Scientific Forecaster Discussion
fxus61 krnk 221845
Area forecast discussion
National Weather Service Blacksburg Virginia
245 PM EDT Mon Jul 22 2019
a cold front extended from a low in Pennsylvania into central
Arkansas. This front will move slowly southeast, reaching the
coast by late Tuesday. High pressure with cooler temperatures
and lower humidity will cover much of the eastern United States
Wednesday through Saturday.
Near term /through Tuesday/...
as of 235 PM EDT Monday...
Models show development of thunderstorms over the foothills and
Piedmont between 2pm-5pm this afternoon moving east to the coast by
late this evening. This area getting a lot of sun, increasing the
instability east of an Appomattox to Reidsville line. Gso 12z
sounding had enough dry air aloft to support enhanced downdrafts in
storms this afternoon. Dcape was already 1000-1200 j/kg in that area.
Precipitable water still at very high values, up to 2.1 inches in
the Piedmont tonight. Plus precipitation potential placement
forecast was indicating the possibility of heavy rain. Will be
including this in the forecast for all of southwest Virginia,
southeast West Virginia and northwest North Carolina tonight. Not as
unstable over the mountains due to more cloud cover today but that
area will get repeated rounds of showers and thunderstorms into late
tonight. At this point not planning on expanding Flash Flood Watch
in time or area, but even the faster moving storms east of the Blue
Ridge will have high rainfall rates.
Upper pattern will amplify tonight and Tuesday with a deep long wave
trough digging over the eastern United States. With trough digging,
strong upper diffluence in the right entrance region of the upper
jet will be over the mid-Atlantic region through Tuesday morning.
Surface and 850mb front will be into southeast Virginia and central
North Carolina by Tuesday afternoon. Main axis of deep moisture
transitions from the mountains to the southeast along with the front.
Air mass above 700mb dries out behind the front Tuesday but low
level moisture will take longer to exit. Timing of the front and
drying behind the boundary is similar on the synoptic scale
guidance. Maximum temperatures on Tuesday will be below normal
behind the front, due in large part to extensive low cloud
Short term /Tuesday night through Wednesday night/...
as of 235 PM EDT Monday...
500 mb heights continue to lower over the eastern United States
Tuesday night and Wednesday. Air mass continues to dry out Tuesday
night and Wednesday. Precipitable water values drop below one inch
Tuesday night. Any rainfall behind the front will be light. Surface
and low level winds become northeast for Wednesday and Wednesday
night with high pressure centered over Pennsylvania. As low level
moisture erodes, expecting much more sun on Wednesday. Even more
noticable will be surface dew points lowering into the 50s on
Tuesday night and Wednesday. Both high and low temperatures will
remain below normal through Wednesday night. Blend of guidance looks
reasonable through the period.
Long term /Thursday through Sunday/...
as of 1025 am EDT Monday...
The longwave trough over the eastern Continental U.S. Responsible for the mid-
week cool down will be shifting east and lifting out by Friday with
h5 heights building back into the 588 to 591 dm range by the
weekend. The negative h5 anomalies will become slightly positive by
Sunday across the most of the eastern Seaboard. The rising
heights will be accompanied by slowly increasing temperatures
and humidity somewhat more typical of mid-Summer, with
temperatures closer to seasonal normals. A surface ridge will
remain centered over the central Appalachians extending north
into New England through the weekend which will keep convection
to a minimum. A slight uptick in convective coverage is
possible with the slowly increasing moisture but there is no
synoptic signal to indicate anything substantial and triggering
mechanisms likely to be at the mesoscale or below.
Aviation /19z Monday through Saturday/...
as of 155 PM EDT Monday...
A cold front over the Ohio Valley will slowly move southeast,
crossing the mid Atlantic region tonight and Tuesday. Scattered
to numerous prefrontal showers and thunderstorm are expected
today and tonight. Storms may organize into line segments with
potential for downburst winds. Very heavy rain will be
associated with the storms, resulting in MVFR visibility. All
terminals will carry vcnty thunder today, with greatest coverage
over and west of the mountains through 20z/4pm. Thunderstorms
will develop east of the Blue Ridge in the late afternoon and
move east to the coast by midnight.
Models were similar with the timing of the front. Average
confidence that the front will be east of klyh and kdan by the
18z/2pm end of the taf forecast period. Ceilings lower to IFR
behind the front as mid and upper levels dry out and the wind
gradually turns to the northwest to the northeast.
Forecast confidence is average for the timing of storms this
afternoon and evening and for the how much ceilings lower behind
the front late tonight and Tuesday morning.
Extended aviation discussion...
The front pushes its way south and east into the Carolinas by
Tuesday evening. Drier weather including widespread VFR
returning for Wednesday through Saturday.
as of 1100 am EDT Monday...
Flash Flood Watch remains in effect for the western half of the
County Warning Area until this evening as bands of showers and embedded storms
make their way eastward in association with the slow- moving
frontal boundary and favorable jet dynamics per the Storm Prediction Center ero.
Some of the cams however, including the NAM nest are showing
strong convection developing later today east of the current
watch area. Any strong storms that do form will be capable of
producing heavy rainfall given the high precipitable waters . 12z krnk was 1.66
inches, above the 90th percentile for the date, although
slightly drier air was in place across the southern Piedmont.
13z National water model near-surface soil saturation analysis
and rfc flash flood guidance are both showing a highly variable
soil moisture pattern across the County Warning Area making runoff predictions
based on variable convection that much more uncertain. Morning
clouds are also suppressing instability across the western County Warning Area
so far and 12z href seems to be latching onto that as well and
showing a relative quantitative precipitation forecast minimum across western Virginia and focusing
higher amount to our west. Will maintain existing watch however,
and continue to monitor trends for a possible short-fused watch
across the eastern counties later.
Virginia...Flash Flood Watch until midnight EDT tonight for vaz007-
NC...Flash Flood Watch until midnight EDT tonight for ncz001-002-
WV...Flash Flood Watch until midnight EDT tonight for wvz042>044-