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