Meteorological Phenomena

Infrared imagery indicates that a Polar Low has developed in the Labrador Sea (below). A Polar Low is a warm core convective vortex similar to a tropical cyclone, they often have an eye feature and spiral bands. This Polar Low is being fueled by the huge temperature contrast between frigid Arctic air and the unfrozen Labrador Sea. Ocean waters are hovering right around freezing (32 F) while the overlying air is 0 to 10 F, resulting in vigorous heat fluxes. This process is slightly different than a tropical cyclone which obtains most of its energy from latent heat fluxes rather than the air-sea temperature contrast. Another difference between Polar Lows and tropical cyclones is that Polar Lows are usually much shallower since the tropopause is much shallower near the Poles than in the Tropics. Structurally they are almost the same though, both being warm core vortices with their most vigorous winds near the surface.

The Polar Low is heading due south towards the coast of Newfoundland. It’s difficult to forecast Polar Lows due to their small size, global forecast models like the GFS barely resolve them. The GFS shows the Polar Low getting near the Newfoundland coast and then curving southeast back out to sea, but it doesn’t initialize the Polar Low in a realistic way.  If the Polar Low were to hit land, which is possible, it could bring tropical storm force wind gusts and heavy snow squalls, so if you happen to be living on the frozen Labrador Sea coast it’s probably a good ideal to keep an eye on this storm.

Visible satellite imagery reveals 2 powerful cyclones in the northernmost reaches of the Pacific this evening. The stronger of the two is centered over the Aleutian Islands and Alaskan Peninsula and is heading slowly northeast. It is causing wind gusts up to hurricane force on the Alaskan coastline and stirring up 20-30 foot waves, and bringing heavy snows to interior locations. The other cyclone is well to the southwest of the westernmost Aleutian Islands, but should start impacting the islands over the next day. Both systems are feeding on the temperature gradient between the frigid Arctic (note the sea ice between Russia and Alaska in the image) and the relatively warm Pacific, which is a common occurrence throughout the Winter.

It’s been a relatively quiet hurricane season for the east coast of the United States thus far, but that may be about to change. Hurricane Sandy has developed in the Caribbean and has maximum winds of 80 mph and a minimum pressure of 973 mb. Visible satellite imagery (below) indicates that Sandy is quickly organizing, with an eye becoming evident surrounding by a thick central dense overcast. Satellite imagery also reveals that Sandy is a large and sprawling storm, with rain bands and clouds extending from the southern reaches of the Caribbean to north of the Bahamas.

Steering currents are driving Sandy northward and it is about to make landfall in Jamaica, followed by a second landfall in Cuba expected late tonight. The mountainous terrain of Jamaica and Cuba may inhibit Sandy from intensifying and even cause it to weaken to a tropical storm, but the large size of the system suggests that it will survive the land interaction and continue northwards into the Bahamas on Thursday. Factors that will help Sandy maintain strength as it interacts with land over the next day are warm sea surface temperatures of ~30 °C and strong upper level divergence associated with a trough in the sub-tropical jet (this is why Sandy’s cloud shield expands so far north, its outflow has linked up with the sub-tropical jet and its using that as a highway to evacuate mass from its center and therefore intensify).

Sandy is projected to remain well offshore of Florida as it passes to the east on Thursday and Friday, but significant impacts like tropical storm force winds, heavy rains, and high surf should be expected in Florida due to the large size of the hurricane. A trough in the polar jet stream approaching Sandy from the west will accelerate the hurricane towards the northeast on Saturday, and Sandy is expected to be passing between the outer banks of North Carolina and Bermuda on Sunday and Monday. Models suggest that Sandy will grow in size and intensify during this time due to interaction with the trough in the polar jet stream. Sandy won’t be a purely tropical cyclone after that point, rather it will become a hybrid between a tropical cyclone and a mid-latitude cyclone (which is a cyclone driven by baroclinicity associated with temperature gradients). This doesn’t mean it’s going to be weak though.

The forecast of what’s going to happen to Sandy after monday is complicated at this point. Some models kick it out to sea, while others show a direct impact on the northeastern U.S. due to a ridge developing to the north of Sandy and blocking it from going out to sea, like the above plot of sea level pressure at forecast hour 144 from the ECMWF shows. It seems like models are starting to come into agreement on the latter scenario but it is by no means a sure thing yet. Sandy probably wouldn’t have winds higher than category 1 strength if it did hit the northeast, but it would impact a very large area since it will be more like a nor’easter than a hurricane at that point. The forecast for the east coast north of Florida should become more clear in the next couple of days.

No tropical cyclones have threatened to impact the United States since Isaac made landfall in Louisiana 2 weeks ago, but that doesn’t mean things are quiet everywhere in the North Atlantic. Tropical storm Leslie is rapidly crossing Newfoundland this morning as shown in the below infrared satellite loop. Leslie is estimated to have maximum winds of 70 mph and a minimum pressure of 968 mb. It appears widespread wind gusts to hurricane force are occurring across Newfoundland,  based on a max wind gust of 82 mph and a prolonged period of strong winds observed in St. John’s, Newfoundland. This is resulting in lots of tree and shed damage based on initial reports.

