969
FXUS61 KLWX 291410
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1010 AM EDT Sat Jun 29 2024

.SYNOPSIS...
As high pressure departs, a warm front lifts through the region
this morning which will bring showers and thunderstorms back to
the region. A cold front will then push through on Sunday,
bringing yet another chance for afternoon showers and
thunderstorms. A strong area of high pressure settles over the
area during early portions of next week leading to cooler and
drier conditions. A more summerlike pattern returns by the
middle of the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Some isolated light showers are forming just west of the
Chesapeake Bay this morning in low level theta-e advection and
weak elevated instability. It appears this chance of showers
should diminish by early afternoon. Low level clouds continue to
hang tough east of the Blue Ridge as they are wont to do, but
visible satellite is showing an increasingly convective/broken
appearance along the I-95 corridor. Given a rather elevated and
weak inversion on the 12Z IAD sounding, a healthy south wind,
and summer solar angle, these low clouds likely mix out by
midday, although scattered/broken clouds will persist. Latest
guidance continues to support a delayed convective threat (mid-
late afternoon in the mountains, evening elsewhere). Therefore
will only be tweaking hourly trends at this point.

Previous discussion:

A stationary front currently analyzed over northeastern
Tennessee down into the Carolinas is forecast to lift
northeastward today. This boundary should push through the local
area during the morning hours which leads to additional
moistening. Forecast dew points rise into the low 70s, which
coupled with highs in the low 90s will support heat indices
between 98 and 103 degrees. This net contribution will increase
instability levels across the area, especially west of the Blue
Ridge. The latest guidance increases surface-based CAPE values
to around 2,000 J/kg, while instability is weaker and more
capped off to the east. Thus, have the initial threat of showers
and thunderstorms confined to west of the Blue Ridge. This will
be accompanied by some risk of severe thunderstorms. However, an
additional round looms which may impact more of the region.

While high-resolution models are somewhat at odds with another
in terms of storm coverage, a few solutions carry a squall line
of sorts tracking into western Maryland by around 8-9 PM. This
forward propagating line is set to track across the Shenandoah
Valley. Areas farther downstream have more unknowns as the
convection may decay while crossing the Blue Ridge. However,
given the degree of heating/moistening of the atmosphere coupled
with some modest perturbations in the flow aloft, have kept a
threat for storms across the I-95 corridor. Confidence is lower
with this aspect of the forecast though. As of right now, the
Storm Prediction Center has a Slight Risk in place from north-
central Maryland back into the eastern West Virginia panhandle
and western Maryland. The main threats are damaging wind gusts,
frequent lightning, and a brief isolated tornado.

Nighttime lows will be very warm with mainly 70s across the
area. 80 degrees is possible over D.C. and Baltimore, with mid
60s to low 70s in the higher terrain.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Some residual showers are possible to start off the morning,
with some cloud cover possibly lingering into the core heating
period. However, do think any cloud cover should erode in enough
time to allow ample heating of the boundary layer. Persistent
warm advection is expected to further moisten the low levels
which yields mid 70s dew points across the region. With high
temperatures rising up into the mid 90s, heat indices could
easily rise to 105 degrees and slightly above. This would
warrant Heat Advisories so this bears watching as the next round
of guidance arrives in the mid-morning.

Besides the excessive heat and humidity, another round of strong
to severe thunderstorms may impact the area. The culprit is a
seasonably strong cold front which arrives during the peak in
the diurnal heating cycle. Forcing aloft is more pronounced than
the previous day, although the better height falls are confined
to upper portions of the Mid-Atlantic. Multiple rounds of
showers and thunderstorms are possible, especially east of the
Blue Ridge. The Storm Prediction Center has maintained a broad
Slight Risk area extending from I-81 eastward. All severe
hazards are possible, but damaging wind gusts stand out as the
most likely threat. The eastward progressing cold front reaches
the Chesapeake Bay during the evening hours before settling
into the coastal Atlantic by late Sunday night. Shower activity
should come to an end as winds shift to northwesterly in the
wake. While most see lows in the 60s, many can expect
temperatures in the 50s to the west of the Blue Ridge.

