-- Discussion containing changed information from previous version are highlighted. --
351
FXUS62 KILM 221039
AFDILM

Area Forecast Discussion
National Weather Service Wilmington NC
639 AM EDT Sat Jul 22 2017

.SYNOPSIS...
Hot and humid conditions will peak today before beginning to
moderate with more typical summertime heat expected next week.
A cold front, approaching from the north, should push into the
area mid week and then remain in close proximity through late
week. The risk for thunderstorms with heavy rain will return
next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 300 AM Saturday...Although the expansive mid level ridge over
much of the central and eastern CONUS will begin weakening today the
overall impact of its weakening will be minimal. Forecast soundings
still show a good amount of mid-level dry air and subsidence.
Isolated cumulus development along the sea breeze is likely but what
does develop will have limited vertical depth. Slightly deeper
convection will develop west of the area, along the Piedmont trough
but even here little more than an isolated storm is likely given the
unfavorable environment. Will carry slight chc pop for inland
locations but the bulk, if not all, of the region will remain dry.

Bigger story today will once again be the heat with both temps and
dew points running a little higher than yesterday. All of the major
guidance agrees on 850 temps approaching 20C this afternoon which
translates to highs in the upper 90s. The sea breeze will make temps
closer to the coast tricky, likely keeping highs in the low to mid
90s. Although low level winds had a southwest component on Fri the
winds at 850 were more westerly. Today the 850 winds will back to
southwest, helping increase low level moisture in the area. Moisture
increase will not be enough to generate a lot of cloud cover or
storms but will lead to higher heat index values. Latest forecast
has all counties in the forecast area peaking between 105-109,
except the immediate coast. It appears even most of the coastal
zones will be close to the 105 mark so will add all zones within a
heat advisory for today. Inland areas will be close to 110 and a few
areas may exceed this thresholds but it currently appears that the
bulk of these areas will remain at or below 109, and not require
issuance of an excessive heat warning. Boundary layer winds and
increased low level RH will keep lows well above climo overnight.
Isolated marine convection may develop but the majority of this
activity will remain offshore.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 300 AM Saturday...The Forecast Area will be in between
two areas of high pressure. One area of hot high pressure will
retrograde westward as the Bermuda High remains to our E. This
will allow an upper trough to slowly dig across the eastern
CONUS. At the surface, the Piedmont trough will remain
prominent through Mon while a cold front slowly slips down the
east coast, likely lying just to our N Tue morning. This
scenario will allow for still humid, but slightly lower high
temps, especially by Mon. Clouds will be more prevalent Mon as
compared to Sun. The risk for thunderstorms will be increasing
and should be highest Mon and Mon night as precipitable water
values climb to near 2.25 inches. At this time, will show small
POPs Sun with POPs trending higher and perhaps becoming numerous
by Mon/Mon night. High moisture content should allow for very
heavy rainfall amounts where thunderstorms are able to train.

Temps on Sun will still be in the lower to mid 90s which will
produce heat index values at or above 105 degrees across most
areas and so a Heat Advisory may be required that day. On Mon,
highs will be in the lower 90s. Heat index values on Mon are
not expected to exceed 105 degrees, but may still top 100. Lows
will be in the mid and upper 70s although if thunderstorms
become widespread enough a cold pool may drop readings into the
lower 70s.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 300 PM Friday...A shortwave trough moving through New
England on Monday night, and a large upper low developing
across southern Quebec will help put a dent in the heat.
Falling upper level heights associated with these systems will
shift the core of the upper ridge (and its associated heat)
farther west into Texas and New Mexico. Don`t expect a cleansing
frontal passage by stretch, (06Z GFS ensemble 850 mb temps
don`t fall below climatology next week) but at least we should
lose the 100+ heat indices for a few days.

Both the GFS and ECMWF show a cold front moving into northern
North Carolina on Tuesday. The GFS then sinks the boundary a
little farther south by Wednesday morning. There are enough GFS
ensemble members showing the front making it into South Carolina
that I`m leaning toward the GFS in this case. This front should
enhance the coverage of t-storms Tuesday night and Wednesday,
and I`ve placed my highest forecast PoPs (50-60 percent) of the
extended period during this period.

With a progressive upper level pattern to our north, surface
high pressure will push off New England and out into the
western Atlantic Thursday and Friday, probably with enough
southerly wind developing over the Carolinas to lift the front
or its remnants back to the north.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
As of 12Z...Patchy IFR stratus and MVFR BR will burn off by 12z.
VFR conditions are expected. However, there is potential for
IFR stratus and some fog to redevelop late tonight. The
seabreeze will again develop with SSW winds this afternoon and
early eve gusting to around 20 kt at KCRE and KMYR. No organized
thunderstorms during the period and a remote pop up shower or
thunderstorm during the heating of the day is too low a
probability for inclusion in the TAFs at this time.

Extended Outlook...Possible IFR in patchy stratus/fog near
sunrise Sun. Brief MVFR/IFR possible in showers and thunderstorms
Sun night through Wed.
-- End Changed Discussion --

&&

.MARINE...
NEAR TERM /THROUGH TONIGHT/...
As of 300 AM Saturday...Southwest flow around west side of Bermuda
High will continue through the period. Gradient along the coast will
be a little tighter than on Fri and onshore flow will be a bit
stronger near the coast. Gusts in excess of 20 kt in the afternoon
and early evening are likely. Seas around 3 ft today will build to 3
to 4 ft later today and remain there overnight. Isolated 5 ft will
be possible in the vicinity of Frying Pan shoals.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 300 AM Saturday...Small Craft Should Exercise Caution
headlines may be required for at least a portion of the time.

A Piedmont trough on Sun will allow for a tightening pressure
gradient across the waters. As upper trough slowly digs across
the eastern CONUS, a cold front will slip progressively south,
likely lying just N of the waters Tue morning. The approach of
this front will keep the gradient rather tight as well. The wind
direction will be SW through the period. Wind speeds will be as
high as around 20 kt Sun afternoon and eve and again Mon
afternoon. Seas will be 4 to 5 ft through the period.

LONG TERM /TUESDAY THROUGH WEDNESDAY/...
As of 300 PM Friday...Bermuda High Pressure well offshore will
lose its influence on our weather by the middle of next week as
low pressure in the Canadian maritime provinces pushes a cold
front down the U.S. East Coast. This front is shown by the
latest GFS and ECMWF models to make it down to near Hatteras on
Tuesday. Models diverge slightly on Wednesday as the ECMWF keeps
the front stalled to our north and the GFS brings it down to
the Myrtle Beach vicinity Wednesday morning. Regardless of which
solution is correct, the frisky southwest winds of the weekend
should diminish by Tuesday within the weaker pressure gradient
near the front. Unfortunately thunderstorm potential should also
increase as the front approaches.

&&

.ILM WATCHES/WARNINGS/ADVISORIES...
SC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for SCZ017-023-024-032-033-039-053>056.
NC...Heat Advisory from 11 AM this morning to 7 PM EDT this evening
     for NCZ087-096-099-105>110.
MARINE...None.

&&

$$
SYNOPSIS...
NEAR TERM...III
SHORT TERM...RJD
LONG TERM...TRA
AVIATION...RJD
MARINE...