CHATTANOOGA WEATHER UPDATE

WINTERSTORM MAY STRIKE TENNESSEE/KENTUCKY/GEORGIA

It just got interesting.............

It just got interesting

POSTED 2:27 PM, FEBRUARY 12, 2015,

We've known for a while now that there could be a “threat” of some wintry weather early next week, but confidence is increasing that a major winter storm could unfold across the South between Monday and Wednesday.

All of the players are on the field, but their positions matter a lot. A surface high over the northeastern US and a low near the Gulf Coast often signal big snows for this area; however, the uncertainty of the exact track of the low casts a big shadow over any pinpointed expectations. If it’s farther north, we get more rain than anything in the Tennessee Valley. It would be areas nearer Interstate 40 that get a big blast of wintry weather. If it’s farther south, the greater threat could be suppressed closer to Interstate 20. There’s a lot of miles between those two major highways, but our longer-range forecast models see that kind of distance as very, very small (margin-of-error kind of stuff).

In fact, if you look at individual members of an ensemble, the positions of the main players may change by hundreds of miles – and that’s within the same “model world!” These three maps are the ensemble means; you could look at it as the average position of all the members of the GEFS ensemble product. That looks awfully wintry for us if it pans out:

PHOTO GALLERY

VIEW GALLERY (3 IMAGES)

Some guidance puts as much as 12 inches of snow over Southern Tennessee by Wednesday morning. An event like that is so rare that it is hard to “forecast.” Forecasts are “best estimates” of the many potential outcomes, and big, record-setting events don’t happen often.

12Z (6 AM) GFS model-derived snowfall at a 10:1 ratio

Things to Watch: We’ll keep close tabs on the trends in the model guidance. If it trends 100 miles north or south, it could greatly change what happens at your house.

If it does pan out, watch the snow accumulation ratio (how much snowfall for an inch of liquid precipitation). It would probably start out around 8:1 (8 inches of snow for every one inch of liquid) and end up around 20:1 when the fresh Arctic air catches the moisture. That could throw totals way off if you just base it on the simple, common 10:1 ratio.

Forecasts like this are never simple, and they change often. Stick with us and we’ll keep a close watch on it for you.

What Do I Really Think? It’s going to be a complicated mess. That’s what I think right now. The scenario we see as of now starts out as some ice and snow on Monday, likely changes to rain for most of North Alabama (stays icy in most of Middle Tennessee) Monday night into Tuesday, then it changes back to some sleet and snow Tuesday night and Wednesday morning. The back-and-forth of the precipitation types make it impossible to “nail” an accumulation amount, but it is not far-fetched at all to expect at least 1-3″ of snow by Wednesday. That could change in a big way (to go higher or go to nothing), but it’s a reasonable expectation given what we know today. Some travel problems are possible, but there is no way to gauge timing and impact yet.

Thursday's Weather Outlook

Santa Rosa, CA 95405

Thursday's Weather Outlook

7:15 PM PST, February 11, 2015

Frigid Arctic air will have residents pulling parkas and winter gloves out Thursday, while plenty of sunshine and unseasonably mild temperatures remain across the rest of the U.S....

The big talk Thursday will be the frigid Arctic air forcing highs across the northeastern corner of the U.S. to plummet. Highs in the single digits to the teens will sink further southward to cover the Upper Mississippi Valley and the Great Lakes, while pushing cold 20s and 30s as far south as the Tennessee Valley. Along with the freezing temperatures, breezy winds and a few light snow showers will bring another taste of winter, from the Great Lakes to the northern Atlantic Coast, while flurries move in across the Upper Mississippi Valley and the Dakotas overnight Thursday.

Elsewhere, a sprinkling of light rain will be possibly across the Rio Grande Valley in southern Texas, while more rain patters over rooftops in western Washington.

The rest of the U.S. can expect to see milder temperatures, with mostly sunny skies shining throughout Thursday. Highs in the 40s and 50s will sweep southeastward from the Northwest down into the Southern Plains and the Southeast, while warmer 60s and 70s will appear across the Coastal West, Central Great Basin, Southwest, northern and central High Plains, the coastal Southeast, the Gulf Coast and the Florida Peninsula. Hot highs in the lower to mid-80s will be possible across the Southern California and the Desert Southwest.

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Do we dare say this term?

