Wednesday, January 28, 2015

...and Now for Something Completely Different

The big news in the weather world this week, of course, has been the huge nor'easter that dumped three feet of snow on eastern Massachusetts, and not so much on New York City. There's plenty to read about that on the web right now, so I'd like to focus on  another weather "event" that has been taking place this week in the central and western U.S. - warm weather.

Temperatures the last few days have been more than 20°F above normal throughout the Central and Northern Plains, and and 10°F to 15°F above normal across the western U.S.

High temperatures have been running 18°F to 25°F above normal from Oklahoma north to the Dakotas and Montana. Low temperatures have been 10°F to 25°F above normal across the same area.

High temperatures yesterday were in the upper 70s and low 80s in Oklahoma and Kansas and in the low 70s as far north as South Dakota. Hard to believe it's late January.  There were 138 max temp records broken and 24 tied on January 26, and more were likely recorded on January 27 but weren't yet available at the time of this writing.

Maximum temperatures for January 27, 2015.

The reason for the warm weather is a strong upper level ridge over the western U.S., a complement to the trough over the eastern U.S. associated with the nor'easter. The ridge trough pattern is clearly evident in this water vapor satellite image from January 27.

Water vapor image at 6:45 a.m. CST January 27, 2015

Residents of the Central and Northern Plains should enjoy the warm weather while it lasts, because it wont last long. A surge of cold air will spread south across the Plains and Midwest this weekend, and by early next week it will definitely feel like it's February.

The 6 to 10 day outlook issued on January 28 for the period February 3-7, 2015.

Sunday, January 25, 2015

Northeast Blizzard - Will History Repeat Itself?

Blizzard warnings are in effect for Monday and Tuesday for an area stretching from northeastern New Jersey and southeastern New York through southern New England into Downeast Maine in anticipation of a major nor'easter that will rapidly intensify during the day on Monday.

Watches, warnings, and advisories as of 10:43 p.m. EST
The seeds of that storm moved through the Midwest today bringing rain and some snow and was comparatively wimpy compared to what it's expected to develop into on Monday.

Surface weather map at 7:00 p.m. EST January 25, 2015
The low in the Ohio Valley will move to and along the Appalachians early Monday, then will redevelop off of the Virgina Capes Monday afternoon and intensify rapidly.

Forecast surface map for 1:00 p.m. EST January 26, 2015

The low is expected to move northeast and stall southeast of Long Island Monday night into early Tuesday. Snow from this system will beginning by morning in southern New Jersey (possibly with a changeover to rain for a period before going back to snow), by afternoon in New York City, and by evening in Boston. Snow will continue to spread northeast through Maine on Tuesday before tapering off in most areas early Wednesday.

Forecast surface map for 7:00 a.m. EST January 27, 2015

Snowfall amounts in excess of two feet are expected from the New York City area to near Boston. Some locations within the heavy snow band could pile up as much as 30 inches of snow before the storm ends.  Snowfall rates could be 2-4 inches per hour at times, and thundersnow is a strong possibility with the heavier snow bands. If the heavy snow weren't bad enough, winds will build to sustained at 30-35 mph with gusts to 50 mph and higher later Monday, whipping the snow in to huge drifts and reducing visibility to near zero at times. Along the coast the storm will produce much higher tides than normal and coastal flooding is expected.

Expected snowfall accumulation ending 7:00 a.m. EST January 28.
This new snow will add to a snowpack that averages about 8 inches across the northeast. There was snow this weekend across much of the northeast and New England which brought 4 to 8 inches to much of the same area that is in the cross hairs of tomorrow's nor'easter.

Snowfall for the 48-hour period ending at 7:00 EST January 25, 2015

The forecast evolution of this nor'easter is similar to that of what is known as the Blizzard of '78 in the northeast. This nor'easter occurred on February 6-7, 1978 burying southern New England in 20 to 30 inches of snow, with a small area of 50+ inches of snow was reported in northern Rhode Island. This storm produced winds gusts up to 100 mph along the coast and snow drifts as high as 27 feet. High astronomical tides coincident with the strong storm winds produced damaging coastal flooding that destroyed dozens of homes. The deaths of 99 people were attributed to the massive blizzard.

