Tropical Storm Ophelia—Tropical Storm Warning for Coastal Mid-Atlantic

[WxWatcher007]

After the incredible pace of tropical activity in the MDR, we're reaching the homebrew portion of the 2023 Atlantic hurricane season. On cue, we have a potential system off the southeast coast that is being tracked by the NHC. 

1. Western Atlantic:
A trough of low pressure located several hundred miles east of the 
Florida peninsula is producing a large area of disorganized showers 
and thunderstorms. A non-tropical low pressure system is expected 
to form within this area by early Friday, and this system could 
acquire some subtropical characteristics on Friday or early Saturday 
while it moves generally northward toward the coast of North 
Carolina. Regardless of subtropical development, this low is likely 
to bring gusty winds to gale force, heavy rain, and high surf to 
portions of the Southeast and Mid-Atlantic United States on Friday 
and into the weekend. Additional information on this system, 
including storm and gale warnings, can be found in High Seas 
Forecasts and products from your local National Weather Service 
office.
* Formation chance through 48 hours...low...30 percent.
* Formation chance through 7 days...medium...40 percent.

 

The guidance, which was split even yesterday on impacts and development of this low, looks to have coalesced into a forecast for an impactful event in North Carolina into the Mid-Atlantic, with reduced impacts possible even further up the coast.

Particularly interesting is that the guidance has trended stronger with the potential low, increasing the odds that this becomes a subtropical storm, and introducing the possibility that it attains tropical characteristics before a landfall on Saturday. Let's take a closer look at the overall setup and potential in the region. 

First, this is a standard way to get subtropical or tropical genesis off the SE coast. A stalled frontal boundary is serving as a focal point for convective activity, which will allow for the formation of a non-tropical (baroclinic) low. 

Yesterday was when we first started seeing the strong convective bursting needed for the development of an eventual sub/tropical low, and it has continued into today as expected. 

Looking at the visible images so far today, you can see the robust convection off the SE coast, but there's still work to do in terms of organizing. To recap, for a tropical system, which is unlikely early on, you need to see persistent convection around a closed low that only acquires its energy from the ocean. For a subtropical system, convection can be removed from the center, but the system needs to essentially be a hybrid that acquires its energy from both the ocean and baroclinic (think nor'easter temperature contrast) sources. With guidance trending stronger, I think this one has a very good shot of becoming our next named storm, Ophelia. 

 

Frequently, we see these boundary systems develop and slide harmlessly OTS on a NE heading, but in this case, as the system develops, it will drift north/NNW around an Atlantic ridge. It's increasingly clear that the center of the system will landfall over eastern NC and then drift into the Mid-Atlantic, before getting shunted ENE and back offshore further north. Watch the Atlantic ridge build in first, drawing the system into the MA. Then another big ridge blocks it from continuing north and the low (or remnants) are driven eastward back offshore and eventually OTS off the New England coast. 

This track, and gradient between the low and high pressure to the north means rain and a lot of it, along with strong winds at the shoreline. Tropical storm force winds look likely at the shoreline, even far away from the low center. Rather than a smaller gif, I want to focus on two images from the GFS below. 

This is fascinating. The first, valid early tomorrow morning, has a 1008mb low that has developed. The exact pressure doesn't matter here really, just look at the depiction and trend with the second image. Friday morning this is a lopsided, but intensifying low. This is when it would be nearing subtropical designation. 

However, note what it looks like 24 hours later, early Saturday morning. This has shown up on a variety of models--the consolidation of the wind field right before landfall in NC. Look at the pressure drop as well which amounts to 15mb in 24 hours. It drops a little more in the next six hours as well. That tells me that as this approaches the coast and crosses the Gulf Stream, it may acquire enough tropical characteristics to be designated as tropical. This is a strong low depicted by the guidance, which enhances rainfall potential IMO as well. That will be something to watch closely, as we've seen a history of these SE lows spinning up quickly on final approach. 

The Bay and coastal areas should see water rise and strong winds that will make this impactful. 

