Friday, February 24, 2012

Potential Severe Weather Event on February 28, 2012 (2/24/12)

I am closely watching the potential for some rare severe weather in the Midwest this upcoming Tuesday. Here's hour 114 of the 12z GFS.

Top left image: The top left image is 700mb wind speeds. The 700mb level is generally considered the area where 'jet streaks' (areas of strong winds within a jet stream) are found. Anyway, we see wind speeds of over to 70 knots, which equates to 80 MPH. 80 MPH is fairly strong for the 700mb level and can very well be conductive for severe weather. Seeing as this wind is flowing northeast, storms that form will be influenced and be moving northeast as well.

Top right image: The top right image is 300mb winds, which are the jet stream winds. In order for severe storms to form, there is usually a strong jet stream In this case, we have a jet stream of up to 130 knots present across a big swath of the Midwest, which equates to 150 MPH. 150 MPH is very strong and is nothing to mess with. To give you an idea of how strong it is, the jet stream present for the devastating April 27, 2011 tornado super outbreak was around 150 MPH.

Bottom left image: The bottom left image is 850mb winds. 850mb winds can detect how fast air flows, as the 850mb level is commonly watched for air temperatures at the surface. 850mb winds in this image show that wind speeds will be up to 70 knots+, which as shown above is over 80 MPH. Strong 850mb winds are usually pretty good for tornadic activity, so this will be something to watch.

Bottom right image: This is precipitation. In a couple images before this, the storms appear to form as a squall line turning into a cluster of storm cells. Multicelled storm clusters are typically the most dangerous and can be more conductive for tornadoes than squall lines. This will be something to closely watch.

We did pull up the best possible analogue picked out by the CIPS Analogue system, and here's what it has for severe weather reports from February 24, 2001.
As you can see, it was a pretty active day for severe weather, with 11 tornado reports, 99 wind reports and 59 hail reports. I can see this happening if it was moved north into the Midwest and Lower Great Lakes.

This could be a pretty serious event and will have to be watched.
Any questions can be asked below.

Long Range Forecast Update (2/24/12)

Prognosis- Just took a look at the 12z GFS. It's looking like the infamous Alaskan Vortex will be returning in a week or so. Until then, expect progressive storm systems and ridges until about a week, when a more solid pattern of a ridge should set up. Thus, some quieter weather is probably in the books in the future.
Personally, this looks to be it for winter. I am seeing some dynamic storm systems that may give the North Plains a good surprise for snow, but other than that, I can't say things are looking too positive.

Any questions can be asked below.

February 22-24 Snowfall Event Wrap-Up

Starting February 22, a strong 'Saskatchewan Screamer' system dropped from Canada and into the US. A Saskatchewan Screamer (SS) system is a storm system that forms in Saskatchewan, Canada, and quickly moves, or 'screams', through the US.

February 22, 2012 Surface Analysis at 03z
This is the surface analysis for 3z February 22, which is 9:00 PM CST February 21. We see the two parts of the storm- the Saskatchewan Screamer just northwest of North Dakota, and the storm system on the Oklahoma/Texas border. At this point, the storm is not of particular interest and not of particular strength.

February 22, 2012 Surface Analysis at 06z.

This is the surface analysis for midnight Feb. 22, CST. The Saskatchewan Screamer (which will now be referred to as System 1) is now racing into South Dakota with a central pressure of 998 millibars. At this point, it is of interest as the system is being strengthened by a jet stream that has winds at roughly 125 MPH. The jet stream of the 0z and 12z timeframes just before and after (respectively) of this surface analysis indicates that a strong trough had developed in the jet stream that had pushed it south. This trough in the jet stream tells me that its southward motion would continue, but the strength of the jet stream to the west would keep the southward movement also at an eastern movement, making for a southeast direction.

February 22, 2012 Surface Analysis at 12z
At 12z (6 AM CST), System 1 has now progressed into Iowa, keeping the main snows behind into the Dakotas. Central pressure indicates the system is at 999 millibars. Of more interest is System 2, now in Oklahoma. The central pressure is at 1005 millibars- not too strong. However, there is now a dry line in place that increases concern for severe thunderstorms. The presence of a warm front over what used to be a stationary front indicates that the warm air mass is now on the move and will now begin to displace other air masses, therefore instigating more thunderstorms along the front. Now, this would be a bigger concern if temperature differences were larger. Temperature readings (red numbers) north and south of the warm front are not too different from each other, thereby significantly lowering the severe weather threat.

After that, everything gets complicated as surface analysis maps do not clearly depict the system differences. However, it does come to point that the main snows begin to come back south towards the Lower Great Lakes.

February 23, 2012 Surface Analysis at 12z
A day later, the main system appears as a 989 millibar system. To watch now is the frontal system stretched from Kansas to West Virginia. Eventually, this cold front will turn into a warm front and move north as the 989mb system moves east into Missouri.

This warm front then becomes the focus for where snow will fall. Tight temperature gradients in the summer are typically the focus of thunderstorms. A temperature gradient is the difference of temperatures between points. This temperature gradient is also a good point for snowfall to occur in the winter. The gradient is also an area where snow banding sets up. This banding is typically where the heaviest of heavy snows fall.

In this situation, the temperature gradient (TG) appeared to set up in the northern half of Illinois. Yesterday afternoon, a line of precipitation moved north across northern Illinois. In areas of lighter precipitation, there was reported rain and a mix of precipitation. In the heaviest precipitation, very large snowflakes were reported to be falling. Accumulations of about an inch resulted.

In the aftermath of this band, it was mentioned by National Weather Service Chicago that this band of precipitation had indicated where the heaviest snow would fall overnight.
And that's where it became foggy.

The National Weather Service and RUC Short Range model were at odds. The RUC placed the heavier snows on the WI/IL border, while the NWS put Northeast Illinois in the heaviest snowfall. In the end, the RUC model won out.

Snowfall amounts were highest on the Wisconsin/Illinois border, where very isolated 8-10 inches were found. The National Weather Service became somewhat erratic in the first couple hours as the storm went on, with NWS Chicago suddenly lowering snowfall amounts and shifting the heaviest snowfall to right up against Lake Michigan.

The system then continued eastward with substantial strength. Again, the frontal positions were of issue, and snowfall amounts may have ended up slightly less than what is shown on here. The only reason that it is unsure is because the snow event is just ending.

If you have any reports, they would be appreciated below.
I am recovering from what may have turned out to be a brush with the flu and am feeling much better. Thanks to everyone who sent well wishes!