Surface map for 7:00 a.m. EST February 6 and 7:00 a.m. February 7, 1978
The February 1978 storm was the worst storm in more than 100 years in Massachusetts, Rhode Island, and eastern Connecticut. It was the worst storm in March 1888 for western and central Connecticut and southeastern New York., and the worst storm since December 1947 in coastal New York, Long Island, and southwestern Connecticut.

There is an entire web site dedicated to this blizzard, Along with descriptions of the blizzard's development, there are photos of the snow and storm damage and personal stories or those who lived through this storm firsthand.

The National Weather Service in Taunton, MA has a presentation online about the Blizzard of '78.

The maps and charts above credited to Kocin/Uccellini came from this presentation. Their description of this storm was published is in Northeast Snowstorms, by Paul J. Kocin and Louis W. Uccellini, American Meteorological Society (AMS), Meteorological Monograph Volume 32 Number 54 in two
volumes, 2004.

Tuesday, January 20, 2015

Try Your Hand at "Forecasting" Winter Precipitation

In my last post I described the different types of winter precipitation we are likely to encounter. Forecasting the type and amount winter precipitation can be tricky. I've already described the many challenges of forecasting snow amounts. Perhaps more challenging are those "borderline" situations where there is a transition zone between rain, freezing rain, sleet, and snow. What area is going to get what type of precipitation and for how long? Will a transition happen quickly or slowly?

I just became aware of a neat interactive applet that lets you explore how temperature and moisture profiles in the atmosphere affect the type of precipitation you see (thanks to NWS Chicago for the "heads up" on this). A second applet lets you "grow" ice crystals in different parts of a cloud to see what type of crystal is produced. These applets were developed by the Cooperative Institute for Meteorological Satellite Studies at the Space Science Engineering Center, University of Wisconsin, and are part of the Explore the Atmosphere page.

Screen shot of the Precipitation Type applet.
The Precipitation Type applet is fun to play with. You can adjust the temperature and wet bulb temperature (a measure of moisture in the air) at the surface and three additional points in the atmosphere. Symbols fall from the cloud indicating the type of precipitation. Snow accumulates on the ground as the simulation continues. If there is snow falling through a warm layer near the ground, the snowflakes change to rain drops. If the mid-level is above freezing and the surface is below freezing, freezing rain and drizzle will be depicted on the ground. Another neat feature of this applet is that it depicts evaporational cooling, which sometimes is a big consideration in a winter precipitation forecast. For example, if the surface temperature is 33°F and the air is dry, the air will cool as precipitation falls through it and some of the moisture is evaporated. This cooling will continue as long as the air is not saturated and there is no introduction of warm air from another source. Evaporative cooling is the same reason you feel cool or even cold getting out of the pool on a warm, dry day in the summer. To see this in action, do the following:
  1. Drag the temperature and wet bulb lines at the first level above the surface so they both read 1.8°C, that is, the air is saturated.
  2. Drag the wet bulb (yellow) line at the surface to about -7.0°C, and the temperature (green) line to 0.9°C (that's about 33°F).
Now, watch as precipitation begins to fall from the cloud as snow and change to rain in the near surface layer. Watch also how the temperature slowly falls to and below freezing (the number will change and the point will move lower). At this point freezing rain starts to accumulate. The temperature will continue to drop until eventually it reaches the wet bulb temperature, at which point it is saturated.

There are any number of scenarios you can set up. For example, if you separate the temperature and wet bulb at the very highest level, let's say, -10°C temperature and -19°C wet bulb, the cloud will disappear - it's too dry.

What's evident after playing with this enough is that it doesn't take much, a degree to two sometimes, to make a big difference in precipitation type. Add in other factors such as the depth of the warm or cold layers and you can see how forecasting winter precipitation can be very tricky.