 

With parts of the inland region in moderate to severe drought, any rain would be beneficial. It's still unclear how far inland the heavy rain gets, but this is the best chance this summer of significant rain further west. Which isn't saying much unfortunately. 

 

We'll see how the trends evolve, but a heavy rain event is looking increasingly likely across part of the region. 

 

 

 

[Eskimo Joe]

06z Euro is well west and yields a steady, soaking rain of 1" - 2.5" west of the Bay. Would be a drought buster for sure.

[WxWatcher007]

Forgot to add the other two images. SSTs are obviously warm and there is OHC near the coast that would support subtropical development and tropical designation near the coast if this system had enough time. In the whole scheme of things, the subtropical/tropical designation doesn't matter, but a tropical designation could mean a more organized rain shield and wind field into the coastal areas. 

[WxWatcher007]

Now designated as Invest 99L. 

[CAPE]

Pretty decent writeup from Mount Holly on the development and likely impacts in their morning AFD. Excerpt below.

Dry air in place across our region will start to give way to a plume of deepening Atlantic moisture wrapping toward the coast around the low as it approaches eastern North Carolina. Enhanced by low-level convergence and some warm-air advection, an initial batch of moderate to perhaps occasionally heavy rain will advance northward across the Delmarva Friday night, and perhaps into southern New Jersey by Saturday morning. However, the details from that point forward remain murky, as models disagree on the northward extent of steadier rain through Saturday, along with the timing and amounts of additional rain as the area of low pressure tracks toward our region on Sunday. The best compromise of forecast amounts suggests that from 0.75 to 1.5 inches will fall through Saturday night northwest of I-95, with 1.5 to 2.5 inches southeast, with the highest amounts right along the coast from Cape May to coastal Delaware. Those amounts have trended somewhat lower than earlier forecasts.

While heavier rainfall amounts than those are still within the range of outcomes, those amounts do not implicitly suggest significant impacts or more than nuisance poor-drainage or localized urban/small-stream type of flooding. Even that would probably be confined to the period of time when the initial batch of rain is lifting northward earlier on Saturday. Precipitable water values do rise into the 1.5-1.75" range on Saturday southeast of I-95, with dewpoints into the upper 60s along the coast, with some potential for an isolated thunderstorm there. Otherwise farther inland, especially northwest of the urban corridor, the atmosphere looks progressively less moist and more stable, which may limit the heavy rain threat. Overall the greater threats from this system look to be along the coast, with the potential for a period of coastal flooding, gales over the coastal waters and rather strong winds along the immediate coast (gusts 40 to 50 mph). Temperatures will be rather cool as well, with highs only in the 60s away from the coast. The wild card in all of this is whether or not the system may take on some subtropical characteristics. That is probably not in much doubt, but whether or not NHC deems it worthy of being named remains to be seen. In either case, the main system will likely weaken as it tracks northward, and it`s effects may actually diminish as it arrives on Sunday, with the pressure gradient between it and the high to the north being the main driver of the inclement weather.

[Eskimo Joe]

Updated morning AFD from LWX:

Quote

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
925 AM EDT Thu Sep 21 2023
.SYNOPSIS...
High pressure lingers nearby through today. A coastal low pressure 
system will bring the potential for gusty winds, moderate to heavy 
rain, and tidal flooding this weekend. High pressure likely builds 
early next week in the wake of the low pressure system.
&&
.NEAR TERM /THROUGH TONIGHT/...
Patchy river valley fog was gradually burning off as of mid morning. Otherwise, high cirrus was streaming northward into the area and is expected to continue to increase through the day.  Highs will top out in the mid to upper 70s for most areas east of the Allegheny Front, with some areas potentially touching 80 degrees for the afternoon. The high pressure will move towards the Northeast during the day today. There may be enough moisture available across our SW VA counties where we could see a stray shower throughout the afternoon and evening. By tonight, conditions continue to remain mostly dry with increasing clouds from onshore flow and lows dropping down into the mid to upper 50s to low 60s for areas east of the Blue Ridge Mountains with upper 40s to low 50s along/further west across the higher terrain.
&&
.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
By Friday, high pressure begins to break down further east and eventually offshore. A surface low is expected to develop off the Carolinas during the day on Friday and deepen as it moves further north over the course of 24 to 48 hours. Friday is expected to remain mostly dry through the early evening hours with afternoon highs topping out in the low to mid 70s for the lower elevations and 60s for the higher terrain. Model spread remains notable, especially being 2-3 days out with respect to potential impacts locally. QPF wise, ensembles have been coming into a bit closer agreement over the last 1 to 2 cycles but there remains some discontinuity between the ECMWF ensemble vs the GFS/CMC ensembles for this system. The 21/00z ECMWF deterministic solution is considerably wetter for the I-95 corridor and points further west compared to the 20/18z run just for example. 

