The former image is just the visible satellite from 1945Z, the later is a microwave pass at 85 GHz (1620Z) which shows that there is a nice eyewall developing under the CDO. Should pop out overnight tonight as it clears out the core, and, combined with decent SST’s and a (fairly) low shear environment, there’s a real chance this might be a major hurricane (115+ mph MSW) before tomorrow is done. SHIPS seems to agree with big percentages for RI (54% for 25 kt, 34% for 40 kt).

Looks pretty healthy on the visible. 850-200 mb shear has weakened today, good outflow, fairly deep convection sitting right over the recon LLC fix. Really would be shocked if the NHC didn’t call this TD1 (or even TS Alex) in the next 12 hours or so. There is no doubt that they have been/will err on the side of caution, especially with it so close to land (I’m sure they’re trying hard not to name a borderline system solely out of the reaction it will garner along the Gulf coast and in the mass media) but I can’t fathom this system not being a TS before initial landfall in 36 hours. No use holding off if that’s the case.

The National Hurricane Center (NHC) warned Friday morning of not one but two weather formations in the Gulf of Mexico, both with the potential to swell into more serious weather systems. Responders in the Gulf of Mexico are eyeing the storm system closely, to see whether it turns towards the Gulf and interferes with ability to mop up spilled oil and cap the leaking well.

There is a 70 percent chance that the low-pressure area centered off the coast of Honduras could become a tropical cyclone during the next 48 hours, warned the NHC, indicating winds as fast as 73 mph. Any faster and the storm could officially become the first Gulf hurricane of the season — and will be named Alex.

Meanwhile, a smaller weather formation just East of the Northern Leeward Islands is also being eyed. NHC describes it as “a large but disorganized area of cloudiness and showers … associated with a tropical wave interacting with an upper level trough.”

Apparently Honduras and the Leeward Islands are now part of the Gulf of Mexico coastline. Good to know. And either Joe Bastardi is even more of a hype-machine than even his biggest detractors thought, or people at Fox have confused the hurricanes with tropical cyclones or the Gulf of Mexico with– you know– the entire Atlantic Basin.

Bastardi believes we’ll see as many as 21 hurricanes in the Gulf area, which means “you may see a naming orgy this season,” he predicted. Hurricane season for the western Atlantic and the Gulf of Mexico begins June 1 and lasts through Nov. 30. That’s when about 90 percent of the storms make themselves present.

FTR, 21 hurricanes would absolutely shatter the Atlantic record of 15 set in 2005. They (he?) meant named systems.

Screenshot (with annotations!) saved below for posterity. Click for full size.

This afternoon it looks like our convection is propagating towards our LLC and our LLC is even retrograding back towards the height falls caused by the convection. A symbiotic relationship that can only happen in a wonderfully low shear environment!

Speaking of shear, we can see the storm’s in a bulls-eye for low 850-200 mb shear (yellow contours, low numbers means less shear = good for tropical cyclone development since the warm core can become vertically stacked). Broad (albeit weak) area of satellite derived cyclonic vorticity in the area (orange contour) which is a good sign as well. However, strong sheer exists across the Yucatan channel (20, 30, 40 m/s yellow contours at the top of the images). Unfortunately for potential Alex or fortunately for Gulf residents, this is really the only conceivable path a landfalling system could follow if it were to develop and make it into the Gulf as anything of any substance. Two models (GFS and CEM) I quickly looked at don’t lift this shear out of the area anytime soon (see below, the pink colors across southern Gulf and Cuba — the two images are for Friday night, but there’s not a whole lot of movement through day 3). Although a few models really blow this system up and show a landfalling cyclone in LA/MS, I just can’t see that unless the models are doing a bad job of the synoptic environment (and they theoretically should have a much better handle on that than the actual cyclogenesis). Should the storm follow the dynamical models that move it NW, any deep convection will be quickly sheared away and the central dense overcast (CDO) will separate from the LLC.

It’s only bet for survival (if the models verify) would be to trek southward (weaker storms are more easily steered by low-level flow anyways) across the Yucatan and into the Bay of Campeche. With this track, it’s unlikely a cyclone could spend enough time over open water to really maintain any symmetric core.

So we might have TD1 or even Alex in the next 24-48 hours. However, unless I see otherwise, I’m highly skeptical of the “doom and gloom” potential SOME people are already trying hard to pick up on. Whatever is forming, it looks highly probably we’ll see it cross the Yucatan, emerge in the Bay of Campeche, and then likely move inland over central Mexico a few days later.

What’s an S/W? Is that a shortwave? What does a shortwave look like on one of the maps that get posted?

We’ll start with the first question. Yes, S/W (sometimes written SW– which is obviously confusing to those of us who also care about directions– which is, well, all mets) means “shortwave.” A shortwave is essentially a “kink” in the large-scale upper level trough/ridge system.

