Thursday, December 20, 2012

A Cold, Stormy End to the Year?

Surface map at 6:00 p.m. CST December 20
The intense low that brought blizzard conditions to parts of the central U.S. and severe thunderstorms and tornadoes to the south is moving through the Great Lakes tonight. The footprint of this winter storm covers the entire country east of the Mississippi River tonight.  Does this storm mark the start of a change in the winter weather pattern? It very well might.

The medium range models are still indicating a change in the upper level pattern in the next week to ten days along the lines of that shown in my December 17th post. The cold air is still bottled up in Alaska and northwest Canada. Fairbanks, for example, just experienced its sixth day in a row with lows -40F or lower and the warmest highs just reaching -19F. It appears that this cold air may be dislodged in the next week or so as an upper level  ridge builds in the eastern Pacific and a trough develops across the U.S.  Both the 6 to 10-day and 8 to 14-day outlooks issued today by the NWS Climate Prediction Center are indicating a higher probability for below normal temperatures across much of the country.

In addition, a series of low pressure systems moving across the U.S. in the next 10 days will likely produce precipitation in the Great Basin and/or the eastern half of the country.

Wednesday, December 19, 2012

Bizzard Warnings from Colorado to Wisconsin

Surface map for 10:00 am EST December 19, 2012
A potent winter storm is spinning up over southern Colorado and northern New Mexico, and the probabilities for high sustained winds and snow have prompted blizzard warnings for an area extending from northeastern Colorado all the way to southern Wisconsin. Winds from 25-40 mph with gusts from 50 to 55 are expected with this storm in the warned area, beginning this afternoon in Colorado and well into Thursday for Iowa and Wisconsin.

NWS Watch/Warning map for 1:26 pm EST December 19
. Counties in orange are under a Blizzard Warning.

Snowfall north and west of the storm center track will be heavy, with  9 to 12 inches or more of snow expected from southwestern Iowa into southwest Wisconsin. Model runs from this morning are indicating the as much as 18 inches of snow could fall in this band.

Probability of at least 12 inches of snow in the 24 hour period ending  6:00 am CST December 20

Forecast snowfall totals by this morning's run of the GFS model.
Image courtesy of Harris WeatherCaster.

This is a dangerous storm, and everyone in the warned areas should take it very seriously. The combination of wind, snow, and low wind chill values will create life-threatening conditions for anyone caught outside. Even where snowfall amounts are expected to be only a few inches the winds will cause very hazardous travel conditions.

Be sure to check your local National Weather Service web site for the latest updates and information on this storm and its impacts on your area. The next round of forecast updates will be done mid to late afternoon.

Tuesday, December 18, 2012

Major Winter Storm on the Way

Residents from the Colorado Rockies to the Great Lakes are bracing for a major winter storm Wednesday and Thursday, while the Pacific Northwest gets a breather between storms today.

Yesterday the large upper trough along the Pacific coast produced more wind, rain and snow in the Pacific Northwest. Winds gusted to more than 80 miles per hour along the coast, and there were blizzard conditions in the mountains where snowfall in the Washington and Oregon Cascades totaled from one to three feet.
CoCoRaHs snowfall map for Washington for December 18, 2012
Forecast surface map for 7:00 EST December 19
This large trough will continue to progress east tonight and tomorrow. By Wednesday morning a surface low associated with that trough will be organizing over Colorado. The surface low is expected to move to northern Oklahoma by Wednesday evening, to central Illinois by Thursday morning, and then to southwestern Michigan by Thursday evening. It will be rapidly intensifying during the day on Thursday and along with the snow will be high winds.

 In the mountains of northwest Colorado up to 20 inches of snow is expected, and to the east significant snowfall is expected from Nebraska to the upper Great Lakes. Another system moving in from the Pacific will being more heavy snow to the Pacific Northwest.

Probability of 4 or more inches of snow for the period from Wednesday evening
to Thursday evening (left), and from Thursday evening to Friday evening (right).
As you might expect, a variety of winter advisories, watches, and warnings are in effect from the Rockies to the Great Lakes.

Watch/warning map valid as of 4:45 pm EST December 18.
Orange is a Blizzard Warning. Pink is a Winter Storm Warning.
Blue area surrounding pink is a Winter Storm Watch.
This is a great time for CoCoRaHs observers in the Plains and Midwest to review snow measurement procedures. Check out the snow measurement videos on CoCoRaHS YouTube channel, and/or the "In Depth" Snow Measurement training slide show on the CoCoRaHS web site.

Monday, December 17, 2012

"It'll be a cold day in..."

Temperature map for 2:00 p.m. EST December 17, 2012
...interior Alaska, for one. The lower 48 has enjoyed a relatively balmy December so far, and many are probably wondering where the cold air has been.  Yesterday the high in Fairbanks was -37°F after a morning low of -47°F with about 20 inches of snow on the ground.  Temperatures in interior Alaska for the last couple days have ranged from lows of -50°F to highs of only -45°F! 

So far this winter the cold air has been bottled up in Alaska, the Yukon and the Northwest Territories of Canada, and the entire lower 48 has experienced very mild December weather. 

 The reason for this is that the upper level wind patterns have been generally zonal this month, that is, the winds have been flowing west to east without many undulations north or south. The map below is the average 500 millibar pattern (about 18,000 to 20,000 feet high) for the first 16 days of December. The winds flow along the lines on the map.

