Strong Winds and Lots of Precipitation Ahead

It is time to find an umbrella, but it needs to be a sturdy one, since a significant blow is ahead of us.

Looking out into the Pacific, an impressive line up of moisture is heading our way, while a small cyclone is threatening Baja California.

Tomorrow, a moderate-strength cyclone (roughly 990 hPa central pressure) will make landfall on the northern tip of Vancouver Island (see sea level pressure map for 1 PM Tuesday).  With relatively high pressure to the south, a very large pressure gradient will occur over western WA and the coastal waters.  The result?  Strong winds.

Let's take a look at the predicted winds, using a graphic from the city of Seattle WINDWATCH site, which shows predicted wind gusts over Seattle.  Major winds ramp up tomorrow morning, with wind gusts hitting around 40 mph during the afternoon and early evening.  That is generally enough to break some branches and produce some power outages.   Not a record-breaking storm.

Winds will start revving up even earlier over NW Washington, where southeasterly winds could hit 40-60 mph overnight.

And then there is the bountiful precipitation.  For the 24-h totals  ending 4 AM tomorrow, moderate amounts over northwest WA as the storm approaches.

But the next 24-h will be sodden, with 2-5 inches on the western side of the regional mountains, but substantial rain shadowing over the western lowlands.

Now, you want to be impressed?  Here is the accumulated rainfall over the next 7 days.  Some locations approaching 10 inches.

And yes, snow.  Initially, the freezing level will be high, but cooler air will move in by Thursday.  The snowfall totals are measured in feet over the next week.  Good for snowpack and skiing.

There is a rumor that the Seattle Times will have a story in tomorrow's paper about the Northwest being in a drought situation that is will be difficult for us to catch up from....😀

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/12/strong-winds-and-lots-of-precipitation.html

Major Potential for Strong Winds and Wildfires in Northern California

Later this week there is the potential for strong Diablo winds over northern California;  winds of sufficient strength to pose a major wildfire threat.

Currently, a far weaker wind event is occurring over northern California--as a result the power utility (PG&E) has shut off power to roughly ten thousand customers, including those near Paradise and Chico, CA.

The sea level pressure pattern this morning (8AM) shows high pressure pushing into the Northwest and southeastward into the intermountain region.  This high pressure is associated with cold air (green colors) in the interior.  Note the lower pressure and warm air over the CA coastal zone.    The result of this pattern is a large pressure gradient over the Sierra Nevada and northern CA that produce strong wind. (Winds accelerate from high to low pressure near terrain).

The maximum observed gusts since midnight (see below, click to enlarge) show winds gusting to 51 mph at Jarbo Gap near Paradise,  54 mph at Mt. Diablo, just east of Oakland, and 46 mph northeast of Santa Rosa.  Such wind speeds, although not really exceptional, were similar to those experienced during the destructive Camp Fire of last year.    But this time the power is out in vulnerable areas, and hopefully PG&E has made strides to upgrade their poorly maintained system.

But today's minor blow is nothing compared to what will happen on Wednesday and Thursday.

By 5 AM on Wednesday, very cold, dense air, associated with high sea level pressure, will surge into the Pacific Northwest (see below).   Yes...that will bring more snow to the Cascades and Rockies. 

At the leading edge of the cold air, there will be a large pressure difference (gradient) the produces strong winds.      The image below for 5 AM shows an intense pressure gradient over northern California.   But it will get stronger and move south.

By 5 AM on Thursday, an intense pressure difference will be found over the Sierra Nevada and northern California (see below).  This will produce very strong winds over the western slopes of the Sierra Nevada and northern CA, far exceeding what is occurring today.  Known as Diablo winds, they are real threat.  I might note that I have a National Science Foundation grant to work on these winds and just finished a paper on it with Brandon McClung.  This situation is a classic.

The sustained surface wind speed forecast (in knots) for the same time (shown below), indicates strong winds over the western slopes of the Sierra Nevada and over the terrain north of San Francisco.  This forecast is from a relatively low resolution simulation (12-km grid spacing).  High resolution forecasts will undoubtedly predict higher winds..and remember these are sustained (2-min average) winds.  Gusts will be substantially more.

These strong winds will be interacting with vegetation that has tried over the long summer.  But quite honestly, even if it rained a few days ago it would not make a big differences for the finer fuels (e.g., grasses).   Strong easterly Diablo winds can dry out such fuels within hours.

