Tuesday, February 18, 2014

A Brief Break in Winter Weather

Forecast max temperatures for February 20.
For almost a week the forecasts for the central and eastern U.S. have been advertising a Februarythaw. The thaw is underway and will continue through the end of the week. As much as the warmer weather is welcome, it will have a down side. The quick warm-up, a deep blanket of snow on the ground, and expected rainfall up to an inch or more will likely cause flooding issues in snow-covered areas of the Midwest and east.

Snow cover on February 18, 2014

A storm system that has moved into the Pacific Northwest bringing rain to coastal areas and snow to the Cascade will move out of the Rockies toward the central U.S. by Thursday. This system will tap into moisture flowing northward from the Gulf of Mexico triggering showers and thunderstorms from the southern Plains into the Ohio Valley. In the cold air north of the low center's path heavy snow will likely fall across the northern Midwest and Great Lakes.  Moisture streaming over the cold air in northern New England may result in some freezing rain.

Surface map forecast for Thursday, February 20.

A broad area from the Gulf Coast to the  southern Great Lakes is in a Slight risk for severe thunderstorms on Thursday. The northern third of this area just picked up 4 to 8 inches of snow yesterday from the last system to move through.

Convective outlook for February 20 issued on February 18

Those of you who are weary of winter will have to hunker down a bit longer.  The latest 6 to 10 -day outlook from the Climate Prediction Center indicates a high probability that temperatures will be below normal through the end of the month.

After a day or two with highs in the 40s and 50s, temperatures will return to normal and then below normal through next week for much of the eastern two-thirds of the country.

Maximum temperature forecast for Wednesday, February 26.

The good news is that the days are getting longer and the sun is getting higher in the sky.  Spring will be here before you know it.

Friday, February 7, 2014

Winter Storm Leaves Huge Footprint

Surface map for 7:00 a.m. February 5, 2014
The winter storm that swept through the central and eastern U.S. the start of this week left its mark on an area stretching from the southern Plains to northern New England. The storm left a broad swath of 4 or more inches of snow from the Texas panhandle to Maine. Snow amounts reached 12 inches at locations in eastern Kansas, Pennsylvania, New York, New Jersey, Connecticut, and Massachusetts.

South of the main band of snow freezing rain coated power lines, trees, and roads with ice in parts of Kentucky, West Virginia, Virginia, Maryland and DC, Pennsylvania, and New Jersey. Officials estimated that at least one million customers were without power on Wednesday as a result of the storm. It could take up to a week to restore power in some areas.

In the wake of this storm another Arctic air mass plunged south all the way into the Gulf of Mexico and spilled west over the Continental Divide .

Temperatures at 6:00 a.m. CST February 6

Light snow and sleet fell in the Houston area yesterday and this morning freezing temperatures were found along the Gulf coast.

Minimum temperatures as of 6:00 a.m. CST February 7.

More than 67 percent of the continental U.S. is now covered with snow, almost 20 percent higher than just a week ago.

Tuesday, February 4, 2014

"The sky is falling! The sky is falling!"

This has been a rough winter for much of the central and eastern U.S. There has been persistent cold and storm after storm.  It has been a few years since this much of the country has experienced a winter of this magnitude.

There have been a couple of occasions this winter when unofficial  forecasts (i.e. not by the National Weather Service) for snow were released and spread like wildfire through social media and in some cases conventional media. One recent example is for the storm that is currently affecting the central and eastern portions of the country. Last Friday, January 31, a forecast of up to 2 feet of snow for Iowa, northern Illinois, and southern Wisconsin for today received more than a little attention in social media.

A forecast through Wednesday, February 5 issued on January 31.

So what was this all about?  This forecast for five days out was someone's interpretation of the results from some computer model. I don't know who produced this forecast nor what they used to produce it.   The bottom line is that the capability to accurately forecast snow amounts and location this far in advance just doesn't exist.

The atmosphere is very complex, as are the computer models that simulate the atmosphere. The accuracy of these simulations depends a lot on the initial conditions the model start with. These initial conditions are usually weather observations, but the density of weather observations varies and assumptions or estimations  have to be made where there is sparse data. If these assumptions are off or incorrect then the performance of the model suffers, and the result (the forecast) gets worse the farther out in time you go.  This is just one aspect of forecasting snow. Snow amounts are tough to forecast. We tend to think of snow as a generally uniform blanket, but snow amounts can vary in much the same way rainfall varies in the spring. That's the reason snowfall forecasts are a range of amounts, for example, "6 to 10 inches". Snow storms can have convective elements, i.e. "thundersnow" that produce intense snowfall rates over relatively small area of the storm's footprint. Bands of snow may sit over the same area producing higher amounts of snow than areas around them.

The Chicago office of the National Weather Service put together an excellent video describing the complexities in forecasting snowfall amounts, including the uncertainty in the computer models and the accuracy of snowfall forecasts made from one to six days out. It's well worth the time to watch.