Spring Starts…But Winter Hangs On



Even though spring officially starts Tuesday at 9:15 am local time, it doesn’t look like the weather is likely to cooperate for a while. Another storm system is moving through the region starting late Tuesday night, bringing a chance of valley rain and mountain rain and snow to our area. This could end up being another fairly wet storm, with brisk winds and warm temperatures on Wednesday that could help eat up some of the valley snow left over from last week. Snow levels could slowly rise to about 8,000’ into Thursday (when the wettest part of the storm moves through) before they drop back to the valley floor Friday into Saturday with another (less wet) storm.

Rain and snow amounts could be significant, even though it looks like the main impact of the storm might be just to the south (although recent model runs have nudged it farther north.) Although rain totals in the valleys could be greater than a half an inch in some places, it’s not likely that we will see any widespread flooding out of all this. Travel over the passes could be tough between Wednesday and late Thursday night, and the National Weather Service has issued a Winter Storm Watch above 7,000’ for that time frame.



A Break This Weekend…But A Wetter Storm Next Week?


As one storm begins to wind down, we will ease into a cool but generally dry weekend… a nice chance to dry out before what could be a very wet system gets lined up to visit us next week. In the short term, scattered snow showers could linger into Saturday morning with only minor accumulations. Temperatures on Saturday will still be cool (low 40s in the valleys) and will only warm into the upper 40s by Sunday with partly cloudy skies. As we start the next work week, temperatures will rise into the 50s on Monday and then bump into the mid-50s on Tuesday.

It’s beginning to look more likely that a wet storm will move into the area Wednesday and Thursday, with the computer models showing a possible atmospheric river pattern which will draw up a moist plume of air from the sub-tropics. Snow levels with this system will start out fairly high (in the 8,000’ range) but could lower below Lake Tahoe (and even possibly close to the valley floor) by the end of the week.



Winter… Then Warmer and Wet


Halfway through March and less than a week from the start of spring, winter doesn’t seem to want to release its better late than never grip on us. A Winter Storm Warning is in effect for both the mountains and the valleys of western Nevada and eastern California through Friday night. Travel over the mountains will be very challenging until later in the day Saturday with snowfall measured in feet on the crests, with nearly as much expected at the lake itself.

In the valleys, snowfall amounts will vary greatly depending on location, with the central valleys getting anything from trace amounts to a few inches, and the western foothills possibly piling up 6-12”. Temperatures will be cool enough to keep the snow around for a while, with highs not expected to get out of the 40s until the first of next week.

After a break in the action Sunday and Monday, a warmer and possibly wetter storm moves through the region, possibly bringing an atmospheric river type of pattern here by the middle of the work week.

Snow Day Friday?



The late winter charge will continue into the weekend, and travel over the mountain passes should stay on the challenging side through Saturday. In the short term, a brief break in the storm pattern occurs late Wednesday night into mid-Thursday, which could allow a brief time of improved travel conditions, but a stronger and colder storm moves in Thursday afternoon, bringing snow levels down to the valley floors.

Winds will be strong enough to produce mountain white-out conditions Thursday evening through Friday, with total accumulations on the peaks a couple of feet or more by Saturday, with Lake Tahoe possibly picking up a foot or more through the same time period. A Winter Storm Warning is in effect from Thursday afternoon to early Saturday morning.

In the valley, the storm system is cold enough to produce snow on the valley floor, but there will be enough shadowing by the mountains to limit valley accumulations, but the foothills on the west side of the valley could pick up substantial amounts, possibly 6-12” while the center of the valley will likely get less than 6”. The Friday morning commute is likely to be strongly impacted.


Winter Storm Warnings



A stormy week should help us recover a bit from our dry winter so far. A series of storm fronts will move through the region for the next five days, giving us a chance of rain and/or snow in the valley with significant snow expected up in the mountains. A mountain Winter Storm Warning is in effect through Wednesday morning, and after a brief easing of the storm, another strong storm moves through late Thursday into early Saturday, As a result, another Winter Storm Warning goes into effect for the mountains over that same time period. By the end of the weekend, five or more feet of snowfall is possible on the crests of the Sierra, with lesser amounts at the Lake itself.

In the valley, snow levels will drop to the foothills Wednesday and then all the way to the valley on Thursday and Friday. A Winter Storm Watch is in effect in the valley starting Thursday afternoon and running through late Friday night. Valley snowfall amounts will vary greatly, but could range from an inch or two in the lower valley to a half a foot to a foot or more on the western foothills.


Don’t Give Up on Miracle March



Don’t give up on a Miracle March yet. This week will be pretty active, especially up in the mountains as a series of winter storms moves through. The first will arrive Tuesday afternoon, with strong winds blowing in rain and high elevation snow. Initially, snow levels will be about 8,000’ but should lower to about 7,000’ by the evening. Subsequent storms through the rest of the work week will lower snow levels to the lake itself Tuesday night and then as low as the foothills around Reno by Wednesday or Thursday. A possible stronger and colder storm moves through Thursday into Friday. Total snow accumulations by the end of the week should be measured in terms of feet in the upper elevations, with lesser amounts at the shores of Tahoe.

