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NWS National Hurricane Center Miami FL AL092020
500 PM EDT Fri Jul 31 2020
...HURRICANE WARNING ISSUED FOR PORTIONS OF THE FLORIDA EAST
COAST...
...TROPICAL STORM CONDITIONS AND HEAVY RAINS CONTINUE TO SPREAD
ACROSS THE CENTRAL BAHAMAS...
SUMMARY OF 500 PM EDT...2100 UTC...INFORMATION
----------------------------------------------
LOCATION...22.6N 75.7W
ABOUT 195 MI...320 KM SSE OF NASSAU
ABOUT 330 MI...535 KM SE OF FREEPORT GRAND BAHAMA ISLAND
MAXIMUM SUSTAINED WINDS...75 MPH...120 KM/H
PRESENT MOVEMENT...NW OR 305 DEGREES AT 15 MPH...24 KM/H
MINIMUM CENTRAL PRESSURE...991 MB...29.27 INCHES
Maximum sustained winds are near 75 mph (120 km/h) with higher
gusts. Strengthening is expected later tonight and early Saturday,
and Isaias is forecast to remain a hurricane for the next couple of
days.
Hurricane-force winds extend outward up to 35 miles (55 km) from the
center and tropical-storm-force winds extend outward up to 175 miles
(280 km). A private weather observing station at Pitts Town Point,
Bahamas, recently reported a sustained wind of 37 mph (59 km/h).
Hurricane Isaias Discussion Number 15
NWS National Hurricane Center Miami FL AL092020
500 PM EDT Fri Jul 31 2020
Deep convection, with occasional overshooting cloud tops of -85C to -90C just north of the center, has continued to develop during the
normal diurnal convective minimum period, which is quite impressive.
The most recent Air Force Reserve recon flight-level wind data,
along with ASCAT surface wind data, indicate that the inner-core and
outer wind field have both contracted in size. Furthermore, radar
data from the Bahamas and an 1810Z AMSR-2 microwave pass also
indicate that a small 10-nmi-wide mid-level eye is forming. The last
recon central pressure was 991 mb and the 700 mb height had
decreased by 30 meters since the earlier maximum height around
1230Z. These data indicate that Isaias is getting better organized.
The initial intensity remains 65 kt based on an earlier 700-mb
flight-level wind speed of 72 kt, which reduces to a 65-kt surface
wind speed using a 90-percent adjustment factor.
The initial motion remains northwestward or 305/12 kt. The 12Z global models have once again made a westward shift due to the ridge to the north of Isaias not weakening as quickly as expected. This is partly due to the ridge being stronger than expected and a shortwave trough over the central United States moving a little slower into the southeastern U.S. than previously indicated. The UKMET and ECMWF
explicitly show Isaias making landfall in 36-48 hours along the
southeast Florida coast, but appear to weaken the system below
hurricane strength. The GFS similarly brings the cyclone close to
the southeast and east-central Florida coasts, but also as a
somewhat weaker system. In the 48 to 60-hour period, the cyclone is forecast to move slowly north-northwestward and northward through a break in the subtropical ridge extending westward from the Atlantic
across Florida and into the northern Gulf of Mexico. By that time,
however, Isaias is expected to weaken below hurricane strength due
to the combination of strong southwesterly vertical wind shear and
interaction the Florida peninsula. Around 72 hours, the cyclone
should accelerate northeastward and possibly strengthen some before
passing over eastern North Carolina on day 4, and across eastern New
England on day 5. The NHC track forecast lies close to a blend of
the consensus models TVCA and NOAA-HCCA and is east of the UKMET
and ECMWF with the system forecast to be stronger than those
models indicate. Due to the westward shift in the NHC forecast
track, a Hurricane Warning and Storm Surge Watch have been issued
for portions of the Florida east coast.
The center of Isaias is now located in the center of an expanding
CDO feature. The improved inner-core wind field and aforementioned
convective structure, along with very warm SSTs near 30C, should
support some strengthening overnight and early Saturday morning.
However, increasing southwesterly vertical wind shear is expected to
cause a gradual decrease in intensity by Sunday and continue into
early next week. The new official intensity forecast is a little
lower than the previous advisory and is near the higher end of the
intensity guidance.
I made annotations on this map to better get a visual. It's all about the upper air pattern and Jet stream. It dictates the weather patterns.
Had the ridge been weaker and the jet stream push into southeast faster, Isaias would of stayed off the coast more.
If it could only move another 200 mile more to the west. Just no chance as the core weather will get blown off well to the east of the state as the upper levels are not gonna be nice to this storm.
The eye closed up earlier which helps fight off the shear but now appears to reopen on the South side again so that may help keep the storm strength in check some.
Here's the 18z GFS rainfall forecast. 18z evening Euro is similar but skinnier line. This may continue to shift left/right. The right side of the storm would have more rainfall (pulls up moisture counter clockwise off the Atlantic).
Hurricane Warning added from Volusia-Brevard Co. line to Flagler/Volusia Co. line. We'll continue to see Watches/Warnings extended up the East coast as they are time based issued (48hrs or less Watch, 36hrs or less Warning).
Hurricane winds extend up to 35miles(55km) from storm center, TS winds 175miles(280km)
No real changes otherwise.
Once it eventually turns more NNE and NE come early next week it should shoot off very quickly into Canada by late Wednesday.
The NOAA G-IV finally got out from Lakeland today and sampled air all around the storm and ahead of it, that'll also help feed models better data. NWS offices in the US are sending up additional weather balloons to help with model data input as well.
On a boat in the keys right now... no more westward movement?
On evening recon it jogged a little to the West recently but overall has been Northwest direction. Per NHC's 11pm update: "The general track forecast philosophy has not changed. Isais should continue to move northwestward on the southwest side of a mid-level ridge through Saturday and begin to turn north-northwestward through a weakness in the ridge on Sunday. " And any Tropical Watch/Warnings from NHC are from just North of Ocean Reef and points Northward at this time.
Storm surge is typically higher North of the storm center (current surge Watch is from near Jupiter and North).
On the recon too it shows us that other then right around the eyewall, the South and SW side of storm is relatively calm today due to the wind shear blowing across it from the SW. Shear looks to continue next few days as storm tries to continuously organize.
recon: https://www.tropicaltidbits.com/recon/
Thank you Psy.... have the boat all tied up and ready to go.. mostly, still needs a little attention if high winds are coming... hoping that can wait till the morning.
Sat 1Aug 2aEDT/AST:
Winds 85mph(135km/h)
Moving NW 14mph(23km/h)
Pressure 988mb
Recon finds wind slightly up but pressure about same so no sudden changes in strength expected next few hours. (pressure fall typ proceeds wind increase, rise typ wind decrease). Satellite presentation not looking as healthy though on IR or Water Vapor. Main rain shield slowly nearing Nassau and Andros Island Bahamas.
Overnight 0z Euro brings weaker storm close to coast and just inland Cape Canaveral to near Jacksonville, then back in Charleston-Myrtle Beach into Carolina's. 0Z gfs similar but just offshore of FL and into SC/NC border region. These slight changes in angle to coast continue which can jump direct impacts around a few hundred miles at a time. Slight wobbles will make a difference. Just one model run.
Last edited by Psychoma; 08-01-2020 at 12:52 AM..
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