To illustrate, below is the 10-hr dead fuel moisture at Jarbo Gap on the western slopes of the Sierra Nevada near Chico, CA for the last 90 days.  10-h fuel moisture represents the moisture level in small branches to large grass--the kind of material that has been the key fuels for recent CA fires.

 For most of the summer, the moisture % was around 6%--plenty dry to burn.  In September,  there were two rainy/cool periods and the moisture level jumped to around 25%.  But note how quickly it came down...and is now back to roughly 6%.   This is true for all of northern CA.  The place is ready to burn--all it needs is wind (which will occur) and an ignition source.

There is real fire potential here and the region must be prepared.    Prepared to quickly evacuate if necessary and prepared to de-energize the power lines as we get closer to the event. 

We have had two catastrophic fires that killed dozens people and cost tens of billions of dollars during the past two years.   With excellent meteorological warnings,  appropriate actions by the population and civil authorities, and a pro-active response by PG&E, hopefully another tragedy can be avoided.




from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/10/major-potential-for-strong-winds-and.html

Temperature Extremes Across the U.S. This Week

Politics is not the only place in which extremes can exist in our country--- the weather is showing that it is no slouch in this domain.

During the last week, there has been an extraordinary contrast of temperatures across the nation, with temperatures hitting 100F or more in the Southeast, but lows in the teens/twenties and heavy snow in portions of the Northwest. 

To illustrate, here are the mean temperatures for October 1-2 (top), with the difference from normal (anomalies) for the same period on the bottom.  Huge warm anomalies over the eastern U.S., reaching nearly 20F.   And nearly equal cold anomalies over the upper Plains and the West down to the Mexican border.

Many records have fallen on both sides of the country:  cold records in the west, warm records in the east.  For example, here are daytime records on October 1.  Wow.  There are not only daily high temperature records, but monthly records as well.  THAT is impressive.

On the other hand, the same morning produced low-temperature records
over the west, including breaking daily records, and in some locations, monthly ones.  Several locations, such as Spokane, broke daily/monthly snowfall records.

So why the simultaneous extremes on differing sides of the nation?  It has to do with a highly perturbed and anomalous upper level wave pattern, which caused a large undulation in the jet stream.

To illustrate, here is the upper level (500 hPa--about 18,000 ft) analysis for 5 AM PDT September 29th (Sunday).  Blue and purple indicate much lower than normal heights (a trough), and orange/red are higher than normal heights (ridge). 
You can see a very disturbed, undulating pattern, with a ridge over the Northeast Pacific, a very deep trough over the west, and a ridge over the eastern U.S. 

Troughs are associated with cold, low-level air, ridges with warm air.

Why?  Such a pattern moves warm air into the western side of ridges (southerly flow) and ridges have sinking (which warms air by compression).  Thus ridges are warm.   Just the opposite for troughs.

The same chart on October 1, the day of the records shown above, is very similar:

 

The pattern today has the same general idea, but a bit weaker.

But I have some news.  It looks like ANOTHER major push of cold air will surge into the Northwest on Tuesday, with more snow at higher elevations (see map below for Tuesday at 5 PM PDT). 

If we don't shake this pattern, November could bring snow to the lowlands.  But don't worry...this weekend should be decent, and Sunday will be perfect.  Sunny, no precipitation, and mid-60s in the western lowlands.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/10/temperature-extremes-across-us-this-week.html

The University of Washington Should Not Censor Faculty Social Media

Hurricane Dorian Will Spare Florida: Why So Much Uncertainty with This Storm?

It is becoming increasingly clear that Hurricane Dorian is going to make a sharp right turn as it approaches the eastern coast of Florida, resulting in only modest impacts to the Sunshine state.  Hurricane force winds (sustained at more than 64 knots) and heavy rains will probably remain offshore.

Early forecasts had predicted landfall of a category 5 hurricane on the central Florida coast,  with potentially devastating impacts, but now the real risk has shifted northward into the Carolinas.

Some critics have already started complaining, suggesting that there were too many cancellations and closures before the storm track was clear.  Folks along the Florida coast have stripped the food stores, gas stations are out of fuel, and even the Orlando Airport was closed.

 Hurricane Dorian at 2:40 PM PDT on 31 August

Was there a failure of weather prediction technology for this event?  Did the European Center Forecast outdo the American FV-3?    Did local governments over-react?

