National Weather Service Weather Forecast Office

• Home
• Site Map
• News
• Organization
• Search for: NWS All NOAA 

-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KLWX 130117
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
917 PM EDT Sun Sep 12 2021

.SYNOPSIS...
-- Changed Discussion --
High pressure will remain anchored offshore through much of the
week. A backdoor cold front will approach from Pennsylvania
Monday night, then retreat northward as a warm front Tuesday.
Low pressure may develop near the Carolina coast Thursday into
Friday as a weakening cold front approaches from the Great
Lakes.
-- End Changed Discussion --

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
-- Changed Discussion --
High pressure settleD off-shore and will remain in place
through most of this week. The positioning of the high will lead
to a continued south to southwesterly flow that will advect in
warm and moist air into our region. Overnight lows will be near
to slightly above normal in the mid to upper 60s to lower 70s.
An low level jet at 850 will increase late this evening and
into early Monday in the western parts of our region. Possible
50 to 55 knots aloft may translate to overnight gusts in the 25
to 35 knots range over our higher elevations.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY NIGHT/...
-- Changed Discussion --
High pressure will remain situated along the eastern seaboard
through the middle parts of this week as weak upper level
ridging remains to the north of our region. A backdoor front is
forecast to drop southward from Canada and stall to the north
Monday through the middle parts of this week. The high to south
of our region will lead to continued advection of warm and moist
air into our region. The warmest weather of September will
occur Monday and Tuesday as highs will peak in the upper 80s to
lower 90s with a few areas potentially breaking daily high temp
records. Overnight lows will be near or above average in the mid
to upper 60s to lower 70s to start this week.

The main question for the Monday through Tuesday period will be
whether showers and thunderstorms will be possible in any parts
of our region. Increasing temperatures and moisture will lead
to increasing instability in our region through the early parts
of this week. Ensembles are suggesting that CAPE values above
2000 will be likely both Monday and Tuesday with shear values
around 25 knots. Although we be relatively unstable to start
this week the threat for convection will be limited due to the
best forcing focused to the north near the frontal boundary
along with increasing heights. High res guidance and global
models do indicate that storms could form along our higher
elevations late Monday evening or a storm to the north could
drop southward into our region. Overall, the confidence remains
low on any widespread thunderstorm threat but a pulse storm
forming off the bay breeze or higher elevations can`t be ruled
especially considering how warm and unstable our region will be
this week.
-- End Changed Discussion --

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
-- Changed Discussion --
By the middle of the week, gradual amplification in the flow
will take shape across the Great Lakes while what remains of
Tropical Storm Nicholas is slated to near the Texas/Louisiana
interior border. A core of the forcing with the approaching
trough should stay north of the Mid-Atlantic while some mid-
level vorticity will peel away from Nicholas and move toward the
Appalachian chain. At the same time, the National Hurricane
Center is monitoring a region of disturbed weather in the
vicinity of the Bahamas. There`s a 50 percent chance a tropical
depression will form over the western Atlantic over the next 5
days. With the mentioned upstream trough approaching the Eastern
Seaboard, this system along the Gulf Stream should stay
offshore. Conditions remain somewhat unsettled with the Mid-
Atlantic region confined to the warm sector. The cold front
attendant to the approaching trough will attempt to move through
Wednesday evening before stalling to the north as a stationary
boundary. The belt of westerlies further retreats into eastern
Canada which leaves more summer-like gradients into next
weekend. If the disturbance over the western Atlantic moves
closer to the coast, some uptick in northerly winds are possible
inside I-95. Upper ridging remains largely in charge to finish
out the weekend with summertime heat and humidity a likely
fixture. This would be accompanied by some chance for daily
showers and thunderstorms, albeit with low confidence in
coverage and intensity.

For Wednesday, expect high temperatures to rise into the upper
80s to low 90s, similar to preceding days. Humidity is expected
to increase a tad into Thursday and Friday as moisture from
Nicholas ejects northeastward from the western/central Gulf
Coast region. An increase in cloud cover should lower
temperatures back into the mid to upper 80s for Thursday and
Friday. Mid 80s are forecast to remain commonplace into next
weekend given the above average heights in place. Multi-model
ensembles do indicate quite a bit of temperature spread with
upper 70s to low 90s being in that 10th and 90th percentile
range, respectively. Overnight temperatures remain mild with
lows in the mid 60s to low 70s east of the Blue Ridge. Farther
west, low to mid 60s will be more commonplace. These forecasts
are easily 10 to 15 degrees above mid-September climatology.
-- End Changed Discussion --

&&

.AVIATION /01Z MONDAY THROUGH FRIDAY/...
-- Changed Discussion --
VFR conditions along with winds mainly out of the south to
southwest are expected through Tuesday as high pressure remains
off to the southeast of our region while a front stalls to the
north. The main uncertainty for our terminal forecasts will be
whether a pop up thunderstorm will form and impact one of our
terminals.

Mainly VFR conditions can be expected on Wednesday and Thursday
given prevailing southerly winds. With a summer-like air mass
in place, diurnally driven showers and thunderstorms may lead to
brief restrictions at local terminals. Confidence is too low to
pinpoint any further details at this time.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
High pressure positioned off the southeast US will lead to
periods of southerly channeling winds. SubSCA conditions
expected through Monday but Small Craft Advisories maybe needed
on Tuesday.

Ahead of the approaching cold front, southerly winds will
continue with gusts 15 to 20 knots at times. This supports the
potential for Small Craft Advisories for Wednesday, perhaps into
the night. The advancing front stalls which keeps southerly
flow in place into Thursday. Winds do come down in strength a
bit. While confidence is low, some daily thunderstorm threat
exists which may require convective-based hazard products.
-- End Changed Discussion --

&&

.CLIMATE...
-- Changed Discussion --
Unseasonably warm weather this week may result in some record
warm temperatures, though at this time we think the probability
is below 50 percent. As a reminder, low temperature records are
for the period 1 AM EDT to 1 AM EDT (midnight EST to midnight
EST).

Record high maximum temperatures
SITE   Mon 9/13    Tue 9/14    Wed 9/15    Thu 9/16
DCA   94 in 1925  95 in 2016  97 in 1930  96 in 1991
BWI   97 in 1952  94 in 1931  97 in 1927  98 in 1991
IAD   94 in 1974  95 in 1998  95 in 1998  97 in 1991

Record high minimum temperatures
SITE   Mon 9/13    Tue 9/14    Wed 9/15    Thu 9/16
DCA   74 in 2008  75 in 2008  75 in 2005  75 in 2005
BWI   74 in 1915  74 in 1915  74 in 1915  73 in 2005
IAD   71 in 2018  72 in 2018  71 in 2005  69 in 2005
-- End Changed Discussion --

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ539.
     Small Craft Advisory until 6 AM EDT Monday for ANZ532-533-537-
     540-541.
     Small Craft Advisory until 9 AM EDT Monday for ANZ534-543.
-- End Changed Discussion --

&&

$$
SYNOPSIS...JMG
NEAR TERM...JMG/RCM
SHORT TERM...JMG
LONG TERM...BRO
AVIATION...BRO/JMG
MARINE...BRO/JMG
CLIMATE...BRO/RCM