Forecast Discussion for Willowsford Farm and vicinity from the NWS in Sterling, Virginia

218
FXUS61 KLWX 210139
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
939 PM EDT Sat Jul 20 2019

.SYNOPSIS...
High pressure will remain over the southeastern United States
overnight into Sunday. A cold front will approach from the
Great Lakes Sunday and Monday before passing through our area
Monday night into Tuesday. High pressure will return for the
middle portion of next week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
During the past few hours, we have had a couple of showers
develop over Hardy and Hampshire Counties in West Virginia. In
that allotted time, these showers materialized into
thunderstorms, becoming severe at times as they crossed into
and over extreme northern and northeastern Virginia. Currently,
there is one severe thunderstorm moving into the northern half
of the District of Columbia and could bring winds over 55 mph,
lightning, heavy rain and perhaps some hail. Additional showers
and thunderstorms are weakening over southern Pennsylvania as
they move toward our CWA. Little to no impacts expected with
this activity.

Upper-level high pressure will remain overhead tonight, causing
very warm and humid conditions to continue. Latest guidance
support current low temperatures forecasts...in the 70s for
most, lower and middle 80s in downtown Washington and Baltimore.
Heat indices for urban areas along the I-95 Corridor may hold
in the 90s.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/...
The ridge will start to break down Sunday, but not til late in
the day. In that aspect, Sunday should be similar to Saturday.
However, convective complexes across the Great Lakes likely to
toss debris clouds toward the Mid Atlantic. In addition, winds
likely to become more westerly as heights start to lower. That
could lead to warmer temperatures but lower humidity. Further,
850 temperatures progged to be a degree or two lower. Based on
latest guidance and very little rainfall relief from this
afternoon`s activity, we have issued an Excessive Heat Warning
and Heat Advisory for a large portion of our CWA on Sunday
between 11am and 9pm, similar to Saturday.

The cold front will continue to drop south approaching the Ohio
Valley Sunday night, and height falls will continue as a result.
A chance of showers and thunderstorms enter the forecast by
early afternoon. Have a slow but steady increase in precip
potential as the dynamic situation improves Sunday evening and
overnight. The potential for damaging wind gusts exists with
these thunderstorms.

The cold front still forecast to pass through the Ohio Valley
Monday and into our area Monday night. It looks like there will
be a pre-frontal trough axis also capable of focusing
thunderstorm activity. Lower heights and a southwest flow aloft
combined with low-level boundary forcing and ample instability
will be more than enough to trigger showers and thunderstorms,
especially during the afternoon and evening hours. Increased
shear profiles along with the high instability suggests that
storms will have the chance to be severe. Monday will not be as
hot as recent days due to more cloud cover and more coverage of
showers and thunderstorms...but that all depends upon how these
thunderstorms evolve. There is still the potential for pockets
of rapid heating.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Tuesday`s weather will be dependent on the southward progression of
the surface front. 12z guidance has trended further south with the
initial positioning of the boundary Tuesday morning, placing it
over far southeastern portions of our forecast area or even further
southeast. As the main trough axis approaches from the northwest, it
will drive the frontal boundary further southeast through the day.
Shower activity will extend well north of the boundary, but with the
front trending more progressive, shower activity should primarily be
confined to locations south and east of DC. The trend throughout the
day will be for showers to gradually move out of the area, with most
locations staying dry during the afternoon hours. It will be much
cooler and less humid compared to previous days, with highs in the
upper 70s to low 80s and dewpoints dropping through the 60s. Even
where conditions remain dry, skies are expected to stay mostly
cloudy as the upper trough axis starts to approach from the
northwest.

High pressure will build to our west on Wednesday, before eventually
becoming centered close to overhead on Thursday and Friday. Mostly
sunny skies are expected, with below average temperatures (highs in
the low-mid 80s) and humidity levels (dewpoints in the upper 50s to
low 60s.

&&

.AVIATION /02Z SUNDAY THROUGH THURSDAY/...
VFR conditions are expected through the valid TAF period. There
is a chance that a thunderstorm this evening near Washington
D.C. could reduce conditions to MVFR or IFR for a period of
around half hour. Otherwise, VFR through the night.

The better chance for thunderstorms come Sunday afternoon and
especially Sunday night. These thunderstorms may produce gusty
winds and brief flight restrictions. Showers and thunderstorms
will be more numerous Monday and Monday night, especially during
the afternoon and evening hours. Some storms may be contain
strong wind gusts.

