Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS62 KCHS 302014

Area Forecast Discussion
National Weather Service Charleston SC
414 PM EDT Sun Apr 30 2017

Atlantic high pressure will give way to a cold front which will move 
through Monday night into early Tuesday. High pressure will 
then bring dry weather through Wednesday night before rain 
chances return late in the week as a stronger storm system 
affects the area. Drier high pressure should then return by the 
end of the weekend.


Subtropical ridging will hold firm for much of the night before
steadily breaking down prior early Monday. A cold front moving 
across the lower Mississippi Valley will trek east across the 
Deep South tonight and is on target to be very near the Alabama-
Georgia border by daybreak. While some pre-frontal warm air 
advection/isentropic assent could help generate a few showers 
across the far western areas during the early morning hours, 
most of Southeast South Carolina and Southeast Georgia should 
remain dry. Gridded pops will be held below mentionable 
thresholds. A strengthening low-level jet will keep the boundary
layer fairly well mixed tonight, which should result in yet 
another warm and humid night for the end of April. Lows will 
only drop into the lower-mid 70s; warmest at the coast. 


Monday: Deep layered ridging will slowly retreat east over the 
Atlantic as a large area of low pressure lifts northeast over the 
Midwest. A southward extending cold front associated with this low 
will make a run toward the Southeast United States, likely losing 
some speed/strength as it becomes further separated from the parent 
low. Ahead of the front, deep moisture advection within a southerly 
flow along with increasing isentropic ascent will help spawn some 
showers inland late morning into early afternoon hours. Sfc temps 
should peak a degree or two cooler than the previous day (low to mid 
80s) due to expanding cloud cover, but sfc heating should still be 
sufficient to produce SBCAPE near 1000 J/kg well inland. These 
values in combination with lifted index values around -6 C and mid-
lvl lapse rates near 6.5 C/km support afternoon thunderstorms away 
from the coast. However, the bulk of precip activity should come 
near or just ahead of the cold front as it progresses through the 
area early Monday night. Given the timing of the front, instability 
will be weaker, but 0-6 bulk shear between 35-40 kt could help a few 
thunderstorms become organized along/near fropa, resulting in strong 
and/or damaging straight-line winds. At this time, the threat for 
greater severe weather coverage is low. Most precip will end early 
Tuesday as the front shifts offshore. Overnight lows will range
in the mid/upper 50s inland to lower 60s near the immediate 

Winds/Lake Winds: The pressure gradient will tighten over the area 
ahead of an approaching cold front Monday, suggesting gusty wind 
potential (near 30 mph) when afternoon heating promotes mixing into 
35-40 mph low-lvl winds fields. A Lake Wind Advisory could 
eventually be needed late Monday morning into Monday evening for
gusts of 25 kt near the beaches of Lake Moultrie. 

Tuesday and Wednesday: High pressure will build over the Southeast 
behind the departing cold front offshore. Expect all areas to be dry 
and temps to be slightly cooler on Tuesday, followed by gradual 
warming on Wednesday ahead of the next system progressing over the 
South Central United States. In general, afternoon temps will peak 
in the lower 80s on Tuesday, then middle 80s on Wednesday. Tuesday 
night lows will range in the mid/upper 50s inland to lower 60s along 
the immediate coast. 


Mainly dry through Wednesday night before rain chances return with 
the approach of a stronger storm system toward the end of the week. 
Although there is a bit better agreement between the GFS and ECMWF 
than 24 hours ago, there are still some timing differences so 
forecast confidence is lower than normal. Conditions look at least 
marginally favorable though for some stronger storms Thursday. Rain 
chances should diminish by Friday night but we kept small rain 
chances into Saturday given the uncertainty regarding the . timing 
of the cold front and strength/timing of the upper trough. 
Temperatures should be near or above normal through at least 
Thursday night before likely dropping below normal Friday through 
the weekend.


VFR for the rest of the afternoon into this evening. Guidance
shows MVFR cigs developing after midnight at KSAV with the main
axis of MVFR cigs remaining west of KCHS. Risk for showers/tstms
will increase after 18z Monday at both terminals, so will not
include any mention at either terminal this cycle. Gusty winds
will impact the terminals for much of the period, possibly 
dropping a bit this evening before increasing again overnight 
as low-level jetting intensifies.

Extended Aviation Outlook: Ceiling and/or visibility restrictions 
are likely Monday evening into Monday night due to low clouds and 
showers/thunderstorms along/ahead of a passing cold front. 
Breezy/gusty conditions are also expected Monday afternoon ahead of 
the front. Another round of flight restrictions is possible with 
showers/thunderstorms Thursday/Thursday night, possibly lasting into 
Friday. Breezy/gusty conditions are also expected Monday, Thursday 
and Friday afternoons.


Tonight: Southerly winds will steadily increase tonight as pre-
frontal low-level jetting takes hold. Speeds are expected to
reach 15-20 kt by midnight and continue through the early
morning hours Monday. In Charleston Harbor, winds will remain 
15 kt, but will be closer to 15-20 kt near the Harbor entrance. 
Seas will also build, reaching 4-6 ft South Carolina nearshore 
waters, 4-5 ft Georgia nearshore waters and 5-7 ft Georgia 
offshore waters by sunrise Monday. Small Craft Advisories have 
been posted for the South Carolina waters and Georgia offshore 
waters for tonight and will remain in force.

Small Craft Advisory conditions will likely be ongoing for all 
waters Monday into Monday night when winds gust near 25-30 kt and 
seas build as high as 6-8 ft. Small Craft Advisories will likely be 
needed for all waters until fropa nears/occurs Monday night into 
early Tuesday. However, 6 ft seas could linger in offshore Georgia 
waters through late Tuesday morning. Otherwise, no significant 
issues are expected over the coastal waters until Thursday when 
another cold front approaches. Much uncertainty exists regarding the 
timing of the frontal passage, but conditions look poor either way 
through Friday.


Observed minimums this morning indicate all three climate sites
should break their respective record high minimums for today.

Record high minimums for 30 April:
KCHS: 69/2014 and previous years. 
KCXM: 73/1912.
KSAV: 72/1953 and previous years.


MARINE...Small Craft Advisory from 8 AM Monday to midnight EDT Monday 
     night for AMZ330-354.
     Small Craft Advisory from midnight tonight to 6 AM EDT Tuesday 
     for AMZ352.
     Small Craft Advisory from midnight tonight to noon EDT Tuesday 
     for AMZ350.
     Small Craft Advisory until noon EDT Tuesday for AMZ374.

National Weather Service Glossary of Abbreviations