wx forecast discussions will continue, as we ramp up our HMT06 activities in the ARB.

A vigorous shortwave trough will translate eastward across the Pacific NW and Rockies tonight and tomorrow. Based on the latest SSM/I water vapor imagery, this system has entrained subtropical moisture from Hawaii to the CA/OR border in a classical atmospheric river (AR) pattern. The AR will sag southward with the approaching cold front across the ARB during the next 24 h. This, coupled with enhanced mountain-normal low-level SWerly flow ahead of the front and jet dynamics aloft, should result in a respectable precip event for the ARB… perhaps 2-3” of liquid equivalent. Snow levels are presently rather low (<4kft, with Blue Canyon snowing) but should rise tonight to at least 6kft as strong pre-cold-frontal warm advection becomes established. Precip will become more showery following cold fropa tomorrow, then abate late in the day as mid-level drying ensues.

The drying will be short-lived, as another shortwave trough approaches the Pacific NW later in the week. In addition, a weak disturbance in the southern branch may phase with this system and entrain another plume of subtropical moisture into CA ahead of the slowly advancing polar cold front. Deep warm advection on Wednesday will likely result in thickening clouds, but hopefully precip will not begin until after the HMT06 field crew performs final field installations during the day. The fact that the atmosphere will dry out following tomorrow’s wave passage may mean that precip with the follow-on wave will be delayed somewhat due to evaporative processes. Regardless, once the precip resumes late Wednesday or Wednesday night, significant liquid equivalent is expected through Thursday and Friday due to favorable orographics coupled with large-scale dynamics. As the cold front makes landfall early Friday morning, precip may be further enhanced. Snow levels may start out at ~5kft Wednesday night, rise to as high as 7kft on Thursday, then crash to ~4kft by Friday evening. Following fropa, cold-core conditions aloft coupled with continued onshore flow may result in an extended period of showery conditions on Friday and Friday night. Total liquid with this storm may exceed 2-4”.

The longer-range outlook for this weekend into next week is a big question mark. It is too early to determine if low-amplitude ridge-building off the CA coast will prevent storms from making landfall as far south as the ARB, or if significant wave energy and moisture will impact our area of interest. Stay tuned.

Paul Neiman