Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 061856
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
256 PM EDT Thu May 6 2021

.SYNOPSIS...
High pressure will build in from Canada through this evening.
Low pressure and its associated cold front will approach tonight
before passing through Friday. High pressure will build toward
the region Saturday through Saturday night before low pressure
and its associated warm front possibly impact the area Sunday. A
cold front will pass through Monday and high pressure will
return for the middle portion of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Today`s air mass is in stark contrast from earlier in the week
with early afternoon temperatures staying in the low 60s
accompanied by dewpoints in the 30s. After a mostly sunny start
to the day, mid to high clouds have been on the increase ahead
of a trough currently traversing the Upper Great Lakes. However,
the forecast will remain dry through tonight, but with a further
increase in cloud cover. In spite of the seasonably cool and dry
air mass in place, temperatures will not fall as much overnight
given the mostly cloudy skies. Widespread 40s are likely with
low 50s closer to the urban city centers.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Surface winds eventually shift to southerly during the morning
hours ahead of the approaching cold front. A band of showers
will break out ahead of this boundary with some of the activity
becoming more moderate in intensity while crossing east of the
Blue Ridge and Catoctin Mountains. High-resolution guidance
suggests the window between 18Z-00Z could be quite wet with a
quarter to half of inch of rain possible, locally approaching an
inch in spots. Precipitable water values should run close to
early May climatology, so do not anticipate any discernible
flood threat. While instability is forecast to be quite meager,
cannot rule out a few rumbles of thunder within the healthier
storm cores. The more concentrated axis of showers should move
into northeastern Maryland by the mid-evening while locations
farther upstream briefly dry out. Daytime highs are likely to
top out in the low/mid 60s in many locations with 40s to low 50s
across mountain locales.

As the shortwave begins to exit off the Delmarva shores,
trailing cyclonic flow moves in as a closed low crosses the
Lower Great Lakes early Saturday. The associated cold advection
with this feature will lead to a chilly night as lows drop into
the upper 30s to low 40s. Near to slightly below freezing
temperatures are possible in western Pendleton/Highland Counties
where a Freeze Watch has been hoisted up. While Saturday will be
drier than the previous day, some instability showers are
expected as the upper low moves through. It is not quite in
phase with the peak in diurnal heating with its track through
the region between 12-18Z. 1000-500 mb thicknesses drop into the
534-540 dm range which is on the lower side for this time of
year. This will make for another chilly day with Saturday`s
highs maximizing in the low 60s, locally into the mid 60s in
central Virginia and southern Maryland. A cold overnight follows
with another chance for freezing temperatures along the
Allegheny Front. Some light warm advection showers could reach
the Potomac Highlands overnight.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
An area of low pressure will approach the region Sunday from the
Ohio Valley. This will bring increased rain chances near daybreak
along the Allegheny Front. Guidance supports upwards of an inch or
two of rain across the far northwest portions of our CWA (Garrett/
Allegheny Counties in MD and Grant/Mineral Counties in WV) which
will need to be monitored for the potential for flooding. A weak
warm front will lift towards the region Sunday afternoon and
evening. The lift provided by the warm front could produce more
showers across the entire region, but do think the threat is
suppressed some east of the Blue Ridge due to the westerly flow
aloft.

A weak cold front looks to move through the region Monday afternoon
before high pressure builds in Monday night into Tuesday. Cannot
rule out a thunderstorm with the frontal passage on Monday, though
showers are favored due to limited instability. Monday`s
temperatures have been increased due to a slightly delayed frontal
passage.

Upper level flow looks to stay zonal (west or slightly west-
northwest), so temperatures do look to stay near or below the (new)
normals across the CWA through midweek. Frost/freeze concerns become
possible Monday and Tuesday night across the higher terrain of the
Allegheny Front if winds relax and clouds break.

Come Wednesday, a trough will move towards the Ohio Valley before
spreading east into the local area Thursday. Ensemble guidance
differs in regard to the exact timing and amplitude of the trough,
and therefore differ in precipitation chances and timing. Some
guidance forms a surface low that impacts the region Thursday. Key
takeaway is unsettled weather returns as early as Wednesday into the
second part of next week with elevated rain chances.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
VFR conditions prevail for the remainder of the day with
northwesterly winds. Winds become lighter overnight before
turning southerly early Friday morning ahead of the next system.
Showers will spread from west to east from the late morning
onward with ceilings likely dropping to MVFR by the afternoon
hours. Cannot rule out an isolated rumble of thunder in the
heavier showers. Behind the front, winds turn northwesterly
Friday evening with a brief drying trend. Additional showers are
possible on Saturday as an upper low crosses overhead. This
could bring brief sub-VFR conditions to area terminals.

MVFR conditions possible due to low clouds and rain showers on
Sunday into Sunday night. VFR conditions likely return Monday with
strong winds out of the NW behind the front.

&&

.MARINE...
Decreasing winds across the waterways will keep sub-SCA
conditions in place through Friday. Winds do shift from
northwesterly to southerly on Friday morning ahead of an
approaching front. Some gusty showers could lead to the
necessity of Marine Weather Statements on Friday afternoon.
Winds turn northwesterly by the evening with the potential for
small craft conditions into the night as well as Saturday.

SCA conditions are likely Sunday through Tuesday, with Gale
conditions possible Monday into Monday evening.

&&

.TIDES/COASTAL FLOODING...
In the wake of yesterday`s cold frontal passage, tidal anomalies
have dropped considerably. Eventually winds turn light and more
variable leading to a marked rise during future high tides. Many
tidal sites move into the Action stage by Friday and Saturday.
In fact, the DC Waterfront is currently forecast to reach Minor
by the high tides on Friday evening and Saturday morning.

&&

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

&&

$$
SYNOPSIS...BJL/BRO
NEAR TERM...BRO
SHORT TERM...BRO
LONG TERM...CPB
AVIATION...BRO/CPB
MARINE...BRO/CPB
TIDES/COASTAL FLOODING...BRO


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