Do we dare say this term? *coughs* Well...it appears the Polar Vortex, or at least a "spoke" of the vortex, will make its presence known this weekend here in Ohio! What does this mean? Well by Sunday morning, some of the coldest air of the season so far will push into the Ohio Valley possibly sending temperatures below zero in some places. Wind chill readings could push to near or colder than 20 below zero. If we do happen to get some snow on the ground, this could help push temperatures even colder. Stay tuned for more on this!

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Santa Rosa, California, Weather Forecasts, National Weather Outlook

The second day of the work week will bring gradually improving skies to storm-weary New England while other parts of the Northern Tier deal with snow and ice. Meanwhile, clouds and light rain will stay socked in across the Southeast Atlantic Coast beaches.

National Outlook.

Early-morning snow will taper off in southeastern New England with sunshine returning by the second half of the day. A few rain, snow and ice patches will give way to blue sky in the Mid-Atlantic. Farther to the south, damp and dreary weather is on the weather menu from the Delmarva Peninsula to the Carolina beaches. A downpour or two could push through places like Wilmington and Cape Hatteras, N.C.

The same storm piercing the Northwest with rain will get caught up in the upper-level wind flow and sweep across the northern Rockies into the Upper Mississippi Valley. Light snow mixed with sleet and freezing rain will cause slippery travel along Interstate 94 in the Dakotas before making headway into Minnesota`s Arrowhead and the western Great Lakes after lunch.

Leftover showers will keep windshield wipers going in the Northwest with light snow showers in the northern Rockies today. High pressure will ensure the remainder of the West, Plains, Mississippi and Ohio valleys and interior Mid-Atlantic stay dry.

Northern New England will be the coldest spot today with highs in the single digits and teens, warming into the 20s and 30s in places like Boston and Hartford, Conn. Highs will also reach to around the freezing mark from the northern Plains to northern Mid-Atlantic.

The West, Plains, Southeast and southern Mid-Atlantic will reach the 50s, 60s and 70s. Only the highest Mountain West peaks will linger in the 30s.

Santa Rosa, California, Weather Forecasts

TUESDAY: Areas of low clouds and patchy fog with some mist at times will linger into the morning, especially for the North and East Bay valleys. Highs should be mainly in the mid to upper 60s, except locally cooler inland where the low clouds take longer to disperse.

THE WEEK AHEAD: High pressure strengthens later in the week which will boost temps closer to near record highs approaching the weekend. Temps near the bay and coastline should climb into the mid to upper 60s with low to mid 70s possible around the valleys by Friday.

Photo's

Sunday’s Storm Update

Posted 10:15pm Saturday evening

Active, Atmospheric River, Convection, Storm, Strong Storm, Thunderstorms, Unsettled, Wet, Windy

It’s been a pretty active past 48 hours across northern California as a potent jet stream has sagged south, directing multiple systems toward/into northern California, along with a very moist atmospheric river originating from the central/eastern Pacific east of Hawaii. This final system – the strongest of the three, is shaping up to be a fairly powerful mid-latitude cyclone, bottoming out down to as low as 984 – 986mb off the California coast Sunday afternoon & evening. As this low rapidly strengthens offshore, it’ll once again draw northward the very moist streak of moisture continuing to surge northeastward toward California in the warm sector of the system Sunday morning and afternoon, ahead of the cold front. A brief period of clearing may occur ahead of the storm’s cloud shield Sunday morning, allowing some surface temperature warming. Clouds are likely to quickly overspread northern California by late morning and afternoon… with frontal precipitation not too far behind.

Given the strong surface cyclone offshore Sunday, a strong southerly low-level flow will develop, transporting warm, moist surface air up the valley, which on top of perhaps a brief period of sun in the morning should allow surface temperatures to climb into the low to mid 60s across much of the lower elevations, from the inland coastal valleys to the central/Sacramento valley. This surface heat beneath increasingly cool mid/upper-level temps should boost mid-level lapse rates to around 6.5 – 7.5c (moderately steep), as well as when combined with relatively high surface moisture (dewpoints in the upper 50s to low 60s) should promote surface-based CAPEs in the 200 – 500 j/kg range on a widespread scale through the afternoon. This instability combined with strong lift and wind energy from the surface upward should promote potential for at least isolated thunderstorms embedded or ahead of the cold front during the morning & early afternoon. Behind the front, which may pass by early to mid-afternoon, a more showery/cloud-broken airmass slides overhead… potentially allowing surface temperatures to climb a few extra degrees locally and boost dewpoints further (locally, once again), and support increased CAPE values on a scattered scale from 500 – 1,000 j/kg, mainly in the Sacramento valley, delta, and coast.