Mean 500 mb heights for December 1-16
As you can see, in general the source of air for much of the U.S. has been the Pacific, not the Arctic, and thus the mild weather so far this month.

Will the cold break loose, and when?  The medium range models have been hinting at a change in the next 10 to 14 days.  There are indicating a ridge developing in the eastern Pacific and deep trough to develop over central Canada and the central U.S.  Should that occur, the dam will finally break and the cold air now parked over Alaska and the Arctic will come spilling south.  Here's a forecast 500 millibar map for New Year's Eve that shows the type of pattern that could mean very cold weather for the central and northern U.S.  Note the nearly north to south lines over and est of the Canadian Rockies indicating northerly winds.

This forecast map for New Year's Eve will change as we get closer to that date, so the usefulness of such a forecast now is that it hints at a change in the upper air pattern. It will be something to keep an eye over the next week or so.  In the meantime, enjoy the mild December weather.

Wednesday, December 12, 2012

Fireballs in the Sky

Location of the Geminid meteor shower.
This image is from
One of the events our family looks forward to in the summer is the Perseid meteor shower in mid-August. We live out in the country and have a good view of the north and northeast sky, which is perfect for viewing this shower.

This week there is another opportunity to view a meteor shower, the last "regular" meteor shower of the year. The Geminid meteor shower will occur from late night on December 13 until dawn on Friday, December 14. This shower often produces 50 or more meteors per hour, up to as much as 100 per hour.  This year, a new moon means a dark sky on the peak night of the Geminid shower (mid-evening December 13 until dawn December 14). The peak usually occurs around 2:00 a.m. local time, but meteors should be visible beginning about 9:00 or 10:00 p.m. local time. The shower is named the Geminid because the radiant (the point where the meteors appear to originate) appears to come from the constellation Gemini. This will be visible in the northeast sky. If you have a smart phone, there is a neat free app called Google Sky Map which helps you find constellations or other astronomical features from your location.

While the new moon is favorable for viewing, the weather in much of the U.S. won't be ideal. A ridge of high pressure will be slipping off of the east coast late Wednesday, and the weather will be most favorable from the mid-Atlantic coast into the Gulf States and then to central Missouri. Clouds associated with a frontal system snaking its way from Quebec through the Central Plains to southern California will muck thing up for areas just south and to the north of the front. Low level moisture being pulled into Texas by the southerly winds on the back side of the ridge will likely result in cloudiness that hampers viewing there.  A trough of low pressure heading out of the Front Range will keep skies cloudy in the Central Plains.

Forecast surface map for Wednesday, December 13, 2012 at 6:00 p.m. CST.
The area within the  yellow line should have the most favorable viewing conditions for the Geminid meteor shower.

Tuesday, December 11, 2012

December Tornadoes in the South

Surface map at 6:00 a.m. CST on December 10, 2021
Eight  tornadoes touched down in Louisiana, Mississippi, and Florida yesterday as severe thunderstorms developed ahead of a strong cold front. This cold front was part of the same system that brought snow to the Northern Plains and upper Midwest over the weekend. Three tornadoes were confirmed in Louisiana between Baton Rouge and New Orleans early Monday morning. Survey teams rated the damage as EF1 for all three tornadoes, and there was also some straight line damage. You read more information about these tornadoes on the NWS New Orleans/Baton Rouge web site.

Two EF0 and one EF1 tornadoes were confirmed in southern Mississippi on Sunday evening and Monday morning. The Jackson, MS National Weather Office web site has more information on these tornadoes.

On Monday afternoon rotating thunderstorms spun up two tornadoes in east-central Florida. An EF1 tornado touched down near Edgewater, and a waterspout was confirmed near Lake Apopka. More information on these storms can be found on the NWS Melbourne, FL web site.

Northern Florida CoCoRaHS map for December 11, 2012
Last night heavy thunderstorms brought 2.00 to more than 4.00 inches of rain to northern Florida. Tonight that cold front lies across central Florida, with thunderstorms expected overnight and tomorrow from Tampa eastward through Orlando and Daytona Beach. One half to one and a half inches of rain are possible in the band through Wednesday night. The front is expected to push out into the Atlantic, and cooler, drier weather is expected for much of Florida Thursday into the start of the weekend.

Surface map for 9:00 p.m. CST December 11, 2012

Sunday, December 9, 2012

Snow and Blizzard Conditions in Upper Midwest

Snow whipped by high winds prompted blizzard warnings for eastern South Dakota and southwestern Minnesota today. A small but intense low pressure system over southern Minnesota coupled with a ridge of high pressure across the northern Rockies produced the tight pressure gradient (note the tight packing of the isobars - lines of equal pressure- in the map below) that resulted in the high winds.

Surface weather map at 12:00 p.m. CST today

Snowfall amounts were heavy, with 12 to 17 inches of snow reported in a band across south-central Minnesota as of this evening, stretching from the South Dakota border to north of the Twin Cities. It is still snowing in some areas and final snowfall totals may be higher. Below is the snowfall map for the 48 hour period ending at 7:00 a.m. local time today

This map is an objective analysis of NWS Cooperative Observer Program (COOP)
and Community Collaborative Rain, Hail and Snow Network (CoCoRaHS) snowfall reports.
The map is produced by the NWS.
The CoCoRaHS snowfall map for Brookings County, South Dakota this morning shows amounts from 10 to 12 inches. Brookings County is in east-central South Dakota along the Minnesota border

24 hour snowfall map for Brookings County, SD for December 9, 2012