NWS Blacksburg, VA wrote its last area forecast discussion before the first event at Jun 16, 6:01 AM EST, 59 minutes earlier.
Area forecast discussion, Jun 16, 6:01 AM EST, 59 minutes before
Area Forecast Discussion
National Weather Service Blacksburg VA
701 AM EDT Tue Jun 16 2020
.SYNOPSIS...
A broad closed off upper low will meander across South Carolina
through Wednesday, then drift northwest back into southwest
Virginia and West Virginia by Thursday. Meanwhile, high
pressure across the northeastern U.S. will keep an unseasonably
cool northeast flow into the Mid-Atlantic and central/southern
Appalachians. This pattern will result in a continuation of
cloudy, rainy, and unseasonably cool temperatures through the
end of the week. By the weekend, the upper low will weaken and
slowly lift north of the area allowing temperatures to warm
closer to normal and result in decreased chances for rain.
&&
.NEAR TERM /THROUGH TONIGHT/...
As of 400 AM EDT Tuesday...
The overall synoptic pattern will change little during the
period. The broad closed off upper low drifted south and
southeast Monday and is currently located across South Carolina.
The upper low will drift toward the SC coast today and then
begin to slowly retrograde back to the northwest Wed-Thu. By
late Thu into Fri, the upper low will be back over southwest VA
and into WV. From here the low will finally begin to weaken and
lift off to the north.
With the upper low now located south of the region, flow aloft
is easterly. In addition, surface high pressure remains across
the northeast U.S.. This combination results in a deep fetch of
easterly maritime flow through a good portion of the
troposphere. Hence, a cool, saturated, stable flow. However, the
best lift and PVA has been relegated to the coastal and
offshore areas from SC to VA. Thus, we should just expect
persistent light rain and drizzle as precipitation streams
westward around the upper low from the western Atlantic inland
and weakens as it moves further west. The net result will be yet
another cloudy, dreary, damp, and unseasonably cool day, more
like early or mid-April than mid-June. Rainfall amounts will be
light as PWATS have dropped to near 1.00 or slightly less.
Instability is nil and diurnal heating will be very limited.
Thunderstorms are not expected and rain amounts will be light
at best and more on the line of very light to light rain and
periods of drizzle. Rainfall amounts today and tonight are
expected to range from a few hundreths of an inch in the far
western parts of the CWA to perhaps 1/3 to 1/2 inch in the
eastern areas. Pops basically chance far west to categorical
east both today and tonight.
Regarding temperatures, look for another unseasonably cool day
with high temperatures averaging 15-20 degrees below normal,
perhaps even 25 degrees below normal in some areas such as the
New River and Roanoke Valleys. On Monday, we saw all five of our
climate stations set or tie their record low maximum temperatures
for the date. Such is possible again today as temperatures are
expected to remain mostly in the 50s west of the Blue Ridge and
creep into the lower 60s east of the Blue Ridge. Normally, we
should see high temperatures in the lower to mid 80s at this
time of year! The MAVMET MOS captured well the extremely low
maximum temperatures today while the gridded NAMNest and GFS did
a fair job. The NBM was overall 10-15 degrees too warm with too
much influence from climatology.
/Confidence in Forecast Parameters/
Temperatures - Moderate to High,
Precipitation Probabilities - High,
Winds - High,
Thunderstorm Threat - None.
&&
.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
As of 4:30 AM EDT Tuesday...
The persistent cut off low remains in our area for this part of the
forecast. Because of it, cool, wet weather will continue.
Wednesday, we start the day with the closed off low starting to
shift northward as it finally starts to get picked back up by upper
level flow: however, this likely won't happen fast enough for many.
Because of its shift Wednesday, we will see more organized/deeper
rainfall rates with maybe a few isolated storms. Regardless, the
greatest rainfall chances will continue to remain in the east.
Overnight Wednesday into Thursday, the low starts to weaken as it
continues shifting northward and retrogrades into the overall upper
level pattern. As this happens, we will start to gradually lose our
easterly flow and become more situated in a southwesterly flow. This
will start to tap into a more unstable airmass with PWATS near 1.5
inches. That airmass come Thursday afternoon, combined with cool air
aloft allowing steeper lapse rates, will give way to better chances
of scattered storms. Likewise, overall rainfall will become far more
diurnally driven, meaning our greatest POPs will occur during the
day. Temperatures by this point will start to make a recovery,
ranging in the 60s and 70s.
Friday, we finally start to be able to wave goodbye to the cut off
low as we assume a more backside trough like pattern with a weak
ridge approaching from our west. Despite the low exiting, another
afternoon of showers and storms are possible. By this point,
instability looks to increase enough, combined with steepened lapse
rates and modest coverage in higher PWATs for us to have a decent
coverage in thunderstorm activity that will fall apart Friday night.
