Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 03/19/21
AREA FORECAST DISCUSSION
National Weather Service Morristown TN
949 PM EDT Thu Mar 18 2021
Currently the strongest convection has moved out of the eastern
Tennesee Valley and diminished in strength across the region.
Still expect lingering showers through the rest of tonight and
into early tomorrow as satellite imagery shows the low spinning
right over the Tennessee Valley. Radar melting layer algorithm and
00z soundings show the 0 degree Celsius layer is somewhere
between 3500 and 5000 feet currently. This lines up fairly well
this afternoons forecast and increases confidence in forecast
snowfall totals in the higher elevations expected tonight. It`s
likely some of the weak reflectivities on radar are already
generating some light snowfall across the southern Appalachian
Mountain peaks, and expect this to continue overnight. Will not
make major adjustments to the snowfall forecast and still looks
like the mountains will get between a dusting and up to 3 inches,
with the amounts increase as you go up in elevation.
00Z TAF DISCUSSION.
Showers and occasional thunderstorms still moving through NE
Tennessee and SW Virginia for the first few hours of the TAFs.
Expect the intensity and coverage of storms to decrease, but won`t
totally go away till early tomorrow morning. Gusty conditions
expected to continue overnight, but the speed of the gusts should
begin to diminish with the setting sun. Low pressure system will
move directly overhead spreading mid and low level clouds across
the entire region. With the gusty winds and the mid level clouds
fog should be kept at bay, but a stray shower could briefly cause
a lowering in visibilities overnight. Low ceilings will remain in
place for the next 24 hours, before conditions really begin to
improve just beyond this set of TAFs.
/ISSUED 315 PM EDT Thu Mar 18 2021/
SHORT TERM...(Tonight and Friday)
1. Scattered showers and thunderstorms will develop the rest of the
afternoon, and some could produce marginally severe hail and gusty
2. Scattered showers continue tonight, especially across NE TN, SW
VA, and the eastern mountains.
3. A changeover to snow will occur across the highest peaks of the
E TN mountains late this evening into Fri morning with 1-3 inches
above 5000 feet.
4. Drier air will filter in during the day Fri with much cooler
temps in the low/mid 50`s.
2 PM satellite, water vapor, and RAP analysis places the large
mid/upper closed low over NW TN and SW KY with the occluded cold
front crossing the Plateau. Breaks in the overcast have allowed
surface temps to warm into the mid 60`s/low 70`s beneath the
increasingly cold pool aloft as the mid/upper low moves E, and this
has resulted in surface based CAPES building to 500-1000 J/Kg per
latest SPC mesoanalysis. New convection has initiated in a broken
line from the N Plateau into NE TN ahead of the cold front, and we
have issued several SPS`s for pea to penny sized hail due to mid-
level lapse rates steepening to 7-8 C/Km, hail CAPE increasing to
300-500 J/Kg, and WBZ heights dropping to 5-6 thousand feet. There
will also be the potential for gusty winds with the strongest cells
this afternoon due to unidirectional deep layer (0-6 Km) shear of 60-
80 kts, significant mid-level drying (DCAPE values up to 800 J/Kg),
and melting hail. The biggest factor that is keeping the coverage
isolated is the dry air started to mix to the surface behind the
morning convection noted by Td`s dropping into the 50`s near the
Plateau early this afternoon, as well as, subtle subsidence behind
the morning convection. Overall, expect the line of convection to
remain broken as it propagates ENE ahead of the cold front with a
couple marginally severe storms possible. Behind this convection as
the cold front moves E, scattered showers are expected to fill in
from late afternoon through the evening as the mid/upper low shifts
overhead and further lowers freezing heights/steepens lapse rates.
Low topped convection with small hail remains possible through the
evening but will quickly wane after sunset. Winds have been gusty
this afternoon due to strong mixing and SW 850 mb flow of 50 kts. We
have seen 30+ kt gusts in parts of the central and S Valley. This
will decrease as temps cool through the evening.
The large mid/upper closed low will cross the S Appalachians early
tonight. 850 mb temps cool just below 0 C tonight, and with the
upper low and associated PVA overhead, as well as plenty of
moisture, scattered showers will continue, especially in NE TN and
SW VA, and this will mix with and change to snow over the highest
peaks of the E TN mountains. As the flow becomes more
northwesterly the second half of the night, some upslope
enhancement will also occur. HREF members suggest 1-3 inches of
snow over the highest peaks in Sevier county, which is just above
the WPC forecast. Used a blend of HREF and WPC giving 1-3 inches
of snow above 5000 ft tonight into Fri morning which seems very
reasonable. Issued an SPS for this light snow accumulation. Lows
tonight will cool into the upper 30`s/mid 40`s.
A few snow showers or flurries will linger Fri morning in the
mountains, but building high pressure from the NW and associated
drier air will dry all areas out during the afternoon. Highs will
only reach the low/mid 50`s.
LONG TERM...(Friday Night through Thursday)
At the beginning of this period our area will be under an upper
northwest flow while at the surface high pressure and a cooler air
mass will be continuing to build into the region. This high
pressure will continue to fill into the region over the weekend,
with Saturday and Sunday looking dry, with slightly below normal
early Spring Temperatures.
By Tuesday we get a surface return flow started and the next weather
system heads our way with showers increasing west to east Tuesday
afternoon and evening. The overall pattern stays pretty unsettled,
with another system coming out of the Gulf around Thursday,
spreading moisture and rain chances towards us. Keeping thunder out
of forecast for now. Temperatures Tuesday through Thursday should
be mild, as minimum temps are limited by higher dew points and
The Thursday system will be worth keeping up with--if it rides along
south of us toward the Atlantic, no big deal for us. However, if it
moves northward toward the Ohio Valley, it would bring a lot of
moisture into our area and would need to be watched for heavy rain
potential and/or severe threat.
.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN 45 56 40 62 38 / 30 10 10 0 0
Knoxville McGhee Tyson Airport, TN 45 54 35 60 35 / 50 10 10 0 0
Oak Ridge, TN 44 54 36 61 36 / 30 0 0 0 0
Tri Cities Airport, TN 42 53 30 60 31 / 80 10 0 0 0
Area Forecast Discussion
National Weather Service San Francisco Bay Area
459 PM PDT Thu Mar 18 2021
.SYNOPSIS...Scattered showers along a cold front pushes southward
through Thursday evening providing light to moderate rain to the
Bay Area and the Central Coast. Rain is expected to taper off
Friday morning with clearing expected Friday afternoon. A warming
and drying trend is expected over the weekend and into early next
.DISCUSSION...as of 2:12 PM PDT Thursday...Widespread rain
showers have returned to the Bay Area and the Central Coast along
the front of a cold front. The front still over the ocean is
slowly moving eastward. The bulk of the rain fall at this time has
fallen over the North Bay Mountains and Valleys. Observations
stations in the Valleys closer to the San Francisco Bay are a few
hundredths to a quarter of an inch, farther inland valleys a
quarter of an inch to a half an inch, with the Wester North Bay
Mountains along the Sonoma Coast between a half an inch to just
under an inch, with rain still falling. Rain has fallen through
the San Francisco Bay, into the East Bay and down the Santa Cruz
Rain will continue through Thursday morning and slowly move
southward toward the Central Coast, leaving lingering scattered
showers in the North Bay, while the bulk of the more constant rain
showers will move southward. Hi-res model HRRR and local WRF keeps
the rain rates below threshold at this time expected through the
evening. Lingering showers can be expected through Friday morning,
with clearing expected by Friday afternoon.
Patchy fog is forecast to develop in more sheltered valley
locations overnight with temperatures into the low 40s and
isolated upper 30s. Temperatures will once again be in the 50s and
low 60s on Friday as the upper level trough moves slowly
southward, with drier conditions developing. By late Saturday, a
ridge over the Eastern Pacific starts to develop bring warmer and
drier conditions leading into the early part of the work week next
week with temperatures approaching the 70s.
.AVIATION...as of 04:45 PM PDT Thursday...For the 00Z TAFs. Radar
shows showers across the majority of the forecast area late this
afternoon. This line of showers will slowly transition southward
through the next several hours. Expect IFR to MVFR cigs and visbys
with wet runways and light S-SW winds as the frontal boundary
driving this wet weather shifts southward. Moderate to briefly
heavy rain is possible as the front moves over the terminals and
with that, brief LIFR to IFR cigs/visbys are also possible.
Additionally, lighter surface winds and a 25-35kt low level jet
at roughly 1500-2000 ft will bring some limited LLWS concerns to
terminals through the evening. Expect ceilings to gradually fill
back in in the wake of the front overnight given ample low level
moisture and cooling temperatures. Patchy fog is possible through
the morning for inland terminals. VFR returns by mid to late
tomorrow morning with FEW to SCT clouds at mid levels.
Vicinity of KSFO...S-SW winds through the evening, minor LLWS
concerns with 25-35kt winds at 1500-2000 ft AGL and potentially
lighter winds near the sfc. Rain and LLWS concerns back off by
02-03Z timeframe this evening though some linger VCSH expected.
For tonight low level clouds redevelop to bring borderline MVFR IFR
cigs btwn roughly 11-16Z, with improving conds to VFR by 17-18Z.
Mid level clouds will linger through midday. Onshore winds
SFO Bridge Approach...Similar to KSFO.
Monterey Bay Terminals...LIFR to MVFR cigs through the majority of
the period, with cigs briefly improving to VFR by late Friday
morning through the early afternoon. A couple of slugs of moisture
will keep intermittent -RA to +RA over the next 10 or so hours,
with the first slug moving over now and the second slated to
arrive late this evening. VLIFR to LIFR conds posbl after rain
finally clears out into the predawn hours of Friday morning with
patchy fog or very low cigs such as OVC002-OVC003.
.MARINE...as of 04:46 PM PDT Thursday...Gusty southerly winds will
gradually diminish in wake of a frontal passage late this evening
and into tonight. These gusty winds will generate locally steep
fresh southerly swell, creating hazardous conditions for small
craft vessels through tonight. Otherwise, a light northwest swell
will be replaced by a larger northwest swell resulting in
hazardous conditions through Friday.
.Tngt...SCA...Mry Bay until 9 PM
SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
SCA...Pigeon Pt to Pt Pinos 0-10 nm from 3 AM
SCA...Pt Reyes to Pigeon Pt 0-10 nm
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
SCA...Pt Arena to Pt Reyes 0-10 nm from 9 PM
SCA...Mry Bay from 3 AM
PUBLIC FORECAST: DK
Visit us at www.weather.gov/sanfrancisco
Follow us on Facebook, Twitter, and YouTube at: