Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
316 PM EDT Friday Jul 25 2014
a stalled front across northeast South Carolina will lift north
and dissipate overnight into Saturday. Building high pressure
will follow...bringing hot and humid conditions over the weekend
and Monday. Another unusually strong cold front may bring strong
thunderstorms late Monday...followed by cooler and drier weather
Tuesday...possibly through much of next week.
Near term /through tonight/...
as of 315 PM Friday...a stalled front in the Georgetown vicinity has
ignited a tremendous convective blowup this afternoon. Large
instability on the order of 3000 j/kg...slow storm movement parallel
to the frontal boundary...and precipitable water values near 1.9
inches are all very helpful for producing heavy rainfall and the
potential for flooding. This convective activity extends westward to
just south of Columbia SC and will likely continue through 5-7 PM
before dissipating as the airmass becomes overturned and stabilized.
Interestingly...a couple reports of hail have been received in the
past half hour...unusual to say the least in a humid airmass
with freezing levels up around 15.5 kft.
To the north it has been an unusually cool day for late July with
temperatures only recently reaching the 80s in Wilmington...
Whiteville...and Florence. Fog and low status that followed the
front south this morning has since lifted into a layer of
stratocumulus with bases between 1500 and 3000 feet.
Models struggled initially with the exceptional southward movement
of this front earlier this morning...and are now struggling with the
front's sluggish return northward. The 12z NAM had very poor
initialization with this boundary. The 12z GFS was initialized a
little better but I am essentially using the RUC...hrrr...and 12z
WRF-arw and WRF-nmm models for wind direction forecasts tonight. As
synoptic high pressure moves east off the middle-Atlantic coast this
should induce the front to return northward...albeit more slowly
than most synoptic guidance is indicating.
With rich low-level moisture and relatively dry air in the
mid-levels...calm winds...and slowly clearing skies...it is a good
setup for fog or low stratus overnight. Visibilities will probably
begin decreasing after midnight with the worst conditions around
daybreak. Lows should reach the lower 70s with some middle 70s near the
Short term /Saturday through Sunday night/...
as of 315 PM Friday...main weekend weather headline is expanding
heat and humidity. Heat indices/apparent temperatures factoring
in elevated dewpoint values...yields 97-102 degree indices for
Saturday...and into the broiler Sunday as maximum apparent temperatures
Sunday afternoon crest to 104-107 degrees. If consistency holds
in the maximum temperature forecast...then we will be looking at prospects
of a heat advisory needed on Sunday.
Pcpn-wise...best chance looks to be Saturday as a decaying frontal
boundary lifts north across Cape Fear and southeast NC. Dry middle-level air
will place limitations of precipitation extent and magnitude Saturday...but
a few showers should be expected on Saturday as low-level moisture
lingers. Sunday looks dryish with westerly flow through much of
the column squelching rain chances. Because of intense heat at
the surface Sunday however...an isolated thunderstorm cannot be ruled
Due to the gradual build up of heat...very mild minimums Sunday
and Monday daybreak are anticipated...essentially middle 70s to
the lower 80s and mildest near the sea...particularly as we see
still SW winds holding up in the overnight periods...especially
late Sunday into early Monday.
Long term /Monday through Friday/...
as of 315 PM Friday...impressive middle-level amplification through the
extended will drive active weather the first portion of the
extended...but quiet and cool conditions will follow the majority
of the upcoming week.
Monday shaping up to be one of the hottest days of the year
locally...as return flow strength increases...and 850mb temperatures climb
above 20c. Combine this with what will likely be a very warm start
to the day (warm overnight lows from sunday)...and Monday will
feature widespread middle 90s across the area...with upper 90s possible
in many spots. These temperatures combined with high dewpoints will bring
apparent temperatures to near heat advisory criteria on Monday. This will
be the last Summer-like day for quite some time though...as an
anomalously sharp trough (up to -4 sd) dives into the eastern Continental U.S..
beneath this trough a strong cold front will cross the local area
Monday evening and night...moving offshore by Tuesday morning. A strong
cold front diving into a very warm unstable airmass will support the
chance for strong convection on Monday...and Storm Prediction Center is already
highlighting the region in the d4 outlook. It is too early to focus
on details of course...but a strong cold front impinging on a highly
unstable airmass with rapid height falls and strong middle-level flow
is unquestionably a solution which supports convection. Will bump to
likely pop Monday for much of the area.
To use the word significant to describe the air mass change from
Monday through Wednesday may be an understatement...as 850mb temperatures
drop from around 21c (+2 sd) to 13c (-2 sd) in just 48 hours! While
this occurs...precipitable waters drop below 1 inch by Wednesday as well...and
thus the Tuesday/Wed/Thu timeframe should be dry locally...albeit with
temperatures well below normal for late July. Although strong cold air advection events in
July are...not surprisingly...rare...expect guidance is overdoing
temperature forecasts middle week and will undercut numbers...with temperatures
likely 5-10 degrees below normal much of next week. Late in the
week...mid-level trough begins to weaken...and strong ridge over the
Atlantic begins to push back to the west driving a retrograding
pattern into Friday...driving warming temperatures but increasing chances
Aviation /18z Friday through Wednesday/...
as of 18z...expect VFR through this evening with increasing chances
for rain showers/thunderstorms and rain through this afternoon...especially near & south of
Myrtle Beach. MVFR/IFR possible late overnight into the early
morning hours due to fog/low ceilings...improving to VFR Saturday late
MVFR/IFR ceilings have persisted across the area..and may continue to do
so at a few of the taf sites for the next hour before improving to
MVFR and then VFR as the cloud layer Burns off. Northeast winds will
become southeast along the coast around 10 kts later this afternoon
with inland sites continuing a light northeast wind. Latest radar
imagery depicts increasing rain showers/thunderstorms and rain near Georgetown and Charleston
SC...and coverage and chances will increase throughout the rest of
the afternoon as the sea breeze and the front interact. Any
rain showers/thunderstorms and rain could produce heavy rainfall or flight Cat
restrictions...and will taper off later this evening. Overnight into
the early morning hours winds will become light and variable and
MVFR/IFR is possible due to fog at all sites...which will dissipate
Saturday morning giving way to VFR with light winds.
Extended outlook...showers/tstms/tempo MVFR possible Sunday and
Monday. Otherwise expect VFR.
near term /through tonight/...
as of 315 PM Friday...a stalled front in the Georgetown SC vicinity
has been the focus for a significant blowup of thunderstorm activity
this afternoon. Look for this activity to continue moving
east-northeastward...affecting large portions of the South Carolina
coastal waters and potentially the waters near and south of Cape
Fear before sunset. Thunderstorm chances are much less after sunset
but an isolated shower cannot be ruled out even after midnight. The
front should begin to move northward overnight...reaching Cape Fear
later this evening and eventually moving northward into eastern
North Carolina. Winds which are still easterly at 3 PM should turn
southerly overnight...but with little increase in wind speed
Seas currently 2-3 feet are evenly split between a 9-second
southeast swell and shorter period waves produced by local winds.
Little change is anticipated overnight.
Short term /Saturday through Sunday night/...
as of 315 PM Friday...the marine weekend headline...increasing SW
winds Saturday through Sunday....with an exercise caution marine
conditions likely developing Sunday or Sunday night....for 15-20
knots SW winds and seas of 4-5 feet. Therefor while Saturday will be
a manageable day for boating...Sunday will become bumpy and a bit
more hazardous as wind-waves continue to grow in persistent and
moderately strong SW winds. Isolated thunderstorms expected on the waters
early Saturday...and potentially late Sunday night offshore.
Long term /Monday through Wednesday/...
as of 3 PM Thursday...strong pre-frontal flow ahead of a cold front
will push winds up above 20 kts from the SW on Monday...driving seas
to 4-6 feet...and a Small Craft Advisory will likely be needed on
Monday. This front will cross the waters Monday night...with a rapid
wind shift to the northwest by Tuesday morning...and speeds easing through
Tuesday to 5-10 kts. Weak area of high pressure will build across
the waters thereafter...causing winds to become NE around 10 kts.
After the Small Craft Advisory conditions subside Monday night...seas will be 2-4 feet
through the remainder of the period.