By this point, our temperatures will be much closer to average for
this time of the year with some areas in the east just starting to
inch into the 80s.
Forecast confidence is moderate to high.
&&
.LONG TERM /SATURDAY THROUGH MONDAY/...
As of 100 PM EDT Monday...
During this portion of the forecast,the persistent upper low over
the mid-Atlantic and southeastern sections of the U.S., is expected
to fill, and open up into a shortwave trough that will eject
northeast towards, then over, New England by Saturday afternoon.
Because of this transition, shower and thunderstorm coverage is
expected to decrease Saturday into Saturday night with
temperatures near to just slightly below average.
Sunday into Sunday night, a shortwave ridge will briefly visit the
region all while the next upper level shortwave trough and
associated surface cold front approach the region from the west. We
are expecting moderating temperatures during this time period, with
diurnal convection across the mountains.
On Monday, we can expect increased coverage of showers and storms,
including locations east of the Blue Ridge, as the upper trough and
surface cold front approach even closer to the region. Temperatures
will continue to inch higher to readings a couple of degrees above
normal for this time of year.
Confidence in the above portion of the forecast is moderate.
&&
.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
As of 700 AM EDT Tuesday...
Poor aviation conditions will continue through the TAF valid
period as a broad cutoff upper low drifts very slowly from
western SC toward the SC coast today, then retrogrades back to
the northwest slowly through the remainder of the week. Surface
high pressure across the northeast U.S. will keep a damp and
cool easterly maritime flow into the region. The result with the
layered clouds will be the continued presence of a strong wedge
for June with much below normal temperatures continuing at
least for a couple more days. As the low drifts to the southeast
today, precipitation will be a bit lighter and less widespread
than yesterday as the bulk of the best lift and PVA is further
east/south and largely along the coast or offshore. Nonetheless,
look for widespread low clouds, fog, drizzle, and periods of
light rain through the TAF valid period with a predominance of
IFR ceilings and MVFR visibilities through 00Z, then prevailing
IFR-LIFR ceilings/visibilities overnight and Wednesday morning.
Winds will continue to be NE east of the Blue Ridge and NE-SE
west of the Blue Ridge at speeds of 5-8kts with low end gusts
east of the Blue Ridge at times between 15Z and 22Z. No
thunderstorms are expected within the CWA or at the RNK TAF
sites due to the stable cool nature of the air mass in place
from the persisting wedge.
/Confidence in Forecast Parameters/
Ceilings - Moderate to High,
Visibilities - Moderate to High,
Winds - High,
Thunderstorm Threat - None.
Extended Aviation Discussion...
The closed upper low will continue to meander about the central
and southern Appalachians at least through Friday, finally
beginning to lift to the north by the weekend. As the low weakens,
the threat for prevailing IFR conditions and persistent -RA will
begin to decrease, especially by Friday and Saturday. Showers
and thunderstorms will tend to become a bit more diurnal toward
the end of the week, but this could also result in increasing
chances for afternoon/evening convection as opposed to just
stratiform rain, drizzle, and fog expected during the first part
of the week.
&&
.HYDROLOGY...
As of 430 AM EDT Tuesday...
Hydro concerns are lessening today as the closed upper low sags
well to our south/southeast. This puts our region in a deep
easterly flow with minimal QPF expected via the cool, stable
maritime easterly fetch and with the stronger PVA/lift shifted
to the coastal and offshore Atlantic areas for the time being.
Thunderstorms are not expected and most of not all of the
precipitation today will be light to moderate at best. 24hr QPF
today/tonight expected to range from 1/2 inch east to just a
few hundredths in the west. Rainfall amounts yesterday were
also mostly 1/2 inch or less. At this time, all monitored
creeks, streams, and rivers are below action stage and expected
to remain there and/or below flood stage through at least
Wednesday. Flooding and/or flash flooding is not expected given
the light rainfall amounts and the long duration of light
rainfall amounts. Main stem rivers are expected to remain below
flood as well.
There could be an increased threat for localized heavy rainfall
later this week or weekend as diurnal heating increases
resulting in a more unstable atmosphere with higher PWATs as the
upper low drifts north of the area and a frontal system
approaches from the west. However, hydrological concerns overall
are expected to remain low through the weekend.
&&
.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.
&&
SYNOPSIS...RAB
NEAR TERM...RAB
SHORT TERM...RR
LONG TERM...DS
AVIATION...RAB
HYDROLOGY...RAB