Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Charleston SC
744 am EDT sun Apr 19 2015

Synopsis...
a warm front will lift into the area today...accompanied by an upper
level disturbance. A cold front will approach the area Monday...
moving offshore Monday night. High pressure will build in Tuesday
through Thursday. A weak cold front will drop through the area
Thursday night...then a storm system will affect the region next
weekend.

&&

Near term /through tonight/...
this morning...widespread clouds and areas rains will shift north
of I-16 and spread through southeast South Carolina as copious Atlantic and
Gulf moisture overrun the sloping warm frontal surface. A few
thunderstorms embedded within some heavier rains to the south of I-16.
There is some local fog as ceilings were close to the ground in
a few spots across southeast South Carolina.

As the boundary lifts north...there may be a break in precipitation
coverage for a wide swath of the forecast area late morning into
early afternoon. Then...on the eastern periphery of an upstream
longwave trough...a shortwave trough and and associated band of
dynamic forcing for ascent/band of enhanced warm conveyor belt
moisture will eject east/northeast across the region...within an
environment featuring some degree of destabilization and 0-6 km bulk
shear 35-45 knots.

The main uncertainties in regard to severe weather today revolve
around surface-based and deep layered instability given proximity
soundings that are literally saturated top to bottom with precipitable waters
nearing 2 inches...this is a normally a significant limiting
factor. The middle level environment between 700-500 mb feature weak
lapse rates and mostly positive showalter indices. The strongly
backing upper flow and late day height falls and increasing deep
layered wind fields all enter the picture during the favorable
diurnal time of the day. Latest high resolution models indicate
deep convection in a linear fashion will break out across southeast
Alabama...SW Georgia and the Florida Panhandle by middle morning then
accelerate eastward across our our region late in the afternoon.
Latest multi- parameter sref products indicate 50-70 percent
conditional probabilities for MUCAPES greater than 1000
j/kg...deep layered shear more than 30 knots and convective quantitative precipitation forecast. The
locations that are most concerning for severe weather potential
appear to be along and west of I-95 where sref supercell composites
show a likelihood for values greater than 3.

Dependent on the timing of the convection and the amount of
diurnal instability it can muster ahead of it...we agree with Storm Prediction Center
that there is a slight chance for severe weather this afternoon
across the forecast area...especially inland areas where high
resolution models painted more robust forecast reflectivities in
regions with the best shear. The NSSL WRF model suggests a very
impressive synoptic scale boundary lifting north through the area
late this afternoon which could enhance the risk for isolated
tornadoes given deep layered shear. Otherwise...large hail seems
unlikely with the deep moisture abound. Complex multi-cell convection
should be the responsible culprit for a risk of locally damaging
winds...again mostly along and west of I-95.

The shortwave will push off the coast tonight...and elevated probability of precipitation
during the evening hours should decrease below 15 percent as
somewhat drier air and temporary subsidence develop overnight into
early Monday. Another in a series of very mild overnights with
temperatures only falling to the middle 60s once again.

&&

Short term /Monday through Wednesday/...
the longwave trough will persist to our west on Monday while a
series of potent shortwaves lift northeast through the local
forecast area. A 145 knots southern stream jet streak is prognosticated to
slide along the central Gulf Coast region during the day...spreading
a region of enhanced upper level divergence into SC/Georgia in the afternoon.
Model time sections show considerable drying above 850 mb early in
the period...with precipitable waters dropping below 1.25" by Monday afternoon.
The main cold front will slowly move east during the day...crossing
the region at some point Monday evening. Given the dry air aloft...
considerable breaks in the clouds and compressional heating ahead
of the front should help push temperatures into the middle/upper 80s.