For now, with spread in guidance continuing for the overnight hours, have maintained similar course as previously forecasted on the Wednesday PM package with a slight increase in QPF for the event, especially across portions of the central VA piedmont and southern MD (2-3 inches). Have stair-stepped PoPs for the event with highest being Saturday afternoon and early evening when the most likely highest shield of precipitation is expected. Regardless of designation of the low pressure (sub-tropical, non-tropical, extra-tropical), there remains a threat for areas of prolonged heavy rain, increased gusty winds (especially over the waters) and tidal flooding. If an uptick in potential QPF is realized, then there may be a need for a Flood Watch but confidence is low at this time with the continued spread in guidance. We will continue to coordinate with the National Hurricane Center on any potential hazards or changes in the forecast. As the threat begins to weaken late Saturday night into early Sunday, low temperatures will drop down into the mid to upper 40s for the Allegheny Front with a gradual warmer setup further east towards the waters.

 

[CAPE]

12z NAM has a more organized/deeper low off the SE coast. Subtropical look.

[CAPE]

[AlexD1990]

How much credence can we lend to the NAM, though? Still eyebrow-raising, nonetheless..

[LeesburgWx]

For impacts further north, we want a more subtropical look though, right?

[nw baltimore wx]

Does anyone have any thoughts on the coastal flooding potential?  Just guessing based on past events, but Alexandria, Annapolis, Fells Pt, etc, might have some issues during high tides over the weekend.

[WxWatcher007]

Wondered why they didn’t go higher earlier.

Special Tropical Weather Outlook...Corrected
NWS National Hurricane Center Miami FL
1000 AM EDT Thu Sep 21 2023

Corrrected time in the product

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

Special outlook issued to raise probabilities and update discussion 

Active Systems:
The National Hurricane Center is issuing advisories on Hurricane 
Nigel, located over the central subtropical Atlantic.

Off the Southeastern United States Coast (AL99):
Updated...A trough of low pressure located several hundred miles 
east of the Florida peninsula continues to produce a large area of 
disorganized showers and thunderstorms. A non-tropical low pressure 
system is expected to form within this area by early Friday, and 
there are increasing chances that this system could acquire some 
tropical or subtropical characteristics on Friday or early Saturday 
while it moves generally northward toward the coast of North 
Carolina. Regardless of tropical or subtropical development, this 
low is likely to bring tropical storm force winds, heavy rain, 
coastal flooding, and high surf to portions of the southeast and 
mid-Atlantic United States coastline on Friday and into the 
weekend. Tropical Storm watches or warnings could be required for 
this system as soon as later this morning. Additional information on 
this system, including storm and gale warnings, can be found in High 
Seas Forecasts and products from your local National Weather Service 
office.
* Formation chance through 48 hours...medium...50 percent.
* Formation chance through 7 days...medium...50 percent.

[WxWatcher007]

PTC 16 coming at 11am now. Escalated quickly lol.

[Random Chaos]

The ASCAT pass I posted in the other Tropical subforum thread shows a closed low as of 4 hours ago. So not surprised they are escalating quickly.

[yoda]

Going to have to do some checking... and I know it's very unlikely, but when was the last time LWX issued tropical products aka HLS?

[yoda]

50kt tropical cyclone in 24 to 36 hours per NHC 

[AlexD1990]

Just now, yoda said:

Going to have to do some checking... and I know it's very unlikely, but when was the last time LWX issued tropical products aka HLS?

