Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
104 am EDT Sat Aug 2 2014
a frontal boundary along the southeast coast will slowly drift
north towards the middle Atlantic coast tonight. The front then
stalls along the coast or just inland over the middle Atlantic
region...keeping unsettled conditions in place through at least
Near term /until 6 am this morning/...
latest msas showing mesoscale low tracking NE across southeastern Virginia and will
continue NE across the ches Bay and Eastern Shore areas through the pre
dawn hours. Expect heaviest precipitation along with scattered thunder across those
areas with lighter precipitation to the west. Second area of upper level energy
will combine with plnty of ll moisture ato continue probability of precipitation acros the western
half of forecast area through 12z...although lighter in intensity.
highly amplified flow over the Continental U.S. And western Atlantic ongoing
this evening...highlighted by longwave troughing over the central
Continental U.S. And longwave ridging over the western Atlantic. Plume of
anomalous moisture visible on water vapor streaming northward in southwesterly
flow from the eastern Gulf and off the southeast coast over the middle-Atlantic
region. At the surface...low pressure resides over southeast Georgia/northern Florida with a
stationary boundary extending northeastward along the southeast and middle-Atlantic
coasts. Warm air advection/overrunning of the frontal boundary as well as small
amplitude shortwaves lifting in southwesterly flow are producing a broad
area of light-moderate precipitation over the local area this evening.
Rainfall rates have been light...with storm total amounts through
930pm ranging from a few tenths to just over half an inch. Latest
GFS 310k Omega is handling the current precipitation over the Piedmont
and central Virginia well. Dry slot overspread southeast Virginia/NE NC earlier this
evening...resulting in a brief lull in the precipitation. However...the
location of the inverted trough/convergence and modest
instability/shear...has resulted in scattered-numerous showers in that
region. Will maintain high probability of precipitation through the evening...but expect
the precipitation to begin lifting northward from the Piedmont and central Virginia by
late this evening. Already beginning to see signs of the precipitation
lifting from the north ctnrl NC.
Models continue to indicate favorable forcing for ascent and
abundant moisture (pwats at or above 2 inches) over the region tonight.
Frontal boundary/inverted trough is expected to nudge to the east as
the upper ridge retrogrades slightly. This places the best
lift/Omega over southeast Virginia/NE NC and over the coastal waters. Will
maintain categorical probability of precipitation along the coast with likely probability of precipitation east of
Interstate 95 (tapering off to low end chance probability of precipitation over the Piedmont
after midnight). Quantitative precipitation forecast amounts of one half inch up to one inch
expected. Outside of urban and low lying areas...flooding is not
anticipated as heaviest precipitation will fall in favorable locations.
Rainfall rates also expected to remain low. Temperatures will generally
hold steady overnight...generally in the middle-upper 60s to low 70s.
Short term /6 am this morning through Monday/...
frontal boundary is prognosticated to straddle the coast from the
Delaware-Maryland-Virginia down along the southeast coast. The interaction of a steady
influx of moisture and shortwave energy with the frontal boundary
will maintain high precipitation chances...once again with the main focus
along the coast and especially NE NC/southeast Virginia. Model soundings for
kecg and korf are very moist...with surface convective available potential energy of 700-1500 j/kg...
and unidirectional SW flow where winds are very light below
700-500mb (at or below 20 kt). This is a favorable set-up for
training precipitation especially along the stalled boundary. The
potential for embedded thunderstorms also increases on Saturday
near the coast. It is very possible that 3-6 hour precipitation totals
could be upwards of 2.00 inches...especially in convection. If
this occurs on top of rainfall received on Friday/Friday night...then
the potential for a Flood Watch become increasingly likely. Will
need to monitor trends and totals Friday evening/overnight to
determine if additional totals on Saturday will exacerbate minor
flooding potential in urban areas...low lying areas...areas of
poor drainage...and in the vicinity of small streams/creeks.