It should be noted that Leslie can barely be considered tropical at this point, with a structure more characteristic of a mid-latitude cyclone than a tropical cyclone. Nearly all of its energy is being supplied by the baroclinicity of the mid-latitudes rather than heat fluxes from the ocean, so dynamically this isn’t much different than a strong Nor’easter. Leslie is expected to rapidly strengthen over the next day as it crosses the Labrador Sea from baroclinic processes, and impact Greenland as a powerful mid-latitude cyclone tomorrow morning.

Isaac was finally upgraded to a hurricane this afternoon after slowly organizing over the Gulf of Mexico the past few days, and now has estimated maximum winds of 80 mph and a minimum pressure of 971 mb, but gusts in excess of 100 mph have already been reported on oil platforms and by aircraft reconnaissance. 60-80 mph sustained winds are occurring up to 150 miles from the center based on reconnaissance data, making Isaac a very large and dangerous storm. Radar gives a similar impression with a huge outer eyewall evident around the relatively calm eye. There is an inner eyewall structure that has been trying to organize near the center but it’s having a hard time getting its act together, so I expect the outer eyewall/band to remain the dominant eyewall.

As can be seen on the radar image, Isaac is now making “landfall” on the tip of the Mississippi Delta. The reason I put landfall in quotations is that the surge from Isaac has put pretty much all the swamps of southeastern Louisiana underwater already, aside from relatively small areas boxed in by levees. Real landfall won’t occur until 6-12 hours from now, and Isaac could continue slowly intensifying until then or at least maintain strength, especially as frictional convergence at the coast helps tighten up the circulation.

The elevation in the swamps of southeastern Louisiana where the greatest surge is expected ranges from sea level to maybe 5 ft at the highest but more generally 3 ft, and already 8.4 ft of surge is being observed at Shell Beach to the east of New Orleans (above). The storm surge will be highest along the eastern shoreline of the delta since strong southeasterly winds are funneling water for hundreds of miles towards that coast, and these winds will continue until at least tomorrow afternoon as Isaac slowly moves northwest. High tide will occur tomorrow morning through early afternoon, so flooding will be especially bad during that time. New Orleans itself is protected by many lines of levees and pumps and is expected to be fine by the Army corps of engineers, but this storm will certainly give the system a test since this is likely to be the highest storm surge since hurricane Katrina. 3.3 ft of surge is being observed on the lakefront in New Orleans right now, water has already overtaken the land between the normal coastline and the levee as can be seen below.

Mississippi, Alabama, and the panhandle of Florida are also dealing with significant winds, rain, and surge. Conditions are expected to deteriorate until Isaac moves well inland tomorrow afternoon.

Tropical storm Isaac is beginning to pick up steam over the Florida Straits after significantly weakening over the terrain of Hispaniola and Cuba, which undoubtedly saved the Florida Keys and South Florida from having to deal with a hurricane today. Currently Isaac has maximum winds of 65 mph and a minimum pressure of 995 mb, and is roughly 50 miles SE of Key West. Radar (below) and satellite indicate that the structure of the storm is becoming more organized, with convection trying to wrap around in all quadrants and a tight circulation developing, as opposed to yesterday when the storm was ragged and the circulation was deformed due to Cuba. Isaac is directly located over the warm waters of the Gulf Stream and wind shear is relatively low (there is some 10-20 kt southerly wind shear which is slightly inhibiting Isaac), so I expect steady strengthening to continue today.

Isaac will be crossing through the lower Florida Keys this afternoon, possibly directly over or just to the west of Key West. Wind gusts of 40-60 mph and heavy rain squalls should be expected across the Florida Keys and South Florida through late this evening, pretty much the conditions that have already been occurring over the region all morning. A 1-3 ft storm surge is also expected to peak in that region this afternoon, which although small it will be an issue for low lying Keys.

After Isaac passes through the Keys it will continue NW towards the Gulf coast, where a landfall anywhere from Louisiana to the Florida panhandle is possible right now. Hurricane watches have been issued for that entire region since the exact landfall point and eventual size of the storm is currently an unknown. It is also very hard at this juncture to forecast landfall intensity, it could be anywhere from a category 1 hurricane with winds around 80 mph to a major hurricane with winds in excess of 115 mph. It depends on how fast Isaac can get its inner core organized, wind shear, as well as coastal upwelling due to Isaac’s wind and waves. Since the waters are relatively shallow around the Gulf coast upwelling easily brings cold waters to the surface, which weakens the storm but it’s hard to forecast how much.

The bottom line is that residents of the Gulf coast from Louisiana to the Florida panhandle need to watch Isaac very closely and follow the advisories issued by the National Hurricane Center, National Weather Service, and local emergency management. Isaac is expected to make landfall on the Gulf coast Wednesday but conditions will likely begin to deteriorate Tuesday.

Tropical storm Isaac is now within 24 hours of making landfall on the island of Hispaniola, which is expected to happen early tomorrow morning. Hurricane watches and tropical storm warnings are in effect for Haiti in anticipation of Isaac’s arrival. Currently Isaac has maximum winds of 60 mph and a minimum pressure of 1000 mb as measured by Airforce reconnaissance. The wind field isn’t tight yet as indicated by the highest winds being measured about 75 miles from the center in the outer bands. Indeed, Isaac has been disorganized most of the past 2 days while traversing the eastern Caribbean, and is only now becoming somewhat better organized as indicated by higher winds and a better satellite presentation (below).

Isaac had multiple vorticity maxima orbiting around a broad center yesterday, making things very confusing for reconnaissance since it was hard to tell which center would become dominant. It appears that Isaac has consolidated into one circulation center today, which is probably why it’s finally strengthening. However, inner core convection remains disorganized with multiple convective plumes competing with each other, so further intensification before reaching Hispaniola should be slow. It’s possible that Isaac could become a minimal hurricane before landfall, but I expect a strong tropical storm with winds of 60-70 mph. Land is another issue that will become a problem for Isaac within the next 12 hours or so as it gets closer to Hispaniola, 2-3 km mountain ranges on the island will begin to exert a frictional drag on Isaac and disrupt the outer circulation.

Isaac is forecast to cross into the Windward Passage later tomorrow morning before making landfall in eastern Cuba and interacting with the mountain ranges there. This one-two punch of mountains could cripple Isaac as seen with past tropical cyclones. Since the mountains extend up to 2-3 km the circulation below that level may be completely disrupted, leaving a mid-level vortex. This mid-level vortex would have to build down to the surface before it can begin absorbing energy from the ocean again, which will likely be a slow process. The mountains will also pump dry air into the circulation as rain falls out on the upslope side and descends on the back side and dries (much like the rain shadow effect which creates deserts), and this dry air would inhibit convection. As if that wasn’t enough abuse for Isaac, the storm is projected to travel right up the spine of Cuba and stay over land until reaching the coast near Havana Sunday morning. This could change of course if Isaac deviates a bit to the north or south.

Thus, as it stands now Isaac will have 24-36 hours of time to re-intensify over water before reaching the Florida Keys Monday, which may not be enough time for Isaac to intensify beyond a tropical storm, if it actually survives Hispaniola and Cuba. If Isaac does make it to the Florida Straits and the Gulf of Mexico it will have a few days to intensify and may be a more serious threat to the Gulf coast, but it is too early to tell. We’ll have to wait and see how Isaac interacts with Hispaniola and Cuba first.

Aircraft reconnaissance flew into a tropical depression to the east of the Lesser Antilles this afternoon, and found that it has intensified into Tropical Storm Isaac, estimated to have maximum winds of 40 mph and a minimum pressure of 1006 mb. Though this is on the weak side as far as tropical cyclones go, conditions look favorable for some fairly quick intensification. Sea surface temperatures are getting progressively warmer as Isaac treks westwards, and upper level flow is mostly favorable, as indicated by the cirrus fanning out in the below visible satellite image. There is some obstruction to the outflow in the eastern semicircle and dry air but that doesn’t appear to be inhibiting Isaac too much anymore. Vigorous convection is evident near the center of circulation, as well as in well defined bands stretching all the way from the SW side of the storm to the NE side.

I therefore expect Isaac to intensify over the next day as it approaches the Lesser Antilles, where tropical storm warnings are already in effect. Isaac is expected to cross the island chain tomorrow afternoon, and could be a strong tropical storm or minimal hurricane by then.

Isaac will then continue west, possibly impacting Puerto Rico and the Virgin Islands Thursday, followed by a potential landfall in Hispaniola Friday. Isaac could be a vigorous hurricane by the time it reaches Hispaniola, so people in that region need to watch this closely. There is still a chance it will miss the island to the south. Whether Isaac hits Hispaniola and Western Cuba or slides to the south will dramatically alter its future. If it hits Hispaniola and/or western Cuba Isaac would likely be significantly disrupted and perhaps less of a threat to the United States as it continues WNW. If it misses both islands the hurricane would remain intact and could be a very serious threat to the Gulf states. It is too early to say which scenario will happen with much confidence since small shifts in the sub-tropical high which is steering Isaac will ultimately determine what happens. The forecast should become much clearer Thursday and Friday for the United States.

Tropical storm Khanun is a weak and disorganized tropical storm, with winds of 50 mph and a ragged infrared satellite signature (below). There is barely any deep convection associated with Khanun, possibly due to dry air being wrapped into the center, as well as cold sea surface temperatures just off Korea (<20 °C). Khanun is expected to head northwards across South Korea, reaching North Korea tomorrow morning. It’s likely that Khanun will quickly weaken from here on out since it is already making landfall, and it probably won’t be a tropical storm by the time it reaches North Korea.

The East Pacific hurricane season has lived up to its name so far this year, with 5 of the 6 tropical cyclones that have developed in the East Pacific intensifying into a hurricane. The latest is hurricane Fabio, which has maximum sustained winds of 105 mph. Visible satellite imagery reveals Fabio has an impressive structure and is no doubt a vicious hurricane. Fortunately Fabio is never expected to reach land, which is usually the case with East Pacific tropical cyclones.