Scientific Forecaster Discussion
fxus62 kchs 231047
Area forecast discussion
National Weather Service Charleston SC
647 am EDT Tue may 23 2017
..risk for flash flooding and severe thunderstorms through
unsettled weather will persist across the southeast through
Wednesday. A cold front will push offshore Wednesday night into
Thursday followed by high pressure prevailing into the weekend.
Near term /until 6 PM this evening/...
a large area of moderate to locally heavy rainfall is moving
steadily northeast across interior southeast Georgia. This
activity will move into southern South Carolina over the next
hour and clear the Santee river by mid-morning. Increased pops
to near 100% along/head of its projected path. Also increased
quantitative precipitation forecast by 0.5 to 1 inch. The rest of the forecast is unchanged.
An impressive moisture plume originating out of the Gulf of
Mexico will remain anchored across the southeast U.S. Today as
the region remains heavily influenced by the increasing
cyclonic flow ahead of a powerful upper low meandering over the
northern Mississippi Valley. Early morning sounder data and
23/00z raobs indicate precipitable waters remain near 2 inches within the
plume which is ~135% of normal for late may. A number of
impulses embedded within the subtropical jet will traverse
southeast South Carolina and southeast Georgia today bringing a
risk for numerous to widespread showers/tstms. Pops 90-100% look
reasonable as most areas will likely see some degree of
measurable rainfall, although it will not rain consistently
through the day.
There are signals in the various guidance that a squall line
type feature could develop across east-central Georgia by early
afternoon, move slowly east and potentially become anchored
across parts of the forecast area as it becomes aligned with the
mean steering flow mid-late afternoon. Should this line
develop, it would bring not only heavy rainfall and a potential
for flash flooding where as much as 3-7 inches of rain has
already fallen in some areas, but a risk for damaging winds and
possible a few tornadoes per output from modified point
soundings. Surface based instability will be somewhat curtailed
by an extensive cloud canopy, but will likely be augmented to
some degree by a strengthening h2-300 mb difluence channel and
increased DPVA ahead of a more significant shortwave which is
progged to pass across the region during peak heating. There
will likely be pockets of higher instability lurking about due
to thinning of the cloud canopy later this morning per latest
rap/CMC total sky cover progs, but pinning down exactly where
these pockets will setup is proving extremely difficult. This
would only serve to locally enhance the severe risk. The risk
for severe weather looks to be the greatest from roughly 2-9 PM.
Short term /6 PM this evening through Friday/...
tonight: convection will gradually wind down and push offshore
as subsidence overspreads the area in the wake of a passing
shortwave. Likely to categorical pops will be held for the
evening hours with pops slowly decreasing with time, highest
along the coast. Lows will range from the upper 60s inland to
the lower 70s at the coast.
Wednesday: an amplifying longwave trough will be over the MS valley.
Naefs indicates the 500 mb heights over the lower MS valley become -
4 to -5 Standard deviations for this time of year, which is very
impressive. A shortwave will round the base of the trough and head
in our direction towards the afternoon. At the surface, low pressure
is forecasted to strengthen across the Ohio River valley. A cold front
attached to this low will stretch into the deep south, pushed
eastward by the aforementioned shortwave. A jet streak strengthening
to ~130 kt will be on the eastern side of the trough and moving over
the southeast in the late afternoon. The 850 mb jet will increase to
30-45 kt in the afternoon while a deep layer of moisture will
persist with pwats approaching 1.8" near record values for the day.
This regime will support numerous/widespread showers. The convective
threat doesn't appear to start off too high due to morning
precipitation and cloud cover. However, some breaks in the clouds
ahead of pre-frontal convection to our west will destabilize the
atmosphere. Deep synoptic forcing will combine with instability in
the form of blcapes approaching 1,500 j/kg and steepening lapse
rates along with 0-6 km bulk shear increasing to 40-50 kt to support
severe weather, especially in the late afternoon/evening. Storm Prediction Center now
has our area under a slight risk. The main threat should be damaging
winds, but given the cool mid-levels, hail is also a concern. A
tornado is not out of the question given the low-level hodographs,
especially if the low-level winds back a little more than expected
in discrete cells. The main line of storms is expected to move cross
late in the afternoon through the evening hours. The cold front will
move through our area overnight, allowing precipitation to wind down
from west to east as the atmosphere stabilizes and drier air builds
into the area. Though, remnant showers could persist along the coast
for much of the night.
Thursday: the trough and surface low pressure will move across the
Great Lakes region stretching into the mid-Atlantic states.
Meanwhile, high pressure will build over the Gulf of Mexico and the
central U.S. The surface pressure gradient is forecast to range from
2-3 mb across the forecast area, meaning west-southwest winds should
remain gusty across forecast area. Models hint at some remnant
showers across the Charleston tri-County, so we have slight
chance pops in the forecast. However, minimal precipitation is
expected, especially when compared to previous days. Despite
mostly sunny skies across the area, thickness values only
support high temperatures in the lower 80s, which is a few
degrees below normal for this time of year.
Friday: the trough will be moving into New England while a ridge and
surface high pressure will be building towards the southeast. Good
subsidence will lead to sunny skies with highs rising well into the
Long term /Friday night through Monday/...
a ridge over the East Coast combined with surface high pressure will
provide dry weather in the long term along with a gradual warming
trend each day. Models hint at maybe some rain on Monday.
Aviation /12z Tuesday through Saturday/...
large area of showers/tstms moving across central Georgia will brush
ksav right around 12z and impact kchs roughly 13-15z. Local IFR
conditions are likely at kchs as this activity pushes through.
More substantial tstm activity will impact the terminals this
afternoon as several lines push across the region. Impact window
looks to be roughly 19-02z at kchs and 18-01z at ksav. Will
highlight prevailing MVFR conditions, but local IFR conditions
will be possible in the heavier convective elements. Gusty winds
could also accompany the stronger tstms. Convection should
gradually push offshore by early-mid evening.
Extended aviation outlook: flight restrictions are expected
Wednesday due to a cold front bringing showers and thunderstorms
to the region. Conditions will improve Wednesday night with VFR
prevailing into the weekend.
today: winds and seas will increase today as low-level jetting
intensifies ahead of a slowly moving cold front. Winds look to
increase to 20-25 kt with seas 4-6 ft by this afternoon across
the Georgia offshore and Charleston County waters so a Small
Craft Advisory will be issued for those legs beginning at noon.
The remaining nearshore legs, including Charleston Harbor, look
to remain just below advisory thresholds during the day. Strong
to potentially severe convection could approach or impact the
marine area late in the afternoon brining a risk for winds >35
Tonight: strong to locally severe convection should push east
with time, but strong low-level jetting will continue through
the night brining 20-25 kt winds and seas 4-7 ft. Small craft
advisories will be expanded to including the remaining nearshore
legs beginning at 6 PM. Will not include the Charleston Harbor
just yet as winds look to remain just sky of advisory
Wednesday through saturday: SW winds will increase Wednesday as a
cold front approaches from the west and then crosses through the
region Wednesday night. A strong surface pressure gradient behind
the departing front will lead to elevated winds on Thursday. As a
result small craft advisories are will remain in effect through this
time period for gusty winds and steep seas, especially beyond 20 nm.
Conditions will improve Friday into Saturday as high pressure builds
into the area.
kclx sta and surface observations show some areas across
southeast Georgia and extending into portions of the lower
South Carolina coast have seen as much as 3-7 inches of rain
over the past 2 days. Ksav observed 6.61 inches of rain just
yesterday and is an indication of the heavy rainfall potential
that could be realized through tonight as stronger upper forcing
and numerous showers/tstms impact the region. Updated gridded
1-hr flash flood guidance values are generally around 3 inches,
which could easily be overcome based on the degree of available
atmosphere moisture and possible convective anchoring due to
mesoscale boundary interactions. A Flash Flood Watch has been
posted for all but McIntosh and coastal portions of Liberty
County through midnight tonight. Although flash flooding will
likely not be realized across the entire watch area, the
potential is there for several pockets of flash flooding per
latest high resolution guidance. Expect general rainfall
amounts of 2-3 inches with localized amount potentially
exceeding 6 inches. Depending on how the convective pattern
evolves today, the watch may be extended into the day on
evening high tide levels will trend higher each day as we approach
the new moon perigee this Friday. The mean lower low water (mllw)
levels are forecast to remain below flood stage. However, it is
possible that thunderstorms could pass over urban coastal areas
during the evening high tide cycle. Rainfall rates could exceed
0.75/hr during any thunderstorm. The combination of elevated MLLW
water levels and heavy downpours could result in shallow freshwater
flooding during this evenings and Wednesday evenings high tide
rainfall records for 23 may:
kchs: 4.86 inches set in 1976.
Kcxm: 5.40 inches set in 1976.
Ksav: 2.41 inches set in 1976.
Rainfall records for 24 may:
kchs: 1.78 inches set in 1979.
Kcxm: 2.23 inches set in 1895.
Ksav: 2.29 inches set in 1938.
Georgia...Flash Flood Watch through this evening for gaz087-088-099>101-
SC...Flash Flood Watch through this evening for scz040-042>045-
Marine...Small Craft Advisory from 6 PM this evening to 8 PM EDT
Thursday for amz352.
Small Craft Advisory from noon today to midnight EDT Thursday
night for amz350-374.
Small Craft Advisory from 6 PM this evening to 6 am EDT