Joe's Weather Blog: 2012

Wednesday, April 25, 2012

Strong Thunderstorms possible over portions of Wyoming Thursday afternoon

Wyoming continues to bounce around under quickly changing weather patterns! We began the week with a strong, upper-level ridge over the region. That, allowed for very WARM AIR to move our way from the desert SW. Every location that keeps records tied or set records. Not only did Monday and Tuesday feature daily highs, but some locations received ALL TIME record highs for the month of April! Sources, click links in BLUE: Central WY & SE WY

The ridge will begin to break down today as a STRONG UPPER LOW over the Pacific begins its journey over the west. Much need, widespread showers will be likely, and they'll give us WELL-NEEDED MOISTURE, mostly Thursday. This low will divide its energy, one of which with dig over the Colorado Rockies (see Image 3). This will also allow a large amount of Pacific Ocean moisture to flow over the state to end the week (Image 1).

The below image shows two significant areas of moisture moving over the west, ahead of that strong, upper low. Moisture plumes shown in the water vapor imagery are shown in the darker colors. This moisture will interact with an incoming low to produce likely chances of showers and t-storms over Wyoming, some of which could go severe. You can see the current water vapor at this link HERE

Image 1: Water Vapor Imagery at 9:45 a.m. Wednesday, April 25

Speaking of severe weather, here is the convective outlook for Thursday, April 26, issued from the SPC based in Norman, Oklahoma. They issued a SLIGHT RISK (or up to 15% chance) of developing thunderstorms reaching severe levels within the yellow area (NE Colorado, western Kansas, SW Nebraska Panhandle & far SE Wyoming). Strong winds of 60 mph and quarter-sized hail will be most likely. A brief tornado can not be ruled out either in these areas.

Image 2: Convective Outlook issued by the Storm Prediction Center (SPC) for Thursday, April 26

To make thunderstorms, you need generally 3 ingredients: (1) MOISTURE, (2) INSTABILITY and (3) LIFT

We have already covered Thursday's system will have plenty of moisture from the water vapor imagery Wednesday morning (see image 1). So let's cover lift next. Lift can come from many sources that range from fronts to even mountain barriers (of which Wyoming has a large supply). But an incoming upper low will be the driver, the creator of the lift, as it creates surface cold fronts, directing moisture to and then up mountain barriers of the intermountain west!

Below (image 3) shows a large upper level low developing over the Colorado Rockies. This feature will bring much lower pressure over Wyoming. To image this happening, you can picture a shovel the size of Maine "digging" a hole in the atmosphere that covers the area over Wyoming. This will create a large "disturbance" over the Rockies that will eventually develop fronts, clouds and then showers over Wyoming. So it's a good "rule of thumb" to correlate relatively low pressure with clouds & showers.

Image 3: NAM data of relative vorticity at 500 hPa, approximately 5600 to 5800 meters above ground

The last ingredient of t-storms & severe weather is INSTABILITY. One measure of this is something called CAPE (Convective Available Potential Energy). Typically to get a good chance of severe weather, CAPE would have to be at least at levels of 2500 Joules per kg. However, this model run is showing value of less than that, and even lower. You can see those maxima of CAPE touching areas of NE and SE Wyoming. Even though this amount of instability is marginal, it is enough to give way for severe thunderstorms, producing damaging downburst winds and large hail.

Image 4: NAM data of CAPE at 6:00 p.m. Thursday, April 26 (00 UTC 27 APR)

So we'll see a great chance of widespread precipitation. Rain, strong thunderstorms and then eventually mountain snow will fall over the state through the late week & early weekend, all associated with the aforementioned upper-low. You can see below the NAM (North American Model) data for precipitation. You can see there's a chance for showers and t-storms to become widespread tomorrow afternoon and evening. This is great because the state is very dry, with even areas of Laramie County currently under a MODERATE DROUGHT. up to 1-2" of liquid precip. could total over local areas of eastern & northern Wyoming! Again, SE Wyoming will have a chance of thunderstorms turning severe Thursday afternoon!

Image 5: NAM 3-hour precipitation at 6:00 p.m. Thursday, April 26

Lastly, I'll mention the likely weather for Friday and through the weekend. That upper-low will eventually travel over the great plains and toward the upper Midwest Friday. Cold, Canadian air will then point to the low, which guides it right into Wyoming. Highs will likely drop into the 50's for lower elevations, east of the continental divide Friday and Saturday. Lingering showers will also wrap behind the low developing mountain snow during this time. Casper last shot for showers appears to be FRI night-SAT morning. Lows could dip below freezing, so snow mixed with rain will be possible for Casper & Casper Mountain but the activity looks very light currently.

Image 6: NAM data of relative vorticity at 500 hPa, approximately 5600 to 5800 meters above ground Friday at 9:00 a.m.

Thursday, January 26, 2012

What Makes a Waterspout

We'll be learning a few topics today. We'll mainly learn about specific waterspout formation on this Thursday, January 26th. We'll also touch on topics like: Doppler Radar, Doppler Shift, Radar Reflectivity, What the RADAR actually reads, Layers of the Atmosphere, Ingredients for Thunderstorm Development, Rear Inflow Notch, Bow Echoes, Bow Echoes and Tornado Development, Conservation of Angular Momentum and the Bookend Vortex. Okay, lets get started!