My conclusion:  this has been an inherently difficult storm to forecast, with more uncertainty than many recent storms, some which were predicted well over a week in advance (e.g., Irma).

The latest forecasts of all major centers are converging on a solution in which the storm approaches the Florida coast, but swings northward just offshore.  Let me show you the usual cream of the crop model, the European Center forecast.

The forecast for 5 AM Monday PDT, shows the storm approaching the Bahamas.

 A day later, it moved to the northwest...quite slowly.

 5 AM Wednesday it is crawling northward.

And Thursday morning it is just off the southeast tip of North Carolina

The strongest winds of a hurricane are in the right front quadrant--to the right of the direction of motion.   And thus the strongest winds are generally kept offshore.  Same thing with precipitation.

The accumulated precipitation totals through Friday morning show the heaviest precipitation offshore.  No problem at all for Florida and Georgia.   An issue for coastal North Carolina.

As I shall describe in a second, this is an inherently difficult forecast situation with high levels of uncertainty, particularly once the storm gets near Florida.  A measure of the uncertainty is apparent in the ensembles--in which multiple forecasts starts slightly differently and are run by both the European Center and the NWS.

To illustrate, here is the latest ensemble prediction of the position of the hurricane center during the next five days..  No much variation (spread) initially as the storm heads west. And then nearly all members make an abrupt right turn....but there is a lot of spread in the exact time when the turns start, resulting in a big difference in the low center positions in five days.  The U.S. ensemble is similar, but closer to the coast.

Why is there so much uncertainly?  It has to do with steering currents.  Hurricanes tracks--or trajectories in time---are controlled by the large scale flow around them....which is called a steering current.

For strong storms like Dorian, the steering flow can be represented by the flow speed and direction averaged from around 5000 ft to 30,000 ft (or roughly 850 hPa to 300 hPa in pressure).  If the flow becomes weak or if there is a large change in steering flow in distance, hurricanes can stagnate or move erratically.  

The current (2 PM Saturday)steering flow analysis by NOAA CIMMS is consistent with westward movement of the storm (the arrows show the flow direction and the hurricane is indicated by the red marker).

But this changes during the next few days, with the flow weakening over the region and the subtropical high moving eastward (steering flow prediction by HWRF model for Monday at 5 PM PDT shown).  The result is a northerly component to the steering flow and the shift to the right.

But let's be clear...this is all on the edge, and it would not take a large prediction error to allow the storm to drift further to the west...and thus onshore.   

The weak steering flows make this inherently a difficult forecast, even for the best of the current models.   A verification of position errors by Professor Brian Tang of U. of Albany suggests the U.S. FV-3 model (dark blue color) and been similar in skill to the European Model (purple color) for 12-96 hr forecasts, with the Euro being significantly better at 120 hr.  The position errors are quite large compared to recent hurricanes. But these are early times.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/08/hurricane-dorian-will-spare-florida-why.html

The First Autumn Storm Makes Landfall on Wednesday

It is very typical for first "autumn-like" storm to reach our region during the third week of August....and this year will not be an exception.  In fact, we have two systems that will be making landfall this week, with the first one being the strongest.

You can see the impact of this "first autumn storm" in the precipitation climatology Seattle.  Below is the  daily  climatological probability of enjoying at least .01 inches at Sea-Tac.  At the end of August the probability jumps to roughly 25%! 

Rain comes in on Wednesday, associated with a robust Pacific front, with the 24 hr total ending 5 PM Wednesday being substantial over the Olympic Peninsula and north Cascades.  On the coast, some locations could get several inches.

The Wednesday system is associated with a front connected with a very intense low center moving into southeast Alaska (see surface weather map at 2 AM Wednesday). Let's say that some of the Alaska cruises will be enjoying some rock and roll that has nothing to do with music.

Another system comes in late Friday and Saturday, with substantial amounts shown in the 24h total ending 5 AM Saturday.

All this rain will be associated with a massive reconfiguration of the atmosphere to a more fall-like pattern.  To illustrate, here is the upper level map for around 30,000 ft ASL, showing the jet stream winds in colors.   At 2 PM today, there is a ridge (higher pressure and heights) stretching from the Aleutians to California, and the jet stream (yellows and light green colors) is to our north.

But by 11 PM next Saturday everything is different, with a fairly strong jet stream heading right into our region.   You won't have to worry about smoke in the Northwest.  You will have to worry about finding your umbrella.

