Wednesday, March 4, 2015

Long Range Outlook for March, Early April 2015

This is the Long Range Outlook for the period of March 2015 into early April 2015.

Paul Roundy
The above image is pretty complicated, but bear with me and it won't be so bad. This chart shows a longitude-by-time forecast for tropical convection anomalies (colored shadings) between 7.5º North and 7.5º South latitude, centered on the Equator. Blue colors correspond to enhanced tropical convection (thunderstorms), while yellows are indicative of suppressed convection. There are a lot of lines here, but the only ones we need to worry about are the red lines, which show the active phase of the Madden-Julian Oscillation in solid (dashed is the suppressed 'phase'), as well as the Kelvin Wave, which is in thin pink.

We currently see an MJO wave evolving from about 75 degrees East, as the red lines in that longitudinal sector show. It is expected that this wave will strengthen in the next week or two as an Equatorial Rossby Wave moves over the region, shown as a light-green delineation. The placement of tropical convection in this part of the world is equivalent to a Phase 4 MJO event. As shown in the image below, temperatures during a Phase 4 event will tend to lean warmer than normal for much of the country, but a deep snowpack across much of the North and Central parts of the US tells me this warm-up could very well be muted around March 12th.

A Kelvin Wave then forms around that March 12th period, and this convectively-coupled Kelvin Wave traverses the Pacific in time for the final days of the month. This very progressive tropical forcing theme tells me the country is likely in for some roller coaster weather patterns for the remainder of the month, despite the MJO wave signal only shifting eastward slowly in contrast to the Kelvin Wave movement. This particular chart has us reaching Phase 5 of the Madden-Julian Oscillation by the final days of March, which would typically favor a cooler than normal pattern for the Northern US.

However, this cold outlook for the end of March is working in contrast to a projected pattern shift in the Pacific, in which ensemble guidance sees a trough positioning itself along the west coast of North America to permit warmer weather in the Central and East US. With warm signals coming from the Typhoon Rule as Japan undergoes a generally-warm pattern in the next two weeks-plus, and tropical forcing not really being effective this past month due to such dominance from the Pacific pattern, I'll opt for a warmer outlook to close March.

To summarize:

- An overall cool pattern should remain in place through about mid-March.
- Model guidance is then indicating, at the very least, the synoptic (overall) weather pattern will undergo some changes for the last half of March. I see this as being a good shot for a warmer pattern.
- This warmer pattern could continue into early April.


Tuesday, March 3, 2015

March 4-5 Potential Ice Storm

There is an increasing likelihood that accumulating ice will pose a threat to residents in the Southern US.

High resolution precipitation-type forecasts show rain impacting the states of Mississippi, Georgia, and Alabama into the Carolinas late on March 4th into the 5th, before widespread icing begins to take hold. Areas affected include northern Mississippi, northern Alabama, central Tennessee, eastern Kentucky, and portions of the Mid-Atlantic. Snowfall may then be expected to the north.

Ice accumulations from the GFS model lay down pockets of close to 0.50" in portions of Arkansas and Tennessee, as well as Texas and Louisiana. Model guidance usually doesn't do well with freezing rain forecasts, but amounts over 0.10" are very possible, if not likely, in northern Mississippi into Tennessee.

To summarize:

- An ice storm may impact the South US on March 4th and 5th.
- Accumulations could reach as high as 0.25" of ice in many areas, with isolated higher amounts.


Friday, February 27, 2015

Long Range Ensembles Predict Incoming Spring-Like Weather

Long range ensembles are indicating a much warmer weather pattern is on the way for much of the country that has been held hostage by brutal cold in the last month.

Click to enlarge
The above image shows 500mb geopotential height anomalies from the GFS ensembles, projected over the Northern Hemisphere and valid for March 1st. In this graphic, we see a massive ridge of high pressure, the same one which has allowed for such a cold and snowy February, meandering westward over the north-central Pacific, as opposed to the northeast Pacific and Gulf of Alaska. This comes as a strong trough drops into the Southwest and Baja California. In response, with the strong upper level low previously positioned over Greenland, now over Europe, a ridge is able to form over the Central and East US. It looks like this ridge will be suppressed a bit, but warmer weather is on the way for those in the Plains, Midwest, Ohio Valley, and Northeast, among other areas.

