It's an active weather period for the continental United States. Our relatively tranquil summer weather pattern has been shattered in the past month as the flow has become far more amplified. Autumn is here. Take a look at this morning's 500mb analysis from the University of Wyoming:
A fairly deep trough is digging through the intermountain west. As this trough begins to cross the Rockies today, strong pressure falls are forecast in the lee of the mountains in the central high plains. Take a look at this morning's 12Z NAM surface analysis:
And then the forecast for tonight at 00Z:
The low pressure center in the central US is forecast to deepen and pressure gradients are on the increase. This means stronger winds. In addition, you can see that there is a sharper contrast between the warmer air to the southeast of the low and the cooler air to the northwest. This is showing us frontogenesis---the strengthening of horizontal temperature gradients into sharp fronts. This points to a continually deepening cyclone, as mid-latitude, extratropical storms like these derive their energy from strong temperature gradients.
What does this mean for weather? The colder air and northeasterly (upslope) winds on the northwest side of the low point to snow for Colorado, Wyoming and the northern Plains. In Colorado, that's not very good for a place still recovering from devastating floods less than a month ago. As the storm moves east over the next 48 hours, that low is really forecast to deepen. Here's the NAM forecast of 500mb heights and 1000-500mb thickness for Saturday morning:
You can see really tight height gradients around that upper-level low, indicating strong winds aloft. These winds are going to drive severe weather chances on Friday into Saturday. Furthermore, strong winds on the back side of the low combined with ongoing snow should deliver blizzard conditions to the northern plains. However, I showed the thickness map to point out that most forecasts still keep the lower atmosphere too warm for significant snowfall as this storm moves away from the Rockies. That solid blue line off on the northern fringes of the map is the 5400m thickness line, usually a good indicator of the rain-snow divide. That's well to the north, though there are so colder pockets near the low. I'm not expecting major snow with this in the midwest.
However, severe weather is definitely on the ticket. The SPC has slight risks for severe weather out for parts of the central plains and into the midwest for today and tomorrow (with a "see text" on Saturday) and even a moderate risk for Iowa tomorrow:
Lots of thunderstorms are expected, and with a height gradient like you see in the thickness map above, there should definitely be enough wind shear to support severe weather.
Finally, not to be outdone in this lackluster tropical year, we have a tropical storm (Karen) that has developed in the Gulf of Mexico. Unlike pretty much every single tropical storm that has formed this year, the upper-level conditions are marginally favorable for development of this storm as it drifts north, though there still is great uncertainty as to where the storm will make landfall and how powerful it will be. It's looking to hit either as a strong tropical storm or a weak hurricane at this point. Here's the HPC forecast track:
Though there remains disagreement even among our best hurricane models. The Hurricane-WRF model run from this morning has the storm making landfall in the Florida Panhandle as a strong tropical storm:
However the GFDL hurricane model has the storm making landfall over in Louisiana, again as a strong tropical storm.