Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 kakq 201728 
afdakq

Area forecast discussion
National Weather Service Wakefield Virginia
128 PM EDT Fri Jul 20 2018

Synopsis...
high pressure will sit along or just off the northern New
England coast today into this evening. Low pressure will track
northeast along the mid Atlantic coast tonight and Saturday,
bringing increasing chances for rain. An unsettled pattern is
then expected to persist through at least the middle of next
week.

&&

Near term /until 6 PM this evening/...
as of 420 am EDT Friday...

Early this morning, sfc high pressure was cntrd just off the
New England coast. The high will maintain dry wx acrs the area
thru this aftn. Meanwhile, a potent upper low moves east from
Wisconsin twd Michigan, but mostly remains un-phased with a
weaker upper trough along the southeast U.S. Coast. This feature will
enable an old sfc trough/boundary to develop into a more
pronounced sfc low along the SC coast during today.

The sky will start out partly to mostly sunny everywhere this
morning, then the sky will become mostly cloudy this aftn into
NE NC. Will maintain some 20-30% pops mainly confined to NE NC
after 18z, as sfc low pressure starting to lift NE along the SC
coast, brings deeper moisture much to the north of that low.
Highs will mainly range into the mid 80s, except lower 80s near
the coast.

&&

Short term /6 PM this evening through Sunday night/...
as of 420 am EDT Friday...

An active period begins tonight, as models do agree on a sfc
low pressure area lifting north-northeast along the coast tonight into Sat
night. There area timing differences with the track of the low
with the GFS the fastest, and the NAM the slowest. The European model (ecmwf) is
in between those two and will favor it with the current fcst
pops. Thus, confidence is higher that deep moisture will
overspread east-southeast zones later tonight into Sat morning, and
reaches into the Virginia nrn neck and Eastern Shore through at least
Sat aftn, with likely pops expected. Chance pops will be
maintained farther inland (w of i95) where the consensus is for
less in the way of deep moisture and lift. Still not particularly
unstable, so mainly just showers with a slight chance for tstms
tonight through Sat aftn. Highs on Sat have been trended down a
few degrees given some easterly flow and low clouds for at
least some of the day. For now, have highs into the mid 80s far
S, to the upper 70s/lower 80s north (though it could end up even
cooler if rain persists through the aftn). Have pops trending
down in the late aftn into the evening for much of the area, as
the sfc low lifts just off the Virginia ern shore and Maryland coasts twd
New Jersey, with the next wave of activity more convective in
nature arriving from the mtns. Pops will be 40-60% in some of
the Piedmont Sat evening/chc pops elsewhere. More humid Sat
night with lows mainly in the upper 60s to lower 70s. Consensus
position of the upper trough sun/Sun night is over the Ohio Valley
swrd into ern Tennessee and nrn GA, with deep moisture into the local
area and diffluent zone aloft. Have 40-60% pops acrs the region
during this time. Partly to mostly cloudy with highs on sun in
the upper 80s se, to the lower 80s W/NW.

&&

Long term /Monday through Thursday/...
as of 330 PM EDT Thursday...

Still expecting daily chances for showers and thunderstorms
throughout the mid-Atlantic during the extended period. An upper-
level low will be located over Indiana/Ohio/Kentucky Sunday evening.
This feature will slowly drift south/southwestward, and will be
located along the Gulf Coast by midweek. With southeast flow at the surface
and southerly flow aloft, ample moisture will be in place on Monday
(gfs forecast precipitable water values are around 2.0"). Coupled with the upper low
nearby, expect scattered to widespread showers and thunderstorms
(with coverage peaking during the late afternoon hours) on Monday.
Given this setup, went ahead and increased pops to ~60% for the bulk
of the County Warning Area.

