Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Charleston SC
350 am EDT Friday Jul 29 2016
Atlantic high pressure along with an inland trough will prevail into
early next week. High pressure will then become suppressed further
to the south which could allow a weak cold front to approach from
the north by the middle part of next week.
Near term /through tonight/...
early this morning: it's hard to fathom, but temps are already
starting out at 80f or greater over many locations near the US-17
corridor and at the coast. And heat indices to the start the day
are above 90f at some coastal sites. It's a strong indication
that it'll be another hot one across the board.
For today: deep ridging will again persist, featuring 500 mb heights
of 5910-5920 meters and the resulting large scale sinking motion
within the troposphere. A Piedmont trough will remain inland while
the sub-tropical ridge is suppressed to the south across Florida.
This will support a solid westerly flow throughout much of the
vertical, and the resulting compressional warming off the
Appalachians, plus 850 mb temps that are near or above the 90th
percentile will generate another hot day across the entire entire.
We stayed close to the low level thickness forecast that shows
highs in the upper 90s or touching the century mark in a few
locations. We'll actually flirt with the record of 100f at kchs, but
will stay at least a few degrees below the kcxm and ksav records for
the date. The sea breeze will only offer the slightest of relief in
regards to Max temps, as it holds shoreline communities to the lower
and middle 90s.
The deep mixing also allows for inverted-v type soundings,
indicative that at least some mixing out of dew points will occur,
especially far inland, and prevent afternoon heat indices from
getting much higher than about 103-106f. However, pooling of the dew
points with the resultant sea breeze near the US-17 corridor and
closer to the coast will equate to some heat indices approaching
110f for about 1-3 hours in the afternoon. This includes such
communities as parts of the Charleston Metro...Beaufort and Hilton
Head and the Savannah Metro. Probably not enough for a heat
advisory, but it'll be close.
Our chances of convection remains less than 15% with poor mid level
lapse rates and large scale subsidence. But given the excessive heat
and humidity and plenty of instability we certainly can't rule out
isolated activity in the afternoon/evening near the sea breeze
boundary where the best convergence would occur.
Tonight: there are hints that isolated storms could sneak in from
the west-northwest in the evening in proximity to the Piedmont trough and a
weak short wave, but probabilities are not high enough to generate
any mentionable pop in the forecast. Overnight there could be a
little activity forming out near the western wall of the Gulf
Stream, but the trajectory forecast would keep anything that forms
offshore. Otherwise it'll be another very warm and sticky night with
mins to struggle to drop to 75-80f inland and not even any lower
than the lower 80s in downtown Charleston and along many coastal
sites. Record high minimums certainly could be challenged at the 3
official climate sites (see climate section below).
Short term /Saturday through Monday/...
Saturday and Saturday night: aloft, the pattern will continue to
display very slight and gradual weakening of the strong and
persistent ridge aloft. The Main Ridge axis will become increasingly
narrow and will setup to become more focused across south Georgia
and the Florida Peninsula. This will result in a bit of a return in
the inland Lee trough as the subtropical surface high is also
further south. Such subtle changes will bring about modestly better
chances for convection and a slightly less hostile environment for
thunderstorm development. Profiles are still quite unimpressive but
should at least allow for isolated thunderstorms. There could be
slightly better coverage further north and away from the ridge axis,
but only enough to carry a 30 pop for the tri-County. Another very
warm day with highs in the upper 90s, combined with lows in the
upper 70s will further contribute to potentially record breaking
temperatures for the month of July (see climate section for details).
The overnight is expected to be dry.
Sunday through monday: aloft, the forecast area will be solidly
sandwiched between broad troughing to the north and the upper ridge
to the south. Through early next week, the inland Lee trough looks
to be increasingly active and with modestly better sounding profiles
across the forecast area, this should result in a much more typical
coverage pattern for thunderstorms. Pop's are generally in the 30-50
percent change range. The overall severe threat doesn't look
particularly high. Temperatures may come down a few degrees into the
mid 90s, but with dew points still in the low to mid 70s heat index
values will still be in the 100-105 degree range.
