Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
742 am EDT Friday Jul 3 2015
a low pressure system will move out of the Ohio Valley and into the
middle Atlantic region. This will bring showers and thunderstorms to
the area...along with continued cool temperatures...as we close out the
workweek. Unfortunately our weather will remain unsettled...with a
series of upper level disturbances bringing a chance for showers and
thunderstorms the region throughout the Holiday weekend.
Near term /through tonight/...
as of 400 am EDT Friday...
Short wave energy entering the mean upper trough over the eastern US will
drive a wave of low pressure at the surface from the Ohio Valley to the
middle Atlantic region. This will take the frontal boundary that pushed
south of our area yesterday and drag it back northward as a warm front
with weak high pressure wedging down the east side of the Appalachians.
Early morning satellite and radar loops show that things are beginning
to come together as the vigorous convection over Tennessee is beginning
to move more toward the east/northeast...which is heading in our
This system has a lot going for it with good dynamic energy and a very
moist airmass in place. Hodographs are small but show a significant
amount of shear as would be expected in the vicinity of a frontal
boundary. The missing ingredient for severe weather today is
instability. Abundant cloud cover combined with the somewhat early
onset time will limit daytime heating and model soundings in BUFKIT are
showing a long and skinny cape presentation which does not favor
vigorous updrafts. Based on these factors...currently anticipate a
seasonable with upper 60s east to lower 60s west.Ealthy area of
showers with embedded thunder will move across the area from west to
east today with activity moving out and winding down this evening/early
tonight. This situation will be monitored very closely since if the
cloud cover is less than currently anticipated and we can generate
greater amounts of instability...especially across the southern and
eastern portions of the area...the severe threat will increase. With
precipitable water values running well above normal heavy downpours can
be expected...but the lack of widespread significant rainfall on
Thursday has eased concern for flooding a bit so will not be issuing a
Flood Watch with this package. Will continue to highlight the potential
for heavy rain and possible Hydro issues in the severe weather potential statement.
As the low moves off to our northeast tonight the frontal boundary will
swing back through the region as a cold front. This will yield some
lingering showers west of the Blue Ridge with some improvement to the
High temperatures today will continue on the cool side with readings in
the lower 80s east of the ridge to the lower 70s west...and lows
tonight range from the upper 60s east to lower 60s west.
Short term /Saturday through Monday night/...
as of 415 am EDT Friday...
The broad upper trough plauging the eastern U.S. Will undergo
some subtle modifications during this period. Sat-sun...the upper
trough will take on a positive tilt as a short wave ridge builds
into the Great Lakes/PA region. Meanwhile...the southern end
becomes cutoff across the Carolinas and lingers across that region
sun...then gradually drifts back northward into eastern Virginia Monday and
northern Virginia Tuesday. Thus...the area remains unsettled and wet...but
with a gradual shift of the better coverage of showers and
thunderstorms toward the eastern part of the County Warning Area with time.
For Saturday...yet again...potential for widespread showers and
embedded thunderstorms to start off the day...just as has been the
case the past two days. However...confidence in this is low at
this point as it will largely be based on upstream convective
complexes...track...and intensity. However...feel that confidence
in ongoing precipitation at 12z is slightly greater than the past
two days as the upper trough shifts east moving the current
corridor of precipitation across the Tennessee Valley eastward into our
region. The extent of clouds and precipitation in the morning will
determine the degree of instability and hence convective coverage
later in the day. Upper flow tends to become more southwest
instead of northwest during this period given the transition of
the weak upper low to the south of the region. This should be
enough to support high chance to likely probability of precipitation into the
afternoon/evening. Storm Prediction Center currently only has general thunder across
the region given the lack of instability. With fairly wet
antecedent conditions...the main concern will be heavy rainfall
and localized flooding. However...it appears unlikely that ffg
will be exceeded as rainfall amounts would need to exceed 2-2.5
inches in 1-3 hours most locations for this to occur. Projected
rainfall for the three day period today through Sunday is mostly
1.5 to 2.5 inches.
For Sunday...the weak upper low drifts back north into the County Warning Area.