0z 4km NAM’s forecast ML (mixed-layer) CAPE valid at 5pm. Light green hues are values in excess of 750 j/kg, and darker greens are 1,000 j/kg+.

Fairly strong south to southeasterly low-level to surface flow combined with a WSW to SW mid/upper-level flow create decent directional shear… not quite a 90-degree angle in difference of winds with height, but not a too far off. Thus, when you add in the strength of these winds from the low to mid-levels, shear in general is quite strong, and should be supportive of rotating storms through the afternoon and into the early to mid-evening before instability wanes. Behind the front, from mid to late-afternoon onward into the evening, is when the threat for the strongest thunderstorms is expected. Given the available shear and potentially decent instability as currently modeled, some thunderstorms could strengthen and take up supercellular structures, rotate, and become capable of producing tornadoes and large hail. It’s also possible that bands of thunderstorms could form, which would be more capable of strong/damaging straight-line wind, as well as hail.

Winds even without thunderstorms will be pretty strong from Sunday morning through the evening as the potent surface low slides northward up the coast toward the Pacific Northwest, with 900mb winds in excess of 45 knots across a good chunk of the region… with locally some 50  – 55kt values being indicated by the 4km NAM. At this level, not every thing makes it to the surface… but a strong thunderstorm could quite easily drag down equivalent speeds to possibly locally higher speeds.

0z 4km NAM’s forecast 850mb winds, valid at 4pm Sunday. Note: these winds are several thousand feet above the surface, thus, winds of this strength likely wouldn’t directly make it to the surface without the aid of a strong thunderstorm.

I’ll likely post another update Sunday morning or afternoon as the event begins to unfold with an update on thunderstorm potential.

Previous updates are below, with still relative information to this event.

Possible tornado activity on Sunday

It may get a lot more interesting around here on Sunday.We are tracking the potential for severe thunderstorms in the Northern Sacramento Valley. The Storm Prediction Center has given us a marginal risk of severe thunderstorms.

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Stormy Weekend on the Way: Update 6

Thunderstorm potential update, posted at 10pm Friday evening

An active weekend is on the way… or I suppose already underway given much of northern California has already experienced at least some rain, wind, and snow – in some cases a lot of each. Offshore lies a parade of shortwaves and developing surface lows – the first of which about 300 – 400 miles off the Oregon/northern California coast as of early Friday evening. This system will continue driving a plume of quite deep subtropical moisture into northern California through night tonight and on into Saturday morning, before much of the moisture and lift exists to the south. The next (second) wave is between California and Hawaii, and should follow this first wave into northern California later Saturday with another wave of showers… but nothing too widespread or heavy like we’ve seen much of today (Friday). The third, and perhaps most potent single system, is still out in the mid-eastern Pacific, and will be off the California coast later Saturday, and begin driving in a fairly powerful cold front supported by a potent jet streak in excess of 100 knots later Sunday into Monday.

Saturday, during most of the morning and afternoon, we’ll be between systems… perhaps allowing some breaks in the clouds to occur and warm up surface temperatures in the lower elevations, while a good amount of the higher elevations may remain socked in with cloud cover as upslope flow continues to wring out showers. Slighty cooler air aloft behind the first system combined with likely warming valley temperatures and a fair amount of moisture in the low/mid-levels should promote at least some modest instability through the morning and afternoon. Lift may be a bit sparse though until later in the way when the second shortwave arrives later in the afternoon, into the evening, which will likely end up being the best timeframe for any thunderstorms. Strong westerly upper-level flow combined with veering surface winds (though, they’ll be much weaker than Friday & Sunday) with the shortwave will promote some decent supportive shear for any storms, but as the shortwave comes inland cloud cover may increase… potentially limiting instability a bit, and keeping lapse rates fairly poor. However, at least slight instability in the form of a few hundred J/KG CAPE to work with and the strong mid & upper-level flow to promote the potential for a few thunderstorms over the course of the afternoon and evening.

