Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 211248

Area Forecast Discussion
National Weather Service Charleston SC
748 AM EST Tue Feb 21 2017

High pressure will linger across the region through the rest of this
week. A low pressure system will pass by just to the south Wednesday
into Thursday. A cold front will sweep across the region on
Saturday. Behind the front, a large Canadian air mass will build
east of the Appalachians on Sunday, likely remaining into mid next


As of 745 AM: Longwave over middle North America was associated
with a band of moisture from the tropics, across the
Mississippi River Valley, north to the arctic. IR satellite
indicated a wide and generally dense band of mid and high clouds
from the Mississippi east to the western Atlantic. Mostly
cloudy to cloudy conditions should remain across the forecast
area through the day. Given the sky conditions and ENE winds, I
will cool high temperatures a degree or two. Otherwise, patchy
fog along and ahead of a weak backdoor cold front will remain
over the Edisto and Savannah Basins until 14z.

As of 645 AM: A weak backdoor cold front slid across KCHS early
this morning. An area of restrictive fog and ceilings was
observed along and ahead of the boundary. 10Z run of the HRRR
indicated that the low values of sfc condensation pressure
deficits will slide south early this morning. I will refine
mention of patchy morning fog.

Previous discussion: Today: Deep high pressure will weaken over
the area as low pressure approaches from the west, although
should remain strong enough to hold off a majority of the rain
to the west. However, cannot completely rule out a few light
showers due to low-level isentropic ascent. As usual the NAM is
more aggressive with the precipitation and since it is often
overdone we prefer to maintain a dry forecast. The best chance
of any light showers/sprinkles looks to be near and south of
around a Allendale to Beaufort line. Cloud cover will generally
be on the increase but we still prefer the warmer MOS guidance
based on recent performance, which means mid to upper 70s for
most locales, except 60s at the coast due to cooler onshore
winds there.

Tonight: Low pressure will slide southeast across the eastern
Gulf of Mexico leaving the deepest moisture and forcing for
ascent south of our area. Still cannot rule out a few showers,
mainly across southeast GA. Also, abundant low-level moisture
could support some fog but cloud cover/wind should limit its
coverage/intensity. Given the uncertainty we didn`t mention fog
in the forecast. Lows remaining well above normal in the mid to
upper 50s most places.


Wednesday, short range guidance indicates that a closed low will
track SE across the Gulf of Mexico, centered over the FL Keys by 12Z
Thursday. Across the forecast area, mid level heights will feature a
broad weakness on Wednesday, likely aiding the formation of a weak
vort max, rippling over the Savannah River Valley Wed afternoon. I
will maintain PoPs between 20-30 percent across the forecast area.
Model soundings show shallow CAPE developing generally below a H7
inversion. The limited instability and forcing should yield only
showers, with thunder chances greater closer to the Gulf Stream.
Mild llvl thicknesses should support high temperatures from the
mid 70s across the SC Lowcountry with upper 70s across SE GA.

Large low pressure will organize over the central Great Plains late
Thursday. A warm front associated with the low will link with a old
cold front across the mid Appalachians. The large warm sector should
provide the region weak instability and moderate mid and low level
moisture. PoPs will remain within SCHC range across the CWA on
Thursday, mentioning scattered showers over the offshore waters.
Temperatures should start the day in the upper 50s to around 60,
rising to the upper 70s by mid afternoon.

On Friday, south-southeast winds will strengthen across the CWA as a
cold front pushes east across MS/AL/TN, especially during the
evening hours. GFS indicates that H85 temps will warm above 10C by
late Fri afternoon. Guidance indicates that high temperatures will
reach the upper 70s near the coast to the low 80s inland. The high
temperatures may challenge record values, see climate section below.
Building instability and moisture could support iso showers


The cold front is expected to remain east of the region by
daybreak Saturday. Min temps are forecast to remain very mild, with
near 60 common Sat AM. GFS shows the passage of the sfc cold front
during the daylight hours on Saturday. Afternoon temperatures should
warm well into the upper 70s. The combination warm temps and dwpts
in the 50s should yield CAPE values from 1000-1500 J/kg, especially
across the SC zones. Given the timing and instability, it appears
that the environment may yield a band of prefrontal convection. I
will mention both showers and thunderstorms with 20 PoPs.

Dry Canadian high pressure is expected to build quickly over the
region Saturday night and Sunday. Cooling thicknesses will yield
lower temperatures, but still remaining around 5 degrees above
normal. The coolest daytime temperatures will occur on Sunday, with
mid 60s across the Santee Basin to low 70s close to the Altamaha
River. Monday through Tuesday, both the GFS and ECMWF indicate that
the sfc high will shift over the western Atlantic, ridging west
below the Mason-Dixon line. To the west, another low pressure will
begin deepen across the southern Great Plains on Tuesday. This
pattern should provide a strengthening return flow during the early
week. High temperature are forecast to return to the mid to upper
70s by Tuesday.


A weak backdoor cold front slid across KCHS prior to the 12z
TAF. An area of restrictive fog and ceilings was observed along
and ahead of the boundary. 10Z run of the HRRR indicated that
the low values of sfc condensation pressure deficits will slide
south early this morning, but not reaching KSAV. However, given
widespread cloud cover across and upstream of the terminals, I
will indicated VFR ceiling through the TAF period. Winds should
remain steady from the ENE this morning, then turning from the
SE-E this afternoon and night. Based on CAMs, light reflectivity
will pass near KSAV during the predawn hours Wednesday, I will
highlight the showers with a PROB30 from 9-12z.

Extended Aviation Outlook: Patchy fog is possible each work day
morning.  Otherwise, no restrictive conditions are expected
through Friday. A cold front will sweep across the region on
Saturday. Dry Canadian air should dominate the region Sunday-


Today/Tonight: Mainly east flow will persist across the waters
as high pressure to the north gradually shifts off the coast.
Winds will be less than 15 kt with seas up to 3 ft within near
shore waters and 4 ft offshore waters.

Wednesday through Sunday: Northeast winds will be common across
the marine zones from Wed-Fri A cold front will sweep across
the region late Saturday, shifting winds from the Northwest.
Cold air advection will occur Saturday night into early Sunday,
likely resulting in gusty conditions. Conditions should remain
below Small Craft Advisory criteria. However, seas will likely
gradually build Friday and Saturday, especially beyond 20 NM.


Record Highs for Feb 24:
KCHS: 81 set in 2012/1996.
KCXM: 81 set in 1930.
KSAV: 86 set in 2012.

Record Highs for Feb 25:
KCHS: 80 set in 1949.
KCXM: 80 set in 1930.
KSAV: 82 set in 1985/1930.

Record High Minimums for Feb 24:
KCHS: 60 set in 1992.
KCXM: 61 set in 1901.
KSAV: 64 set in 1980.

Record High Minimums for Feb 25:
KCHS: 62 set in 1992.
KCXM: 59 set in 2011.
KSAV: 63 set in 1992.




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