Please refer to hydrology section below and severe weather potential statement for additional
details. Additional rainfall and threat for minor flooding
continues into Sunday as overall synoptic conditions remain
unchanged. Precipitation coverage starts to diminish from west to
east on Monday as the front gets pulled offshore by Tropical Storm
High temperatures will run about 5-10 degrees below normal due to
precipitation and cloudy skies. Highs Sat middle-upper 70s...sun around 80...
Monday low-middle 80s. Lows will be near normal with readings in the
middle 60s northwest to lower 70s southeast.
Long term /Monday night through Friday/...
the nearly stationary front and associated plume of Atlantic
moisture should finally nudge offshore Monday night as a final
wave of low pressure lifts NE along the boundary. This wave then
pulls off the coast Tuesday. 30-40% probability of precipitation will be maintained over
southeast portions in closer proximity to the surface boundary. A brief
dry period is expected Tuesday night into early Wednesday as
shortwave ridging passes overhead. A cold front then approaches
from the northwest Wednesday and drops into the area Wednesday night.
This will bring a chance of afternoon/evening showers/tstms. The
front is prognosticated to drop into NC Thursday. 20-30% probability of precipitation will be
included for southern portions to account for any uncertainty with
respect to the frontal location. The front should eventually lift
back north later in the week...but confidence with respect to timing
and location is low...so minimal probability of precipitation will be continued. Highs
Tuesday should average in the upper 80s...with upper 80s/
around 90 Wednesday ahead of the front. Highs should drop back
into the middle/upper 80s Thursday and Friday behind the front. Low
temperatures should average from the middle 60s to low 70s.
Aviation /06z Saturday through Wednesday/...
showers with light/occasionally moderate rain continue to push north
into the region...affecting all akq taf sites. Ceilings continue to
lower overnight and could see several hours where they drop to 1-2
k feet overnight...with kric/ksby having highest chance for IFR
ceilings/ below 1 kft. Some brief embedded thunderstorms possible as
well...most likely at kecg/korf/kphf...but probability too low to
include in tafs.
Outlook...threat for showers/thunderstorms persist Sat/sun as moisture
with stalled boundary along/just inland from the coast. Periodic
flight restrictions will be possible in heavier showers/thunderstorms both
days. Overall coverage of precipitation will gradually diminish Monday/Monday
an inverted trough will remain off the coast tonight and steadily
drift inland through the weekend. No flags are expected as wind
speeds should remain at or below 15kt (direction averaging east/se) and
seas/waves should remain below 5ft/4ft. Nevertheless...persistent
onshore flow should build seas to around 3ft...with occasional 4ft
seas out near 20nm. Waves in the mouth of the Bay should build to
2-3ft...with 1-2ft elsewhere in the Bay. The system pushes
offshore early next week as high pressure returns to the region. A
cold front approaches from the northwest during the middle of next week.
Overall sub-Small Craft Advisory conditions are expected to prevail during the
early to middle portion of next week. However...some long period
swell could encroach the area depending on the strength/track of
ts Bertha. Current 01/12z wavewatch guidance brings 3-4ft swell
out near 20nm Tuesday/Wednesday of next week.
persistent rainfall along a slow-moving frontal boundary will
bring steadily accumulating precipitation to the region the rest
of today through at least Monday...with best accumulations focused
over NE NC and southeast Virginia. Across the aforementioned southeast areas...
storm total precipitation amounts of 2.50 to 3.50 inches (locally
up to 4.00 inches) are anticipated over the next 48-72 hours. This
may potentially cause minor flooding of urban areas...areas of
poor drainage...low lying areas...and to rises of small
streams/creeks. A Flood Watch may be needed later this evening or
overnight depending on upstream precipitation trends. Will
continue to highlight flooding potential in the severe weather potential statement (wbchwoakq).
Maryland...moderate rip current risk from 8 am EDT this morning through
this evening for mdz025.
NC...moderate rip current risk from 8 am EDT this morning through
this evening for ncz102.
Virginia...moderate rip current risk from 8 am EDT this morning through
this evening for vaz098>100.