T-storms will still be possible on Tuesday and Wednesday, as we will
still be under southerly flow aloft (with southeast flow near the surface).
Coverage will likely be highest over the western third of the cwa,
but scattered (diurnally-driven) showers/T-storms will be possible
throughout the area. Shower/T-storm chances continue through
Thursday, with the 500mb through axis approaching the region from
the west as a large upper low moves eastward across southern Canada.
Went with pops around 60% over the far western County Warning Area during the day on
Tue/Wed, with 40-50% pops elsewhere. We continue to monitor the
potential for some areas to receive several inches of rain next week.

Not much in the way of temperature changes from Monday through
Thursday. Highs will only be in the mid to upper 80s with the daily
shower/T-storm chances. Expect overnight lows in the low-mid 70s.

&&

Aviation /18z Friday through Wednesday/...
as of 130 PM EDT Friday...

VFR to start off the 18z taf period with onshore winds, high
pressure offshore and low pressure developing near the southeast coast.
The low slides up the Carolina coast while strengthening
tonight, leading to lowering cigs and increasing rain chances.
For now, kept any mention of rain until after 00z at kecg and
03-04z at korf/kphf. Rain starts later for the other taf sites.
Could be some low vsbys in heavier showers as well. For cigs,
expecting mainly MVFR with areas of IFR psbl, but not confident
enough to include IFR this far out in the tafs attm. Increasing
moisture/pcpn chcs continue into Sat aftn at all taf sites (with
lower cigs and vsbys), as low pressure tracks nwrd along or
just off the mid Atlc coast. Best chcs for widespread showers
and possible tstms will be mainly near the coast, but scattered
showers and an isolated tstm will be possible area-wide. An
unsettled pattern looks to persist sun-Tue with scattered to numerous
showers/tstms expected (highest coverage focusing in the
aftn/evening).

&&

Marine...
as of 415 am EDT Friday...

High pressure off the New England coast will provide one more benign
day over the area waters with east to southeast winds of 10 - 15 knots.
But as the area of high pressure retreats the two low pressure
systems will affect the area for the weekend. The first low is a
coastal low that should form off the Georgia coast this afternoon and
lift northward along the coast tonight and pass over eastern
portions of Delmarva on Saturday. This low will enhance the
easterly flow to near Small Craft Advisory levels, but expect with the longer fetch
that the seas will be more than 5 feet on the ocean and 4 feet on the
southern Bay. Now the timing on this low is still in question with
the 00z GFS the fastest and the 00z NAM the slowest. So have split
the timing difference with the European model (ecmwf) and have the low near the NC/Virginia
border around 12z and out of the area by 00z with the flow turning
from east ahead of the low to SW by Sat evening. Have raised Small Craft Advisory flags
mainly due to the increase in the seas but winds appear to be near
Small Craft Advisory levels as well. Did leave the northern two Bay zones and the
river zones out at the moment as the winds appear to be marginal and
with the uncertainty on the strength and timing of the low not as
confident for this zones along with the hazard being a 3rd period
advisory.

The coastal low will lift away from the area on Saturday night with
conditions beginning to settle down so will have the Small Craft Advisory go away
from south to north during the overnight hours. The second low that
is moving from the Great Lakes today into the Ohio Valley by Sunday
will increase the flow again out of the south Sunday afternoon and
into early next week. While the winds generally remain below Small Craft Advisory
levels, the persistent southerly to southeasterly flow the will
develop as the surface low settles into the Tennessee River valley
and the strong Bermuda high that develops will help push seas up to
Small Craft Advisory levels so additional headlines may be needed Sunday night into
early next week.

&&

Equipment...
kdox radar is inoperable due to a failure of the motor. All
parts will not arrive until Tuesday July 24. Return to service
is unknown at this time.

&&

Akq watches/warnings/advisories...
Maryland...none.
NC...none.
Virginia...none.
Marine...Small Craft Advisory from 8 am to 4 PM EDT Saturday for anz632-
634.
Small Craft Advisory from 8 am Saturday to 1 am EDT Sunday for
anz654-656-658.
Small Craft Advisory from 8 am Saturday to 4 am EDT Sunday for
anz650-652.

&&

$$
Synopsis...tmg
near term...tmg

National Weather Service Glossary of Abbreviations