In Reno, there will be appreciable shadowing with the storms, so rainfall on the west side of the valley should be greater than in the central or east. As the week progresses, temperatures will steadily fall from the upper 50 Tuesday to the lower 40s by Friday, and some snow in the valley is possible by late in the work week.


Mild Weekend…Stormier Next Week…And What in the World is a hPa?



A cloudy, but generally mild and dry weekend will slowly progress into a stormier pattern by the middle of next week. Valley temperatures will range from the upper 50s to the lower 60s through Tuesday, after which a cold front will drop highs into the 40s by Thursday. Storms accompanying next week’s cold front could be significant, with snow levels lowering to Lake Tahoe level at the least.


Marci wrote me to tell me that her son just got a new weather set. She knew that the barometer needed to be set to Reno’s elevation, and wondered how to calibrate it. I told her to set it to 29.89 inches of mercury (the pressure at the time), and Bob’s your uncle, Luke will be on his way to meteorological harmony.

Marci thanked me, but then said “It’s marked in ‘hPa,’ not inches. Does that make a difference?”

hPa chart

Indeed it does. hPa stands for hectopascal, or 100 Pascals. A Pascal is the standard metric unit of pressure. One atmosphere of pressure (the approximate pressure you will find at sea level) equals just over 100,000 Pascals. So a hectopascal is roughly equal to one thousandth of an atmosphere, and is exactly equal to one millibar.

It actually makes more sense to use hPa for pressure when it comes to the international community, but we Yanks just have to be different, and we’ve adopted inches of mercury as our public standard.


A Few Storms Lined Up…And How Does A Rain Shadow Work? (Final)



The weather picture will stay mildly warm and slightly unsettled for the next week or so. Friday should give us some clouds with an approaching cold front, but most of the showers from that front will likely stay to the north. Another wave of moisture comes up from the south late on Saturday, bringing a better chance of showers to the Reno area. A few scattered showers Sunday will lead to a dry Monday, then a series of stronger storms bring a better chance of valley rain and mountain snow starting late Tuesday and going through the middle of the week.


When you are talking about a rain shadow, many people mistakenly think that the Sierra “block” or “stop” the moisture from getting over the mountains. That’s not really how it works. Just as lifting air causes condensation/clouds/precipitation, descending air heats up, which evaporates the clouds and suppresses precipitation. As the air flows up the west side of the mountains, it increases rain and snow, but as it flows down the back side into western Nevada, it heats up and kills the precipitation producing process, unless some other lifting process is strong enough to overcome the backside drop. So the mountains don’t block the moisture… it just turns back into vapor and stays in the air, bypassing us until it gets lifted by some other process further to the east.

rain shadow 1


Warm Week…and How Does A Rain Shadow Work? (Pt 3)



The next week will be mild to warm temperature-wise, with occasional showers here and there, although the mountains will likely capture most of that meager amount. High temperatures will range from the upper 50s to the lower 60s through the middle of next week. After a dry Thursday, there’s only about a 20% chance Friday through Tuesday any day will have any showers in the valley, with somewhat higher odds you will get some rain showers in the mountains. There is some indication a stronger storm pattern will set up starting the middle of next week.

rain shadow 1

Yesterday, I mentioned that air that moves upward can produce clouds and wind. In fact, much of my job as a meteorologist is wrapped up in trying to figure out where the upward vertical motion is and will occur. Air moves up… you can get rain. So how does this forcing upward happen naturally? This can occur several ways. A cold front will burrow under warmer moist air, lifting it. Converging airmasses at the lower levels of the atmosphere will have nowhere to go but up, and diverging airmasses in the upper levels of the atmosphere will draw up air from the lower levels. But land can also play an important part in vertical motion of the atmosphere. The Sierra Nevada will take moist air coming in off the Pacific and lift it, greatly increasing precipitation in the mountains.

I’ll finish tomorrow.




Sorta Warm and Sorta Wet…And How Does a Rain Shadow Work? (Pt 2)



Temperatures will be a little more seasonable for the next week with highs expected to stay in the 50s throughout. In terms of precipitation, several weak to moderate storm systems will move through over the next week, none of which at this time look particularly strong or wet. Most of the precipitation we get over the next week will be trapped up in the mountains (and we will be mostly rain shadowed in the valley), but an occasional valley shower could happen.

Scanned by: Retouched by: DT-RV
QC'd by: DT-AU

Scanned by: Retouched by: DT-RV QC’d by: DT-AU GRADE : – A QC by CWL

I mentioned the rain shadow, caused by the mountains. Before we can understand how a mountain can cause a decrease in precipitation, first we need to talk a bit about what causes rain and snow in the first place. In an oversimplified nutshell, precipitation can occur if you take moist air and cool it to the point to where the water vapor in the air condenses back to a liquid (or solid…ice…but for the sake of this discussion, we’ll combine both states here.) This forms clouds, which if the condensation continues, will form droplets big enough to fall out as rain. The best way to cool air is to force it higher up in the atmosphere, where the drop in air pressure causes a drop in temperature (see the gas laws).


Tomorrow, I’ll tell you where this is all going.


Get more at mikealger.net