Widespread showers and thunderstorms appear likely...but the
extensive cloud cover will again limit instability. Once
again...localized heavy rainfall will be the main concern...but
given that the heaviest rainfall should remain mostly east of the
mountains...this further reduces concerns of flooding.
However...the pattern does once again support likely probability of precipitation across
much of the County Warning Area...especially eastern sections.
Monday promises to be yet another wet day as the remnant upper low
drifts into northern Virginia. Once again...the best coverage of
precipitation should be across the eastern sections...but
differential heating across the mountains should result in an
uptick of afternoon convection there and further north compared
to sun. Again...high chance to likely probability of precipitation appear in order.
Instability once again appears marginal and the main concern will
remain that of localized heavy rainfall.
Maximum temperatures should remain well below normal through the period
thanks to cloud cover and precipitation...with highs mostly in the
70s...to lower 80s east if a few breaks in the overcast can occur. On
the other hand...low temperatures will remain above normal thanks
to high precipitable waters /dewpoints and considerable cloud cover.
Long term /Tuesday through Thursday/...
as of 430 am EDT Friday...
By Tuesday...there is fairly good agreement among the models that
the cut off upper low lingering across the region will split in
two...with the part across northern Virginia lifting northeast toward
New York/New England...as the parent upper low sinks southward toward
the Florida Panhandle/south Georgia. Meanwhile...a short wave and associated
front will approach from the northwest by Tuesday...then drift
into the region and stall Wed-Thu. At this point...Tuesday should be
the day with the lowest probability of precipitation overall...and likely lower than any
prior days or succeeding days. Once the front drifts into the
region Wednesday and stalls...waves of low pressure will ride along the
front and keep good chances of showers/thunderstorms across the
region. However...compared to the current week with northwest
flow...upper flow will be more west-southwest as broad upper
ridging is noted across the southern states. Precipitation overall
should be slightly more diurnal in nature...I.E...less
nocturnal...as a result and temperatures will certainly be
trending warmer...but remain mostly 70s mountains and 80s Piedmont
as 850mb temperatures top out in the +16c to +18c range.
Aviation /12z Friday through Tuesday/...
as of 730 am EDT Friday...
Poor aviation conditions expected through the taf valid period as
broad upper trough and embedded disturbances continue to track
across the region. Confidence is low in timing of convective
complexes and degree of instability for heavier afternoon/evening
convection given widespread morning showers. Area of showers and
even a few thunderstorms at this hour spreading to the western
part of the region already. Models show this tracking from
southwest to northeast through the morning...then another area of
convection...possibly stronger...arriving during the
afternoon/evening...before shifting to the east of the County Warning Area by
Saturday morning. Have advertised widespread MVFR ceilings and visibilities
east of the Blue Ridge in rain showers and occasional thunderstorms and rain through the
evening...with MVFR-IFR ceilings/visibilities west of the Blue Ridge through
the evening. Overnight...heavier precipitation should shift
east...but lingering showers will remain. In addition...conditions
appear favorable for widespread IFR-LIFR ceilings/visibilities in fog and low
clouds...especially blf/lwb/bcb. Winds generally NE-southeast 3-7kts through
the day in advance of a weak area of surface low pressure tracking
along the quasi-stationary front draped across the region. Winds
should become south-southwest-SW 3-6kts behind the heavier rain showers/thunderstorms and rain activity
Low to medium confidence in ceilings and visibilities through the taf valid
Low to medium confidence in thunderstorm potential through the taf
Medium confidence in wind direction/medium to high confidence in
wind speed through the taf valid period.
Extended aviation discussion...
Poor aviation conditions likely to continue into early next
week...but windows of opportunity for VFR conditions increase as
we move into Mon/Tue. Upper low/trough will linger west of the
Virginia/NC coastal plain into the southeast states through the
weekend...then begin to drift into northern Virginia by Monday.
Meanwhile...a cold front will approach from the Ohio Valley by Wednesday.
This leaves a very unsettled/wet pattern across the region through
the period. Expect widespread late night/early morning low
clouds/fog with IFR-LIFR cigs/vsbys. Overall...conditions should
improve to MVFR-VFR outside rain showers/thunderstorms and rain activity during the
afternoons. At this point...Tuesday appears to offer the best
opportunity for more widespread VFR conditions.