Area Forecast Discussion
National Weather Service Peachtree City GA
434 AM EDT Thu Jun 17 2021
...Morning
AFD Discussing Heavy
Rainfall Potential with Possible Weekend
Tropical System...
.SHORT TERM /Today through Friday/...
Short term forecast largely quiet with a transition to a more
active
weather pattern expected by late Friday. Though still under the
influence of the
ridge, models continue to show a weakening trend
with a relaxed pressure
gradient through late week. Temperatures
return to at or slightly above
normal today with highs in the upper
80s and low 90s and lows not quite as cool Friday.
By Friday afternoon, have introduced
slight chance pops in the far
southern
CWA as tropical
moisture creeps back into the area. This
begins the shift to a wetter pattern in the extended.
31
.LONG TERM /Friday Night through Wednesday/...
The long term forecast remains dominated by what will be happening
with current AL92, which as of 2AM EDT has an 80% chance of
formation in 2 days and a 90% chance in 5 days. Air force
hurricane
reconnaissance is now scheduled to begin investigating the
disturbance this afternoon if deemed necessary. System remains a
very broad low as is typical with disturbances produced as part of a
Central American Gyre. It is very
likely that by the time the long
term picks on Friday night this system will be either a tropical
depression or
tropical storm Claudette and will be located within
the western Gulf of Mexico.
The 00Z runs of the GEFS and
GFS are interesting in that they
show development of the system occurring a bit further north than
in previous forecast runs. Using the
GFS as a proxy, given both
are running the same dynamical core, this development seems to
happen due to an area of stronger
convection associated with a
local vortmax on the northern edge of the broader low in the gulf.
This highlights in part why there is still uncertainty around the
system despite getting closer and closer to the period of
development. Initial development may be contingent on correctly
timing convective processes, something long term guidance will
always struggle a bit more with given vertical resolution and
reliance on general convective parameterization schemes. This
development being a bit further north would bump up the timing of
the system a little bit, potentially bringing the heavier
rainfall
into the
CWA on Saturday night into Sunday morning, rather than
other guidance which still leans toward a Sunday into Monday
timing. For
PoPs, have chosen to go ahead and bump things up a bit
Saturday night into Sunday, as even if the system is on the later
Sunday to Monday timing, copious
moisture will be streaming in
from the gulf as the door to the
tropics will be wide open.
As the system moves through this weekend, two threats continue to
remain - the potential for heavy, flooding
rainfall and the
potential for tropical tornadoes. To the first, the exact location
of the track with
likely have large impacts. Generally expecting to
see a widespread 3-5" of rain, but some models are beginning to hint
a the potential for a band of heavier precip that could fall
somewhere over the area, with GEFS
QPF 90th percentile being in the
5.5" range for ATL and
numerous other locations across the
CWA. A
band like this would certainly lead to some
flash flooding as well
other river based problems. Additionally,
orographic lift may see
some much higher totals over the mountains of northeast Georgia.
This all may be exacerbated even more by a cold
front moving in
early next week which is discussed below, bringing in another
round of heavier
rainfall ahead of it. See the hydro portion of
the
AFD for more info on the flooding potential. Models still
continue to advertise high
SRH values and enough in the way of
instability to support the low topped supercells that are common
in landfalling tropical systems, another potential threat that
will need to be monitored and will depend on timing and track
location.
Even with the passage of the system, the tropical airmass will hang
around with PWATs in the 1.5-2"+ range. Some significant timing
differences exist between the Euro and
GFS regarding a long wave
trough located over the Great Plains on Monday that become more
negatively tilted Tuesday into Wednesday as it moves into the NE
CONUS, with the
GFS being much faster than the Euro.
GFS also digs
the
trough a bit further south. All this leads to differences in
when a cold
front moves into the area and how far south it gets.
GFS moves it through Monday night into early Tuesday and clears it
through most of the
CWA, whereas Euro stalls it further north.
This will have large impacts on our weather, as the
GFS solution
would be mostly dry Tuesday into Wednesday, but the Euro would
bring continued
rainfall with
likely continued flooding. Have
chosen to leave some lower end
PoPs in on Tuesday evening and
Wednesday, as I`m wondering how much the earlier timing of the
tropical system is playing a role in how the
GFS progresses the
upper level pattern, as a faster moving
trough may be the result
of the faster progression of the tropical low. Many questions
regarding the forecast may be answered once our tropical system`s
progression becomes more clear.
Lusk
&&
.
HYDROLOGY...
Concerning weather pattern in the extended periods with a
combination of the Gulf tropical system followed shortly after by a
cold
front. This one-two punch of activity is currently advertised
to produce 5 to 7 inches over northeast Georgia, and 3 to 5 along
and north of the
fall line. There is a considerable amount of
uncertainty with these amounts given the lopsided nature of the
tropical low, the large spread of model solutions regarding the
timing and position of highest
rainfall amounts, and the overall
disagreement between the operational deterministic models and the
ensemble means. At this time, WPC has taken a weighted blend of
the model solutions, leaning more heavily on the 00Z
ECMWF which
seems to have the best handle on the situation. Expect these
rainfall amounts to be more finely tuned in the next few days, but
should these totals remain this high, north and central Georgia
is in for a widespread
flash flooding event, particularly in the
Chattahoochee, Coosa/Etowah, and Tennessee River basins.
Please note that the Southeast
River Forecast Center continues to
use 48 hours of future
rainfall in their hydrologic models and
therefore the hydrographs for many area rivers may not fully
consider the entirety of the event.