Thunderstorm potential map for Saturday.

Sunday could perhaps be the most interesting day, as a powerful sub-990mb surface low slides northeast off the California coast. Ahead of the low’s potent precipitation shield, some cloud breaks may occur Sunday morning as warm air advection occurs ahead of the cold front… which combined with cooling mid/upper-level temperatures directly following the warm air advection ahead of the front could lead to fairly substantial instability when it comes to California’s standards. For instance, the 4km NAM and it’s 12km counterpart both indicate SBCAPE values in excess of 700 – 800 j/kg across a fairly large chunk of inland northern California – with potential for 1000 – 1500 j/kg values in the valley and delta. A more conservative analysis of forecast SBCAPE generates 300 – 500 j/kg, and this analysis comes from the SREF’s mean… and usually these values increase as the event nears. In any case, both of these solutions, bullish and conservative, either one supports the idea of sufficient instability for thunderstorms to develop and sustain themselves. Strong flow across all levels, including a 50kt+ southerly 850mb jet, beneath a SW to WSW 500 – 300mb flow creates a moderately strong directional shear setup on top of strong speed shear.

0z 4km NAM’s forecast (surface-based) CAPE values, valid at about 1pm Sunday.

As the cold front begins to push inland Sunday afternoon into the evening, believe the increasing lift on the southeast side of the low will favor potential for pre-frontal convective development, as well as convection directly along the front where convergence is maxed. Pre-frontal thunderstorms, if any develop, would have the best potential to rotate and potentially become tornado-producing in this environment, as well as the best potential overall to become severe. Along the front, a line of thunderstorms or embedded thunderstorms could develop, and sweep through a good chunk of interior northern California. Storms along the front would have a lower chances of producing any tornadoes, certainly not a zero chance, though, but typically bands of storms like this are more known for strong winds and flooding rainfall rates. The thunderstorm threat Sunday appears that it’ll be maximized between around 2pm and 9pm, when the pre-frontal zone begins entering (~2pm) and the front exists much of the region (~9pm). As a side note, the Storm Prediction Center (SPC) of the National Weather Service/NOAA has also expressed interest in Sunday across northern California thunderstorm wise – and may even introduce severe probabilities in later outlooks.

Thunderstorm potential graphic for Sunday.

Colder upper-level air moves overhead Monday, with another round of isolated to scattered thunderstorms possible. The flow Monday begins to flip more northwesterly, which typically isn’t very favorable for severe storms… but will be monitored as we get closer to the day.

I’ll bring more updates Saturday & Sunday regarding thunderstorm potential this weekend with an emphasis on Sunday it appears.

Previous post, from Wednesday:

A fairly significant precipitation event is still on the to northern California, and confidence has increased in that double-digit precipitation totals in some places are likely through early next week as a series of storms travels along a belt of very moist air. This belt of very moist air, also known as an atmospheric river, will drive precipitation amounts up significantly compared to what we may normally see from these systems without the significant plume of moisture.

The first in the series of systems traveling along a raging west to east jet will traverse into the Pacific Northwest & British Columbia late Thursday, spreading precipitation into northwestern California through the afternoon, and into the rest of northern California early Friday. The belt of moisture ahead of the system’s cold front will remain aimed at northern California through the day Friday, in fact increasing in intensity through the day into the evening as a another system moves upstream, strengthens, and slams into the west coast late Friday night into early Saturday, intensifying lift and increasing precipitation intensity across the region.

Lift weakens as this system moves inland and the moisture plume sags southward into central and even southern California through the day Saturday; meanwhile, cooler air behind the front, still packed with decent moisture, swings inland during the day Saturday. This cooler airmass will lead to a more unstable environment across northern California during the day Saturday… thus, some convection appears possible. All layers of the jet remain aimed at northern California through Saturday, thus there’s good mid/upper-level support for thunderstorms to form and perhaps get their act together… that is, if some pockets of sunshine can break out and boost instability/buoyancy further to increase potential for more robust updrafts given the presence of decent shear. This will be looked at closer in the coming days.

Saturday’s forecast thunderstorm potential.