Screen shot of the Grow Snow applet.
Often the type of snow crystals that form and fall can make forecasting snow amounts difficulty. A fluffy snow with large flakes and lots of air in between will accumulate rapidly. Snow crystals that are elongated and more compact will accumulate more slowly to to the greater density. The Grow Snow applet lets you grow your own ice crystals in different parts of the cloud to see what results.

Have fun with these!

Wednesday, January 14, 2015

Sorting Out Winter Precipitation

Now that we are the mid-point of January Mother Nature has seen fit to provide most of the country with some winter weather. When most people in the colder part of the country hear "winter precipitation" they are most likely to think of snow. One of the challenges to winter precipitation measurements is the menu of precipitation that can occur.

The precipitation can be broken into two categories, frozen, and freezing or unfrozen precipitation. In the frozen category we have snow, graupel, ice pellets (sleet), and snow pellets. In the unfrozen or freezing category we have freezing rain or drizzle, and "plain" rain or drizzle. Let's look at these one at a time.

Snow is precipitation of ice crystals, mostly branched in the form of six-pointed stars. The shape and size of snowflakes can vary, from large "fluffy" flakes to smaller ice crystals. The type of snow and the snow density is dependent on many complex factors. Among these are are the temperature and water vapor within the dendritic growth zone in the clouds (where the temperature is -12°C to -18°C), and the depth of the dendritic zone. Dendrites are beautiful, ornate snow crystals that most people are familiar with. Additional factors affecting snow density include the shape of the snow crystals, vertical motion in the clouds, the amount of liquid water in the cloud,and the thermal profile of the layers from cloud level to the surface. The large "silver dollar" snowflakes tend to occur when the air is warmer and moisture is plentiful. What appear to be huge snowflakes are at times multiple snowflakes that have collided and stuck together. Go here for more information on snow density and the snow-to-water ratio.

Snow pellets/graupel
Rime on tree branches
Snow pellets are white, opaque grains of ice, with diameters about five times the size of snow grains. The real distinction between snow grains and pellets is that snow pellets are brittle, crunchy, and bounce upon hitting a hard surface. Sometimes they will break on impact. The are usually opaque white, as opposed to the clear ice pellets.  Also, they fall in the form of showers, from mainly cumulus congestus clouds.They typically form when snow crystals come into contact with supercooled droplets, and these droplets freeze onto the surface of the crystal. The process is similar to that which you see when water droplets in fog freeze to the outer surface of objects, forming ice crystals (riming).

Credit: Wikipedia

Snow Grains
Snow grains are very small, white, and opaque grains of ice, the solid equivalent of drizzle (or as I like to call it, "snizzle"). They are flat and elongated, with very small diameters. They fall in small quantities, and never in the form of a shower. When these grains hit the surface, they neither bounce or shatter.

Ice Pellets
Commonly called sleet, ice pellets are transparent or translucent pellets of ice which are round or irregular. Typically they are hard grains of ice consisting of frozen raindrops or largely melted and refrozen snowflakes. It often looks like it's raining, but the ice pellets often bounce when they hit a flat surface and they accumulate. Driving on a a few inches of sleet is like driving in sand or on tiny ball bearings.

All of the above precipitation types fall as frozen precipitation and are reported as "new snowfall" on the CoCoRaHS Daily Precipitation report.

Freezing Rain and Freezing Drizzle
The challenge of freezing rain.
Freezing rain or freezing drizzle occur when rain becomes supercooled and freezes on impact with objects on the surface. Typically snow falls though an elevated warm layer where the snow melts and becomes rain. The rain drops then fall through shallow layer of cold air near the surface, where they become supercooled. When the supercooled but still liquid rain drops come in contact with objects below freezing (the ground, trees, power lines, etc.) they immediately freeze. The resulting glaze of ice, if heavy enough,  can cause extensive damage and make travel difficult if not impossible.

Since freezing rain falls as liquid, it is reported the same as normal rain after melting the ice on the inside of the gauge and measuring the liquid.

Wednesday, January 7, 2015

An Eventful First Week of January

January 2015 is off to a rousing start in the weather department based on what's happened this first week.

