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Watching the Remnants of Hurricane Irma Over Georgia

September 11, 2017/by Steven Copertino

Good Evening!

Hurricane Irma battered the Florida peninsula yesterday, first making landfall on the Florida Keys as a Category 4, then again later in the day near Naples as a Category 3. Though information is still rolling in on the extent of the damage in the state, it is pretty obvious that the entire state of Florida has been affected to some degree. After its final landfall in Florida, the storm began to expand its radius of tropical storm force winds quite a bit as the mid level system from Canada began to interact with the hurricane and started some extra-tropical processes. This only worsened the already massive storm surge that the hurricane carried with it during its long trek across the Atlantic. Places as far north as Charleston experienced storm-surge related damage, as well as power outages from Irma’s massive circulation. As is common with many intense hurricanes when they hit land, the amount of momentum that the storm had built up over the past two weeks will take some time to spin down, and that has been very evident today on radar and satellite. If we look at radar images from the south, we can see a large area of rotation associated with the decaying low level circulation of Irma, as well as the more pronounced mid level circulation. This mid level circulation has been responsible for mixing down strong wind gusts to hurricane-force today across portions of the southeast, causing tree damage and numerous power outages.

Irma will continue to weaken over the next day or so, as it makes its way into the Tennessee River valley by Tuesday afternoon. The storm will be fully-extra-tropical (meaning that other process are sustaining the wind and convection and the storm in no longer fed by its warm-core) and quite large in nature. Showers and thunderstorms should extend all the way from Arkansas to portions of the southern Mid-Atlantic states like Virginia and Maryland. Some of the stronger showers and storms may be able to mix down some residual tropical storm strength gusts, and will also be capable of producing tropical downpours which may caused localized flooding of poor drainage areas and perhaps on roads. There is also a risk of tornadoes with any cells coming in from the feeder bands over the Atlantic. There have been some strong signatures observed over water during the past 24 hours, and a tornado watch has been issued for portions of the Southeast.

Please check with your local NWS office to stay on top of any watches or warnings that may be issued once the NHC declares the storm “post-tropical” and hands over the tracking to the folks at the Weather Prediction Center.

Hi-res Base Reflectivity imagery of the remnants of Hurricane Irma over the Southeast US

As Irma winds down late Tuesday and into Wednesday as the low begins to fully occlude and fill in, a warm front will progress from west to east over the Mid Atlantic states, and eventually into the Northeast. This front will begin to slow as it moves through Pennsylvania on Wednesday, due to the residual high pressure system just off the coast of the Mid Atlantic. This will cause numerous showers to form over the area, but as tropical moisture begins to fade, these showers should be on the more typical side, with steady rain and generally cloudy conditions expected.

As of right now, the most unsettled day across the eastern half of the nation appears to be on Thursday, when a little more surface heating may take place just ahead of the main area of mid level energy, and this could create a marginally unstable air mass supportive of weak thunderstorms and isolated downpours. Friday may feature more showers and possibly some thunderstorms over much of the Mid Atlantic and Northeastern states, but the overall area of coverage will depend on just how strong the mid level energy remains as it meanders over the next few days. Dry conditions may return this weekend, as an area of high pressure begins to build in from the west, which should kick the remnants of Irma out of the area and allow for more stable and clear conditions.

Evolution of the mid level remnants of Hurricane Irma over the next few days as it meanders across the eastern third of the country

Hurricane Jose Wandering Over the Open Atlantic

Hurricane Jose is still out and about over the Atlantic, located to the north of Hispaniola and Puerto Rico. Jose has been experiencing significant wind shear due to the very rapid expansion of Irma’s outflow once it began to undergo an extra-tropical transition. Even in the face of this shear, the storm has been able to maintain hurricane intensity, and currently has winds of around 100 mph. The storm may continue to weaken as it begins to execute a very complex loop over the next few days due to a break-down of the steering currents from the remnants of Irma. This pattern is forecast to last until Friday or so, when the hurricane will once again be located just north of the Bahamas. It is at this point that the models begin to diverge significantly. Some models have the storm taking a hard right and heading out to sea, and others have Jose maintaining a more westerly motion by this weekends, which would put it uncomfortably close to the southeast US. This scenario would result in the storm also encountering more favorable conditions for re-intensification as an upper level high would be allowed to rebuild overhead.

While it is much too early to say what the overall outcome will be, there is plenty of time to watch this storm as it churns over the open waters of the Atlantic, with the only concerns being increased wave action and rip-tides. We will have more on this storm regardless if it becomes a legitimate threat to the east coast over the next few days!

Latest GOES 16 IR imagery showing Jose getting beaten up by shear from now Tropical Storm Irma’s outflow (courtesy of weather.us)

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Have a great evening!

Steve Copertino

Irma a powerful Category 5 Hurricane moving toward Caribbean

September 5, 2017/1 Comment/by John Homenuk

Hurricane Irma maintained immense strength and power per this evenings 8:00pm National Hurricane Center Advisory, with maximum sustained winds of 185mph. The Hurricane is in the upper echelon of Atlantic-based storms, packing the second highest sustained winds on record since the satellite era of hurricane observations began. Irma will continue on a west/northwest heading this evening, on course for potentially catastrophic impacts in parts of the Caribbean.

Unsettled and Cooler Conditions Dive In, Irma Becomes a Category 4 Hurricane

September 4, 2017/by Steven Copertino

Good Evening and Happy Labor Day!

Today was a rather big improvement from the dreary and cool conditions that we experienced this weekend over much of the northeast, with mainly sunny skies and light winds throughout the day. A weak area of mid level ridging over the east provided adequate southwesterly flow, which ushered in highs ranging from the upper 70’s to middle 80’s across the area. This pleasant weather should last into the evening as a weak surface high pressure system associated with the mid level ridge begins to move off the Atlantic coast and into the West Atlantic. This will allow for winds from the south to persist into the evening, but conditions will be supportive for some radiational cooling to take place. Expect temperatures to drop into the lower 60’s to middle 50’s across the area, with locations well removed from the coast a good 5 degrees cooler.

This evenings latest high resolution GOES-16 visible satellite imagery, regional radar mosaic, and surface observations showing a rather tranquil and pleasant Labor Day (Courtney of Simuawips)

Tuesday into Wednesday

Tomorrow should start off rather calm for much of the area, with partly cloudy conditions likely taking over by mid morning. A large upper level trough will be moving down from Canada and into the northern portions of the Northeast. This cold front will likely spark some showers and thunderstorms over portions of the Northeast during the afternoon hours, however this feature will likely take a while to reach the area. Otherwise, more strong south-westerly flow just ahead of the impressive trough will overspread the area. This will once again lead to warm mid level temperatures, which should be able to support highs into the middle to upper 80’s by late tomorrow afternoon. As with many high mid level temperatures/questionable cloud cover days, the areas that see more sunshine during peak heating will likely wind up a few degrees warmer than the areas that stay mostly cloudy. Precipitable water values and dewpoints will also begin to steadily increase during the day, and this will make for a more “muggy” feel to the day tomorrow with heat indices possibly reaching into the lower 90’s.

As we move later into the day, the cold front will begin to move to the east with time-likely setting off numerous showers and thunderstorms over Pennsylvania and New York. PWATS will be quite high just ahead of the front, and a very impressive upper level jetstreak will promote expansive upper level divergence, which will work to enhance convergence at the surface. Additionally, the upper level trough moving in will be quite large and powerful for this time of year, and will have some decent wind shear associated with it. When all these ingredients come together later in the day on Tuesday, very heavy rainfall will be possible with any showers or thunderstorms that develop west of the immediate NYC metro area. If a less-cloudy solution works out tomorrow, we could see instability rise over the area, which would be supportive for more strong-severe thunderstorms with strong winds, frequent lightning, small hail, and torrential downpours all possible.

These showers and thunderstorms should gradually work their way towards the NYC metro area by the mid to late evening hours tomorrow. The activity will likely be on the downswing due to a loss of daytime, so they will likely be below severe limits, even though gusty winds, frequent lightning and heavy rains will still be possible. There is a minor concern for flash flooding as the north to south orientation of the trough will allow for the showers to train over the same locations as the night goes on. The one positive thing about this scenario will be that the increased shear values will allow for the showers and storms to move rather quickly. This should limit the overall flood threat to roads and poor drainage areas.

By Wednesday morning, the cold front should be along the Mid Atlantic coast, but beginning to slow down significantly as it starts to bump into the west Atlantic ridge. The overall upper level trough will also begin to nudge up against the ridge and this should cause the associated upper level jet to expand and intensify as well. This will set the stage for a much cooler and unsettled day across the area as a Canadian airmass overspreads the area, and the lift from the impressive upper level jet provides support for showers and isolated thunderstorm development. Gusty winds will be possible with any of the stronger showers and thunderstorms, as they may be able to mix down stronger winds from high up in the atmosphere. Overall, expect temperatures to be limited to the middle 60’s across the area, which is a good deal below normal for this time of year.

There will likely be some residual rain on Thursday, with the core of the trough passing nearby. We will have more on this and this weekends conditions on Wednesday!

This afternoons NAM model showing the expansive full-latitude trough moving in and over much of the East.

Hurricane Irma Update

Hurricane Irma has intensified this afternoon after a Hurricane Hunter flight investigating the storm found winds up to Category 4 strength (130mph). The storm appears quite healthy on this afternoon/evenings satellite imagery , with very deep convection firing within the eyewall of the storm and upper level outflow expanding outward from the hurricane. The presence of upper level cirrus clouds has even increased in the northern half of the system, which has been lacking over the past two days due to a cutoff low pressure system imposing some northerly winds onto the system. Regardless, the system appears to be in a conducive environment for further strengthening over the next few days , characterized by low shear, very warm sea surface temperatures, and little to no dry air in the vicinity.

Irma will be moving towards the northern Leeward islands, with tropical storm conditions possibly affecting the area as early as tomorrow night. Irma will likely be a strengthening Category 4 hurricane by that time, and conditions are likely to rapidly deteriorate as it moves closer. The models have converged on the idea that the center of Irma will be over the US Virgin Islands during the day on Wednesday, and will likely bring extremely strong sustained winds, torrential downpours, life-threatening mud slides, as well as very dangerous waves/surge. With 36 hours left until direct impacts are felt, those in this area should rush preparations to completion as soon as possible! A Hurricane Warning has been issued for most of the northern islands, with a Hurricane Watch in effect for Puerto Rico.

12z HWRF showing a very dangerous Hurricane Irma over the northern Leeward Islands (Valid 8am Wednesday)

Irma will likely move to the north of Puerto Rico late Wednesday, likely bringing tropical storm, to even hurricane conditions to the island. The hurricane will likely remain an extremely dangerous storm through this time, much of the same hazards-if not all apply. Irma should begin to move away by early Thursday morning and will likely continue heading west-northwest, just north of Hispaniola. This is around the point that some models begin to diverge on the exact track of Irma, with some keeping the storm moving towards the west and into the southern Bahamas, and others showing a stronger mid level ridge over the system attempting turn the hurricane a little south of due west and into Cuba. This will have to be monitored over the next few days, due to the fact that Irma would likely weaken a good deal if it were to interact with the mountains of eastern Cuba (which would have some impact on the eventual track). Interestingly enough, the later part of the forecast seems to be much clearer than it originally was, with multiple models and their ensembles showing Irma lifting north once its near the Cuban coast by this weekend. The main question will be just how far west does the storm get before its begins to move north? With near-continuous observation by NOAA and USAF planes, special soundings, and the activation of Critical Weather Days, we will be getting a massive amount of data ingested into the models that should help with the forecast of the next week or so.

Regardless, it can be said that there is an increasing hurricane threat for portions of south Florida, and residents in that area should closely monitor the progress of Irma over the next few days and begin to review their hurricane plans.

Category 4 Hurricane Irma over the Central Atlantic this evening as seen by GOES 16.

We will have more on Hurricane Irma on Wednesday!

Have a great evening!

Steve Copertino

September 2017 Outlook

August 22, 2017/by Doug Simonian

We are already rapidly approaching a new time of year — Meteorological Fall — which starts on September 1st. As the seasons begins to turn, we always find a little extra excitement in issuing our September outlook. There is something inherently more significant about it; maybe the colder trend in temperature, the approaching change in wavelength, or the growing hum of conversation about the winter ahead.

The beauty of meteorology is that each year, month, week, and even day have their own flavors that make every forecast a unique challenge. At the same time, there are also some general themes and correlations that one can continually follow with reliance as the forecasting process unfolds. One of these correlations (which will appear in our monthly and seasonal outlooks quite frequently, but remains especially important here) is the MJO — the Madden Julian Oscillation. Meanwhile, one of the unique flavors will be an inherently increased propensity for ridging in the West and some high-latitude blocking in Canada. The goal is to combine the correlation and the unique flavor to create with discernible ideas on tangible, sensible weather effects.

Without getting into too much technicality, MJO represents the activity of thunderstorms and convection at or near the Equatorial latitudes. As latent heat gets released from this convection, it can have a big effect on the associated ridges and troughs across the entire globe. The MJO is broken down into 8 phases, which depend on the location of this convection.

A plot correlating where convection is located and the associated phase of the MJO (CPC).

In the above image, the blue colors represent areas of enhanced convection, and the red colors represent enhanced subsidence. Let’s compare this to where we may potentially be headed to start September — the first panel displays today, the last panel (bottom) displays the September ideas amongst forecast model guidance.

The MJO forecast for the next two weeks shows most of the convection being focused in the Indian Ocean (Michael Ventrice).

Again, the blue colors represent enhanced convection, and the red colors represent more subsidence or sinking air. The above panel shows most of the convection being focused in the Indian Ocean, which is associated with MJO Phases 3, 4, and 5, based on the prior image. It does appear that the convection will be focused a bit east of where phase 3 historically resides, so we can focus on phases 4 and 5 with more confidence.

Now that we can at the very least start September with the MJO in phases 4 and 5, we can correlate these phases of the MJO with the general ridge and trough patterns across the globe.

These are composites showing the MJO phases 4 and 5 and their associated 500mb ridge and trough patterns across the globe (Meteo Network).

MJO phase 4 analogs show a clear, large ridge in the Western US, with troughing in the Midwest — pretty similar to the pattern that is currently developing, though there are some subtle differences (our current pattern has more blocking in Canada). MJO phase 5 analogs show the ridge in the West progressing east towards Southern Canada — possibly the result of a decaying Omega Block in Canada as well, which also is part of the current pattern progression we are seeing in medium-range model guidance.

The fact that we are already seeing hints of the MJO phase 4 and 5 patterns developing without necessarily being in those phases today shows that there are other facets to the pattern that may be “teaming up” with the MJO to amplify these effects. This lends more credence to these general ideas going forward to September. But the aforementioned MJO forecast image only runs through September 5th — what about the rest of the month?

Although on different time scales, convection in the Indian Ocean is generally associated with a La Nina, which thus adds credence to the idea that there are La Nina aspects to the developing pattern. Further examples of this are seen from the AAM — Atmospheric Angular Momentum. Again, without getting overly technical, the positive phase of the AAM is generally associated with El Nino, and the negative phase of the AAM is generally associated with La Nina.

The GFS ensemble forecast strongly suggests the AAM is heading into a negative state, which is consistent with La Nina conditions (Albany/Nicholas Schiraldi).

The above GFS ensemble forecast strongly suggests that the AAM is going to be headed negative, and perhaps staying there for quite some time. It is, undoubtedly, evidence that the pattern is going to be shifting towards a general La Nina base state, even without an official La Nina designation. This can help provide a feedback mechanism to enhance the MJO in its currently evolving phase 4 and 5 state as well as keep it there in the longer term. Thus, while the MJO may at times briefly show some phase 6 characteristics as it “progresses”, the La Nina base state developing will generally mean that phase 4 and 5 forcing should be the general rule, at least for a good part of September, rather than just the first week of the month.

This again lends credence to the idea that September should feature, on average, a strong ridge in the Western US which may at times bleed east and interact with an Omega block in Canada, encouraging further amplification. In other words, or laymen terms, the overall ridge axis near the Rockies appears likely to remain stagnant for much of September, despite the possibility of some variation from west to east at times.

When considering a few other technical factors (QBO, summer wavelength trends, stratosphere), we expect a ridge to generally be over the West in the means — perhaps for about two thirds of the month. The other third of the month may see this ridge shift a bit further eastward in the means as it readily interacts with the high-latitude blocking pattern in Canada. This will at times create a trough West –> ridge Central US –> trough East pattern. Additionally, high-latitude blocking in general, regardless of the exact ridge positioning, may allow some lower heights to progress eastward towards the Great Lakes and Northeast, underneath the blocking and downstream of the ridging.

Forecast model guidance is already catching on to the idea of a Western USA ridge and Central USA trough through September.

This actually suggests, despite initial forecasts and forecast models, a somewhat dry pattern for a large portion of the Corn Belt — more so the western half of it — as it oscillates from being on the upstream side of a trough (cool and dry) and on the downstream side of a ridge (seasonable and dry). It leads to a warm pattern for much of the West, and somewhat cool anomalies in most of the Central US as well, though with perhaps the core of the cool anomalies shifting a tad east into the eastern Corn Belt/Missouri/Tennessee/Ohio Valleys late.

On the downstream side of these troughs — in the Deep South, the eastern Corn Belt and Ohio Valley and even parts of the Northeast at times, precipitation events can be focused — though the immediate Eastern Seaboard may be a tad east of the core of the action. Temperatures on the East Coast are likely to remain transient and nearer to average than areas to the west, as periodical southwest flows develop allowing for warmer anomalies to offset the cooler air otherwise in place.

Some variables remain uncertain — especially including the tropics, which obviously cannot be fully accounted for in precipitation outlooks throughout the month. However, our confidence on this forecast is higher than moderate, especially given the overall wavelength pattern and spatial distribution of anomalies. Maps are included below.

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