Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
802 PM EDT Wednesday may 27 2015
high pressure off the southeast coast will maintain a broad
southerly flow of moisture into the region through the remainder
of this week. A weak upper level disturbance and associated
frontal boundary will slide into the region early Thursday. This
weather feature in combination with daytime heating and
instability will result in periodic rounds of showers and storms
over the next few days.
Near term /through Thursday/...
as of 630 PM EDT Wednesday...
made some adjustments to probability of precipitation for this evening into tonight.
Used a blend of hrrr and rnk wrfarw. Drier air pushing in from
the west...was hindering thunderstorm development. Given limited
instability expected convection to diminish with the loss of solar
heating this evening. Low temperatures will be mild overnight with
readings from around 60 degrees in the mountains to the middle 60s
in the Piedmont.
As of 350 PM EDT Wednesday...
Convection struggling to get going so far this afternoon and will
likely remain isolated through this evening...mainly in mountains and
foothills...given limited instability and drier air pushing in from
the west behind the weak short wave that has lifted northeast over
the northeast U.S. Any widely scattered convection that does
Managua to develop the remainder of the afternoon should diminish
quickly with loss of heating. Will need keep an eye on more
organized convection well to the west as it moves into eastern Kentucky
and Tennessee...but all indications are that this will die quickly well
before it reaches western slopes of Appalachians...which would be
near midnight if it did survive. Keeping a slight chance pop in
the west for this small possibility... as well as along Blue Ridge
mainly in northwest NC given some lingering instability and convergence
per the hrrr...but otherwise dropping probability of precipitation after midnight.
Some suggestion in the guidance that patchy fog may be more likely
late tonight...but with general lack of rainfall expected remainder
of the day...and some residual middle deck overnight...think this is
not likely and have chosen not to include it.
For Thursday...we finally have some ingredients coming together to
support more widespread coverage of convection...with weak upper
wave moving into Appalachians early in the day and associated weak
frontal boundary...or really more of a trough axis...which will add
to some weak convergence already geographically favored near the
Blue Ridge. Models suggest modest instability in part because of
warmer afternoon temperatures expected...but still nothing super
impressive...and perhaps enough dry air at middle levels to support
some gusty downdrafts. At this point think severe potential is
limited...but much better than the last couple of days. However with
weaker steering flow and well above average precipitation water values
expected...locally heavy rain will be the main concern. Given dry
antecedent conditions...not all that concerned about a flash flood
threat...but still something to keep an eye on with isolated areas
if any storms become locked on terrain. This will still mainly be
over the mountains...with coverage justifying some likely probability of precipitation...but
storms may drift into the Piedmont by late afternoon...so middle chance
probability of precipitation by late in the day there as well.
Short term /Thursday night through Saturday night/...
as of 330 PM EDT Wednesday...
Upper pattern will be in transition during the short term with
flattening of the East Coast ridge and a trough developing into a closed
low over the Mississippi Valley. This will keep US in a weakly forced
warm/moist/unstable environment with a diurnal trend and mountain bias
for showers/thunderstorms. A weak cold front will be approaching from
the northwest by the weekend but uncertainty will be increasing as
guidance differs in handling the speed of this feature. Believe the
scenario will lead to increasing chances for for convection during the
latter portion of the weekend. Biggest concern will be the possibility
of locally heavy rain given precipitable water values of 1.0 to 1.5
inches across the region. Temperatures will remain above normal with
readings trending up a few degrees through the weekend... generally
looking for middle/upper 80s east of the Blue Ridge lower 80s west with
the usual cooler readings at the higher elevations.
Long term /Sunday through Wednesday/...
as of 330 PM EDT Wednesday...
By Sunday night the upper pattern will be well on its way to developing
a split flow regime across the northern tier of states as a closed low
develops over the Mississippi Valley and slowly sinks southward through
the middle of next week. Some ridging will also be taking place over
the middle Atlantic region and set up a sluggish blocky pattern. At the
surface high pressure will build through the northeast and wedge down
the eastern Seaboard...bringing cooler/near normal temperatures to the
region. However...the pattern will keep US moist with deep southeasterly
flow and expect enough forcing between orographic upslope and diurnal
effects to keep some shower activity...and possibly a thunderstorm...
in the forecast through the period.
Aviation /00z Thursday through Monday/...
as of 745 PM EDT Wednesday...
Generally VFR conditions expected through the taf valid
period...with some caveats to consider. First issue is potential
for overnight clouds and fog. With respect to clouds...upstream
convection lingers across Kentucky associated with next upstream
disturbance as upper trough remains back in the Midwest.
Additional strong convection evident across Alabama/middle Tennessee...moving
east-northeast. This should bring convective debris middle/high clouds across
the region later tonight...so have indicated so in the tafs...all
VFR. Next issue is fog. Models are very robust in generating IFR-
LIFR fog tonight at most taf sites. Synoptically...this does not
make sense. Even though the air mass is moist with precipitable waters around
1... have had little to no rain for several days and middle-
level drying further dried out the air mass today allowing for
abundant insolation and a large T/dew point spread at most sites. If any
fog develops...cannot see much worse than 4-5sm br...mainly at
lwb/bcb/lyh...but for now have left out of the tafs.
For Thursday...lingering boundary and mcvs from upstream convection
should provide for better coverage of convection beginning around
17z. GFS looks overdone...but certainly would expect more than we
saw today. Have introduced vcsh/thunderstorms in the vicinity at most taf sites with still
MVFR ceilings at this point. Not enough confidence to include tempo or
prob30 just yet...and not enough confidence to say that one site
will be more favored than another...other than kdan...which should
see the least convection at this point...but even there it is a
possibility. Again...brief periods of MVFR-IFR visibilities/ceilings possible
in thunderstorms and rain...but not enough confidence to advertise this far out.
Winds look very chaotic through the taf valid period and
confidence in any one solution is very low. SW winds early will
diminish to near calm and potentially become light southeast early
Thursday...but model solutions are all over the Compass and with surface high
pressure generally situated over the area...this scenario is
possible. A preponderance of the model wind guidance is
southeast...so have leaned in that direction after 13z.
Still...speeds mostly 7kts or less through the taf valid period.
Medium confidence in ceilings through the taf valid period.
Medium to high confidence in visibilities through the taf valid period.
Low confidence in wind direction...medium to high confidence in wind
speed through the taf valid period.
Low to medium confidence in thunderstorm potential Thursday afternoon.
Extended aviation discussion...
For the remainder of the period...a moist unstable air mass
remains across the region with weak triggering mechanisms and
dynamics. A frontal system will approach from the north by the
weekend...but will likely remain well north of the County Warning Area through
Sunday. Thus...no change in air mass or the overall synoptic
pattern is expected through the period.
Early morning low clouds/fog and associated IFR-LIFR ceilings/visibilities become
increasingly more likely through the period...especially following
any late day rainfall. Otherwise...expect mostly VFR-MVFR ceilings
outside heavier showers/thunderstorms.