HOW WE GET OUR WINTER WEATHER WARNINGS
by Philip Lutzak – February 2006
INTRO: What Provokes a Weather Warning in Wintertime?
Winter at its worst: An ice storm at Troutdale, Oregon 1996. Courtesy National Weather Service, Portland, Oregon
When hazardous weather threatens any area of the United States, no matter what the type or time of the year, the meteorologists at the Storm Prediction Center (SPC) in Norman, Oklahoma are already on top of it.
The SPC is a division of the National Weather Service, and the meteorologists who work there are one of our first lines of defense against dangerous weather. They monitor every potentially serious weather situation, and then issue advisories or warnings if they feel it may become dangerous. Although it is most well known for its severe thunderstorm and tornado advisories, the SPC issues many statements and advisories for hazardous winter weather as well. This article will concentrate on one particular example of the advisories it issues in winter, and what methods and tools it uses to produce it.
One of the first things the SPC does when it sees a potential weather danger is to alert weather forecasters around the nation. It does this by issuing a special weather statement called an MCD or Mesoscale Discussion. The word "mesoscale" is derived from the Greek word mesos, meaning middle. So mesoscale weather events are ones that are in between the microscale (such as mist over a pond), and macroscale (such as a thousand mile long front of cold air advancing towards the eastern U.S. coast in winter.) Typical mesoscale weather events in the warmer months include severe thunderstorms, tornadoes and hurricanes. In the winter they include blizzard conditions, heavy snow, and ice storms (a result of freezing rain.) Although available to the general public, MCDs are rarely read by them. That’s because they contain a discussion of the impending weather problem using advanced terminology, and are intended primarily to alert forecasters or public officials in emergency management, who can easily assess the situation and then issue their own local warnings to their public.
The goal of this project is to allow you to understand how the Storm Prediction Center helps protect us from severe winter weather by my presenting a typical discussion (MCD) that it issues in the wintertime, and then explaining how and why they produced it. I chose a discussion from January of 2004, for two reasons: 1) this event included, besides heavy snow in the mountains of the Pacific Northwest, a classic case of freezing rain that results in what we know as an “ice storm”, an obviously dangerous weather event that can bring a substantial area of the country to a complete standstill and 2) since I’ve been studying winter mesoscale weather events, I’ve learned about the dynamics of an ice storm and realized how little I understood them. So I’m going to try to see if I can get you to read an MCD about an imminent heavy snow or freezing rain event and understand the gist of what it's saying. Most of the abbreviations are fairly obvious(!), but I’ll make sure I explain them. The most important point is to explain the meteorological terms so that you can understand what message the SPC is trying to convey to the forecasters and others who read them..
The Discussion From January 2004:
MESOSCALE DISCUSSION 0011
NWS STORM PREDICTION CENTER NORMAN OK
1119 PM CST MON JAN 05 2004
AREAS AFFECTED...NWRN OREGON...WRN WASHINGTON
CONCERNING...WINTER MIXED PRECIPITATION
VALID 060519Z - 060915Z
...SNOW WILL INCREASE IN INTENSITY WEST OF THE CASCADE RANGE OVER THE NEXT FEW
HOURS. FREEZING RAIN WILL ALSO SPREAD NWD ACROSS THE WILLAMETTE VALLEY...
BROAD WARM CONVEYOR BELT OF DEEPER PRECIPITATION IS SHIFTING EWD WITH LEADING
EDGE OF THIS ACTIVITY NOW SPREADING ONSHORE ACROSS PORTIONS OF WRN ORE/WA. STRONG
PRESSURE GRADIENT DUE TO POLAR HIGH EAST OF THE CASCADES WILL CONTINUE TO FORCE COLD
AIR INTO THE WILLAMETTE VALLEY REGION BENEATH GRADUALLY WARMING MID LEVEL COLUMN.
ALTHOUGH MUCH OF THE INITIAL PRECIPITATION WILL BEGIN AS SNOW ACROSS ORE...