A brief break in the action appears to be a good bet come Saturday night and (very) early Sunday before the next system arrives. This next wave, by the way, will consist of a fairly deep mid-latitude cyclone off the California coast riding the jet (stream) on northeastward into the Pacific Northwest, spreading a frontal shield of rain across northern California once more by Sunday morning along the coast, and into the interior by afternoon into the evening. The main frontal band scoots southward fairly quickly compared to the previous firehose event Thursday – Saturday, but showers will remain possible through Monday as another bout of cooler, unstable air slides overhead… thus, can’t rule out more thunderstorms Monday morning/afternoon. The front will dump a decent shot of precipitation across the region to add to Thursday – Saturday’s accumulations. But after this storm, it appears we’ll be done for a while once more, unfortunately.

Wind

Fairly tight pressure gradients between the train of surface lows off the west coast and the surface high(s) in the Great Basin and southwestern U.S. combined with periodically heavy precipitation late this week, periods of gusty winds are likely… with the strongest winds expected over the sierra. The strongest winds are currently expected during the day Friday into Friday evening, as a surface low intensifies off the west coast. 850mb winds are currently forecast in the 55 – 75kt range from the valley into the sierra Friday afternoon into the evening, which also when precipitation is expected to reach peak intensity across a good chunk of the region. Thus, believe some gusts in excess of 40mph are likely in the lower elevations Friday, with >60mph in the sierra.

Wind gust forecast based off a mean of model solutions.

Orographics/terrain enhancement

Given the strong south to southwesterly flow slamming into the mountains, the higher elevations will pick up substantially more precipitation than the lower elevations. South and southwest facing slopes will pick up the most, as the flow parallels these slopes and precipitation becomes maximized. The great flow stacking up against the mountains will also create a banking effect that extends into the east part of the Sacramento valley, where precipitation amounts will be higher compared to the west side due to banking of moisture against the west slope of the sierra/foothills.

Precipitation

Essentially every major global model except for the ECMWF indicates pretty substantial precipitation amounts across northern California through Monday. A blend, or mean of these solutions including that of the somewhat less-wet ECMWF generates fairly generous amounts of precipitation, and these amounts are as follows: 2 – 5″ for much of the Sacramento valley with upwards of half a foot or so in the north end of the valley and parts of the eastern valley, 1 – 3″ in the northern San Joaquin valley, 4 – 8″ across much of the foothills & mountains, 2 – 5″ along a good chunk of the immediate, 4 – 8″ in the coastal mountains (locally higher amounts possible along south/southwest facing slopes), near the same in the northern mountains, and upwards of 8 – 12″ in orographically favored slopes inland, such as the higher elevations of Butte, Plumas, Yuba, Sierra, Nevada, Shasta, and Siskiyou counties.

Rainfall amounts based off a mean of model solutions.

Next update should be out later Thursday, or Friday if no significant changes are needed.

Updated....4

Wondering where all of our wet weather in the coming days is originating from/currently positioned? Here's the answer to that!

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aircraft scouring the atmosphere off the California coast today

NOAA aircraft scouring the atmosphere off the California coast today seeking data, similar to what they do with tropical systems, ahead of our looming big precipitation event. 

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12:10 PM Storm Update

US National Weather Service San Francisco Bay Area/Monterey California

2 hrs · Edited · 

12:10 PM Storm Update -- Everything is still on track for a stormy end to the week, although we have slightly increased rainfall forecast for all locations. The Friday commutes could be very challenging, so please allow for extra time. Remember, roads are very slick during the onset of rain after long dry spells!

Thank you for continuing to follow us. We are here to answer your questions about the storm.

":A wet and windy storm is forecast to move into the area later today(Thursday) and continue through the weekend. Rainfall totals across the urban San Francisco Bay Area are expected to be 2 to 4" of rain, 3 to 6" in the North Bay and coastal mountains. Locally up to 10" is possible on the higher peaks of Marin and Sonoma counties. Locally up to 8” are possible for the coastal mountains in Santa Cruz and Monterey counties. Lower rain amounts of 1.5 to 3” can be expected for the remainder of our area."

Atmospheric River Late Thursday..........

Atmospheric River Late Thursday – Saturday: Update 3

A fairly significant precipitation event is still on the to northern California, and confidence has increased in that double-digit precipitation totals in some places are likely through early next week as a series of storms travels along a belt of very moist air. This belt of very moist air, also known as an atmospheric river, will drive precipitation amounts up significantly compared to what we may normally see from these systems without the significant plume of moisture.

