Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
715 PM EDT Tuesday Sep 30 2014
high pressure will build into the region from the north through
Thursday...before a strong cold front moves through late Friday
and Friday night. High pressure will then build over the region
through early next week.
Near term /through Wednesday/...
other than some lingering cumuliform clouds over the far SW corner
of the County warning forecast area and near the Santee river this evening...skies will
be or become clear/mostly clear area wide tonight. Winds will
decouple quickly and this sets US up for almost optimal
radiational cooling. This will be tempered a tad by lingering
surface moisture from recent rains...but with a fast drop to temperatures
this evening it/ll lead to overnight lows in the upper 50s and
lower 60s just about everywhere. The exceptions will be near and
along the coast where middle and upper 60s will be the rule.
Radiation fog seems a good bet for after midnight under this
regime. Most locales will achieve their cross-over temperatures and fog
stability indices are down in the teens or lower 20s. We have
tweaked the coverage of the fog a bit from an earlier
forecast...showing areas of fog mainly over the inland counties.
We have trimmed the coverage of patchy fog to no longer inland the
barrier islands...but still across our coastal counties and mainly
near and west of the intra-coastal.
Short term /Wednesday night through Friday/...
Wednesday and Thursday...surface high pressure will build from
the north during the middle week period. At the same time...upper
level ridging will amplify along the East Coast downstream of a
potent shortwave trough. Precipitable water values remain below
one inch...indicative of the dry air mass in place. Dry air and
subsidence under the deep layer ridge will confine any isolated
shower potential south of the forecast area...closer to deeper
moisture and some convergence associated with return flow along
the periphery of the surface high. Inland temperatures will
rebound into the middle to upper 80s under mostly clear to partly
cloudy skies each afternoon...several degrees above seasonal
normals. As wind directions steadily veer more solid onshore middle
week...expect a decent temperature gradient between the coastal
counties and inland locations...as well as milder nighttime
minimums in the low to middle 60s.
Thursday night into Friday...a highly amplified upper level trough
will shift through the Mississippi Valley Thursday night...then
become negatively tilted while swinging toward the East Coast on
Friday. The upper ridge and surface high over the forecast area
will be pushed into the Atlantic as a result...allowing the
predominantly east flow to continue veering to the south late
week. Numerical model solutions continue to slow the progress of
the strong cold front ahead of the upper level system...possibly
delaying its passage through the forecast area until Friday
evening. Initial isolated precipitation could occur along the far
southern edge of the forecast area as early as Thursday
night...mainly due to the aforementioned return flow and perhaps a
subtle warm front lifting through the region. Shower and
thunderstorm chances will then increase from west to east through
Friday afternoon...with best chances later into the day toward the
evening time frame. Considering the trend in delaying the frontal
passage until late day or after sunset...the potential for the
bulk of the convection to occur during the higher instability of
the afternoon is diminishing. Although bulk shear values will
increase to 30 to 40 knots...there is some uncertainty as to whether
instability will be sufficient to support strong to severe
thunderstorms. Prefer to keep mention of any organized strong to
severe weather out of the forecast and hazardous weather outlook
due to these timing issues.
Long term /Friday night through Tuesday/...
medium range guidance is in very good agreement with the overall
synoptic pattern...leading to higher than normal confidence in the
forecast for the long term period. A deep upper level trough will
lift out to the northeast over the weekend and get replaced by a
more broad upper level trough which will encompass much of the
eastern United States through early next week. Meanwhile at the
surface...a strong cold front will move across the area Friday
night...with high pressure then building over the southeast states
into early next week.
As for expected weather...scattered showers and thunderstorms will
accompany the passage of the cold front Friday night...with
rain-free conditions and significantly cooler temperatures then
expected through early next week.
Aviation /00z Wednesday through Sunday/...
VFR. Shallow ground fog possible at both airfields late tonight
through middle morning on Wednesday...with flight restrictions
Extended aviation outlook...VFR conditions are expected to prevail
through much of Friday outside of any late night/early morning
patchy fog and/or stratus. Flight restrictions will be possible
Friday night into early Saturday within scattered showers and
thunderstorms associated with a passing cold front.
tonight...a 1007 mb low centered out near 31n and 75w this evening
will pull east and high pressure builds inland. The pressure
gradient will slacken some and land breeze circulations will back
winds around to the north at generally under 10 or 12 knots. Seas
will start the evening as high as or 4 feet...then drop about a foot
Wednesday through Sunday...winds/seas will remain well below Small
Craft Advisory levels as high pressure builds from the north through
middle week. In general...onshore winds will remain less than 15 knots
with seas no higher than 1 to 3 feet. Southerly flow will then
strengthen on Friday ahead of a cold front that pushes offshore
Friday night...however conditions still look to remain below
advisory criteria. Offshore flow will then prevail on
Saturday...with a slight northeast surge possible Sunday behind a
weak reinforcing cold front.
repairs to the downtown Charleston weather sensor at Waterfront
Park /kcxm/ are complete. Hourly observations are once again
available. Daily climate products will resume Wednesday afternoon.