NWS Blacksburg, VA wrote its last area forecast discussion before the first event at Apr 11, 9:31 PM EST, 2 hours 29 minutes earlier.
Area forecast discussion, Apr 11, 9:31 PM EST, 2 hours 29 minutes before
Area Forecast Discussion
National Weather Service Blacksburg VA
1031 PM EDT Sat Apr 11 2020
.SYNOPSIS...
A strong storm system will impact our weather Sunday into
Monday bringing strong gusty winds, heavy rain, and the
potential for strong to severe thunderstorms. High pressure
returns for the middle of the week associated with colder than
normal temperatures.
&&
.NEAR TERM /THROUGH SUNDAY/...
As of 1020 PM EDT Saturday...
Mid and upper level clouds have spread into the area and cloud
cover will continue to increase overnight. This will limit
radiational cooling early Sunday morning. A weak short wave
entering the Mid Atlantic region was producing some light
returns on radar but the 00Z/8PM soundings from the region were
dry at low levels, mainly below 750MB. Most of the mid level
rain will evaporate before it reaches the ground. Have a few
hours with isolated sprinkles in overnight. Made only slight
adjustments to overnight lows base on current temperature/dew
point and cloud trends.
Models holding off on any bonafide rain threat until Sunday.
Mid and upper level moisture will be on the increase, associated
with a developing storm system over the middle U.S., but the
low levels of the atmosphere are forecast to remain dry which
should preclude any measurable rainfall. And this appears that
it will ring true for much of Sunday morning...suggesting Easter
Sunrise Services will be precipitation free. If anything, the
increasing cloud cover from the southwest may set the stage for
a brilliant sunrise, with the sun glow underneath the mid/upper
level cloud layer at daybreak.
For Sunday, a major spring weather system will be spinning up
over the nations mid-section, the associated warm front moving
north through our region Sunday afternoon and evening. We are
anticipating this initial wave of moisture to begin as light
rain, gaining coverage and intensity with time as we progress
through Sunday afternoon and evening with southeast upslope wind
flow resulting in higher rates and qpf along the spine of the
Blue Ridge.
Temperatures Sunday are expected to be slightly cooler than
normal with highs averaging in the 50s to mid 60s...although
once the rain begins they should tail off a few degrees due to
evaporational cooling.
&&
.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
As of 200 PM EDT Saturday...
By this point in the forecast period, the low pressure system
bringing showers and storms will be negatively tilted and increasing
in strength. This intensification is going to result in a strong 850
jet of 50-60+ kt winds. Because of this, winds will be increasing
Sunday night and through the day Monday, even after the cold front
passes.
Early Sunday night, before the cold front's passage, warm, saturated
air is allowed to funnel into the region bringing in an unstable
airmass, though not overly impressive with MU CAPE topping of
generally in the mid 1000s. Rather than the severe storm threat
being driven by thermodynamics, it looks like this will be a
dynamically driven event with a large amount of shear and
helicity. Because of this, the primary threats look to be spin
up tornadoes and damaging winds. Location wise remains generally
on the southern half of the CWA where the exit of the 850 jet
will be, and thus the location of the greatest shear. Timing
wise, it looks like it is going to be early Monday morning for
the front's passage, starting between 4-6AM for the east and
8-12PM for it's exit from the west.
With this system also comes the concern of hydro issues. Overall,
the final rainfall amounts out of this event do seem slightly lower
based on guidance, but with PWATs of 1.5+ still in guidance, the
general belief of 1-3" of rainfall is still in place. Highest
amounts will be along the southern Blue Ridge. The main flooding
threats would be flash flooding with deep convection being able to
quickly over saturate basins. Current river guidance shows only a few
select areas possibly entering action stage, and no rivers entering
flood stage. The Greenbrier river is forecasted to come the closest
to reaching flood stage since overall ground moisture there is the
highest. Likewise, overall vegetative growth in the west is the
farthest behind of the rest of the CWA, so the most runoff will be
available.
After the front's passage, we start to dry out. At this stage, the
main concern will just be strong gradient winds as the 850 jet will
still be overhead. They will eventually slack by late Monday as the
actual cold front with this system arrives, bringing NW flow and
cooler temperatures. A disturbance late Tues into Wednesday AM bring
a new chance of rain to the region. Current guidance suggests that
moisture will arrive just as we start to cool Tuesday night,
starting snow showers for parts of the west. However, only
accumulating snowfall will occur in the higher elevations with
current totals of generally half an inch to an inch.
&&
.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 100 PM EDT Saturday...
A cold front should depart offshore on Saturday, which will allow
the rain showers to taper along and east of the Blue Ridge. However,
lingering upslope snow showers could remain in southeast West
Virginia through Saturday night as an upper level trough crosses
over the Mid Atlantic. This trough will move off the coast on
Thursday as high pressure builds overhead to provide drier
conditions. The flow aloft should become more zonal on Friday, and
high pressure will head toward the Atlantic Ocean by Friday evening.
Model spread increases notably for Friday night and beyond between
the GFS and the ECMWF. A low pressure system will develop across the
central Plains and head toward the Appalachian Mountains, but the
models differ considerably on the timing and track for the eventual
approach of this low pressure system. Given the better performance
of the ECMWF over the GFS this far out in time, this forecast will
steer closer toward the ECMWF solution, which is also supported more
favorably by other global models such as the Canadian. As a result,
the end of the week appears increasingly milder with rain chances
arriving by Saturday.