On January 1st record cold affected the southwestern U.S. from California east to Texas.  Then a day later a very strong cold front swept through the Hawaiian Islands producing damaging winds at the surface and blizzard conditions near the summits on the Big Island.

As the system that brought cold and snow to the southwestern U.S. moved east and northeast on the 2nd and 3rd, it drew moisture from the Gulf of Mexico to produce 3 to 6 inches of rain from eastern Texas and Louisiana through central Arkansas. Severe thunderstorms erupted ahead of this system in the southeast, producing at least four tornadoes and wind damage from southeastern Alabama across southern Georgia.

Total precipitation for the period January 1-7, 2015

A clipper system brought snow to a wide swath of the northern and central U.S. this first week. Snow accumulated  across much of Montana, with 12 to 18 inches in and around Billings. As this system moved southeast snow spread from the Dakotas across Iowa, the northern two-thirds of Illinois, Indiana, Michigan, and to the mid-Atlantic coast. Several inches of snow falling during the morning rush hour tied Washington DC traffic into knots.

72-hour snowfall ending the morning of January 7, 2015

At the end of this week snow covered about 46 percent of the lower 48 states, the highest since the unusual snow in mid-November.

Snow depth on January 7, 2015

Finally, a massive Arctic high pressure system dropped into the northern U.S. early this morning.

Surface weather map at 9:00 CST January 7, 2015

Temperatures at 9:00 a.m. CST January 7, 2015
The strong pressure difference between the high and a low over southern Quebec/New England produced bone-chilling winds across the eastern half of the U.S., prompting wind chill warnings for parts of the Midwest and Northeast, and wind chill advisories for most of the remainder of the eastern half of the country. Hard freeze warnings are in effect for tonight and tomorrow morning along the Gulf Coast from Houston east to northern Florida. Blizzard warnings are in effect for Thursday for northeastern North Dakota and northwestern Minnesota, and a blizzard watch for northeastern South Dakota.
Watches, warnings, and advisories in effect as of 9:00 p.m. CST January 7, 2015

The cold Artic air streaming across the Great Lakes has switched on the lake-effect snow machine, and it will continue for the next 36 to 48 hours.

Satellite visible image of the Great Lakes today at 1:15 p.m. CST. The flow of cold air across the Great Lakes can be clearly seen in the bands of clouds in and downwind of the lakes. The dark areas are clear skies on the lee side of the lakes.

A CoCoRaHS observer near Watertown, New York (NY-JF-25, Watertown 1.3 WSW) reported 23.5 inches of new snow for the 24 hour period ending at 4:30 a.m. EST this morning. Watertown is located at the east end of Lake Ontario.

CoCoRaHS snowfall map for northern New York for the morning of January 7.

So how strong is the high pressure system? Very strong - high pressure records were set in a number of locations in the central and northern Plains today. Aberdeen, South Dakota reached a pressure of 1055.4 millibars (31.17" of mercury) this afternoon, a new record at that location.

Photo of the barograph (recording barometer) at Aberdeen, SD today. The actual air pressure was literally "off the chart". Photo credit:  NWS Aberdeen, SD
Omaha's air pressure reached 1055.3 millibars today (31.16 inches), breaking the old record 1053.7 millibars (31.12 inches) on December 22, 1989. A new record was also set at Grand Island, NE. It's likely that by tomorrow we will be seeing a number of reports of new record high pressure readings that occurred in the Plains today.

Saturday, January 3, 2015

Hawaiian Blizzard

While much of the U.S., including Alaska, has been short on snow this winter, snow is making some headlines in the least likeliest of places, Hawaii. On Friday night the National Weather Service in Hawaii issued a blizzard warning (yes, blizzard as in snow and wind) for the Big Island summits above 11,000 feet. Winds were expected to gust to 130 mph along with up to a foot of snow, producing near zero visibilities and white-out conditions. The blizzard warnings extended through 6:00 p.m. HST today. High wind warnings were issued for most of the Hawaiian Island chain last night through early this morning.