Figure 1. Waterspout image courtesy NWS - Miami

Tornadoes not only develop over land. That statement may be common sense to many. However, they easily occur over many locations that are deemed "safe" due to popular weather folklore. Here I will be talking about tornado development over water ("waterspouts"). So, if you find yourself with a tornado staring you in the face, with nothing but a river between you two; I still recommend you find cover asap!

I will be using today's weather set-up (1/26/12) as the foundation of the waterspout explanation. Waterspouts really are just weaker versions of land-spouts. To put it simply, you need 3 things to develop a tornado-producing thunderstorm: moisture, wind shear, and a trigger. So below (Fig. 2) I show you RADAR imagery scanned 1643 Z (10:43 AM - Alabama time). Below you see a well-developed, squall line feature over the Gulf of Mexico, with a heavy, sporadic organization of heavy rain/t-storms spreading NE over the gulf states. By the way, RADAR stands for RAdio Detecting And Ranging!

Figure 2. RADAR Base Reflectivity Imagery of states bordering the Gulf of Mexico at 1643 Z

What I am highlighting in the image above is the "Rear Inflow Notch" feature. This is the name given to when a line of thunderstorms "bulges" farther eastward (usually) from the parent area of the squall line. This bulge is creating what's called a "Bow Echo." The bow echo is the portion of the squall line the takes on a bow-shape. A rear inflow notch develops when quickly moving air enters the backside of a storm in the middle levels of the troposphere. Inflow notches are related to severe weather if they carry high momentum, dry air into the storm. If the air is dry, evaporative cooling will case the air to cool. This will increase the negative buoyancy of the air and it will accelerate toward Earth's surface. This acceleration can also create damaging wind gusts.

I also highlighted the approximate location of the waterspout (blue triangle) over the portion of the squall line that is bulging into an arrow-like shape. On the north-side of the bow echo, the winds will often rotate north and south through the bow. The winds rotating to the north will cause a counter-clockwise flow to develop here. This is often called a "Bookend Vortex" where tornadoes can develop. The possible waterspout in this case study is likely the result of a north-side bookend vortex. Below (Fig. 3) shows a typical evolution from squall line to Bow Echo & Bookend Vortex.

Figure 3. The evolution of a bow echo due to strong wind shear, courtesy of COMET

One thing to really understand about tornado movement is that they don't just propagate horizontally over the ground (or water in this case). They also rotate! You can measure the direction and magnitude of that rotation with one of my favorite (and necessary) tools, the Doppler RADAR. Over the northern hemisphere, within our atmosphere, tornadoes almost always (but not always) rotate counter-clockwise. In the displays we create from Doppler RADAR data, we can show what's called the Doppler Shift of particles that are scanned within parent thunderstorms. If these particles (rain drops, hail, etc) are moving towards the RADAR, the display shows the particles in GREEN. If they are moving away, they are RED. Below (Fig. 4) I show you the apparent rotation over the Gulf due to the inflow notch's affects on the squall line. You can see there is counter-clockwise rotation! Within that area of rotation would be the best approximation of the waterspout!

Figure43. Base Velocity Display taken at 1643 Z, courtesy of Mobile, AL Doppler RADAR

Monday, January 9, 2012

Why I think Casper will see its first measureable snowfall of 2012 this week!

This Wednesday morning, we’ll see a cold front put an end to the 40’s and even low 50’s early this week. This is a stronger front that will have enough strength behind it to push south across the entire state and much of the region. This front should not only drop temperatures Wednesday and Wednesday night; but it will also drop a chance of accumulating snow.

A trough of low pressure will be digging south over the intermountain west Tuesday into Wednesday morning. You can see the position of the trough at 2 A.M. Jan., 11 below in fig. 1.

Figure 1. Map of NAM model 500 hPa heights and geo. abs. vorticity at 2 A.M. Jan., 11 - Image Courtesy Golden State Weather Service

Ahead of the trough, there should be substantial upward motion, even as the energy becomes spit between the trough and the cut-off low over California. That upward motion should promote cloud and snow growth over Wyoming.

We can also look at the jet stream at the same time using the same model run (12Z – 5 A.M. January 9^th).

Figure 2. Map of NAM model 300 hPa heights, isotachs and Jet Stream at 2 A.M. Jan., 11 - Image Courtesy Golden State Weather Service

I want to show you the jet stream position at this same time as well. A dip in the jet stream will position the jet streak (generally the strongest winds within the jet stream) right over Wyoming. The Right-Entrance Quadrant of the encircled jet streak (shown above in fig. 2) is found to promote the best growth of snowfall. A source on that is below in figure 3. The entrance regions of jet streaks are found to be excellent sources of enhanced winter weather.

Figure 3. Basic dynamics of a jet streak - Photo courtesy of MetEd.

The jet streak position matters to where you could potentially see the heaviest snowfall. I will compare the right-entrance region of the jet streak in figure 2 with the NAM forecasted snowfall below in figure 4.

Figure 4. NAM forecasted 3-hour precipitation at 2 A.M. January 11 - Image courtesy IPS Meteostar

Okay, here is the nitty-gritty…the snowfall forecast. The below images show the NAM forecasted snowfall for the Wyoming area. Generally, 1-3” is possible for areas north and east of a mountain range (Casper, Lander, Sheridan, Buffalo, Glenrock, Douglas, Lusk, Wheatland and Torrington). This model run is also going strong for snow on Casper Mountain (up to 8 inches). The timing looks to be through Wednesday morning. The snow will quickly pass and should be done by the afternoon.