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.

PSU
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.

PSU
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.

PSU
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.

Andrew 

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.

Andrew 

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.

Andrew 

Thursday, February 19, 2015

Long Range Outlook (Made February 19, 2015)

This is the updated long range outlook, made on February 19th, and valid for late February into the first half of March.

Tropical Tidbits
Click to enlarge
We're currently seeing a portion of the semi-permanent Arctic low pressure vortex intruding into the United States, with anomalously low 500mb geopotential height values appearing in the colored shadings and analysis in the graphic above. This anomaly has been forced by a strong ridge pushing into Alaska from the northeast Pacific and west coast of North America. Over the next few days, cold air intrusions should continue as this ridge flexes its muscles and persuades additional Arctic air masses to collapse into the lower latitudes.

ESRL
I want to now go over the teleconnections over the next two weeks, which can help us diagnose the pattern heading into the 14-31 day period.

Top left: PNA Forecast
Top right: NAO Forecast
Bottom left: WPO Forecast
Bottom right: EPO Forecast

A quick refresher on the PNA, NAO, WPO and EPO...
The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US.

The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits.

The WPO (West Pacific Oscillation) and EPO (East Pacific Oscillation) are very closely related. In the negative phase of the WPO, a strong ridge exists over the Bering Sea, which can allow for sustained cold weather in the Central and Eastern United States. The negative phase of the EPO gives similar results, though the ridge is positioned in the Gulf of Alaska instead. The positive phase of both the EPO and WPO see warm weather prevail in much of the US, as stormy weather replaces the ridges in each respective region.

The positive PNA and negative EPO have worked in tandem to indicate this strong ridge blossoming into the Gulf of Alaska and general northeast Pacific. In the upcoming couple of weeks, model guidance is indicating we see the ridge shift further offshore to the west, as the PNA dip from positive to negative shows. Over time, ensemble guidance is telling us that this ridge could keep shifting west, resulting in the EPO actually moving positive for the first time in a while. The positive EPO signal, however, is weak, and will have to be watched for a 'false positive', both literally and metaphorically. If this forecast does verify, we can expect a warm-up in the early days of March.

Tropical Tidbits
Ensembles indicate we will see a ridge of high pressure pushing into Japan on the morning of February 21, as the graphic above shows. Using the Typhoon Rule, which states weather phenomena occurring in Japan is reciprocated in the United States 6-10 days later, we can anticipate a warm-up for millions that have undergone a brutal February in the final days of the month, likely into early March. From there, confidence decreases, but a return to an average or cooler than normal pattern may be expected.

You'll notice I haven't used tropical forcing in this post; that's because the ridge in the West is just so overpowering that the Madden Julian Oscillation can't do much of anything.

To summarize:

- A cold pattern is expected to round out February.
- A warm-up is expected for the last days of the month into early March.
- The first half of March should be characterized by a generally average to slightly below-average temperature pattern. There are hints of some stormy weather in early March.

Andrew

Monday, February 16, 2015

Chicago, Milwaukee Bracing for Severe Arctic Cold

Midwestern cities such as Chicago and Milwaukee are bracing for cold that could drop temperatures well into the double-digit negatives. This sort of cold has been seen recently east of Lake Michigan, but will make its first appearance of the month in more western regions this workweek.

Tropical Tidbits
Click images to enlarge
The above image shows the latest GFS model forecast for air temperatures on Thursday morning at 6AM, fresh off the presses. We see temperatures near Madison, Wisconsin plummeting to -15 degrees Fahrenheit, with Minneapolis seeing similar readings. Chicago looks to be under the gun for -10 to -15 degree temperatures, while Springfield, IL may see relatively warmer values.
There is model discrepancy with respect to how cold this air mass will be. The GFS model is favoring a much colder solution, as we see above, while the short-range NAM model (not shown) is favoring a warmer solution. One model has to cave at some point; it's a waiting game to figure out which one. For now, better safe than sorry.

Tropical Tidbits
Friday morning is the tricky forecast for these Midwestern cities. Skies will be clear for swaths of the Midwest, and with much of the area under a snow pack, temperatures could drop even lower than Thursday morning readings. However, in addition to the NAM model favoring a warmer air mass, warm air advection will be ongoing across the Plains into the Midwest. Discrepancies remain with respect to how fast this warmer air will push into the Midwest, and how intense it will be. For now, I'm taking more of a wait-and-see approach, but preparing for the worst. Again, better safe than sorry.

Purely for eye candy, as in this will not verify, check out the Canadian model air temperature forecast for Friday morning.

Tropical Tidbits
To summarize:

- The Midwest may undergo its most severe wave of cold weather this season, with cities such as Chicago, Milwaukee and Minneapolis going below the -10 degree F benchmark.
- Unusually high uncertainty still exists surrounding the intensity of this cold air on both Thursday and Friday. Any updates to this post will be made on either our Facebook or Twitter pages.

Andrew