Maybe Isaias or Elsa?

[AlexD1990]

I can't remember the last time they moved so quickly through the stages. We went from a 40% orange to advisories on a PTC in what, an hour??

[WxWatcher007]

1 minute ago, alexderiemer said:

I can't remember the last time they moved so quickly through the stages. We went from a 40% orange to advisories on a PTC in what, an hour??

I’ve seen this happen a few times recently with homebrew systems. Kind of surprised the NHC wasn’t higher given the cross guidance signal on the models, particularly in the last 24 hours.

[Random Chaos]

3 minutes ago, alexderiemer said:

I can't remember the last time they moved so quickly through the stages. We went from a 40% orange to advisories on a PTC in what, an hour??

Was even faster - the 40% was in the 2-7 day period. It was only 10% in the 48hr period just 3 hours ago.

[AlexD1990]

1 minute ago, WxWatcher007 said:

I’ve seen this happen a few times recently with homebrew systems. Kind of surprised the NHC wasn’t higher given the cross guidance signal on the models, particularly in the last 24 hours.

Agreed. From the standpoint of the general public(if they're even paying attention), it would seem to be confusing..

[yoda]

Potential Tropical Cyclone Sixteen Advisory Number   1
NWS National Hurricane Center Miami FL       AL162023
1100 AM EDT Thu Sep 21 2023

...LOW PRESSURE AREA EXPECTED TO BRING TROPICAL STORM CONDITIONS
AND THE POTENTIAL FOR LIFE-THREATENING STORM SURGE TO PORTIONS OF
THE SOUTHEAST AND MID-ATLANTIC COASTS...


SUMMARY OF 1100 AM EDT...1500 UTC...INFORMATION
-----------------------------------------------
LOCATION...28.7N 75.9W
ABOUT 370 MI...595 KM SE OF CHARLESTON SOUTH CAROLINA
ABOUT 450 MI...725 KM S OF CAPE HATTERAS NORTH CAROLINA
MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H
PRESENT MOVEMENT...N OR 10 DEGREES AT 9 MPH...15 KM/H
MINIMUM CENTRAL PRESSURE...1012 MB...29.89 INCHES


WATCHES AND WARNINGS
--------------------
CHANGES WITH THIS ADVISORY:

A Tropical Storm Warning has been issued from Cape Fear, North
Carolina, northward to Fenwick Island, Delaware, including
Albemarle and Pamlico Sounds, and the Chesapeake Bay south of
Smith Point.

A Storm Surge Watch has been issued from Surf City, North Carolina
to Chincoteague, Virginia, and the for the Chesapeake Bay south of
Smith Point, including Albemarle and Pamlico Sounds.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT:

A Tropical Storm Warning is in effect for...
* Cape Fear NC to Fenwick Island DE
* Chesapeake Bay south of Smith Point
* Albemarle and Pamlico Sounds

A Storm Surge Watch is in effect for...
* Surf City NC to Chincoteague VA
* Chesapeake Bay south of Smith Point
* Albemarle and Pamlico Sounds

A Tropical Storm Warning means that tropical storm conditions are
expected somewhere within the warning area within 36 hours.

A Storm Surge Watch means there is a possibility of life-
threatening inundation, from rising water moving inland from the 
coastline, in the indicated locations during the next 48 hours.
For a depiction of areas at risk, please see the National Weather
Service Storm Surge Watch/Warning Graphic, available at
hurricanes.gov.  

For storm information specific to your area, including possible
inland watches and warnings, please monitor products issued by your
local National Weather Service forecast office.

 

[yoda]

Warnings go up to the LWX/AKQ border on the waters

[WxWatcher007]

Going tropical as of that advisory. 

[Eskimo Joe]

10 minutes ago, yoda said:

Going to have to do some checking... and I know it's very unlikely, but when was the last time LWX issued tropical products aka HLS?

[AlexD1990]

WOW. Definitely making up for lost time on that advisory!

