Cold, warmth and the PNA

The weather patterns for the past month over the continental United States have been remarkably persistent.  Though we've been hearing a lot about the continued cold and snowy weather in the eastern US, not as much has been said about the unusual warmth of the western US.  Here's a map from the Southeast Regional Climate Center showing the percentile of the average high temperature over the past month compared to climatology.

In the east, particularly the northeast, you can see a lot of places where the high temperature in the past month has been in the lowest 1-3 percentile of the past 30 years.  The cold continues out in the midwest and southeast where the average high temperature has been in the 15-20% range of normal high temperatures.

Contrast this with the west, particuarly west of the Rockies.  Incredible warmth out here!  Many pleases are in the 99th percentile for high temperatures over the past month.  Extremely warm.

We can see this in the large scale pattern as well.  Here are the 90-day (three month) anomalies of 500 hPa heights over the Northern Hemisphere:

There has been a very persistent pattern over the past few months; rarely do you have such large anomalies that show up in the 90-day climatology.  Notice the pattern---anomalously low heights (associated with troughing and often colder air) in the central Pacific, anomalously high heights (associated with ridging and often warmer air) over the western US and up into Alaska, followed by another belt of lower heights over northeastern Canada and slightly into the eastern US, and then another area of high heights in the central Atlantic. This means that the atmosphere has had persistent ridging over the west. This ridging has acted like a protective shield, deflecting significant storms and cold air outbreaks to the east.  No wonder it has been so warm and pleasant in Seattle this winter...

This pattern of alternating troughs and ridges arcing across North America is something that is called a Rossby wave train and this particular pattern is known as the Pacific-North American pattern (PNA).  The PNA pattern is suspected to be caused by variations in heating along the equator.  Here's an example of what the PNA pattern is supposed to look like during the winter of a strong El Nino year:

This looks a fair bit like our anomaly pattern above, though our exact pattern is shifted a little further south and west. Because our current pattern is rather similar to this classical PNA pattern, we would say that the current pattern projects strongly on the PNA.  We can measure the correlation between the upper-level pattern of ridges and troughs every day with the classic PNA pattern.  Here's a time series of what that has looked like over the past few months from the Climate Prediction Center:

The more closely the current pattern matches the PNA pattern, the more positive the index here. We can see that, at least since mid-December, this PNA index has been slightly positive for the most part.  Remember that the cannonical PNA pattern in the map above is most prominent during El Nino periods.  Despite some signs last year that an El Nino was going to develop, we have not developed a strong El Nino this winter.  Here's the Nino 3.4 index over the past two years:

You can see that though we've been in a weakly positive (El Nino) phase for much of 2014 and into 2015, the values have not gotten up to 1.0 yet, which is a commonly-used threshold for declaring an El Nino event.

So we've been stuck in an "almost" pattern for the past several months---a weak El Nino and a weak PNA pattern.  Notice, however, in the plot of the PNA index above that there are several red lines extending into March that represent different long-range forecasts of the PNA index.  Notice that most of the forecasts are actually predicting a significant swing into a negative PNA phase.  This would mean that the pattern is more like the opposite of the PNA: more troughing in the west and ridging in the east.  Our longer-range weather forecasts are also hinting at this.  So, we may finally be getting out of this rut...

Snowpack and Surface Temperature

Here's an interesting case today of an area in central Illinois that somehow has managed to miss out on all the snow.  Here we have the NWS National Snow Analysis of snow depth:

We can see that extraordinary swath of snow that the most recent set of storms this weekend has brought to the mid-Mississippi and the Ohio River Valley---much further south than the snow we've seen so far this season.  North of that, repeated rounds of storms have brought a lot of snow to the upper midwest and the mid-Atlantic and New England. But in between them is a narrow sliver---from southern Iowa through central Illinois and Indiana and into Ohio---between the two snowy areas.  You can see that area on the visible satellite this morning over central Illinois:

There are clouds developing to the east over central Indiana, but there is a clear gap in central Illinois between the northern snowpack and the southern snowpack.

