650
FXUS61 KLWX 170741
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
341 AM EDT Fri May 17 2024

.SYNOPSIS...
An area of low pressure will approach from the south and west later
today bringing renewed chances for showers and thunderstorms.
Widespread rain and thunderstorm activity is expected Saturday into
Sunday as low pressure pushes towards the Carolina coast. Drier
conditions with a slight warming trend are expected Monday and
Tuesday as high pressure pushes back into the region. A potent cold
front looks to bring additional showers and thunderstorms to the
area by the middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
It`s a dry start for many this morning with patchy fog confined to
the river valleys along and west of the Blue Ridge. Fog formation
has been stunted a bit as a result of cloud cover (low and mid level
stratus) advecting in from the filling low pressure offshore and
mid/high level clouds pushing east ahead of an open wave of low
pressure over the Tennessee/lower Ohio River Valley. Meanwhile, at
the surface level lies a weak boundary draped between the
Allegheny/Potomac Highlands into the Shenandaoh Valley and
central/southern VA. This boundary will become the focal point for
slow moving shower and thunderstorm activity this afternoon and
evening. Some minor flooding issues are possible especially in areas
that due see repetitive rounds of slow moving convection.

This is especially a concern for portions of the Potomac Highlands
(Hardy and Hampshire counties in WV) southward into the Shenandoah
Valley and central VA (Rockingham, Madison, Nelson, Augusta, and
Albermarle counties). Current CAMS (HREF, RRFS, ARW, HRRR and FV3)
and 00z synoptic(NAM/GFS/ICON) guidance continues to paint a
bullseye of 2-4" of rain across the Potomac/Allegheny Highlands
south and east toward the central/southern Shenandoah Valley. 3-6
hour flash flood guidance in these areas sits between 2-3 inches
with 1.5-2 inch values across the urban centers (i.e Harrisonburg,
Waynesboro, Staunton, and Charlottesville). With all that said, WPC
has highlighted this area in a Slight Risk for excessive rainfall
within the Day 1 outlook

Given the consistency will go forth with a Flood Watch to encompass
the threat later this afternoon and evening. Saturation of column
(i.e PWATS around 1.5-1.7"), training convection, terrain, and
consistency amongst guidance further confidence for the watch
issuance. Any convection should wane as we head into the late
evening and overnight hours. Light to moderate rain will remain as
shortwave energy pivots east and the incumbent upper level low
approaches from the Tennessee River Valley/southern Appalachians
region.

High temperatures today will be slightly cooler in the upper 60s and
low to mid 70s under increased cloud cover. The warmer air will be
down across central and southwest VA where a few breaks in the
clouds will occur through midday before filling in this afternoon.
Lows tonight will fall into the mid to upper 50s.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...

It`s another stormy and soggy weekend ahead across much of the Mid-
Atlantic region. This marks our 2nd weekend in row during the month
of May and 4th weekend since mid/late April for wet weekend weather.
Once again an upper level trough and slow moving low pressure system
are to blame for the increased precipitation chances across the
region.

By Saturday, the resultant upper level low pressure system will be
ejecting out Tennessee River Valley and toward the Carolina
coastline Saturday night into Sunday. Ample moisture will continue
to advect in off the Atlantic ahead of low on southeast flow. This
will aid in a bit more stability during the afternoon and evening
hours especially along and north of I-66/US-50. Even with that said,
numerous to widespread showers and thunderstorms are likely late
Saturday morning and into Saturday afternoon. The highest likelihood
for thunderstorms looks again to be confined to areas south of I-
66/US-50 and in particularly south of I-64 due to the proximity of
the upper level low. Severe weather does not appear to be a concern
at this point given limited CAPE and shear with ample cloud cover
overhead. Localized flooding will remain a concern given repetitive
rounds of heavy rain from Friday and rich moisture (high
PWATS)funneling in. Some locations could see one to two inches of
rain with locally higher amounts throughout the weekend period. WPC
and CSU learning probabilities continue to highlight the area in a
Marginal Risk for excessive rainfall per the Day 2 outlook. Highest
rain totals will be confined to areas along and west of the Blue
Ridge and back across the central/southern VA Piedmont.

The upper level low pressure will scoot south of the region Saturday
night into Sunday. This will allow for north to northeast flow to
advect slightly drier air down the spine of Appalachian Mountains.
Shower and thunderstorm coverage and intensity will decrease as
result.