Sub-VFR conditions are possible Tuesday morning, but conditions
should improve late Tuesday through the middle of the week as a
frontal boundary progresses off to the southeast and high pressure
builds in from the west.

&&

.MARINE...
High pressure to the south will likely cause a west to southwest
flow through Sunday. Wind speeds should remain below Small Craft
Advisory criteria for most areas. Mesoscale guidance suggesting
southerly channeling possible again tonight. Signals don`t look
as strong as yesterday, which didn`t pan out to the extent
requiring an Advisory. Will raise forecast gusts this evening
but keep forecasts below SCA thresholds.

In addition, a stray thunderstorm is not out of the question
this evening. There is a better chance (isolated to scattered
coverage) for thunderstorms later Sunday afternoon and Sunday
night as a cold front approaches. Some storms may produce gusty
winds. The cold front will cross the waters Monday night. By
that point, precipitation likely to to interfere with mixing
processes.

Sub-SCA level winds are expected on Tuesday, along with the
potential for some lingering morning showers. Conditions should
improve Tuesday afternoon through the middle of the week as high
pressure builds in from the west.

&&

.CLIMATE...
Here are the warm temperature records for this weekend:

Record Daily High Temperatures
        Jul 20       Jul 21
DCA    106 (1930)   104 (1926)
BWI    102 (1930)   104 (1930)
IAD    101 (1980)   101 (1991)

Record Daily Warm Low Temperatures
         Jul 20       Jul 21
DCA     82 (2015)    82 (1987)
BWI     80 (1930)    83 (1930)
IAD     75 (2015)    77 (1987)

Highest July Temperatures
        Highest Max       Warmest Low
DCA   106 (7/20/1930)   84 (7/24/2011, 7/23/2011, 7/16/1983)
BWI   107 (7/10/1936)   83 (7/21/1930)
IAD   105 (7/22/2011)   78 (7/24/2010, 7/8/2010)

All-time Highest Temperatures
        Highest Max                 Warmest Low
DCA   106 (7/20/1930, 8/6/1918)   84 (7/24/2011, 7/23/2011, 7/16/1983)
BWI   107 (7/10/1936)             83 (8/5/1930, 7/21/1930, 6/6/1925)
IAD   105 (7/22/2011)             79 (8/8/2007)

Longest stretches of consecutive days with high temperatures at or
over 100 degrees at IAD:
2 (7/7-8/2012, 7/21-22/2011, 7/6-7/2010, 8/16-17/1997, 7/16-17/1988
and 7/20-21/1980)

Longest stretch of consecutive days with low temperatures at or over
80 degrees at DCA:
5 (7/16-20/2013)

Longest stretch of consecutive days with low temperatures at or over
80 degrees at BWI:
4 (8/8-11/1900)

Temperature records for the Washington DC area have been kept at
what is now Ronald Reagan Washington National Airport (DCA) since
1941. Additional temperature records observed downtown extend the
period of record back to 1872.

Temperature records for the Baltimore MD area have been kept at what
is now Baltimore-Washington International Thurgood Marshall Airport
(BWI) since 1950. Additional temperature records observed downtown
extend the period of record back to 1872.

Temperature records for the Sterling/Dulles VA area have been kept
at what is now Washington Dulles International Airport (IAD) since
1960.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Excessive Heat Warning from 11 AM to 9 PM EDT Sunday for
     DCZ001.
MD...Heat Advisory from 11 AM to 9 PM EDT Sunday for MDZ502.
     Excessive Heat Warning from 11 AM to 9 PM EDT Sunday for
     MDZ003>006-011-013-014-016>018-503>508.
VA...Heat Advisory from 11 AM to 9 PM EDT Sunday for VAZ025>030-
     036>040.
     Excessive Heat Warning from 11 AM to 9 PM EDT Sunday for
     VAZ031-050>057-501-502-505-506.
WV...Heat Advisory from 11 AM to 9 PM EDT Sunday for WVZ050-051-055-
     502-504.
     Excessive Heat Warning from 11 AM to 9 PM EDT Sunday for
     WVZ052-053.
MARINE...None.

&&

$$
SYNOPSIS...KLW
NEAR TERM...HTS/KLW
SHORT TERM...HTS
LONG TERM...KJP
AVIATION...KLW/KJP
MARINE...KLW/KJP
CLIMATE...KLW

NWS LWX Office Area Forecast Discussion