Flooding in Southern California

1-minute GOES-16 Infrared Window (10.3 µm) images; with hourly reports of surface weather type plotted in yellow [click to play MP4 animation]

1-minute GOES-16 Infrared Window (10.3 µm) images; with hourly reports of surface weather type plotted in red [click to play MP4 animation]

An onshore flow of moisture (MIMIC TPW) in tandem with forcing for ascent with the approach of an upper-level low and a surface cold/occluded front brought heavy rainfall and some higher-elevation snowfall (NWS LOX/SGX | WPC) to much of Southern California on 09 January 2018. To help monitor the event, a GOES-16 (GOES-East) Mesoscale Sector was positioned over the region, providing images at 1-minute intervals. “Clean” Infrared Window (10.3 µm) images (above) showed the colder clouds associated with periods of moderate to heavy rainfall. Some of this precipitation fell over burn scar areas from wildfires that occurred in December 2017 — including the Thomas fire, which was the largest on record for the state of California — resulting in numerous mud/debris slides that caused at least 17 fatalities, destroyed/damaged hundreds of homes, and closed many streets and highways.

GOES-16 “Red” Visible (0.64 µm) images (below) showed some of the features which helped produce heavier rainfall and snowfall during the daylight hours on 09 January.

1-minute GOES-16

1-minute GOES-16 “Red” Visible (0.64 µm) images; with hourly reports of surface weather type plotted in red [click to play MP4 animation]

The circulation of the upper-level low was easily seen on GOES-16 Mid-level Water Vapor (6.9 µm) images (below).

1-minute GOES-16 Water Vapor (6.9 µm) images; with hourly reports of surface weather type plotted in red [click to play MP4 animation]

1-minute GOES-16 Water Vapor (6.9 µm) images; with hourly reports of surface weather type plotted in red [click to play MP4 animation]

===== 10 January Update =====

Suomi NPP VIIRS True-color and False-color RGB images [click to enlarge]

Suomi NPP VIIRS True-color and False-color RGB images [click to enlarge]

On the following day, a toggle between Suomi NPP VIIRS True-color and False-color Red-Green-Blue (RGB) images from RealEarth (above) showed (1) the large burn scar from the Thomas Fire (shades of reddish-brown), and (2) snow cover in the higher terrain (darker shades of cyan) on the False-color image. The True-color image revealed sediment from runoff flowing into the nearshore waters from Santa Barbara to Oxnard (shades of brown to light green).

A closer look at the Thomas Fire burn scar was provided by 30-meter resolution Landsat-8 False-color RGB imagery (below), which showed thin filaments of muddy sediment just offshore, as well as fresh snow cover (shades of cyan) along or immediately adjacent to the northeastern edge of the burn scar (in the Hines Peak area). On 10 January, the fire was listed as 92% contained (100% containment was declared on 12 January).

Landsat-8 False-color RGB image [click to enlarge]

Landsat-8 False-color RGB image [click to enlarge]

===== 11 January Update =====

Suomi NPP VIIRS True-color images on 10 January and 11 January [click to enlarge]

Suomi NPP VIIRS True-color images on 10 January and 11 January [click to enlarge]

A comparison of Suomi NPP VIIRS True-color RGB images on 10 January and 11 January (above) showed that sediment was flowing farther offshore from the Thomas Fire burn scar area.

Farther to the south, offshore sediment transport was also seen in the San Diego area (below).

Suomi NPP VIIRS True-color image on 11 January [click to enlarge]

Suomi NPP VIIRS True-color image on 11 January [click to enlarge]

Lessons in Snow Level Forecasting

It's worth taking a look at temperatures this morning as the overnight southwesterly flow has brought in mild air.

At about 1440 UTC (0740 MST) temperatures at Snowbasin were around 35ºF at the base, 35ºF at the Middle Bowl observing site, and 29ºF at the top of Mt. Ogden.  This puts the freezing level, the level at which temperatures are 32ºF/0ºC at about 9000 feet. 

Source: MesoWest
In the central Wasatch, temperatures are above freezing at the base of all the ski areas.  It's currently 38ºF in town at Park City, 36ºF at the base of Alta, and 36ºF at the bases of Solitude and Brighton.  Here, the freezing level sits at right around 9000-9500 feet depending on the local topography. 


The snow level is typically lower than the freezing level for a number of reasons.  As snow falls and begins to melt, it extracts heat from the atmosphere.  This often results in a layer of constant temperature that is near 0ºC.  The evaporation of snow also can cool the atmosphere some or slow the melting of snow.  Eventually, the snow turns into a mixture of snow and wet snow (or slush) and eventually wet snow and rain, before becoming all rain.  The layer in which this occurs is called the transition zone


Because of the effects of melting and evaporation, the snow level (and freezing level) can yo-you depending on precipitation rate, lowering when precipitation rates are high, and there is greater cooling due to evaporation and melting, and rising when precipitation rates are low, and there is less cooling due to evaporation and melting.  This may be noticeable if you elect to don the garbage-bag look and are skiing today. 

To estimate snow levels, meteorologists use soundings extracted from numerical forecast models.  In addition to temperature, humidity (or dewpoint) are also used, along with estimates of precipitation intensity.  Typically, one uses the temperature and humidity profiles to estimate the height of what is known as the wet-bulb-zero level, the height at which the wet-bulb temperature is 0ºC.  The wet-bulb temperature is the temperature of the air if it were cooled by evaporation to saturation and helps to account for some of the cooling effects noted above.  In Utah, one often lowers this level by 1000 feet for an estimate of the snow level, although there are times when one might fudge it by less or more.  For example, in very high precipitation rates, the one might lower it more.

We extract the hight of the wet-bulb-zero level from the NAM and provide them from the latest forecast at http://weather.utah.edu/text/COTTONWOODS.txt.  Below is an example of the tabular output from last night 0600 UTC initialized NAM forecast.  The wet-bulb-zero level was forecast to fluctuate between 8700 and 9000 feet through 8 AM this morning.  It remains between 8700 and 9300 feet through 5 PM this afternoon.  Thus, expect snow levels to be near or around 8000 feet today. 


Lowering of the wet-bulb zero begins slowly after about 10 PM tonight, with a more sudden drop very early Wednesday morning with the approach of the cold front.

Also available in that table are temperatures and winds for Mt. Baldy, estimates of snow-to-liquid water ratio/water content based on an algorithm developed by Trevor Alcott and myself, water equivalents produced by the NAM, and estimates of snowfall based on the water equivalent and the snow ratio estimates.  Handy, if you keep in mind it is just guidance from the NAM model.  Through 7 AM this morning, this NAM run produced 0.35" of water and 2.5" of snow at Alta-Collins, which compares fairly well to the 0.34" and 4" observed.  I can assure you that most forecasts aren't that good!

Snow in Mountains, Wind, and Ample Precipitation: A La Nina-Like Pattern This Week

For those worried about the Cascade snowpack, salvation is at hand:  the atmospheric circulation is taking a turn towards more of a La Nina pattern, with cooler/wetter/windier conditions prevailing this week.

The upper level flow pattern for Wed and Thursday at 1 PM is predicted to include an offshore ridge and northweserly flow over the NW, with some disturbances moving SE in the cool flow.  Much more La Nina-like than we have seen in a while.



As a result the models are forecasting a major snowfall over the Cascades, BC, and NE Washington as illustrated by the 72 hr snow total ending 4 PM Thursday. We are talking about 1-3 feet above 3500 ft.


And we could get blustery on Thursday as la ow center moves into southern BC during the afternoon, with large pressure gradient established over WA (see below).    Haven't seen  too many of such vigorous systems this winter.


Precipitation? Substantial...but not enough for real flooding.  The 72 hr total ending 4 PM Thursday, show 2-5 inches over the mountains and about 1.5 inches over Seattle.