A strong dome of surface high pressure advances from the Upper
Midwest toward the Great Lakes. A seasonably cool air mass
arrives on Monday with 850-mb temperatures falling to around
10-12C. The well-mixed boundary layer yields dry adiabatic
profiles down to the surface. This favors high temperatures in
the upper 70s to low 80s which is around 5 to 10 degrees below
average for early July. Expect plenty of sunshine with northerly
winds gusting up to 20 mph. Cooler nighttime conditions push
into locations to the east with lows in the upper 50s to mid 60s
east of the Blue Ridge. This is accompanied by clear skies as
the surface high is positioned over the northeastern U.S.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
The long term period will start will begin with mild and dry
conditions as high pressure and upper level ridging build over the
region. Temperatures will be slightly below normal with highs in the
mid 80s for most(70s at higher elevations) and lows in the upper 50s
to low 60s. Temperatures warm to the upper 80s to mid 90s Wednesday
and Thursday as high pressure shifts off the east coast and
southerly flow ushers in warm and moist air. A nearby warm front
will bring renewed precipitation chances, mainly to those along and
west of the Alleghenies on Wednesday.

For the Fourth of July, a nearby cold front will bring precipitation
chances to the forecast area. There is a chance of showers and
thunderstorms throughout the area, with chances peaking in the
afternoon as instability increases with daytime heating. Shower and
thunderstorm chances continue Friday as the aforementioned frontal
boundary remains nearby. Precipitation chances peak in the afternoon
with the greatest chance of precipitation remaining in the
northwestern portions of the forecast area.

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
Low stratus persist east of the Blue Ridge, although visible
satellite shows an increasingly convective/broken appearance
from DCA/BWI southward. Expect these clouds to continue to mix
out through midday, although scattered/broken VFR clouds will
likely persist. An isolated shower is also possible through
midday at BWI/MTN, but should not present any impacts. Southerly
winds are also increasing this morning, with some gusts to 20 kt
possible this afternoon.

Any convective threats likely are delayed across the terminals,
mainly after 00Z. Have placed a 4-hour VCTS group at all
locations, generally during the late evening to early overnight
hours. Some showers possibly linger through the night.

Sunday brings another potential active weather day, all ahead of
an approaching cold front. Restrictions are again possible as
these storms races across the area. Right now the afternoon to
early evening would be the most likely timing of such activity.
Any convective activity should diminish by later in the evening
with northwesterly flow ensuing in the wake. For Monday, VFR
conditions are expected with northerly gusts up to 15 to 20
knots.

VFR conditions and light winds are expected at all terminals Tuesday
with high pressure overhead. There is a slight chance of
precipitation at KMRB on Wednesday with all other terminals expected
to remain dry.

&&

.MARINE...
Made several adjustments to the Small Craft Advisory. Southerly
channeling gusts continue to affect the narrower portion of the
bay north of Drum Point through midday. It appears all waters
except perhaps the Potomac from Indian Head to Cobb Island could
experience gusts up to 20 knots throughout the afternoon and
early evening. The stronger winds will gradually contract to the
main channel tonight. Some convective potential late in the
evening to early overnight may require Special Marine Warnings.
However, confidence is low at this time.

Small Craft Advisories may be needed early Sunday and again
behind a cold front late Sunday into Monday. In between, strong
to severe thunderstorms could impact the waters during the late
afternoon to early/mid evening. Special Marine Warnings become
likely with this activity before the convection pushes off to
the east overnight.

Light and variable winds Tuesday morning before southwesterly in the
afternoon and are expected to remain below SCA criteria. Small Craft
Advisories are possible Wednesday as southerly winds gust over 15
knots in the afternoon.

&&

.TIDES/COASTAL FLOODING...
Tidal anomalies rise today in southerly flow. Winds shift to more
southwesterly Sunday afternoon, allowing for tidal anomalies to fall
slightly. During southerly flow, most tidal sites will reach Action
stage while Havre de Grace and Annapolis reach minor flood stage
during the high tide cycle early Sunday morning.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 AM EDT Sunday for ANZ530-534-537-
     539>541-543.
     Small Craft Advisory until 6 AM EDT Sunday for ANZ531>533.
     Small Craft Advisory until 9 PM EDT this evening for ANZ535.
     Small Craft Advisory until 1 AM EDT Sunday for ANZ538-542.

&&

$$

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