Strong-to-Severe Convection Possible Late Afternoon/Evening 09/25/2018

Two windows will exist for thunderstorm development through Wednesday evening and the potential will also exist for a few strong-to-severe thunderstorms. The first window will develop later this afternoon into the early overnight hours as a warm front slowly lifts northward and the second window will be tomorrow evening as a cold front approaches.

A warm front is currently positioned south of Long Island back southwest into NJ. This warm front will continue to slowly lift northward through the afternoon. South of the warm front the airmass is characterized by rich theta-e air/low-level moisture and rather high dewpoints. The combination of rich theta-e air and dewpoints well into the 70's south of the warm front is yielding mixed-layer CAPE values between 500-100 J/KG. A very strong low-level jet is also present with 925mb winds in excess of 40 knots across CT. Forecast models indicate this could strengthen upwards of 45-50 knots through the afternoon.

With the warm front approaching CT and a strong low-level jet in place, strong speed and directional shear is present within the 0-6km layer which is yielding enlarged and curved hodographs. As the warm front lifts northward instability will begin to increase across portions of the state where the warm front lifts through. Forecast models indicate an overlap of the strongest wind shear and instability will occur across CT late afternoon into the early overnight hours. The image below is the forecast hodograph for Waterbury, CT from the 12z run of the NAM (on the left) and the HRRR (on the right) for early this evening. The NAM shows 0-3km helicity values just over 300 m2/s2 with 0-1km helicity values well over 200 m2/s2 (not shown) while the HRRR shows 0-3km helicity values over 400 m2/s2 with 0-1km helicity values just under 400 m2/s2 (not shown). What is also noteworthy is the 0-1km CAPE values just over 100 J/KG:

CAMS (convective allowing forecast models) have been rather consistent and aggressive in developing convection later on this afternoon and given the projected instability parameters and shear there is the possibility that any convection  may develop supercell characteristics. With this said we will have a window of opportunity for an isolated tornado/damaging wind gusts into the early overnight It is also possible that this convection may not contain thunder or lightning.

In addition to the potential for an isolated tornado/damaging wind gusts we will see localized pockets of flash flooding with heavy rain continuing through the day.

 A separate blog post will highlight and discuss the potential for severe weather tomorrow a bit later on. 

Florence Discussion: East Coast Threat?

All eyes are on Tropical Storm Florence which is currently located out in the tropical Atlantic about 935 miles east-southeast of Bermuda (as of this writing). Florence was recently as strong as a powerful category 4 hurricane; however, some dry air and strong wind shear has led to rapid weakening of Florence over the past day. As we enter the weekend, Florence is expected to leave the area of dry air and enter an environment which is less hostile wind shear wise and with this the expectation is for Florence to rapidly strengthen during the weekend. Model Intensity Guidance shows strong support/agreement on this rapid strengthening occurring this weekend with several pieces of guidance suggesting strengthening to a category 3 or perhaps even a category 4 hurricane come the beginning of next week:

The big questions are; does Florence hit the east coast? If so, where does Florence hit? And what will the intensity of Florence be? There are growing indications that Florence may indeed hit the east coast with a potential landfall along the Carolina or VA coast…or at least if not a landfall, impacts occurring across these areas:

There are a few pieces of the puzzle and these pieces will be extremely critical with regards to strength and track of Florence and we will analyze all the information below.

High Pressure

Forecast models are in excellent agreement regarding an amplifying positive PNA (Pacific-North American teleconnection pattern) trough across western Canada and the western United States coast as next week progresses. The response to this amplifying trough is for ridging to begin developing across the eastern United States and western Atlantic. The graphic below is from Friday’s morning run of the GFS ensembles valid for 12z (8:00 AM EDT) Wednesday morning showing 500mb height anomalies:

The key piece here is the high pressure which is outlined. Before we explain why this piece is critical it is important to note that this projected atmospheric pattern configuration combined with the current latitudinal positioning of Florence is NOT FAVORABLE FOR AN EAST COAST LANDFALL. What this high pressure does is block the system from recurving and going well out to sea. In the Northern Hemisphere, the flow around high pressure systems is clockwise. Given the position of the high pressure with Florence tracking south of the high, the steering flow would push Florence into the east coast of the United States as it has nowhere to go with the high pressure to the north. So, this is set in stone, right? Well not necessarily but the indications are growing for this possibility to become a reality. The best thing to do is to further explore this high pressure.