The warnings were prompted by an unusually strong low pressure system and cold front moving through the islands. The strong pressure gradient around the low produced the high winds near the surface, while a strong upper level trough maintained the strong winds at the higher elevations.

North Pacific surface analysis for 8:00 a.m. HST January 2.
Last night at 6:00 p.m. HST winds at Mauna Kea on the Big Island were west sustained at 79 mph gusting to 139 mph with the temperature at 28°F. The highest wind gust recorded early this morning was 155 mph near the summit of Mauna Kea at the Canada-France-Hawaii Telescope weather station
Timer trace of the wind speed and direction near the summit of Mauna Kea.
The passage of this weather system left thousands without power on most of the islands as the strong winds toppled trees and power lines. Precipitation was also heavy, with amounts from 0.50 to 1.50 inches across the islands. The CoCoRaHS observer at HI-KI-6, Kilauea 0.5 SE measured 2.77 inches of rain this morning.

CoCoRaHS observations for Hawaii the morning of January 3.

Colder air in the wake of this system spread across the island chain today. Island residents were able to observe cold air cumulus today, a common occurrence here in the continental U.S. in the spring and fall but very rare in the tropical Pacific. These clouds develop as much colder air at middle and upper levels creates in a vertical temperature profile that favors the vertical formation of cumulus. As the surface warms cold weather cumulus quickly form and often fill in to result in overcast conditions.

Overcast skies from cold weather cumulus on the north shore of Oahu at 10:30 a.m. HST January 3.

A dry trade wind pattern is forecast to redevelop across the islands early next week.

Thursday, January 1, 2015

Frost on the Roses

Temperatures at 6:00 a.m. PST January 1
Those arriving early this morning in Pasadena, CA to find good viewing spots for the Rose Bowl parade contended with record cold. The temperature bottomed out at 32°F in Pasadena, and the low at the Los Angeles International Airport dropped to 36°F, a new record. The old record was 38°F set in 1972. A winter chill was common throughout southern California and the southwestern U.S. this first day of 2015. Temperatures were in the single digits in the Ventura Mountains and in much of the mid 20s in many of the interior valleys. The cold also was common across most of the Desert Southwest. Tucson, AZ reached a maximum temperature today of only 41°F, a new record low maximum temperature. The previous record was 43°F in 1960.

The same system that ushered in the cold weather also dropped snow on most of Arizona, with up to 20 inches reported in the mountains in northern Arizona, with 20 inches at Mountainaire and 17.3 inches reported at Flagstaff. 

24 hour snowfall ending at 7:00 a.m. local time January 1, 2015
 The CoCoRaHS observer at Pine 0.9 SW in Gila County reported 14.5 inches of new snow, with six other Arizona observers reporting 10 inches or more of snow.

CoCoRaHS snowfall for the 24-hour period ending at 7:00 a.m. PST
The strong upper level wave that brought a chilly start to 2015 became closed off over southern California on December 31 and moved into Arizona last night. It tapped into the cold air being fed into the Rockies and Plains both at the surface and aloft resulting in the uncharacteristic winter weather for the southwestern U.S.

500 millibar map for 4:00 p.m. PST on December 31, 2014.
Cold weather will continue across southern California and southwest Arizona again tonight and Monday morning. Hard freeze and freeze warnings are in place for large portions of these areas. Winter weather advisories and winter storm warnings are in effect for eastern New Mexico and northwestern Texas as precipitation spreads into the cold air with freezing rain, sleet, and snow expected.

Watches, warnings, and advisories in effect as of 8:39 p.m. PST January 1.

The upper low will continue to move slowly east across Arizona and New Mexico the next 48 hours. Precipitation will spread from the Southwest through Texas and eventually into the Midwest and the eastern U.S. over the weekend. Precipitation will be more than a half inch in much of the eastern U.S. By Monday snow is likely across the upper Midwest and the Northeast.

Quantitative Precipitation Forecast for the 72-hour period ending 6:00 p.m. CST Sunday, January 4

24-hour, 2 inch snow probability for period ending 6:00 p.m. CST January 4.