The first in the series of systems traveling along a raging west to east jet will traverse into the Pacific Northwest & British Columbia late Thursday, spreading precipitation into northwestern California through the afternoon, and into the rest of northern California early Friday. The belt of moisture ahead of the system’s cold front will remain aimed at northern California through the day Friday, in fact increasing in intensity through the day into the evening as a another system moves upstream, strengthens, and slams into the west coast late Friday night into early Saturday, intensifying lift and increasing precipitation intensity across the region.

Lift weakens as this system moves inland and the moisture plume sags southward into central and even southern California through the day Saturday; meanwhile, cooler air behind the front, still packed with decent moisture, swings inland during the day Saturday. This cooler airmass will lead to a more unstable environment across northern California during the day Saturday… thus, some convection appears possible. All layers of the jet remain aimed at northern California through Saturday, thus there’s good mid/upper-level support for thunderstorms to form and perhaps get their act together… that is, if some pockets of sunshine can break out and boost instability/buoyancy further to increase potential for more robust updrafts given the presence of decent shear. This will be looked at closer in the coming days.

Saturday’s forecast thunderstorm potential.

A brief break in the action appears to be a good bet come Saturday night and (very) early Sunday before the next system arrives. This next wave, by the way, will consist of a fairly deep mid-latitude cyclone off the California coast riding the jet (stream) on northeastward into the Pacific Northwest, spreading a frontal shield of rain across northern California once more by Sunday morning along the coast, and into the interior by afternoon into the evening. The main frontal band scoots southward fairly quickly compared to the previous firehose event Thursday – Saturday, but showers will remain possible through Monday as another bout of cooler, unstable air slides overhead… thus, can’t rule out more thunderstorms Monday morning/afternoon. The front will dump a decent shot of precipitation across the region to add to Thursday – Saturday’s accumulations. But after this storm, it appears we’ll be done for a while once more, unfortunately.

Wind

Fairly tight pressure gradients between the train of surface lows off the west coast and the surface high(s) in the Great Basin and southwestern U.S. combined with periodically heavy precipitation late this week, periods of gusty winds are likely… with the strongest winds expected over the sierra. The strongest winds are currently expected during the day Friday into Friday evening, as a surface low intensifies off the west coast. 850mb winds are currently forecast in the 55 – 75kt range from the valley into the sierra Friday afternoon into the evening, which also when precipitation is expected to reach peak intensity across a good chunk of the region. Thus, believe some gusts in excess of 40mph are likely in the lower elevations Friday, with >60mph in the sierra.

Wind gust forecast based off a mean of model solutions.

Orographics/terrain enhancement

Given the strong south to southwesterly flow slamming into the mountains, the higher elevations will pick up substantially more precipitation than the lower elevations. South and southwest facing slopes will pick up the most, as the flow parallels these slopes and precipitation becomes maximized. The great flow stacking up against the mountains will also create a banking effect that extends into the east part of the Sacramento valley, where precipitation amounts will be higher compared to the west side due to banking of moisture against the west slope of the sierra/foothills.

Precipitation

Essentially every major global model except for the ECMWF indicates pretty substantial precipitation amounts across northern California through Monday. A blend, or mean of these solutions including that of the somewhat less-wet ECMWF generates fairly generous amounts of precipitation, and these amounts are as follows: 2 – 5″ for much of the Sacramento valley with upwards of half a foot or so in the north end of the valley and parts of the eastern valley, 1 – 3″ in the northern San Joaquin valley, 4 – 8″ across much of the foothills & mountains, 2 – 5″ along a good chunk of the immediate, 4 – 8″ in the coastal mountains (locally higher amounts possible along south/southwest facing slopes), near the same in the northern mountains, and upwards of 8 – 12″ in orographically favored slopes inland, such as the higher elevations of Butte, Plumas, Yuba, Sierra, Nevada, Shasta, and Siskiyou counties.

Rainfall amounts based off a mean of model solutions.

Next update should be out later Thursday, or Friday if no significant changes are needed.......

UPDATE! Here is the afternoon issuance of our precipitation forecast.