FORECAST SOUNDINGS STRONGLY SUGGEST SNOW WILL CHANGE TO FREEZING RAIN AROUND
07Z FROM SOUTH...NEAR EUGENE...TO NORTH...AROUND PORTLAND BY 12Z. ALL BUT EXTREME
SWRN COASTAL AREAS OF WA WILL MAINTAIN SNOW PROFILE THROUGH 12Z. SNOW RATES WILL APPROACH
1 INCH PER HOUR IN THE VALLEYS WITH HEAVY ICING EXPECTED ACROSS THE SRN WILLAMETTE VALLEY.
48362262 47342190 45242197 43672279 44322402 46472399
Figure 1: Storm Prediction Center MCD #0011
All MCDs use the same format. At the top is a map of the area of concern, with an overlay of symbols and phrases that illustrate what the impending weather problem is. The curving symbols with a dot in their left side (in red here) are the National Weather Service’s symbol for freezing rain. The symbol with four blue asterisks signifies heavy snow. The dashed red line and arrow show the advancing edge of the freezing rain and the direction in which it’s moving. The graphic depiction is followed by the text which explains the circumstances leading up to this particular discussion. Because the text is not especially lengthy in this case, I’ve included the entire discussion as it normally appears in Figure 1. The title information is fairly obvious. For the curious, “NWRN” means northwestern and “WRN” means western, and the time used is Greenwich Mean Time (or “Z” time), which translates to 8PM – 4AM Pacific Standard Time on the 6th of January. Meteorologists frequently use UTC or Z time, so if you’re curious how it works, check out this explanation of UTC and ZULU time.
The last four lines refer to the name of the meteorologist issuing the report and the date issued, followed by the weather stations involved and the
map coordinates bordering the areas affected.
What They Are Trying To Convey:
...SNOW WILL INCREASE IN INTENSITY WEST OF THE CASCADE RANGE OVER THE NEXT FEW HOURS.
FREEZING RAIN WILL ALSO SPREAD NWD ACROSS THE WILLAMETTE VALLEY...
Because they refer to specific regions of the Pacific Northwest here, it’s time for a brief look at the area they
are talking about, and why meteorologists there are being cautioned. The following photos should get you acquainted:
FIGURE 2: (above left) Pacific Northwest, with the Willamette Valley highlighted in red.
FIGURE 3: (above right) Willamette Valley & Columbia River close-up. At the top of the map is the Columbia River, running from east to west through the north end of the valley. Below it is the Willamette River, with all of its forking tributaries, running from south to north between the Coast Range to the west and the Cascade mountains to the east. Both photos ©2004 Matthew Trump.
Note in the topographical maps of figures 2 and 3 that the Columbia and Willamette valleys are both deep valleys situated between very high mountains. The Willamette Valley has the Cascade range to its east and the Coastal range to its west. The Columbia river cuts a deep gorge through the Cascades before running through the Willamette Valley towards the Pacific Ocean. In figure 2, we can see the deep greens representing the low-lying valleys, and the yellows and grays representing the higher terrain and mountains. Figure 3 is a close-up view, showing the four biggest cities lying in the Willamette Valley. Now picture very cold air approaching from the north in winter. This frigid air (High pressure of Polar or Arctic origin) is quite shallow and dense, so you can think of it as oozing southward towards these valleys, and then settling into them, somewhat like syrup poured on waffles. The cold winds come from the north and east, blowing into the low areas north and east of Portland first, then funneling through the Columbia River valley westward and finally southward into the Willamette Valley (see figure 4 below).
Notice also in figure 4, the thin brown lines that curve around the HIGH approaching the Pacific Northwest. These are lines of equal pressure, and when they are very close together as they are here, they form what is called a strong or tight "pressure gradient". This results in faster winds which help push the gelid air through the mountain passes and into the valleys.
Now picture a storm coming in from the Pacific Ocean, with winds blowing from the south over the ocean ahead of it, where ocean temperatures at this time of year are in the 45 to 50 degree Fahrenheit range. The air over the ocean acquires roughly the same temperature as the ocean below it, and the air for thousands of feet above it is also relatively warmer because it's approaching from the central Pacific, a warmer wind direction than from the north. This relatively warmer air coming from the southwest at low and middle levels of the atmosphere, often referred to as a “conveyor belt” of moisture, is loaded with rain producing clouds and when it hits the coast it gets forced up over the coastal mountains, and then ON TOP of the shallow frigid air in the valleys (see Figure 5 below).
So we have below-freezing air at the surface and warmer, above-freezing air in the air columns above it. The rain falling out of the warmer air aloft doesn’t have enough time to re-freeze in the thin layer of cold air underneath it, so it stays as rain and lands on below-zero surfaces such as trees, roads and homes, and freezes on contact. This is freezing rain. As you might imagine, the duration of the event and how hard it falls usually determines how much of an ice coating you are going to get. Even a few hours can have a severe impact.
Here’s a great satellite view of the area, where you can clearly see the streams of clouds and moisture coming into Oregon from the southwest.
Now this excerpt from the MCD should make sense:
BROAD WARM CONVEYOR BELT OF DEEPER PRECIPITATION IS SHIFTING EWD WITH LEADING EDGE OF THIS
ACTIVITY NOW SPREADING ONSHORE ACROSS PORTIONS OF WRN ORE/WA. STRONG PRESSURE GRADIENT DUE
TO POLAR HIGH EAST OF THE CASCADES WILL CONTINUE TO FORCE COLD AIR INTO THE WILLAMETTE VALLEY
REGION BENEATH GRADUALLY WARMING MID LEVEL COLUMN.