The big question on Monday is how much convective coverage develops.
The deep dry layer above the surface may limit the initial
convective development. However...models are hinting at an upstream
convective cluster moving into western areas later in the afternoon.
Thermodynamics and kinematics are quite impressive on Monday. 40-50
knots of 0-6 km bulk shear...convective available potential energy of 1500-2000 j/kg and downdraft
convective available potential energy 1000 to 1500 j/kg. A fairly robust sea breeze is expected to
develop in the afternoon given such warm temperatures inland and prevailing
southerly flow. This may come into play late in the day if upstream
convection moves in from the west.

Given the somewhat drier trend in the latest models...we minimized
probability of precipitation Monday morning with values increasing to 30-40% in the
afternoon...especially across interior SC/GA. As the activity ahead
of the front moves through our area it should be gradually
weakening. Then some additional activity...likely weaker...could
accompany the actual front later in the evening or overnight. We
plan to maintain severe weather potential in the hazardous weather
outlook.

High pressure will build in behind the front with dry weather and
seasonable temperatures Tuesday and Wednesday. Highs will climb to
near 80 both days with overnight lows in the middle 50s.

&&

Long term /Wednesday night through Saturday/...
a zonal pattern will exist much of the period. A weak back door cold
front drops through the area Thursday night. The front will stall to
the south before lifting back north as a warm front Friday night and
Saturday. A pronounced shortwave embedded in the quasi-zonal flow
will move through Saturday night accompanied by a cold front.
Precipitation possible starting Friday night and continuing into
Saturday night. Temperatures will be moderated by increasing cloud cover
and weak cold advection.

&&

Aviation /12z Sunday through Thursday/...
deep moisture across the taf corridor today. Layered clouds this
morning difficult to pin down as recently low stratus decks developed
in the warm frontal zone developing across the region. Models
indicate that IFR/LIFR ceilings and IFR visibilities should be confined to
the morning hours with MVFR to VFR ceilings developing in a bit of a
drier slot this afternoon ahead of convection that should sweep
through the terminals in the 19z to 00z time frame. Thunderstorms are
possible with heavy downpours and perhaps gusty and shifting
winds. Mainly VFR tonight but MVFR/IFR ceilings are possible late.

Extended aviation outlook...scattered showers/thunderstorms expected at both
terminals Monday afternoon and evening with brief MVFR ceiling/visibility
restrictions possible. Otherwise VFR conditions expected.

&&

Marine...
a warm front will lift through the waters today...followed by
intensifying south/southwest flow. Light and variable winds early
this morning will become southeast prior to the warm front which will
shift flow SW to S tonight. While speeds increase tonight and seas
slowly build...the average speed could reach only 15 knots by
daybreak over near shore waters with seas 3 to 4 feet.

The potential exists for thunderstorms with strong winds over the
waters this afternoon evening. Very heavy downpours reducing
local visibilities below 1 nm are also possible with the strongest
storms moving offshore late in the day and early this evening.

A prevailing south-southwest flow will exist Monday morning as a cold front
approaches from the west. The gradient will strengthen during the
afternoon...especially along the coast where a strong sea breeze
develops due to middle/upper 80s temperatures over land. The latest GFS and
NAM MOS at Folly Beach SC show surface winds exceeding 20 knots late Monday
afternoon when the sea breeze is strongest. This seems fairly
reasonable given the pattern and the warmer inland temperatures expected. A
Small Craft Advisory may eventually be required for Charleston
Harbor Monday afternoon for winds/gusts in the 20-30 knots range.

A brief period of Small Craft Advisory conditions may develop over
offshore waters Monday night as the cold front moves through and
winds gust to 25 knots with some 6 feet seas also possible.

Tuesday through Friday look fairly quiet over the waters as high
pressure prevails.

&&

Tides/coastal flooding...
there is a chance that the high tide could reach or slightly
exceed 7.0 feet MLLW on Charleston Harbor this evening.

&&

Chs watches/warnings/advisories...
Georgia...none.
SC...none.
Marine...none.

&&

$$
Synopsis...
near term...
short term...jrl
long term...jrl
aviation...
marine...jrl
tides/coastal flooding...

National Weather Service Glossary of Abbreviations