Long term /Monday night through Thursday/...
in the long term period through Thursday, the relatively broad
trough will finally break down much of the upper ridge with it
splitting into one part over the south-central Continental U.S. And the other
over the Atlantic. Towards the end of the period the orientation of
the ridge to the west will help turn the large scale flow more
northwesterly across the area which may allow for more embedded
shortwave energy to approach the area. As such, we may need to keep
more of an eye on upstream convection and how it evolves and
potentially heads into the region. Overall convection changes are
higher through this period, thanks in part to the presence of a weak
front situated to the north and the slightly cyclonic flow aloft.
Also of note, temperatures should exhibit a slight cooling trend
with primarily low 90s by Thursday.
Aviation /08z Friday through Tuesday/...
VFR. Both ksav and kchs will be gusty at times during the late
morning into the afternoon with deep mixing profiles.
Extended aviation outlook: mainly VFR. Brief restrictions will be
possible with afternoon and evening showers/thunderstorms into
early next week.
early this morning: the strong low level jet that has allowed for SW
winds as high as 20 or even 25 kt will slacken, and winds will veer
enough to the west to allow for the ongoing Small Craft Advisory for the amz350 waters
to come down at 8 am.
The the rest of today: the local waters will remain sandwiched
between a trough inland over the southeast and Atlantic high pressure that
extends west across Florida. There is some temporary relaxation of
the gradient during the mid and late morning, before the pressure
packing tightens again in the afternoon. That along with the sea
breeze influences will allow for west and SW winds to back to the SW
and S in the afternoon as speeds climb again up near 15 or 20 kt.
Frequent gusts will exceed 20 kt in the Charleston Harbor during
the afternoon, but gusts of 25 kt or greater are not anticipated.
Thus no Small Craft Advisory should be required. Seas will average 2 to 4 ft in the
ocean, with waves building to 1 to 2 ft in Charleston Harbor.
Tonight: the low level jet will again impact the maritime community,
although geostrophic winds are a bit lower than they were last
night, so we'll probably not require any Small Craft Advisory headlines. However,
winds and seas will be elevated for the end of July with enough
pinching between the inland trough the sub-tropical ridging to the
south. Expect SW winds of 15 or 20 kt, even some higher gusts at
times, with seas of 3 or 4 ft. Maybe a few showers/T-storms out
near the Gulf Stream after midnight.
Saturday through wednesday: the overall pattern will generally
remain unchanged into early next week and will feature a trough of
low pressure inland and high pressure over the Atlantic. This will
drive a persistent southwest flow across the local waters, which
will top out in the 15-20 knot range each afternoon/evening. Seas
will be in the 1-3 ft range for the most part, with some 4 ft seas
possible around 20 nm and beyond during the times of strongest
Record high minimums for 29 July...
kchs: 80 set in 1999.
Kcxm: 85 set in 1999.
Ksav: 82 set in 1878.
Record high maximums for 29 July...
kchs: 100 set in 1993.
Kcxm: 99 set in 1999.
Ksav: 103 set in 1993.
Record high minimums for 30 July...
kchs: 80 set in 2011.
Kcxm: 82 set in 1999 and previous.
Ksav: 81 set in 1878.
***Top 5 warmest months on record by average temperature***
Charleston International Airport - kchs (records back to 1938)
1. 86.1 - July 1986
2. 85.9 - July 2016 (3 days left)
3. 85.5 - July 1993
4. 83.9 - July 1992
5. 83.8 - July 1998
Savannah International Airport - ksav (records back to 1874)
1. 86.7 - July 1993
2. 85.9 - July 2016 (3 days left)
3. 85.7 - July 1986
4. 85.7 - July 1875
5. 84.8 - August 2011
***top 5 driest months on record for july***
Savannah International Airport - ksav (records back to 1871)
1. 0.82 - 1888
2. 1.21 - 2016 (3 days left)
3. 1.25 - 1881
4. 1.33 - 1949
5. 1.35 - 1972
Marine...Small Craft Advisory until 8 am EDT this morning for amz350.