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

249
FXUS61 KLWX 210232
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
932 PM EST Wed Nov 20 2019

.SYNOPSIS...
High pressure over the Ohio Valley will build eastward over the Mid-
Atlantic through Thursday, then pull offshore Thursday night. A cold
front will follow for Friday. Low pressure is expected to develop
over the Mid-Mississippi Valley Friday night, then track across
the area this weekend. High pressure will likely return early
next week before another area of low pressure develops to the
west by the middle portion of next week.

&&

.NEAR TERM /THROUGH THURSDAY/...
High pressure centered over the Ohio Valley this evening will
slowly progress southeastward towards the Carolinas Thursday
afternoon. At 02Z, satellite shows a patch of clouds extending
from Central PA to the Potomac valley, which have been stubborn
to dissipate this evening. Expecting a gradual erosion of these
clouds with clear skies by daybreak. Winds east of I-95 have
also stayed up, as the pressure gradient between the high to the
west and the low pressure east of the Canadian Martimes still
have influence with NW sfc flow.

On Thursday, high-to-mid clouds will encroach from the west
after Noon as the high pressure slides from the Ohio Valley to
the Carolinas, putting the CWA under return flow from the S/SW.
No impactful weather through Thursday, other than bright
sunshine for Thursday morning commute.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
The surface high will slide to the south and east Thursday as
low pressure moves toward the Great Lakes. The day will be dry,
but mid level moisture will result in increasing clouds. High
temperatures will be seasonable in the 50s. Clouds will increase
further Thursday night, holding temperatures in the 40s for most
areas. The low will move toward Quebec, and a few showers along
the trailing cold front may approach the Potomac Highlands late.

The cold front will move into the area on Friday, but it will
likely be able to warm up into the upper 50s to lower 60s before
cold advection can begin. The best forcing/lift will move north
of the area, while a secondary vort max will slide across
southern Virginia. Thus expecting the best chance of rain
showers to be in the Potomac Highlands and then extending across
central Virginia to southern Maryland. Amounts should be light
with westerly downslope flow promoting drying. The front will be
to the south Friday night and most of the night should be dry.
Isentropic lift ahead of the next low pressure system
approaching from the Tennessee Valley may spread some light
precipitation toward the southwestern corner of the CWA late,
but most of it will hold off until Saturday.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
The latest model guidance is struggling with finer scale details
regarding potential trough interaction (or lack thereof) during the
first half of the upcoming weekend, though models generally agree on
the large scale pattern.

An upper-level low is expected to be departing over the Canadian
Maritimes early Saturday. However, significant model spread is noted
with regards to the strength of the upper-low, which affects the
strength of cold northerly flow into the Mid-Atlantic in its wake.
Meanwhile, the jet stream will remain split into two branches over
the U.S., with a low in the southern branch ejecting from near the
Four Corners region across the Plains into the Mississippi and
Tennessee Valleys at the start of the weekend. Models are also
exhibiting notable differences with the evolution of this system,
particularly on exactly when it ejects eastward. Lastly, there`s
also substantial timing and strength differences with a system
trailing in the northern branch of the upper jet. This all results
in differences in the track of a surface low and associated
precipitation shield which will likely affect the area Saturday
afternoon through the first half of Sunday in some form (exact
timing TBD given aforementioned timing spread).

At this time, it appears best to stick with a general model blend
since there doesn`t seem to be any one model that has an advantage
over another in the current pattern. This blend results in the
highest precipitation chances from Saturday afternoon through Sunday
morning, with temperatures generally 5 to 10 degrees below normal
for late November. The resultant precipitation type should be mainly
rain given the current projections, but there is at least a low-end
freezing rain threat over the climatologically favored higher
elevations (i.e. Allegheny Front, Blue Ridge, Catoctins). A period
of upslope snow showers is possible in northwest flow behind the
departing surface low Sunday over typically favored west-facing
slopes of the Allegheny Front.

High pressure is slated to build in from the west later Sunday
through Tuesday. Thereafter, a very strong and sharp upper trough
will deepen across the midsection of the country, sending a front
towards the Mid-Atlantic by Wednesday. Long range model guidance has
shown above average agreement and consistency regarding this system
in a broad sense, but has wavered some on details locally. A general
average of the last couple day`s worth of model runs yields
widespread precipitation amounts of around a half an inch, which is
rather robust for a forecast a week away. But the latest run of
models seems to have backed off a touch. Either way, it seems more
likely than not that our region will be on the "warm" side of the
storm with a subsequently lower risk of wintry precipitation.
There`s plenty of time for this to change back and forth, though, so
considering the vulnerability due to increased travel and higher
likelihood of an unsettled weather pattern, it`s prudent to monitor
later forecast updates heading into the holiday next week.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
Mid-BKN clouds will persist through midnight at KBWI, KIAD and
KDCA. Northwest winds will be NW at all terminals through Noon
Thursday, but much lighter than earlier today.

VFR conditions and southerly winds less than 10 kt are expected
Thursday and Thursday night as high pressure slides across the
area. A cold front will slide across the area Friday. The metro
areas have a relatively low chance of seeing any showers or
impacts to ceilings/visibility, with a greater chance of brief
rain at CHO. Gusty northwest winds are possible in the front`s
wake.

Conditions may drop to IFR on Saturday as low pressure passes near
or over the region, increasing the chance of lower ceilings and
visibility in rain. Condtions should return to VFR Sunday as low
pressure exits to the east, leaving behind drying but gusty
northwest winds in its wake.

&&

.MARINE...
Dropped prior SCA as observations and trends showed decline with
sunset in bay above North Beach and north of 301 Bridge on tidal
Potomac. However, SCA is continued for our southernmost
Chesapeake and lower Potomac zones until 6 AM. Lighter winds
are expected Thursday under high pressure.

SCA conditions may occur in southerly channeling on the Bay
Thursday night. A cold front will cross Friday, and SCA
conditions are probable in its wake, with decreasing winds by
Friday night.

The wind speed forecast is a little uncertain Saturday and will
depend upon the strength and track of a nearby surface low. There is
higher certainty in gusty northwest winds approaching or exceeding
Small Craft Advisory thresholds on Sunday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 AM EST Thursday for ANZ533-534-
     537-543.

&&

$$
SYNOPSIS...ADS
NEAR TERM...Lee
SHORT TERM...ADS
LONG TERM...DHOF
AVIATION...ADS/DHOF
MARINE...ADS/DHOF

NWS LWX Office Area Forecast Discussion