[yoda]

ZCZC MIATCDAT1 ALL
TTAA00 KNHC DDHHMM

Potential Tropical Cyclone Sixteen Discussion Number   1
NWS National Hurricane Center Miami FL       AL162023
1100 AM EDT Thu Sep 21 2023

A broad non-tropical area of low pressure has formed well east of 
the east coast of Florida this morning.  Although this system is 
forecast to remain non-tropical during the next 12-24 hours, the 
dynamical model guidance indicates that it will likely acquire 
tropical characteristics late Friday and early Saturday as it 
approaches the coast of North Carolina.  Although it is unclear as 
to whether the cyclone detaches from a front that is forecast to 
extend northeastward from the center, the guidance suggests a 
tropical-cyclone like core and structure when it nears the coast.  
As a result, advisories are being initiated on Potential Tropical 
Cyclone Sixteen to issue Tropical Storm Warnings and Storm Surge 
Watches for portions of the coasts of North Carolina, Virginia and 
Maryland. Winds associated with the front farther north are being 
covered by non-tropical products issued by local National Weather 
Service offices and the Ocean Prediction Center.  Additional 
tropical watches and warnings could be issued for other portions of 
the Chesapeake Bay later today. 

Since the low is still in its formative stage, the initial motion
estimate is a highly uncertain 360/8 kt.  The model guidance
suggests that center reformations are likely to occur during the
next day or so, but the overall motion of the system is expected to
be a little east of due north. As the system interacts with a 
mid-latitude trough that it becomes embedded within, a bend toward
the north-northwest is forecast.  That motion should bring the 
center over eastern North Carolina within the warning area Saturday 
morning.  The NHC track forecast follows a blend of the various 
global models that are in good agreement. 

The system is forecast to gradually strengthen during the next 24 
hours. After that time, the guidance suggests it is likely to form 
a smaller inner core with additional strengthening expected until 
the center reaches the coast.  The NHC intensity forecast follows 
the ECMWF and GFS models trends. 

Key Messages:

1. An area of low pressure is forecast to develop off the 
southeastern U.S. coast later today and will bring 
tropical-storm-force winds, storm surge, heavy rain, and high surf 
to large portions of the southeast and mid-Atlantic United States 
coast beginning Friday and continuing into the weekend. 

2. There is the potential for life-threatening storm surge 
inundation from Surf City, North Carolina to Chincoteague, Virginia, 
the Pamlico and Albemarle Sounds, and the lower Chesapeake Bay. 
Residents in these areas should follow advice given by local 
officials. 

3. Tropical storm conditions are expected within portions of the 
southeast and mid-Atlantic coasts within the Tropical Storm Warning 
area beginning on Friday and continuing into Saturday. 

4. Heavy rainfall from this system could produce localized urban 
and small stream flooding impacts across the eastern mid-Atlantic 
states from North Carolina to New Jersey Friday through Sunday.

FORECAST POSITIONS AND MAX WINDS

INIT  21/1500Z 28.7N  75.9W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 12H  22/0000Z 30.0N  75.5W   35 KT  40 MPH...POTENTIAL TROP CYCLONE
 24H  22/1200Z 31.7N  75.4W   45 KT  50 MPH...POTENTIAL TROP CYCLONE
 36H  23/0000Z 33.2N  75.6W   50 KT  60 MPH...TROPICAL CYCLONE
 48H  23/1200Z 35.1N  76.4W   50 KT  60 MPH...TROPICAL CYCLONE
 60H  24/0000Z 37.3N  76.4W   40 KT  45 MPH...TROPICAL CYCLONE
 72H  24/1200Z 38.8N  76.1W   30 KT  35 MPH...POST-TROPICAL
 96H  25/1200Z 40.4N  71.8W   25 KT  30 MPH...POST-TROPICAL
120H  26/1200Z...DISSIPATED

$$
Forecaster Brown

[Eskimo Joe]

Looks like AQK will at least be issuing some tropical products for lower Delmarva.

[yoda]

 

[Random Chaos]

Waiting for CBOFS tide forecasts to get in range. Wind direction is going to be NE initially which will push water toward the western side of the Bay. This is NHC: