Northeast Weather Blog

Tropical Update



Dolly



Latest satellite image of Tropical Storm Dolly.


Tuesday afternoon update

Dolly now a hurricane.

At the 5pm EDT (Tuesday) National hurricane Center advisory the center of Hurricane Dolly was located at 24.6°N 95.3°W, or about 165 miles east-southeast of Brownsville, Texas. Maximum sustained winds are around 75mph and the minimum estimated central pressure is 986mb. Previous reasoning from this morning applies to the current forecast.


Tuesday morning update

At the 5am EDT (Tuesday) National hurricane Center advisory the center of Tropical Storm Dolly was located at 23.3°N 93.8°W, or about 295 miles southeast of Brownsville, Texas. Maximum sustained winds are around 60mph and the minimum estimated central pressure is 997mb. Dolly is moving to the west at 15mph. Dolly has become much better organized during the overnight, developing a central dense overcast within a structurally improving inner-core. Banding has drastically improved over the northern and western semi-circles and it now appears that Dolly is on the verge of becoming a hurricane. Outflow is excellent in the northern and western quadrants with a well-defined outflow channel noted venting clockwise around the upper ridge over the central Gulf of Mexico down into the Caribbean. Outflow is restricted in the western quadrant and non-existent in the southern quadrant. The upper low the has been hindering Dolly's development thus far is weakening over Old Mexico, allowing for this sudden burst of intensification.

Dolly should continue to intensify over the next 18-24 hours while over the warm waters of the Gulf of Mexico. In fact, Dolly may become a strong category 2 storm at peak intensity. As Dolly approaches the coast oceanic heat content decreases and the cyclone will begin to ingest some dry continental air, likely putting Dolly in a steady state or slight weakening trend in the hours before she comes ashore. Upon moving inland rapid weakening should occur and Dolly should be nothing more than a heavy rain producing tropical depression by late Thursday afternoon.

Dolly should continue on a just north of due west track over the next 6-12 hours. Thereafter a slightly more northerly component to Dolly track should ensue as the storm grows stonger and begins to feel a weakness in the ridge to her north. Dolly should also slow her forward speed to around 10mph or slightly less during the evening into the overnight tonight. It is still expected that Dolly will make landfall over the Rio Grande Valley region along the international border sometime during the day on Wednesday. As Dolly loses her vertical structure after landfall the storm will bend back towards the west in the low-level flow into Old Mexico.

Previous Discussion



At the 8am EDT (Monday) National hurricane Center intermediate advisory the ill-defined center of Tropical Storm Dolly was located at 21.6°N 88.7°W along the northern coast of the Yucatan Peninsula. Maximum sustained winds are around 50mph and the minimum estimated central pressure is 1005mb. Dolly is moving to the west-northwest at 16mph. Tropical Storm Dolly continues to look very disorganized on satellite imagery with the mid-level center about 40miles west-northwest of the low-level center along the northern coast of the Yucatan. Outflow remains good to the north and east of the cyclone but fair to poor in the southern and western quadrants. The atmosphere surrounding Dolly is quite moist as satellite derived precipitable water soundings are upwards of 2.5". The upper level low which has been moving in tandem with Dolly has also begun to outrun the cyclone. and is now moving towards the west-southwest into the southwestern Bay of Campeche. Given the favorable outflow pattern and highly moist environmental conditions and deep warm waters along her projected path, the only factor holding the storm from intensifying is a poorly-defined inner structure of the storm.

Dolly should gradually get her act together over the next 24 hours as expectations are that she'll finally be able to develop a well-defined center of circulation. Dolly should also slow her forward speed giving the system more time to develop before reaching the Rio Grande River Valley region. In all likelihood Dolly will become a hurricane by Tuesday evening or early Wednesday morning and make landfall at hurricane strength Wednesday evening. Dolly should continue on her west-northwest track right up until landfall then bend to the west as it moves under the upper level ridge over the southern Plains. Despite drought conditions which have persisted since last autumn over the Rio Grande Valley, heavy rains from Dolly combined with recent heavy rains from several tropical waves that have come ashore in this region could lead to flash/river flooding. Moisture from Dolly will eventually become incorporated into the newly dubbed ‘North American Monsoon’ bringing additional rain to the Southwest. This moisture should eventually find the westerlies and could bring MCS activity to the northerntier of the country by next weekend.


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Current watches, warnings and advisories.




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Regional Forecast




High impact rain event for the Northeast is underway. The first in a series of low pressure waves is currently moving through Pennsylvania and New Jersey. This wave is accompanied by a line of strong to severe thunderstorms preceding a large area of heavy rainfall. A classic setup for heavy rain over the Northeast has shaped up as a strong sub-tropical ridge (Bermuda high) builds and moves to a position a few hundred miles south of Newfoundland while a digging upper level trough over the Great Lakes/Ohio Valley region goes into negative tilt as it slowly pushes east. Shortwaves rounding the base of this trough will spawn several areas of low pressure that will track northward along a nearly stationary surface frontal boundary that will extend from southeastern Pennsylvania to the Lake Champlian region. Deep tropical moisture connection will be present which will enable each of these waves the potential to drop 1-3"of rainfall. Where tracks of heavy rainfall overlap, rainfall total have to potential to drop close to a half foot of rain with locally higher amounts. This amount of rain falling in a 36-48 hour period will lead to flooding concerns of both flash flooding and river flooding.


Instead of trying to concentrate on each wave I'll just give a general overview of the synoptic set-up and let the chips fall where they may. Trying to pinpoint exact times and locations in such events where a large area stands to see the potential for flooding rains is nonsensical and, quite frankly, a waste of time.

Focus will be on the severe threat first since that will be the biggest initial threat. mlCAPE values over eastern Pennsylvania, New Jersey and eastern New York are in the 500-1,200J/kg range, so the atmosphere is still plenty unstable even at what should normally be diurnal min. Vertical shear ranges from 30-40kts across this region as well, leading to storm organization and maintenance along a line of storms moving north through Pennsylvania and backbuilding cells just south of Lake Ontario in New York. Triggers for these cells are the remnants of an old frontal boundary across New York and a rapidly northeastward advancing 500mb shortwave pulling through Pennsylvania. Severe threat comes from a 40-50kt 500mb flow out of the southwest that any of the stronger cells should easily be able to transfer to the surface. Combined with anomalously cold temperatures at 500mb (-11 to -13°C) not only will severe wind gusts be a threat, but also nickel-sized hail. Some turning of the mid-level flow due in part to differential vorticity advection will be able to support stronger updrafts adding to the hail threat.

This afternoon, as diurnal effects take place, additional storms will increase in coverage and intensity once again. A train of mid-level impulses will make their way into the Northeast serving as a triggering mechanism for convection. There's a question as to how much insolation will take place to destabilize the atmosphere but at the time thinking is that enough sun should break through to sufficiently create modest instability. Models prog mlCAPE values to approach 1,500-2,000J/kg by afternoon along with 6-7°C/km mid-level lapse rates and 30m height falls @500mb. Deep southerly unidirectional flow along with 30-40kts of bulk shear will promote the development of bowing line segments of storms. However, as the afternoon progresses and the low-level jet increases to ~45kts due to the tightening pressure gradient, development of supercell storms are possible capable of damaging winds and hail, along with the threat of isolated tornados.


As for the heavy rain threat, favorable jet dynamics will play a role in enhancing rain amounts as the region from eastern Pennsylvania to western New England will find itself under the right rear entrance region (and remain there) of a 90-100kt jet maxima over western New York and southern Ontario. In addition, the tightening pressure gradient between the strong ridge over the western Atlantic and the upper trough over the Ohio Valley will increase the southerly low-level jet, helping to transport a deep tropical airmass from the Gulf of Mexico and the tropical western Atlantic over the region. Precipitable water values approach and exceed 2" focusing along the surface frontal boundary and zone of highest 850mb convergence axis. Given the convective nature of the precipitation, rainfall rates will range from 1-2"/hr in the heavier cells, leading to flash flooding problems of creeks, streams and urban areas. Large scale lift/ascent for heavy rainfall will be provided by the many impulses riding north into this tropical airmass, along with the favorable jet dynamics and theta-e ridge forecast to line up with the Hudson River Valley. Orographics will also come into play along the south facing slopes of the Green and Whites, Catskills, Berkshires, Taconics, Adirondacks, Poconos and Litchfield Hills of Connecticut. Rain will come in waves with and out ahead of each impulse moving north in the deep southerly flow. The frontal boundary will slowly push east by Thursday afternoon bringing the heavy rain threat into New England, but by then the frontal boundary, albeit slow. will finally show signs of progression as the ridge over the western Atlantic begins to give way to the slowly advancing trough. Total rainfall from this series of storms will approach 3-6" with locally higher amounts in the hardest hit areas, especially in the areas that will benefit, or in this case suffer, from orographic lift. Outside of this region rainfall amounts of 1-2 are possible with extreme western sections seeing generally under half in inch. Temperatures will be kept near normal given the cloud cover and rainfall with slightly below normal lows and slightly above normal highs. Of note, the NAM model has consistently shown southern New York to receive upwards of 10" of rain with this event. While highly unlikely, this possibility will be closely monitored.



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Radar: Northeast Region Loop




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Local SST's




Current SST's off the Northeast Coast.



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July Daily Weather Statistics


July 1st - 75°F/55°F.....Trace....60%
July 2nd - 80°F/54°F.....0.00"....90%
July 3rd - 79°F/54°F.....0.31"....20%
July 4th - 71°F/63°F.....Trace.....5%
July 5th - 80°F/59°F.....0.00"....40%
July 6th - 81°F/63°F.....0.00"....50%
July 7th - 82°F/66°F.....0.00"....50%
July 8th - 84°F/67°F.....0.00"....60%
July 9th - 79°F/64°F.....0.21"....50%
July 10th - 75°F/57°F....0.00"....80%
July 11th - 79°F/52°F....0.00"....70%
July 12th - 86°F/64°F....0.00"....80%
July 13th - 80°F/65°F....0.72"....20%
July 14th - 77°F/63°F....0.03"....80%
July 15th - 79°F/55°F....0.00"....80%
July 16th - 85°F/54°F....0.00"....95%
July 17th - 84°F/60°F....1.18"....60%
July 18th - 88°F/62°F....0.00"....75%
July 19th - 89°F/64°F....0.00"....60%
July 20th - 90°F/65°F....0.52"....60%
July 21st - 82°F/64°F....0.03"....40%
July 22nd - 81°F/59°F....1.12"....40%