“SOUNDINGS”: How They Know About The Warm Air Aloft.
Meteorologists now have a huge amount of data to help them improve their forecasts. Among the most important tools they use are pictures of what’s going on in the upper air columns above us, called “soundings”. Weather station operators take sensors that can measure temperature, humidity and air pressure (among other things) and launch them upwards with weather balloons or drop them via parachutes from airplanes in order to get these soundings and find out what’s going on in the upper regions of the atmosphere. When they are represented on a graph, they look like the following example in figure 6, which shows the atmosphere over Salem, Oregon, while the freezing rain was occurring. I have added some overlays to show where the warm and cold layers are.
You are looking at the air over Salem from a side, cross-sectional view. The bottom of the chart represents sea level or ground level. The top line is roughly the top of our atmosphere. The red line represents the air temperature over Salem, and the blue line is representative of how moist the air is. When the red and blue lines are close (as they are here), there usually is precipitation falling. Anywhere on this chart that the red line is to the right of the slanted green line, the air is above freezing, or the melting point of snow and ice. So snow falling from above 8,000 feet falls into the above freezing layer, melts into rain, and then falls through a thin layer of frigid air before it can change back to snow or ice. The result is freezing rain.
Note that the snow gradually melts to rain as it falls from 8,000 to 3,000 feet. So the higher, mountainous areas surrounding the valleys (roughly 6,000 to 8,000 feet) will remain mostly all snow, while the areas from 6,000 down to about 4,000 or 5,000 feet will have rain and snow mixed. This zone of mixed precipitation often shifts up and down the mountainsides by a few hundred feet or more during a storm like this.
Also note that this warm middle layer of air was not there when the storm first started moving in. It gradually wedged in between the frigid air over the valleys and the cold air above 8,000 feet. So even though all of the precipitation was originally snow, when the warm air started to sandwich in between the upper-level cold air and the cold air trapped in the valleys, the snow falling in between the two cold layers began to melt into rain, and fall into the cold air trapped in the valleys below it before it could turn back into snow.
Now here’s the rest of the MCD text:
ALTHOUGH MUCH OF THE INITIAL PRECIPITATION WILL BEGIN AS SNOW ACROSS ORE...FORECAST
SOUNDINGS STRONGLY SUGGEST SNOW WILL CHANGE TO FREEZING RAIN AROUND 07Z FROM SOUTH...NEAR
EUGENE...TO NORTH...AROUND PORTLAND BY 12Z. ALL BUT EXTREME SWRN COASTAL AREAS OF WA WILL
MAINTAIN SNOW PROFILE THROUGH 12Z. SNOW RATES WILL APPROACH 1 INCH PER HOUR IN THE VALLEYS
WITH HEAVY ICING EXPECTED ACROSS THE SRN WILLAMETTE VALLEY.
Unlike the actual sounding pictured above in figure 6, forecast soundings are, as you would imagine, a graph similar to the one above, but a forecast of what the atmospheric conditions will look like at specific future time periods. So we know that the meteorologists at the SPC were concerned because all the forecast soundings predicted a gradually increasing wedge of warm air moving over and on top of the frigid air in the Columbia and Willamette Valleys and under the normally frigid air above, setting them up for a freezing rain event. Of course, the local meteorologists quickly took over and issued their own winter watches and warnings to alert the public to the danger. I’ve added a brief epilogue to show you the results of such an extended event of severe icing.
EPILOGUE: The January 6th – 8th Storm
The following diagrams (figures 7, 8 and 9), called “meteograms”, are a type of report from a weather station showing what conditions have occurred over a previous 24 hour period. The three shown here cover Portland, Oregon from 4PM on January 5th to 3PM on January 8th. Look at the line near the middle left marked “WX”. Next to it, under each vertical line (hour), is the symbol showing what weather was occurring at the time. Where there’s no symbol, no precip was occurring. The “xx” and “xxx” symbols represent snow. If you remember the symbol for freezing rain from the beginning of this article (the curving symbol with a dot in its left side), you can see that freezing rain fell over Portland for a long time. From the charts (I noted the time span in Portland local time beneath each graph), we can see that it started at 1PM on the 6th and lasted until 8AM on the 8th, , meaning that they had almost continuous icing for 43 hours!
The results were disastrous. In many places, the ice fell on top of 3-5 inches of snow, but almost all of the Willamette Valley received 1 to as much as 2 inches of ice. The storm snapped trees in half, halted Portland's transit system, and even coated airplanes at the airport so badly that the airport had to shut down. If you’d like to see more coverage and some photos of this storm, here are a few links to websites that document the damage:
These sites contain news accounts and some great photos of the storm:
For the technically ambitious, here are some links to excellent meteorological references:
2) National weather Service WFO in Pendleton, Oregon. A case study of this storm examines how better grid spacing in forecasting models produced better results: