Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS62 KCHS 230009
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
809 PM EDT Mon May 22 2017

.SYNOPSIS...
A series of disturbances will track across the Southeast through 
midweek. A cold front will push offshore Wednesday night into 
Thursday. High pressure will then prevail this weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Numerous showers and tstms will continue through this evening as
shortwave energy moves through and a plethora of mesoscale
outflow boundaries continue to interact. PWs are hovering near
1.9" this evening, which combined with residual SB CAPE of
1500-2000 J/kg are allowing convection to continue beyond
sunset. There may be a lull across inland areas late this
evening and early Tue morning, but another round of showers and
tstms are expected later tonight as another strong shortwave
pushes in from the SW. Localized heavy rainfall and some minor
flooding is the main concern.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
On Tuesday, a cold front is expected to run across the Piedmont
of the Carolinas and GA. Short range guidance indicates that a 
front wave will develop along the cold front across GA or SC. 
The front wave is forecast to track NE along the front during 
the daylight hours on Tuesday. Model forecast soundings suggest 
that LFC levels will remain below 2 kft with PW values around 
1.9 inches. In addition, a wide field of 1500-2000 J/kg CAPE 
will develop along and east of the front/wave. Convection should
easily form near the frontal wave, supporting categorical PoPs 
for showers and thunderstorms. Shear between 0-6 km will likely 
range around 40 kts, yielding loosely organized clusters or 
bands of thunderstorms. It is possible to see a few damaging 
wind events with the stronger storms. Perhaps more likely, 
rainfall rates may become quite heavy during the passage of 
thunderstorm clusters, see tide section below. High temperatures
should verify within a degree or two of 80 degrees on Tuesday. 
Based on the timing of the frontal wave, coverage of deep 
convection should shift NE during the evening hours. It appears 
that a noteworthy decrease in coverage of showers and 
thunderstorms will occur across forecast area Tuesday night, 
highlight with PoPs decreasing to CHC. Lows Tuesday night are 
forecast to range in the upper 60s to low 70s.

Wednesday, deep sfc low pressure should organize across the Ohio
River Valley. Short range guidance indicates that a cold front 
will approach from the west during the daylight hours. At the 
mid level, H5 trough will swing across the deep south during the
afternoon, GFS1deg shows a wide field of H5 Q-vector convergence
will spread west to east across the CWA. The environment ahead 
of the cold front is forecast to see CAPE values pool to 1500 
J/kg with SWEAT peaking near 400. I would expect that a band of 
deep convection will form along and ahead of the cold front. 
Given the synoptic scale forcing, instability, and shear, the 
severe risk could peak Wed afternoon and evening. High temps 
should range very close to values expected for Tuesday. Dry air 
will spread east across the region Wednesday night.

Thursday, large low pressure will circulate across the Mid West
and Great Lakes regions as high pressure builds over the Gulf 
of Mexico. The sfc pressure gradient is forecast to range from 
2-3 mbs across the forecast area. WSW winds should remain gusty 
across forecast area, especially during the morning.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
A ridge building across the East Coast combined with surface high 
pressure will provide dry weather in the long term along with a 
gradual warming trend each day.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
Another round of convection expected to lift through KSAV
through 03Z with mainly light to moderate stratiform rain
affecting KCHS during this time. Some upstream MVFR ceilings may
affect KCHS briefly this evening, otherwise the strengthening
southerly low-level jet should maintain mostly VFR conditions.
Cannot rule out a shower at any time overnight but chances too
low to warrant mention. We put VCSH Tue afternoon at both sites
when another round of more widespread convection is expected to
fire.

Extended Aviation Outlook: An unsettled weather pattern will bring 
periods of flight restrictions through Wednesday night.

&&

.MARINE...
Low-level jet strengthens later tonight as shortwave energy
approaches the area. Sustained winds will increase to 15-20 kt
later tonight with some gusts approaching 25 kt over outer
portions.

Tuesday through Wednesday night: Weak low pressure will track NE 
along a sfc front across during the daylight hours Tuesday. SW winds 
across the marine zones will increase through day. Gusts across 
AMZ350 and 374 are forecast to exceed 25 kts by the afternoon. Wave 
heights are forecast to build to 3-4 feet within 20 nm and around 5 
feet across AMZ374. Small Craft Advisories may be needed for 
Tuesday. In addition, rounds of thunderstorms are forecast across 
the marine zones on Tuesday. Winds may decrease slight Tuesday 
night, however, wave heights are forecast to gradually build through 
the night.

Wednesday through Wednesday night, SW winds will increase as a cold 
front approaches from the west. SCA conditions are expected. Marine 
zones should see common gusts to 25 kts within 20 nm and around 30 
kts beyond 20 nm. Wave heights are forecast to peak Wednesday night, 
with 6 feet possible within 20 nm and up to 9 feet across eastern 
AMZ374. Then, winds/seas should relax significantly by Friday.

&&

.TIDES/COASTAL FLOODING...
Evening high tide levels will trend higher each day 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 coast 
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 
deep street and sidewalk water.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...None.

&&

$$
NEAR TERM...JRL

National Weather Service Glossary of Abbreviations