One very interesting aspect to note is the rapid development and strengthening (as well as placement) of this high pressure system early next week along with the projected strengthening of Florence. We will illustrate this by looking at the GFS projected sea-level pressure at 0z Wednesday (8:00 PM EDT) and at 0z Thursday and 500mb height anomalies at 0z Wednesday and 0z Thursday. Notice how the high pressure has strengthened while the sea-level pressure has decreased (indicating a stronger system):

Tropical systems are huge heat machines. They are composed of an incredible amount of moisture with vigorous evaporation and condensation occurring. The process of condensation releases heat and this heat is referred to as latent heat. It’s very tough to illustrate this (well at least for me as I’m not artistic) but try to visualize this…you have a very large system with a tremendous amount of condensation going on...the result is a tremendous amount of heat being released. When you have large and strong tropical systems they can release so much heat that the atmosphere has a response and that response is for rising heights A.K.A ridging. It is very possible that the forecast models are really hitting this process hard which could explain the rapid strengthening of the high pressure/subsequent ridging. The question is…is this correct? This very well could be overdone and even if it’s not where this high pressure develops and where it strengthens is extremely critical. If this happens farther east, there is now an escape route for a rapid re-curvature of Florence…this isn’t to say there wouldn’t be any impacts, but a landfall would become much less likely. If this happens farther west well the system likely goes straight into NC, SC, or VA coast.

There are other questions regarding the ridging as well. There are times where models have a bias with the strength of high pressure systems and the structure of the high pressure systems in the medium range. Perhaps very strong high pressure does develop, but perhaps it doesn’t configure as modeled and there are weaknesses within the high pressure…that is not totally uncommon, and that weakness would act as an escape route.

How this high pressure is handled and how it eventually evolves is extremely critical in the overall result of Florence.

Sea-Surface Temperatures

We can’t talk tropical without discussing good ole sea-surface temperatures (SSTs). I mean after all, it’s the warm waters which fuel these tropical systems and give them the energy they need. There is no question water temperatures are supportive of a hurricane. The minimum threshold for water temperatures is about 80-82°F. Sea-surface temperatures are running much above-average (noted by the SSTA (sea-surface temperature anaomalies) chart on the left in the image below) as water temperatures are into the lower 80’s as far north as the NJ coast (noted by the chart on the right:

Sometimes, however, sea-surface temperatures alone don’t tell the whole story. Especially considering this is just looking at water temperatures at the surface…well what about below the surface? Are these warm waters very shallow in nature or do they penetrate deep? This is extremely important when considering the process of upwelling. The top of the ocean surface is always going to be warmer than water 10…15…20+’ below as the surface of the water is most exposed to the heating from the sun. Upwelling (the rising and mixing of ocean water) brings colder water from below the surface towards the top while the warmer waters from the top go beneath the surface. This results in the colder waters warming and the warmer waters cooling. With tropical systems which can generate big waves the result is for tremendous upwelling…waters from well below the ocean surface which are quite cold mixing to the top. This will quickly result in lowering water temperatures. The map below shows the depth of the 26°C isotherm to be around 50 to perhaps as much as 75 meters off the east coast which isn’t terribly deep. As Florence really ramps up this weekend big waves will begin to generate well ahead of it likely promoting upwelling and the lowering of sea-surface temperatures…enough to lead to any weakening of Florence…that remains to be seen:

This is also important because with a high pressure to the north and ridging across the eastern United States there is nothing to really promote an accelerated forward movement of Florence. Typically, when we see hurricanes near the east coast their forward speed will accelerate (usually enhanced by steering flow around the high and being captured by a trough across the Midwest or Ohio Valley (this is what you usually want to see when talking about a landfalling system). The slower Florence moves, the greater the duration of the upwelling and sitting over a location for too long will also result in cooling water temperatures due to evaporation and rain falling.

Wind Shear

Wind shear is extremely critical to the strengthening and structure of a tropical system. Unlike thunderstorms, tropical systems hate strong wind shear. Strong wind shear disrupts the physics of a hurricane. Currently, Florence is in an area of strong wind shear which has resulted in the weakening mentioned in the opening, however, as you’ll see in the map below Florence will be moving into an area of much less wind shear (Florence is located within the red contours which constitute strong wind shear while off the east coast we see green contours indicating weak wind shear):

The Wild Card…

In baseball they have wild cards and hey…why not in weather? In this case there may be an underrated or overlooked wild card and that is a circulation of clouds and convection in the western Atlantic well off the south east coast.:

This is sure to not only influence Florence but will likely wreak havoc on forecast models and could really yield forecast models to struggle with the development of the atmospheric pattern configuration across the Atlantic. When there is a great amount of convection present forecast models sometimes struggle vastly and this is due to the physics of the models and the complexities of convection.

As it stands there is an increasing likelihood for impacts along the east coast with these impacts more likely to occur across the Carolina or VA coast. There is even a possibility that the mid-Atlantic coast seems impacts but no landfall as Florence begins to quickly re-curve and this is a scenario which I think will unfold. There is an extremely minimal likelihood for any impact (outside of waves and maybe eventual leftover moisture) here across southern New England.

An update will be conducted later in the weekend.