By March 7th, ensembles are indicating the ridge remains in the north-central Pacific, but an upper level low has now developed in the Gulf of Alaska. Unfortunately, this then results in a ridge forming over the Western US, as the graphic above shows. Consequentially, the semi-permanent Arctic long wave trough (better than using 'polar vortex', apparently) is being pushed back into North America, setting up another period of cold weather for the Central and East US. This would likely last for a handful of days, right between February 5-10 or so.

But... finally... finally!... relief comes.

In the long-long range, to March 11th, ensembles see our ridge forcing itself north in the north Pacific into the Aleutian Islands, along the jet stream, where the "polar vortex" is stationed just off to the north. The net result for us is another trough setting up over the western section of North America. You weather enthusiasts know this as a negative Pacific-North American (PNA) pattern, with a trough in the west and warmth in the East.

This looks to be our new pattern, going strictly by these ensembles. Stormy weather in the West with predominantly warm weather out east.

To summarize:

- A brief warm-up is expected in the final days of February and early days of March.
- Another cold blast can be expected around March 7th.
- The pattern may then shift to a warmer set-up for the middle half of March.


Wednesday, February 25, 2015

March 1-2 Potential Winter Storm

Model guidance has been indicating a winter storm will track across the middle of the country, bringing accumulating snow to those further south of where much of the wintry weather has occurred this winter.

Tropical Tidbits
The image above shows the GFS forecast for precipitation type and intensity, valid for Sunday night. We see light to moderate, possibly even heavy snow falling across the Midwest, Great Lakes, and Ohio Valley. A classic signal of winter finally on its way out, the rain/snow line is displaced pretty far north in comparison to where it has been in recent days and weeks, with rain being a concern in southern Illinois into southern Ohio. A bit of mixed precipitation could impact those in the central Plains.

Instant Weather Maps
Snowfall projections have been waving back and forth with the axis of heaviest accumulations, but the latest GFS pegs a swath of 6-10" accumulations across the central Plains into the Midwest and central Great Lakes, with Kansas, northern Missouri, central Illinois and northern Indiana receiving the most snow.

To summarize:

- Model guidance is anticipating a winter storm to affect the country over the next few days.
- Snowfall on the order of 6-10" may be expected.
- While the track of this storm is still not nailed down yet, the central Plains and Midwest ought to see the most intense snow accumulations.


Sunday, February 22, 2015

Second Round of Harsh Arctic Air Flowing Southward

The second round of intense cold air in under a week is on the way for millions in the Northern United States.

Tropical Tidbits
Click images to enlarge
The above image shows the latest GFS model forecast for Monday morning. On this chart, we see temperatures in the negative-teens across Wisconsin, Illinois, Minnesota, Iowa, Michigan, Indiana, and Ohio. The most intense cold is projected to hit northern Wisconsin and southwestern Michigan, but that doesn't take away from the cold in the other aforementioned areas.

Tropical Tidbits
The GEM model, with its notorious cold bias, agrees with the GFS model. Here, we see a swath of bone-chilling cold in southern Minnesota and Iowa, where temperatures are forecasted to approach -30 degrees. This is more than likely overdone, but the general trend of very cold temperatures similar to the GFS stands in Wisconsin, Illinois, Michigan, Indiana, Ohio, and even into the Dakotas.

Other model guidance, mainly the NAM model, is favoring a warmer solution than the two depicted above. It comes out to the GFS/GEM models and their ensembles, against the NAM/SREF models/ensembles. The National Weather Service appears to be favoring the NAM model / warmer forecast, even though morning observations in the Dakotas and Minnesota support a more GFS-like evolution. Regardless of what happens, another very cold morning is in store tomorrow.

To summarize:

- Another bout of harsh Arctic air is on the move south.
- Temperatures could drop into the negative-teens in some spots.
- Uncertainty still exists with how cold it will get.