Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
743 am EDT Tuesday Jul 28 2015
a diffuse frontal boundary will linger across the region through
Wednesday. A new cold front will move into the area from the
northwest Thursday. Scattered showers and thunderstorms will
continue in the forecast until the front moves south of the region
Friday. Otherwise...it will remain very warm and humid through
Thursday...slightly less humid into the weekend.
Near term /through tonight/...
as of 400 am EDT Tuesday...
There has been little change in the synoptic pattern as an upper ridge
over the eastern US keeps US in northwest flow aloft...while a diffuse
frontal boundary remains draped over the region. This will yield an
overall scenario quite similar to yesterday with some early isolated to
scattered shower and thunderstorm activity moving through the region...
followed by redevelopment later this afternoon as diurnal instability
increases with heating. And again we will see modest instability
combine with weak dynamics and high precipitable water values to bring
convection with slow moving very heavy downpours while the overall
threat of severe weather from wind/hail remains low. The storms on
Monday were indeed quite prolific precipitation producers with local
amounts over 3 inches found on some mesonet sites...and several flood
advisories issued. This will likely be the case again today as local
flooding from downpours remains a distinct possibility. Activity will
be on the decline this evening with the loss of heating after
sunset...but expect some activity will be stewing around until around
midnight if not a bit longer.
Morning cloud cover and expected early afternoon onset time of
convection should be enough to keep temperatures today within a degree
or two of readings yesterday...with upper 80s east of the Blue Ridge to
low/middle 80s west. Tonight will stay muggy with lower 70s east to middle
Short term /Wednesday through Friday night/...
as of 430 am EDT Tuesday...
A weak and very diffuse frontal boundary will remain across the
region Wednesday with a large subtropical ridge...anchored across
Texas...still ridging into the middle-Atlantic region. Weak convergence
and resultant moisture pooling near this boundary will again
support diurnal convection. Most models point to the Piedmont and
areas east of the Blue Ridge as the most favored area for
convection...but given the overall environment...pulse
thunderstorms cannot be ruled out anywhere. Weak steering currents
will continue to support slow moving showers/thunderstorms...with
very heavy rainfall in isolated locations. Other spots will
receive no rainfall...just as has been the case the past couple of
By Thursday...a new cold front will move into the area from the
west...supported by a deep upper trough sliding eastward across
central into eastern Canada. Subsequent retrogression and
amplification of the subtropical ridge into the southwest U.S.
Will result in strengthening northwest flow aloft across the
eastern U.S. This should allow this second front to be pushed
further south than the early week front...potentially reaching
Georgia/SC before stalling in that region late Friday. Instability
looks to be a tad better with the Thursday front than was the case
with the early week front. Thus...would expect slightly better
coverage of showers and thunderstorms as a result.
However...timing of the front could be problematic...arriving in
the western areas around 12z Thursday...then pushing east of the Blue
Ridge by afternoon. This could again result in the bulk of the
convection being across the Piedmont...with morning cloud
cover...convective debris...and overnight lingering dissipating
showers/thunderstorms western areas. As noted above...instability
is better than we have seen over the past few days...albeit middle-
level wind fields are not remarkable at all...northwest 10kts at
best during this time frame. Storm Prediction Center has outlooked the entire area for
a marginal risk of severe thunderstorms...noting that the favored
type of storm will be pulse with isolated severe wind gusts
possible. Confidence in this occurring given the synoptic pattern
and very weak dynamics is low. The slightly stronger middle-level
flow should...however...reduce the flood potential.
For Friday...the models have come into better agreement now...as
noted above...that the front will push south of the area...enough
so that probability of precipitation have been dropped below 15 percent for Friday and Friday
night. Dewpoints drop on average 5-7 degrees into Friday...allowing
min temperatures to drop back closer to normal values for early
August...mainly 55 to 60 mountains and 60s Piedmont. The maximum temperature
drop will only be a couple of degrees overall with less cloud
cover... lower dewpoints...and greater amounts of insolation.
Long term /Saturday through Monday/...
as of 445 am EDT Tuesday...
The main focus for the extended period will be on a broad upper
trough remaining across eastern Canada and its potential deepening
into the eastern U.S. Another concern for this period will be a
weak area of low pressure aloft...most likely initially an
easterly wave...developing across the Florida/GA/western Gulf area and
combining with the stalled late week frontal boundary located
across the southeast states.
With respect to the upper trough...the GFS is much more agressive
than the other models in deepening this trough across the eastern
U.S. Consequently...it advertises a fairly dramatic drop in 850mb
temperatures from the current readings around +20c to as low as the +7c
range by the middle of next week. The European model (ecmwf) is not on board with
this at all...instead showing a large-scale broad upper ridge
remaining in place pretty much across the entire U.S. South of 40
degree latitude. At the same time the GFS has 850mb temperatures in the
single digits...the European model (ecmwf) shows readings in excess of +20c. Feel
that the GFS is overdone with the deepening of the eastern U.S.
Upper trough and its related dramatic cooling in the eastern
With respect to the pseudo-tropical low developing across the
eastern Gulf...southeast states...models are in better
agreement on this issue than the eastern Canadian upper trough...with
the European model (ecmwf) still the most agressive...as it has been for
sometime...in developing a closed surface low...and then tracking it
northeastward just inland from the Georgia/SC/NC coast into early next
week. A weak frontal boundary is slated to arrive with the
deepening of the upper trough...at least in some fashion...by
early next week...which could...depending on timing...either pull
the southeast U.S. Low pressure area further inland across the
Piedmont or help shove it east of the area. At any rate...rain
chances should be on the increase again by early next week. The
best rain chances will remain across the Piedmont with this
scenario. However...the weekend for the most part looks dry with
seasonable temperatures and humidity levels.
Aviation /12z Tuesday through Saturday/...
as of 735 am EDT Tuesday...
Showers and thunderstorms from the earlier morning hours have
dissipated...with a remnant shower or two trying to drift into the
far southwest corner of Virginia. Otherwise...increasingly moist
atmosphere and moist ground from fairly widespread rain showers/thunderstorms and rain late
Monday has left areas of low clouds and fog this morning.
Lwb/bcb/blf all LIFR in fog...with IFR ceilings in/out at lyh...and
MVFR br/clouds elsewhere...including Dan/roa. These conditions
should all improve to VFR by or after 14z.
Not much change with the synoptic pattern for today as the
frontal boundary will linger in the area and help kick off more
showers and thunderstorms this afternoon. Isolated activity will
be floating around throughout the morning but things really dont
get going until the afternoon so will confine mention of thunder
to after 16z. Environment continues to be weakly forced so expect
locally heavy rainfall with a low severe threat. Just like the
past few days...activity will be hit and miss and not enough
predictability to include anything more than a thunderstorms in the vicinity in the tafs.
Activity will wind down with loss of heating after sunset with
lack of any synoptic scale forcing. Once again...expect late
night/early morning fog with areas of IFR-LIFR ceilings/visibilities in fog.
Winds mostly light and variable through the taf valid period.
Medium confidence in ceilings/visibilities through the taf valid period.
Low confidence in wind direction through the taf valid period...high
confidence in wind speeds 5kts or less most areas through the taf
valid period. Medium confidence in thunderstorms and rain potential today and
Extended aviation forecast...
Wednesday will be another day of MVFR to IFR valley fog in the
morning and scattered afternoon and evening showers and
More widespread showers and thunderstorms are expected on Thursday
as a cold front crosses through region from the northwest. This
front should push south of the area and allow for drier and mostly
VFR conditions Fri-Sat.