Shower chances continue into Sunday although coverage will become a
bit more scattered throughout the day as low pressure pulls away
from the region. Highest probabilities for showers will be across
central and southern portions of the forecast area with
sprinkles/drizzle further north. Rain chances finally diminish Sunday
night as drier air filters in. Temperatures were kept to the cooler
side of model guidance due to the abundant cloud cover and ongoing
chances of showers and thunderstorms Highs will struggle to get out
of the mid to upper 60s (low 60s across the PA/MD line and
mountains) Saturday afternoon. Lows Saturday and Sunday nights will
fall back into the low to mid 50s.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
To start off the work week, a positively-tilted southern stream
trough should slowly pull away from the Carolina coast. The
resultant surface pattern does maintain low pressure off the
Carolinas and high pressure across the northeastern U.S.
Consequently, a net easterly onshore flow is maintained which does
keep temperatures slightly below average on Monday. However, a broad
mid/upper level ridge eventually settles over the Eastern Seaboard
through roughly mid-week. The building of heights will allow for a
marked warm up as high temperatures return to the low/mid 80s by
Wednesday. Ample subsidence underneath this ridge should squash any
shower chances through Tuesday. As return flow increases by mid-
week, humidity levels begin to creep back up as dew points rise into
the low/mid 60s by Wednesday.

A frontal system approaching from the west will increase shower and
thunderstorm chances by mid-week. The net result of warming
temperatures and increasing humidity levels should raise surface-
based CAPE values to around 1,000 J/kg or so. This increasingly
unstable air mass may interact with a series of perturbations
tracking through in the southwesterly flow aloft. While global
models struggle to capture convective processes this far out in
time, the overall signal and pattern recognition suggests an
increasingly convective pattern. The approaching cold front is not
slated to pass through the local area until late Wednesday. Thus,
any convective threats could fester into the overnight hours. In the
wake, expect drier conditions into Thursday, but with similar
temperatures. Ensemble systems do agree on decreasing humidity owing
to the shift to west-northwesterly flow on Thursday.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Currently sandwiched between an increasing stratus deck advecting in
off the Atlantic and mid/high level clouds pushing in from the
Ohio/Tennessee River Valley ahead of an approaching low pressure set
to impact the area this weekend. Predominantly MVFR conditions are
expected across the terminals this morning with intermittent pockets
of IFR scattered across mountain terminals and terminals close to
large bodies of water. The encroaching cloud cover has limited
patchy fog formation to the river valleys this morning and terminals
mainly along and west of the Blue Ridge. Any fog should quickly mix
out before daybreak as mid/high level cloud cover takes over for a
majority of the day.

VFR to MVFR cigs will start the afternoon before deteriorating flight
conditions move in late afternoon and into the evening hours. Any
shower and thunderstorm activity should be relegated to terminals
west of a line from KMRB south to KCHO/KSHD beyond 18z/2pm this
afternoon. As for the corridor itself, any thunderstorm activity
should wane as precipitation moves east. Did leave a mention of VCTS
in at KIAD with the higHer confidence toward KCHO/KSHD/KHGR/KMRB
later this afternoon and into the late evening hours.

MVFR to IFR conditions are expected tonight as widespread light to
moderate rain overspreads the terminals. Sub-VFR conditions will
continue through much of Saturday into Saturday night as numerous
showers and thunderstorms pass across the region. This is due in
part to an upper level low pressure system that will push off the
Carolina coast Sunday. Additional MVFR/IFR conditions will likely
remain during this time period.

Winds will remain light out of the north/northeast this morning
switching to the southeast this afternoon and evening. Winds will
turn toward the east Saturday before switching back to the north and
northeast Sunday. Speeds of 5 to 15 kts are expected through the
weekend period.

Depending on how long the easterly onshore flow keeps marine stratus
in the picture, the D.C. and Baltimore terminals could see MVFR
ceilings to start off the day on Monday. Expect improving conditions
for the second half of the day and into Tuesday. A period of VFR
conditions are likely through Tuesday, accompanied by dry weather
and a shift from east to southeasterly winds.

&&

.MARINE...
Lighter winds are expected today out of the northeast switching to
the southeast this afternoon as weak high pressure pushes off the
southeast U.S coast.

SCA conditions return Saturday under increased onshore east to
northeast flow. Low pressure will pass south of the waters Saturday
night into Sunday leading to a continuation of SCA conditions. This
will be especially true over middle and lower portions of the bay
and lower tidal Potomac where a bit more of the gradient winds will
be realized. Numerous showers and perhaps a few thunderstorms will
accompany this system Saturday into early Sunday. Winds Saturday
night into Sunday will gradually shift to north-northeasterly as the
low slides south of the region. Northeasterly winds diminish Sunday
night and become light and southeasterly heading into Monday
afternoon and evening.

The relatively tight gradient formed by low pressure off the
Carolina coast and high pressure over the northeastern U.S. may keep
small craft conditions in place for Monday. This is especially the
case over the more southern waters. Such gradients weaken into
Monday evening/night and Tuesday. Initial easterly flow gives way to
southeasterly winds by Tuesday, accompanied by gusts staying
under 15 knots.

&&

.TIDES/COASTAL FLOODING...
Persistent onshore flow through the weekend will raise tidal
anomalies across the waters. This brings a number of locations into
Action stage, with the more sensitive sites pushing into Minor.
Water levels should stay elevated into early next week before
onshore winds eventually abate.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

$$

SYNOPSIS...EST
NEAR TERM...EST
SHORT TERM...EST
LONG TERM...BRO
AVIATION...BRO/EST
MARINE...BRO/EST
TIDES/COASTAL FLOODING...BRO