486 
fxus62 kchs 201727 
afdchs 


Area forecast discussion 
National Weather Service Charleston SC 
1227 PM EST Thursday Nov 20 2008 


Synopsis... 
weak high pressure will persist over the region today. A dry 
reinforcing cold front will move across the area early Friday...then 
high pressure will build in through Monday. Another cold front is 
expected to move across the area Monday night into Tuesday... 
followed by high pressure through Wednesday. 


&& 


Near term /until 6 PM this evening/... 
a cold front north of the region...will continue to drop southward 
toward the forecast area. Model relative humidity fields continue 
to show little advection advection of moisture into the 
region...so anticipate sunny skies to continue across the area. 
Low level thickness values suggest based on the kchs 12z sounding 
suggest highs around 60 to the lower 60s. Will tweak temperatures 
slightly higher than these values given low level thicknesses 
increasing in advance of the approaching cold front. Daytime maximum 
temperatures should top out in the lower 60s north to middle 60s 
south. 


&& 


Short term /6 PM this evening through 6 PM Friday/... 
a strong shortwave will dive southeast through the Ohio Valley 
into the middle-Atlantic on Friday...bringing a backdoor front and 
another cold high pressure system into the area. Before the 
front...geostrophic winds will be less than 10 knots with much of the 
area likely decoupling for several hours of good radiational 
cooling. Inland areas may well freeze due to a relative lack of 
middle-clouds ahead of the front. Winds along the coast will pick up 
late and moderate overnight lows. 


A cool breezy day is in store for Friday as 850 mb temperatures drop to 
between 0c far south to -8c north by 18z. Our current forecast of 
highs in the lower to middle 50s may be a tad too warm if the cold 
advection kicks in earlier than currently forecast. However the 
trend on the NAM for the last few runs has been slightly slower 
with the best cold air advection during the afternoon hours after a morning with 
full sun and downslope. 


&& 


Long term /Friday night through Wednesday/... 
an expansive area of high pressure at the surface centered over the 
Tennessee Valley will build over the region in wake of the departing 
cold front Friday night...with strong cold air advection expected to 
be ongoing. It does appear that the gradient should weaken enough 
during Friday night to allow winds to decouple...at least across 
inland areas. Excellent radiational cooling in locations where winds 
manage to decouple will allow temperatures to plummet...with lows 
possibly in the lower to middle 20s across many areas away from the 
coast. 


An unseasonably cool and very dry airmass will remain in place 
through the weekend as the high pressure shifts eastward toward the 
middle-Atlantic and Carolina coast. Despite full sunshine...thickness 
values suggest that MOS guidance is too warm with respect to high 
temperatures and have factored that into the forecast. 


The high will shift offshore on Monday while a cold front approaches 
from the northwest...allowing a southerly flow to develop across the 
region. Warm air advection will cause temperatures to moderate 
some...but they should still remain just below normal for this time 
of year. Moisture from the Gulf of Mexico will be tapped long enough 
prior to the arrival of the cold front to moisten the column 
sufficiently to warrant continued mention of precipitation in the 
forecast Monday night into Tuesday. At this time however...will 
limit probabilities to a slight chance across the area. Another cool 
and dry high pressure system will build in following the passage of 
the cold front late Tuesday into Wednesday. 


&& 


Aviation /17z Thursday through Monday/... 
VFR conditions will persist at both terminals through 18z Friday. 
Winds will decouple under clear skies tonight leading to light and 
variable winds at both terminals. Gusty northwest winds between 
14 and 22 knots are expected at both terminals by 15z tomorrow as 
strong cold air advection enters the southeast and allows higher 
speed winds to reach the surface. 


Extended aviation outlook...VFR conditions will prevail. 


&& 


Marine... 
winds and seas are expected to continue to diminish this afternoon 
over the coastal waters as low level wind fields decrease. Some 
nocturnal surging expected this evening and overnight as weak cold 
advection continues on a west-northwest 925 mb flow. However the real surge 
will occur behind a backdoor cold front late tonight with another 
powerful blast of cold air moving in behind. Currently show winds 
increasing into Small Craft Advisory criteria over the offshore Georgia 
waters late tonight and continuing through Friday. The remainder of 
the waters may begin gusting to 25 knots sometime on Friday...although the 
best surge will be later. Have initiated Small Craft Advisory for 
only the offshore Georgia waters at this time. Later shifts may need to 
expand this as time GOES on. 


Strong cold air advection will be ongoing across the coastal 
waters Friday night in wake of the departing cold front. Small 
Craft Advisory conditions will remain present over the outer 
Georgia waters...and winds and/or seas could approach Small Craft 
Advisory levels across the near shore waters during this time as 
well. Winds should diminish significantly on Saturday as cold air 
advection wanes...but a north/northeast flow will keep seas 
elevated...especially over the outer Georgia waters. Conditions 
will improve further Sunday into Monday as high pressure passes 
offshore north of the area. 


&& 


Chs watches/warnings/advisories... 
Georgia...none. 
SC...none. 
Marine...Small Craft Advisory from 4 am Friday to midnight EST Saturday 
night for amz374. 


&& 


$$