Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Blacksburg Virginia
912 PM EDT sun Mar 29 2015
a cold front will approach the region from the northwest overnight
before passing across the area Monday morning. A second cold front
and associated clipper system will pass across the region on
Tuesday...followed by high pressure for the middle of the week.
Moisture ahead of another stronger cold front will increase by the
end of the week bringing the next chance for showers and possible
storms on Friday.
Near term /through Monday/...
as of 845 PM EDT Sunday...
Main axis of deeper pre-frontal moisture remains just west of the
Ohio River this evening with lots of very dry air still over the
area in advance of this boundary. This seen via the lingering low
precipitable water and middle level dryness seen off the evening rnk sounding as
well as the continued low dewpoints at the surface per latest observation.
Most solutions including the latest hrrr and hires-arw do bring some
precipitation into the far northwest shortly after midnight...then south and
east to the Blue Ridge with best lift/moisture working down the
western slopes into the northwest NC ridges before daybreak. Shower band
may then tend to jump east of the mountains late or early Monday
but iffy given deep westerly flow aloft seen off evening radiosonde observations.
Since guidance remains too fast with precipitation into the dry air from
earlier...slowed down probability of precipitation again overnight in the west while
increasing some over the SW toward daybreak where running with
brief Cat/likely probability of precipitation for low quantitative precipitation forecast. Left in some low probability of precipitation out east
as well but doubtful as to how far east of the Blue Ridge much
more than sprinkles make it overnight. Very dry air and temperatures cooling
into the 30s could support a bit of sleet west at the onset so added
while keeping the snow mention at elevation across the northwest where could
see some light accumulation. Otherwise lowered temperatures early over the
west for some wet bulb cooling at the onset before readings rise
late as mixing picks up just ahead of the front by dawn.
Previous discussion as of 329 PM EDT Sunday...
1030 mb surface high pressure centered over South Carolina this
afternoon will move southeast this evening into tonight.
Low pressure over the Great Lakes with a trailing cold front
will move east tonight into Monday.
Cloud cover will spread east across the area this evening into
tonight in advance of an approaching cold front. Models continue to
trend slower with the arrival of the precipitation in the region.
With a dry airmass in place as noted by precipitable water at 0.05 on this morning
12z rnk sounding...slowed our arrival time a little. Despite the dry
air...moisture and lift will increase late tonight into Monday
morning. Models are still showing precipitable water rising above
0.50 inch. For tonight probability of precipitation used a blend of hiresw-arw east...rap
and NAM...then leaned towards the NAM and GFS blend Monday.
Believe ther is enough cold air to allow rain showers to become
mixed with snow showers in the higher elevations. A light snow
accumulation less than a inch may be possible in the northwest
highest elevations. Wpc has painted a small area of snow in the
winter weather day 1 outlook just to our north.
Winds pick up ahead of the cold front but BUFKIT showed strongest
winds ahead of the front just above the surface and remaining above
the inversion. Low temperatures tonight will range from the lower 30s
in the northwest mountains to around 40 degrees in the Piedmont.
A cold front passing across the area early Monday...will trigger
showers as it moves east across the Piedmont. Showers will diminish
quickly behind the front as winds shift west to northwest as high
pressure builds in from the Central Plains. High temperatures Monday
will range from the upper 40s in southeast West Virginia to the
lower to middle 60s across the Piedmont.
Short term /Monday night through Wednesday/...
as of 305 PM EDT Sunday...
Next shortwave/clipper slides from northern Illinois Monday evening into
central Virginia by Tuesday afternoon. Models are similar in keeping main
threat of showers north of Interstate 64...so not going with probability of precipitation
any higher than 25. A little leftover cold air Tuesday morning could
allow for some wintry mix north of Lewisburg and Hot Springs...but
We will see high pressure work eastward across the middle Atlantic
Wednesday with southeast flow developing late in the day.
Temperatures will be at and above seasonal norms. The west flow Tuesday
ahead of the clipper should enhance temperatures across the Piedmont
barring any extensive cloud cover...with near 70 possible from
Long term /Wednesday night through Sunday/...
as of 200 PM EDT Sunday...
Models continue to vary on speed/amplification of next system
affecting our area the end of the week...though in decent agreement
that there will be a threat of showers Thursday-early Saturday...with
Friday looking the most unsettled.
Warm advection pattern will kick in once the high moves off the
coast Wednesday night...though am thinking enough southeast flow and in
situ wedge may hold temperatures down briefly before stronger low level jet moves
across the mountains expect a chance of rain/rain showers by Thursday
with better convergence west of the mountains...with some overrunning
into the southern County Warning Area.
Thursday night into Friday frontal boundary works slowly southeast into PA-
Kentucky line with main surface low well into Canada. Another wave or two of
low pressure is expected to form along the front Friday
afternoon/evening. Models show instability in the warm sector and
added some thunder in most places Friday. Still not going to have
likely probability of precipitation yet...as models are not consistent with potential for
seeing a break in the action in between waves. As we head into
Friday night-Saturday...the European model (ecmwf) kicks the front east fast by about
5-8 hours...with GFS slowly edging it southward with weaker low.
Will maintain shower chances with higher probability of precipitation Friday night before we
start to see drying by Sat afternoon. A brief shot of cooler/drier
air...though closer to normal follows this front Sat night-Sunday.
Aviation /01z Monday through Friday/...
as of 650 PM EDT Sunday...
High pressure to the south of the region will give way to a cold
front that will approach from the northwest overnight before
passing through the area Monday morning. Moisture well in advance
of the front will overspread the taf sites from the northwest by
midnight...with deeper moisture and precipitation closer to the front
arriving during the early morning hours of Monday. This should
bring a few hours of MVFR to locations from kbcb westward into southeast
West Virginia after midnight and perhaps as far east as kroa by daybreak
Monday. Light rainfall may reduce visibilities to low end VFR at times by
early Monday with occasional MVFR possible around kblf. Given the very
dry air...some sleet may also occur across the mountains at the
onset but should be brief and on the light side. Ceilings should
gradually improve to mostly VFR heading into Monday afternoon as
much drier air aloft works in behind the front.
Southwest winds at 10-15 kts will gradually become more westerly
by early Monday and then shift to the northwest behind the front Monday
afternoon when gusts to 25-30 kts may occur over the west.
Medium to high confidence in ceilings...visibilities and winds
during the taf period.
Extended aviation forecast...
There is also potential for strong gusty winds to continue Monday
night behind the front...and again on Tuesday as another clipper
slides across from the northwest.
Weak high pressure should maintain mainly VFR conditions Tuesday
into Wednesday with only lingering mountain upslope driven MVFR
ceilings and spotty showers on Tuesday. On Thursday...as a warm front
lifts north across the region with the possibility of sub-VFR
Moisture will increase ahead of a stronger cold front Friday with
MVFR/IFR conditions possible in convection.