This pattern change will have a huge impact on wildfire potential, greatly reducing the changes of new fire initiation or the growth of old, smoldering burns.




from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/08/the-first-autumn-storm-makes-landfall.html

Saturday Morning "Deluge" Shows the Limitations of Modern Weather Prediction

This morning was very wet around Puget Sound, with some locations getting 3/4 of an inch or more.  And eastern Washington is being hit quite hard today with vigorous thunderstorms.

To illustrate, here is the total precipitation over the past 24 hr, ending 4 PM Saturday (click on figure to expand).  About 2/3rds of an inch in Seattle...close to the usual amount for the entire month of August.   Several locations in the southern Cascades and the eastern slopes of the mountains got over an inch.

Over western Washington, precipitation was associated with a deformation band north of a land-falling low center to our south.  What is a deformation band?  It occurs then two streams of air converge together, often north of a low center, resulting in upward motion--and thus clouds/precipitation.  Here is the forecast winds for 8 AM this morning at 700 hPa (about 10,000 ft)...you can see the deformation pattern.

The result was a band of precipitation that hung over Seattle for several hours, thoroughly wetting the area.  A radar image around 7 AM  shows the band very well.

Unfortunately, although the models had shown that a band would set up somewhere in western Washington, they universally got the timing and location wrong.  For example, the 24h precipitation forecast from the NOAA/NWS HRRR model, their best at the short-range, kept the band too far south, with less than .25 inches over Seattle. 

This afternoon the action moved to eastern Washington, where surface heating released a great deal of instability, resulting in strong thunderstorms, lightning, and lots of rain.   Take a look at the radar for 5 PM.  Wow.  The red areas are very heavy rain or hail. Lots of places over the eastern Cascade slopes are getting hit hard.

These storms are really impressive in the visible satellite imagery at the same time (see below).   You see the high high ice clouds extending northwestward from the storms?  These are anvils--the high cloud blow off from the thunderstorms.     Some of this anvil cloudiness is spreading into western Washington!

The big issue, of course, is the potential for lightning from these storms causing more fires.  Will have to be watchful.

Sunday should be a bit drier, but eastern Washington will get more thunderstorms, and a few showers might well drift over the west.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/08/saturday-morning-deluge-shows.html

Alberta Smoke Spreads to the Atlantic

The latest NASA MODIS satellite image is startling:  smoke from the fires in northern Alberta have spread eastward across the entire continent (see below).

Although a lot of this smoke is staying loft, the latest AIRNOW air quality index map shows  some of the smoke reaching the surface in southern Alberta and Montana, substantially reducing air quality there.

According to a recent run of the NOAA/NWS HRRR-smoke modeling system, the smoke will move southeastward during the next day.   The Northwest will continue to enjoy clear skies.

The fires of concern are found in the boreal forest in northern Alberta (see below), with the major burns starting in mid May.   The biggest fire, the Chuckegg, began on May 12th and is now over a half-million acres in size.

The weather conditions associated with the northern Alberta fires is of interest.   Interestingly,  temperatures during the May have been cooler than normal most of Alberta , in contrast to warmer than normal conditions over BC (see below)

On the other hand, conditions have been drier than normal over the same period, with some parts of northern Alberta receiving 40% or less of normal precipitation.

Having big Alberta fires in May is not unusual.  Many of your might remember the huge Fort Murray wildfire that started on May 1, 2016 or the May 2011 Slave Lake fire.

May is a vulnerable month for fires in northern Alberta.  That area is downstream (in the rain shadow of) the Rocky Mountains, and winters tend to be dry.  To illustrate, here is the average monthly precipitation at Edmonton, Alberta, a location that receives about 18 inches per year (Seattle enjoys twice that).  May is the beginning of the thunderstorm season and after a dry winter and with rapidly increasing temperatures, the forests are ready to burn.

But there is a something more.   Many decades of fire suppression has created a very dangerous situation in which the northern Alberta forests are unnaturally dense with growth and ready to burn catastrophically. 

You have heard that story before.  Climate change may well contribute to more fires as the century progresses, but the key issue today is not global warming, but mismanagement of forests.

One more thing---and a good one.  The latest NOAA/NWS GFS run has a lot of precipitation falling over the next two weeks over Alberta and BC  (see below).   So we should have a good moistening and cooling pattern that will work against new fire starts to our north and northeast.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/06/alberta-smoke-spreads-to-atlantic.html