&&
.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
As of 800 PM EDT Saturday...
Overnight into Sunday morning will see VFR conditions, but clouds
will fill in the mid-levels as a springtime system approaches and
brings moisture into the area. As the day wears on Sunday, cigs will
begin to lower and eventually enter MVFR conditions. When a warm
front passes over from south to north Sunday afternoon, wind
direction will change to come almost directly out of the south
later in the afternoon. This is accompanied by a strong (60+ kt)
low level jet that will create some low level wind shear that
lasts beyond the 24 hour period.
Confidence in ceilings is moderate to high.
Confidence in winds and low level shear is high.
Extended Aviation Discussion...
A storm system will approach from the southwest Sunday with
deteriorating aviation conditions with sub-VFR ceilings and
widespread rain expected Sunday afternoon into early Monday. A
risk for strong to severe thunderstorms exists Sunday night and
with the passage of the front around daybreak Monday. There is a
strong, 60 kt LLJ associated with this front, so anticipate
LLWS late Sunday night and Monday morning prior to the frontal
passage.
After the frontal passage Monday morning, turbulent mixing will
result in strong westerly winds which will continue into Monday
night. Improvement with respect to cloud bases is expected
behind the front for Monday afternoon.
Clearing is expected to be temporary. Ceilings will vary at
times as another low forms along the coast Tuesday- Wednesday
between VFR to MVFR.
&&
.HYDROLOGY...
Incoming hydro event looks to be roughly 18-hour duration from
around Sunday 18z to Monday 12z. Models are generally
consistent in throwing down 1 to 3 inches of QPF with some
variations to be expected. Highest amounts look to be focused
along the southern and central Blue Ridge. Question is will this
be enough to cause flash flooding and/or river flooding?
To answer the more temporally limited question of flash flooding
first: The WPC is carrying a Day 2 (ending 12z Monday) Moderate
risk of flash flooding along the NC Blue Ridge into parts of SW
VA and Slight risk for most of the CWA. Ingredients for heavy
rainfall appear to be in place as discussed the Excessive
rainfall outlook and elsewhere in this AFD. Whether there will
be flash flooding will depend on receiving sustained higher
rates of rainfall, approaching 1 to 1.5 inches per hour and over
the same area for some duration. Flash flood guidance (FFG), a
proxy for antecedent moisture conditions, reflects what we have
seen in terms of rainfall for the past 30 to 60 days, wet in the
west and actually fairly dry in the east (foothills and
piedmont). Gridded 6-hour FFG, perhaps the most applicable
duration for a synoptic-scale event with embedded convective
elements is 3 to 4 inches east of the Blue Ridge but as low as 1
to 1.5 inches across parts of the west, most notably in the
Bluestone River basin (much of Mercer County, WV). As we have
seen many times in this area, an inch or so of rainfall followed
by a few hours of more intense, convective-type rain can cause
what amounts to flash-flooding type problems that may not meet
the classic definition of a flash flood, i.e. short-duration
rainfall leading to flooding. Still, at this point it appears
flash flooding will likely be isolated but lower-end runoff
issues are probable in more flood prone areas, including urban
areas.
River flooding: Operational forecasts (AHPS) from SERFC include only
48-hr of QPF (through 12z Sunday) so forecast points on the Yadkin,
Dan and Roanoke Rivers showing no response, same thing for LMRFC and
Tennessee basin. OHRFC (New, Greenbrier) and MARFC (James) using
longer QPF duration show rivers just starting to respond by early
Monday. So at this point, still reliant on MMEFS ensemble forecasts.
GEFS (GFS-ensemble) currently showing the most robust response with
median QPF near 2.50 inches (basin-average) in the New and
Greenbrier producing a 30 percent Moderate flood stage probability.
James, Roanoke and Dan are lower, near Action stage or Minor FS.
NAEFS (GFS + Canadian ensembles) less bullish on any flooding but
lot of Action Stage type rises. Such solutions will bounce around
some from run-to-run but the bottom line at this point is that
bankfull to minor river flooding seems quite probable under this
scenario. Looked at some National Water Model (NWC) output presents
a similar picture, with river rising but remaining below flood stage
at a half-dozen or so sites. Even the Watauga River near Sugar Grove
where some of the highest QPF is expected, and one of our most
responsive and flashy rivers, gets up to just around Minor flood
stage. Some of the experimental NWC output shows the most impressive
rises in several western basins most notably the Bluestone River as
well as Walker Creek in Giles County.
Interestingly as well, as of this morning the top CIPS Analog for
this event (using the most recent GFS forecast) was March 1-2, 2007.
This was also basically a 24-hr rain event (with some snowmelt in
the mountains) with 2 to 3.5 inches of rain along the Blue Ridge and
1 to 2 inches further west in the mountains that produced rises to
Action stage on Greenbrier, Dan and lower James. Several river flood
warnings were issued none of which verified. There was also some
fairly minor small stream flooding, with several Small Stream Flood
Advisories issued and no reported flash flooding. This same
March storm resulted in 68 tornado reports, nearly all in the
southeastern U.S. Only 1 severe weather report (wind) occurred
in the Blacksburg CWA however.
&&
.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.
&&
SYNOPSIS...PM
NEAR TERM...AMS/PM/VFJ
SHORT TERM...RR
LONG TERM...PW
AVIATION...PM/VFJ
HYDROLOGY...PC