Springfield, Illinois happens to sit in that gap.  Springfield also happens to be the current forecast city for the national WxChallenge forecast competition.  Nearly everyone (including most of the models) thought that the clouds would stick around today and keep the sun out.  Furthermore, it's unclear whether our weather models actually started out thinking there was snow on the ground in this region.  For one or both of these reasons, the high temperature today was forecast to be between 20-22 Fahrenheit.  It's already up to 30.

The SPC Mesoanalysis shows a clear pocket of 25-30 Fahrenheit temperatures (the purple contours) over central Illinois in this "gap" in the snowpack:

It's pretty amazing how much having snow on the ground keeps down the temperatures, even in broad sunlight.  Much of it has to do with the albedo of snow versus bare ground.  Snow, being white, has a higher albedo, meaning it reflects more of the incoming solar radiation.  Bare ground, on the other hand, tends to be much darker in color and therefore has a lower albedo, meaning it absorbs more solar radiation and has a tendency to warm up because of that.  These simple differences in the color of the surface can make a world of difference when it comes to temperature.

We can look at this morning's sounding from Lincoln, IL, just to the northeast of Springfield to see what the temperature profile looked like:

There's a pretty sharp inversion just above 850 hPa.  This actually leaves a pretty deep layer below that over which a lot of mixing has to have taken place for the temperature to warm up to nearly 0 Celsius.  Furthermore, if the sunshine continues and we don't have clouds move in (which we may see soon from the west), following a dry adiabat down from that inversion suggests that if it completely mixes out, it could get as high as 3-4 Celsius (37-39 Fahrenheit).  However, it's already mid afternoon there, clouds may move in shortly, and there is a fairly brisk northwesterly wind that's helping to advect in cooler air from off the snowpack to the northwest.  It has still gotten up to 34 Fahrenheit at some surrounding coop sites...

Anyhow, an interesting case of how small-scale variations in the snow can have big implications for the temperature.

A different blizzard track

Looking at the National Weather Service's hazards map for today, it's clear where the story is:

Another powerful snowstorm forecast to hit the northeast.  Let's run through a quick overview of what the current model forecasts are doing with this storm.

Here's the GFS sea-level pressure (black contours) and relative humidity at the surface (colors; not as important here...) starting with a 24 hour forecast for tomorrow morning at 12Z.  I get these graphics from Weather Underground's Wundermap utility, which not only lets you zoom in on various areas but also is the only place to get full-resolution ECMWF images for free...

We can see that there's a 1000mb low over northeastern Lake Huron and central Ontario.  Note that this is very different from the first blizzard at the end of January, which moved up the coast from the south.  This sort of storm that comes out of the west-northwest is commonly referred to as a "clipper" system, as they tend to be rather fast moving.

Twelve hours later on Saturday evening, the low is forecast to basically be centered over New Jersey.

It hasn't deepened too much, but the storm is about to move out over the northern Atlantic.  Here's a map of the current sea-surface temperatures off the east coast from NOAA/NESDIS:

You can see right near the coast the temperatures are around 4-5 Celsius (39-41 Fahrenheit) but they rapidly warm into the 15-20 Celsius (59-68 Fahrenheit) range the further you get from the coast.  There's a sharp gradient of temperature there, and that's exactly what a storm like this can feed on to grow.  Storms form as the atmosphere's response to temperature and pressure imbalances.  The atmosphere wants to try and "smooth out" the temperature as much as possible.  Therefore, when a developing storm meets an region like this (a "baroclinic zone" if we want to have a technical term), it tends to deepen quickly, and that's exactly what we see.  Here's the forecast for Sunday morning at 12 Z:

An explosion of contours!  The low has deepend to below 976 mb as it feed on the baroclinic zone off the coast.  In just 12 hours the low has deepened nearly 24 mb---clearly meeting the 24mb in 24 hours deepening rate we use for "bombogenesis".  This deepening tightens the pressure gradients and will increase the winds.  Here's the GFS wind forecast for Sunday afternoon:

Strong, 50 kt. winds forecast over Cape Cod and along the new England coast.   The winds along New England and the Mid-Atlantic states at this point have strong northerly-northwesterly components (offshore winds) and they will be bringing in much colder air at the low levels.  The flow above, however, is wrapping around the low from the east over Nova Scotia and into New England, bringing moister, oceanic air over this low-level cold air and setting the stage for snow.  Here's the GFS precipitation forecast for Sunday afternoon:

Heavy precipitation along the New England coast.  The ECMWF forecast has slightly less precipitation at this time, but the position of the low is very similar:

All in all, it will be another blizzard for the New England coast...

More snow doesn't mean more liquid

Keeping on the theme from the other week, I wanted to look at the snowfall to date this year in Chicago and Boston given their recent near-record snowfall.  Fortunately the National Weather Service in Chicago makes it easy with their year-to-date climate plots.  Here's the most recent one from O'Hare Airport (KORD) in Chicago.

In the top panel you're seeing the daily temperature ranges as the dark blue bars, then in the background are the normal daily range (light green) stretching out to record highs (the light red) and record lows (light blue).  While bouncing around a bit, the temperatures in Chicago actually have been pretty near normal overall, with cold spells in early January and early February and a warm spell in mid-late January.  The next plot down is the precipitation so far this year in inches and the plot below that is the snowfall so far in inches.  The yellow lines in both plots are the "normal" precipitation up to that point in the year.  Now, precipitation doesn't smoothly fall in even amounts every day, so we never expect the actual precipitation to look like that.  Instead, it tends to follow a stair-step pattern, jumping every time there's a big event.  Anywhere you see dark green, the precipitation at that point was greater than the normal for the year at that time.

Here's what's interesting about the plots above.  You can see in the bottom panel that the total snowfall since January 1st is at 33.8 inches--almost 250% of normal.  The big snowstorm of January 31-February 2 shows up quite starkly as the abrupt jump in the snowfall.  But what about the actual liquid precipitation?  In the panel above, you can see that this is still right about normal: we've had 2.44 inches so far this year.  So, despite having extreme snow depth amounts, the actual liquid content of that snow wasn't anything to go on about.  It must have been extremely fluffy snow.

In fact, you can estimate the snow ratio from the last event.  Looks like the total gain in snowfall was 20 inches of snow (~14 inches to 33.8 inches) but the gain in precipitation was only about 1 inch (adding up the daily observations).  So the snow ratio was around 20:1---twice as big as the commonly-assumed 10:1 snow ratio we use as a ballpark figure.

The Boston forecast office doesn't seem to have nice plots like the one I showed above (in fact, most of the climatology data on the website seems to not have been updated since 2002 or, in some cases, since 1995 (!)). From the climate data, though, in January KBOS had 34.4 inches of snow (compared to a normal of 12.9 inches).  For liquid precipitation, they had 3.57 inches compared to a normal of 3.36 inches.  So, despite having over 250% of the normal snowfall, there was only slightly more liquid equivalent than normal.  The snow ratios were lower--about 10:1--but that seems to be more usual for Boston.  Regardless, incredible snow depth totals doesn't mean incredible snow liquid totals.  Thinking down the line, this means that when the spring melt-out time comes, these heavy snowfalls don't portend any unusual flooding...

In fact, the January precipitation was near normal for most of the areas hit hard by snow, according to the Climate Prediction Center.

The two areas that stand out, though, are the continued drought in much of the western US (particularly California) and the wetter-than-normal conditions in Arizona through west Texas that were caused by a few cut-off lows that brought a lot of moisture to that region.  The effects of these anomalies will be watched in the coming months.