UPDATE! Here is the afternoon issuance of our precipitation forecast. Still on track for heavy precipitation Thu through Mon with the heaviest affecting the region on Friday. The higher terrain of northern CA will see the greatest precipitation by Tuesday morning with wettest locations reaching or just surpassing 10 inches. Central CA will also see precipitation, but not as much as areas from Interstate 80 northward. To see the impacts on area rivers, check out our webpage at 

www.cnrfc.noaa.gov

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Atmospheric River Event Likely Thursday into the Weekend: Update #2..

The long stretch of dry, boring weather is nearing its end – at least temporarily, as our ridge continues sliding southeastward into the southwestern U.S., allowing a large set of lows in the eastern Pacific to nudge eastward toward the west coast. A long fetch of westerlies across the Pacific has, and will continue to tap into a significant amount of subtropical moisture east of Hawaii, transporting it to the western U.S. coast by Wednesday night in the Pacific northwest as one of many deep lows in the eastern Pacific/Gulf of Alaska begin rotating into British Columbia. By later Thursday, the long fetch of moisture ahead of the cold front begins to slide into northern California, with moderate to heavy rain spreading inland through the day into the night.

It currently appears that the stream of moisture will remain aimed at northern California through at least early Saturday as potential lows develop along the jet transporting this moisture into the west coast; possibly through much of Saturday if some model solutions are correct. Each low/wave that develops along the jet will aid lift along the front and continuing to support moderate to heavy precipitation over the 24 – 48 hour period across northern California, in addition to a SW to NE oriented 130kt+ upper-level jet… and perhaps more importantly, a 40 – 60kt mid/low-level jet blasting south out of the south into the foothills and mountains. These strong southerlies will create a perfect orographic enhancement across south and southwest facing slopes across the foothills and mountains, as well as help precipitation bank up against the higher elevations and boost totals along the east side of the valley, too.

12z GFS’s forecast 300mb winds and heights, valid at 4pm Friday, with a long belt of strong westerlies streaming into the West...

The strong southerly mid-level jet will support a period of gusty winds across northern California during peak precipitation intensity, which should occur Friday afternoon into the evening. 850mb winds increase during this period into the 45 to 65kt range, and believe there will be enough groundward force from heavy rain to drive some 35 to 45mph gusts to the surface across much of the region. In the higher elevations (sierra/parts of the northern mountains), gusts in excess of 50 to 60mph will be possible. Given these winds (in both the higher and lower elevations) combined with periods of moderate to heavy rainfall, some scattered down trees are likely, and thus probably some power outages here and there.

A second and fairly deep cyclone will traverse out of the central/eastern Pacific toward the west coast late Sunday into Monday – sending another cold front into northern California Sunday night into Monday morning with another, but much quicker wave of moderate precipitation to add on top of Thursday – Saturday’s event. There’s still some fairly notable differences with this system between models, with the GFS for instance remaining the wetter model, bringing the low further south, and the ECMWF taking the low further north, limiting precipitation.

Rainfall amounts through late Monday still vary by model, but a blend between major models generates 1.5 – 3.5″ across much of the Sacramento and northern San Joaquin valley, 3 – 6″ across much of the foothills, mountains, coast, and coastal mountains, with 6 – 12″ in orographically favored slopes, including much of the higher elevations of Butte, Plumas, Shasta, Sierra, Yuba, and Sierra counties. I would like to point out that the amounts I’ve mentioned are fairly low compared to what some model output suggests… for instance, the GFS suggests 4″+ for the east side of the valley from Sacramento northward, and 15″+ in the highest elevations of Butte, Plumas, and western Shasta counties… quite a bit more than I’ve personally went with. But, this is a good base to begin with for now, with room to increase or decrease a bit if needed in the next 24 – 36 hours before the event begins.

850mb temperatures along the belt of moisture look like they’ll be in the 3 – 8c range, which will generally support snow levels well up around 6000 – 7000ft, possibly a bit higher in the south and on the lower end of that range in the north. Thus, while snow levels will be high and this event won’t greatly contribute to the snowpack in the more frequented parts of the sierra, the high sierra (>7500 – 8000ft) should pick up a couple feet of snow, at least, through Monday.

Things dry out by Tuesday, and warm up rapidly through the week as the pattern completely flips once again to an extraordinary warm pattern as the flow switches offshore. For now, lets just focus on the fun part of the forecast: Thursday through the weekend, before we get to the more boring pattern we don’t need.

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