For examples, I’ve pulled some output from the last 84-hr NAM run. Below is the 300 mb map at 30-36-42 hours from model spin-up (18Z Wed, 00Z Thu, 06Z Thu) which is Wednesday afternoon through about 1 AM Thursday AM. Color contours are wind speed, arrows are wind direction (length corresponds to magnitude) and the line contours are height contours (you can think of higher heights being higher pressure and lower heights being lower pressure– they are the height in geopotential meters that you find the 300 mb pressure level for a given X/Y (lat/lon) location).

There are a couple prominent shortwaves in the trough. One begins in ID and makes its way down to the NE/CO/WY border by the end of the run. The one we’ll focus on begins in KS/OK and makes its way up through IL into IN by 42hr.

Here’s a better look; from now on we’ll freeze at 36hr (00Z Thursday).

You can see the counterclockwise kink in the white (height) contours very well in this picture. You can see hints of it as far north as Minnesota and it almost parallels the Mississippi River all the way down to the gulf coast. Shortwaves (like this one) are usually formed by cool pools aloft and upper level fronts.

There are phenomenon associated with shortwaves that are evident at all layers in the atmosphere. Next is the 500mb map, which shows color contours of “vorticity.” Vorticity is essentially the amount of rotation an infinitesimal parcel of fluid has.

We can see a brightly colored area right in the region of the most “kinkage.” This is conceptually expected. The smaller radius of curvature at the bottom of the kink implies more vorticity since the an air parcel following the curved contours would need to rotate more than one following non-curved contours. Also, from now on I will refer to positive and negative vorticity. In this graph the “warmer” the colors (reds, oranges, and yellows) the more positive the vorticity. The way I tell most people to keep it straight is (in the NH) positive vorticity = good for rising motion = exciting weather (storms) = cyclonic = counterclockwise. On the downwind (eastern) side of the kink we get what is called “positive vorticity advection.” For those not familiar with advection, it is (essentially) the transport of a fluid property from location X to location Y. Here, we see the wind is blowing from Kansas through Arkansas and up the Ohio River. Positive vorticity advection is occurring around the Mississippi River (east of the big yellow bulls-eye) because the highly positive vorticity is being transported (via wind) eastward. In simplistic terms, advection can be thought of as the derivative (d/dx) or gradient of a field. If you have no gradient, you have no advection because you’ll just replace your parcel of air with another parcel of air with the same properties (vorticity, temperature, moisture, whatever property you want to advect). However, if the gradient is steep, then you will notice rapid changes as air travels to and then through your location because you are transporting air that is very different than what was previously in the region (high magnitude of advection).

This kink also causes another type of advection. Temperature advection. Meteorologists typically like to look at the 850 mb map to see this.

Here we same the same thing as vorticity. The color contours are temperature (warm colors are higher) and the white arrows are wind. In the black circle we see the wind “blowing” from warmer air to cold. Therefore, in regions like Arkansas, warm air advection (WAA) is occurring due to the shortwave– it’s causing warm air to be transported into a region where cooler air previously existed. Over time we expect to see temperatures rise. You are also seeing cold air being advected BEHIND the kink (KS/OK). This is called cold air advection (CAA). The WAA and CAA are what cause the thermal gradients more commonly interpreted as “warm fronts” and “cold fronts.”

So why is this all important? Well, in very simple forecasting terms, both WAA and positive vorticity advection (PVA) are associated with rising motion in the atmosphere (there are a lot of very complicated dynamical reasons this is the case; I have spent 6+ years learning all of this so it’s not something I can really delve into the “why” of on a blog). Not surprisingly, our shortwave is associated with both of these lifting mechanisms– lifting implies latent heat release through condensation and we typically see precipitation. Well do we? I’ll show one last chart. This is the surface map with QPF (precipitation) in color contours.

Voila. We have precipitation in the area of our shortwave! Now there’s a couple small issues with this. One, this precip is actually the accumulated rainfall over the past 6 hours (30hr-36hr). For it to “line up better” with our other charts above we would have to average the “before” and “after” (36hr-42hr) QPF to get one centered on 36hr. Two, the shortwave is part of a complicated system; it may the main driver, but it isn’t working alone and is always interacting with the atmosphere. For instance; we see the heaviest precipitation a bit further south than we’d expect given the vorticity plots. Well from just a quick glance at the maps (don’t worry– I didn’t point this out earlier so as not to confuse the topic at hand) there is some upper level jet divergence near the Gulf Coast which is enhancing the vertical motion (and hence rainfall). We also expect QPF to be highest near the warm Gulf waters because there is a greater moisture flux from the surface to the atmosphere there (which means higher vapor pressures, more rapid condensation and more precipitation).

If all these features persist as the shortwave moves and they continue to work the way they need to; we increase baroclinic instability: instability driven by temperature gradients in the atmosphere (which is a result of the WAA/CAA as well as the rising motion induced by factors such as vorticity advection). This creates an energy source that can intensify tiny shortwaves into powerful mid-latitude cyclones. Now, this is an extremely simplified conceptual model; and isn’t EXACTLY how big nor’easters form (which is obviously more complicated; otherwise you could get a met degree in 6 weeks ); but it should give everyone a broad understanding of the dynamics at play and how to spot a few of these features on either weather maps or model output.