Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 130759

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
359 AM EDT Thu May 13 2021

High pressure will gradually drift eastward from the Midwest
today and Friday, reaching the east coast by Saturday. A warm
front may lift toward the area early next week and stall nearby.


High pressure remains centered over the Midwest this morning. A
nearly latitudinal shortwave trough is drifting south into the
area, which is causing scattered mid level clouds to develop.
Between the clouds and a lingering 5 mph northwest wind in
spots, temperatures are quite variable this morning. The
greatest chance for frost/freeze conditions remains in the
advised areas.

The pattern will shift little today, with the shortwave trough
axis very slowly drifting southeastward. Scattered to broken
clouds will remain possible, but there should be enough sunshine
to allow temperatures to rise into the mid 60s to lower 70s (50s
in the mountains). Skewed toward warmer guidance due to recent
cool bias in guidance. Some guidance tries to develop a few
showers over the terrain this afternoon and evening, but
forcing, moisture, and instability will be limited, so have kept
the forecast dry.

Calm and dry weather is expected to continue into tonight.
Troughing will remain overhead, so can`t rule out some cloud
cover persisting. That, along with slightly higher dew points,
should result in a lower threat for frost/freeze conditions.
However, it could be close in some of the valleys in the Potomac
Highlands if conditions become a bit more ideal. Most areas will
be in the upper 30s to mid 40s.


High pressure will remain west of the area Friday. Recent
guidance has trended toward the 500mb trough consolidating near
the coast or offshore. While a shortwave at the upper levels
will still be moving through, overall forcing will likely be a
little weaker. This will keep shower development isolated to
widely scattered and generally focused near the terrain. A few
thunderstorms are possible if instability is deep enough.
High temperatures will likely rise another degree or two. The
surface high will be nearly overhead by Friday night. While
radiational cooling will be a factor, a further increase in dew
points should keep low temperatures in the upper 30s to mid 40s.

The center of the surface high will be near the east coast on
Saturday. Forcing will be more subtle, but there could still be
some vorticity advection in northwest flow aloft, and moderate
mid level lapse rates will result in weak instability. Any
shower or thunderstorm development should remain isolated to
widely scattered. The very slow warming trend should continue,
with high temperatures reaching the lower to mid 70s. Most of
Saturday night should remain dry and a touch milder. A small
subset of guidance brings showers produced by isentropic lift
into the mountains late, but greater precipitation chances will
hold off until Sunday.


The high pressure system that will be in place over the eastern US
this week will shift off-shore on Sunday. This will lead to winds
becoming more south to southeasterly leading to increase advection
of warm and moist air. Over the past few days, the models have been
trending wetter for the Sunday late afternoon and into evening
period. There is model disagreement on the extent of the
precipitation threat on Sunday. GFS/GEFS is the wettest solution
while the Euro and its ensemble is more bullish with the
precipitation. It seems the main disagreeing factors is the extent
of moisture advection along with the strength of the upper level
shortwave. I have adjusted up Sunday`s precipitation chances to
account for the wetter trends.

A frontal boundary will become stalled over our region on Monday and
remain in the area through the middle parts of next week. This
boundary will become the focus for the formation of showers along
with the possibility for thunderstorms. Temperatures through the
middle parts of next week will trend upwards with highs reaching
back up into the upper 70s to lower 80s by Wednesday. The main
question during this period will be where the front ends up
settling. If the boundary is further north, our region would observe
more warm air advection and an increase thunderstorm threat. A
further south solution would see less moisture and heat advection but
our region would see increased overruning precipitation. I don`t
believe flooding is a concern at this time considering our region
hasn`t see a lot of precipitation over the past month. If the
precipitation threat continues through the end of the week, we could
start to see an isolated flood threat.


High pressure over the Midwest will result in VFR conditions and
light northwest winds through Friday. The chance of showers has
decreased for Friday, though there could be an isolated one near
MRB/CHO. Light south winds develop by Saturday as high pressure
nears the coast. There could be a shower near MRB due to an
upper level disturbance.

Sky cover will become broken to overcast on Sunday and remain mostly
overcast through the middle parts of next week. Showers will be a
continued threat at all terminals Sunday through Wednesday. SubVFR
conditions can`t be ruled out during this period. Winds will be
mainly out of the south to southwest on Sunday but the wind
direction will become more uncertain next week as it will be highly
dependent on the location of a stalled boundary.


Northwest winds will continue through Friday with high pressure
over the Midwest. Still some potential for a brief surge of
higher winds over the middle portions of the bay this morning,
but did trim the end time of the SCA to 9 AM. Will be evaluating
if this can be cancelled altogether. Otherwise, sub-SCA
conditions are expected through the end of the week, with winds
turning southerly by Saturday.

Showers and isolated thunderstorms will be a continued threat for
our marine waters Sunday through early next week. SubSCA conditions
expected at this time.


MD...Frost Advisory until 9 AM EDT this morning for MDZ501-502.
     Freeze Warning until 9 AM EDT this morning for MDZ001.
VA...Frost Advisory until 9 AM EDT this morning for VAZ025>027-029-
     Freeze Warning until 9 AM EDT this morning for VAZ503.
WV...Frost Advisory until 9 AM EDT this morning for WVZ050-055-502-
     Freeze Warning until 9 AM EDT this morning for WVZ501-503-505.
MARINE...Small Craft Advisory until 9 AM EDT this morning for ANZ534-


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