Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 11/30/20
Area Forecast Discussion
National Weather Service Jacksonville FL
903 PM EST Sun Nov 29 2020
...ISOLATED SEVERE THUNDERSTORM THREAT OVERNIGHT TONIGHT...
Water vapor imagery shows a strong mid/upper[er level trough to
our west with good divergence at 250 mb headed our way. Mid to
high clouds are spreading downstream over our region at this
time. The warm front appears to have lifted north to near the
FL/GA state line, with the cold front located over AL from a 1007
mb sfc low, then south-southwest into the Gulf of Mexico. Sfc
based CAPE continues to be very limited so far in our area is
below 200 J/kg. JAX sounding at 00z showed a stable airmass but
fairly strong veering and a 0-6 km bulk shear of 54 kt.
Based on latest HRRR model guidance, timing of heavier showers
and potential storms, along and ahead of the cold front, looks to
be about 11pm- midnight over our western GA zones and then pushing
east about 25-30 mph through the night. This places the front and
precip along the east coast about 7AM-9AM Monday. As far as the
isolated severe t-storm threat, there is a narrow corridor of
instability right along and just ahead of the front. Here is
where a damaging wind gust or a brief spin-up of a tornado will
be possible, with the main threat area from inland southeast GA
southward into the Suwannee Valley area. Further east, instabilty
will be weaker but nonetheless, strong wind gusts in convection
cannot be ruled out and wind gusts upwards of 40-60 mph will be
possible. Clearing skies and gusty westerly winds expected by mid
morning on Monday as the cold front pushes further offshore. Given
the quick moving front, rainfall amounts will generally be less
two-thirds of an inch. Main updates this evening were to delay
precip a bit for this evening and lower t-storm probabilities for
the next 3-4 hours given the weak instability thus far.
.PREV DISCUSSION [650 PM EST]...
.NEAR TERM [Tonight through Monday]...
Currently, a warm front is situated roughly along the I-10 corridor.
As the front has lifted northward this afternoon, the cirrus shield
has slowly eroded allowing weak destabilization across NE Florida,
evident by a patchy cumulus field. These updrafts will battle with
dry air aloft and strong mid-level (3-6 km) shear, which is likely
to be more destructive than beneficial at this point. That said,
outside of an isolated shower, conditions should remain quiet south
of the front through the evening. North of the warm front,
overrunning showers and overcast conditions will continue for the
next several hours. The warm front will continue to lift across GA
through the evening hours.
A deepening surface low near the MS/AL coast will deepen and race
northeastward tonight as the associated upper low is absorbed by an
upstream trough. As it does, the lower tropospheric wind fields will
increase resulting is very strong deep shear and sufficient helicity
parameters across the region this evening. The cold front trailing
behind the surface low will press across the region late this
evening through pre-dawn hours Monday. A line of strong and isolated
severe thunderstorms will be possible for across the inland SE GA
zones and Suwannee River Valley areas late tonight as an axis of
instability develops amid strong wind shear ahead of the cold front;
risk for isolated tornadoes will be highest in these locations. The
line of showers and thunderstorms will continue eastward toward the
Atlantic coast but will start to fracture as instability begins to
decrease early Monday morning. Though the severe risk will start to
wane with weakening instability, an isolated damaging straight-line
wind threat will exist, especially with an localized bowing segments
on the line. Expect the line to clear out of SE GA shortly after
Sunrise Monday and NE FL by noon or earlier, leaving breezy west
winds and clearing skies in it`s wake.
Temps will remain in the low to mid 60s overnight ahead of the
approaching cold front/line of thunderstorms. For SE GA the warmest
temperatures on Monday will occur in the morning, as cold air
funnels in behind the front. By 4 PM Monday, temps will fall to the
low to mid 50s across inland SE GA and the Suwannee River Valley and
to the upper 50s and low 60s along the coast and north-central FL.
Temps will continue to fall Monday night, bottom out in the mid-30s
for across inland SE Georgia and Suwannee valley and to the upper
30s and low 40s along the coast and north-central Florida.
.SHORT TERM [Tuesday through Wednesday]...
Much colder with inland freeze potential as surface high pressure
builds eastward across the Gulf Coast states. Below normal highs
will only range in the 50s to upper 40s for parts of inland SE GA
Tue, with some moderation of temps into Wed as upper level WSW flow
develops ahead of the next frontal system.
The main weather impact this periods will be an inland freeze Tue
night for SE GA and the Suwannee River Valley of NE FL where lows in
the upper 20s are expected. For coastal SE GA generally west of the
I-95 corridor a light freeze is expected extending southward across
inland Nassau and Duval counties, western Clay-Putnam and Marion
counties. There could be some patchy frost, but at this time airmass
looks too dry to support significant frost formation. Another light
inland freeze is expected across parts of inland SE GA Wed night but
moisture and temps will increase from the east as onshore flow
develops as the surface high builds NNE of the local area. Wed night
may actually be the better night for some patchy frost as lows fall
into the mid 30s inland.
.LONG TERM [Wednesday Through Sunday]...
Temperatures further moderate back toward near climo values Thu as
high pressure shifts farther offshore of the mid-Atlantic coast in
advance of a stalling frontal system. This boundary will gradually
settle southward across the SE region Fri into Sat while beginning
to stall as stronger upper level forcing lifts NE. Elevated moisture
and forcing near the meandering front return rain chances to the
local area Fri. A stronger upper level low passes quickly across the
deep south on Saturday, bringing the potential of thunderstorm
activity. Ultimately depends on where the stalling front lingers in
terms of rainfall amounts and t`storm outlook and confidence in the
details remains too low at this time.
A much drier and cooler airmass returns Sunday under drier post-
frontal deep layer WNW flow with surface high pressure building
eastward from the TX Gulf Coast early next week.
[Through 00Z Tuesday]
A warm front continues to slowly push northward over northeast FL
this evening. Brief period of cig restrictions looks possible from
SGJ northward this evening but overall chances will diminish by
about 04Z-05Z as the front lifts northward. Later on tonight
cloud cover will increase and southerly winds increase as a pre-
frontal line of gusty showers and isolated tstorms approaches as a
cold front moves into the area. Showers and VCTS are indicated in
the terminals for now after 08z through about 12Z-14z. In addition,
some LLWS is noted in the terminals from JAX metro area northward
as low level jet increases to 40-45 kt at about 1500-2000 ft.
Breezy south- southwest winds will increase into the 10-13 knots
before sunrise ahead of the front. Showers and the front
will push offshore 13Z-15z with just lingering light showers and
low clouds through mid to late morning or so. Gusty post-frontal
westerly winds expected and developing VFR. Some guidance suggests
some wrap around clouds late Monday aftn but too low confidence
to include any cig restrictions.
Potential for elevated fire danger Tue due to breezy WNW winds
combined with low humidity in at least the lower 30s.
.PRELIMINARY POINT TEMPS/POPS...
AMG 55 61 32 49 27 / 80 30 0 0 0
SSI 61 69 37 52 34 / 60 60 0 0 0
JAX 62 69 36 53 32 / 60 60 0 0 0
SGJ 65 73 39 55 38 / 40 70 0 0 0
GNV 62 69 35 53 29 / 70 60 0 0 0
OCF 65 71 36 55 31 / 70 70 0 0 0
AM...Small Craft Advisory until 11 PM EST Tuesday for Waters from
Altamaha Sound GA to Fernandina Beach FL from 20 to 60 NM-
Waters from Fernandina Beach to St. Augustine FL from 20 to
60 NM-Waters from St. Augustine to Flagler Beach FL from 20
to 60 NM.
Small Craft Advisory from 3 AM Monday to 11 AM EST Tuesday for
Coastal waters from Altamaha Sound to Fernandina Beach FL
out 20 NM-Coastal waters from Fernandina Beach to St.
Augustine FL out 20 NM-Coastal waters from St. Augustine to
Flagler Beach FL out 20 NM.
Area Forecast Discussion
National Weather Service Nashville TN
738 PM CST Sun Nov 29 2020
FOR EVENING DISCUSSION.
A deepening surface low is currently moving northeastward across
the state of Alabama, with an inverted trough extending
northward from the low and bisecting Middle Tennessee. The 00Z
sounding from OHX shows a nearly saturated column extending from
near the surface to almost 500 mb. While there is no instability
to speak of, precipitable water is a robust 1.10" and a strong
southwesterly 120-knot jet is centered around 300 mb. The
aforementioned low will continue along its present track into the
Appalachians later tonight, and this will result in a tightening
pressure gradient on the back side of the low and also plenty of
wrap-around moisture to go along with the cold advection. Also
tomorrow, an unusually strong low height center will descend from
the upper midwest and keep our upper-level pattern highly
amplified. The latest HRRR shows most of tonight`s precipitation
exiting the mid state by 12Z, after which light echoes resulting
from the wrap-around moisture and closed upper low will begin to
fill in across Middle Tennessee. At any rate, the current forecast
appears to be in good shape. The hourly grids are holding up
reasonably well and no changes are needed for now.
00Z TAF DISCUSSION.
Rain will impact the terminals overnight with heavier activity
expected by 03z as surface low pressure lift NE closer to the
area. There should be a break between this activity and the wrap
around activity that will begin to impact CKV by 12z. Temperatures
will fall through the morning and afternoon and rain will
transition to a rain/snow mix before ending as all snow. CSV will
experience the heaviest snow after 21z which could reduce vis to
less than 1 sm. Northwest winds will increase overnight to 10-15
kts with gusts up to 25 kts at times.
Winter Weather Advisory from 3 PM Monday to 6 AM CST Tuesday for