Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 210514

Area Forecast Discussion
National Weather Service Blacksburg VA
114 AM EDT Sat Apr 21 2018

High pressure will be over the area through the weekend.
Low pressure will approach from the southwest Monday and
slowly track northeast to the Virginia coast by Wednesday.
This will bring widespread rain and below normal temperatures
to the Mid Atlantic region the first half of next week.


As of 1230 AM EDT Saturday...

High pressure will continue to build overhead keeping skies
clear and winds light. The 00Z RNK sounding illustrates the
magnitude of dry air in place with a PW value of 0.13 inches,
which is the lowest PW value for 00z/April 21 for RNK (records
back to 1995).

While these very favorable radiational cooling conditions will
likely lead to low temperatures falling below guidance values,
the very dry airmass will limit the potential for widespread
frost. Still, will continue Freeze Warning across the southern
Shenandoah Valley where temps will drop below freezing for
several hours early Saturday morning. As for the mountains,
temperatures will again dip into the 20s,continuing to post-
pone the growing season and delay green- up.

Forecast lows tonight are a few degrees above record values for
the 21st of April.

After a cold start, Saturday will be another dry, fair weather
sort of day with seasonably cool temperatures. The April sun
angle should offset the cooler thicknesses in spite of
temperatures being 5 degrees below normal. The airmass is very
dry so looking at afternoon humidity minimums of 20 to 30


As of 330 PM EDT Friday...

Surface high pressure will shift east of the region Saturday night
into Sunday, allowing our low level wind flow to transition from
calm/light and variable to southeast at 5 to 10 mph. We will also
see an increase in cloud cover thanks to increasing dew point and a
weakening cap aloft allowing for better chances of cumulus
development. Also, higher in the atmosphere, we will start to
experience cloud cover associated with a southern stream area of low
pressure advect into the region from the south. Low temperatures
Saturday night will range from a mix of mid 30s to around 40 across
the mountains and lower to mid 40s across the Piedmont. On Sunday,
high temperatures will range from around 60 to the mid 60s across
the mountains with mid 60s to upper 60s across the Piedmont.

Sunday night through Monday night our weather pattern has a decent
amount of uncertainty to it. While the deterministic solutions bring
the Lower Mississippi Valley upper low eastward through the Deep
South, there are notable differences in the timing, track, and the
extent to which its associated rainfall spreads into our region. The
NAM is the most progressive with the entire region receiving a
generous rainfall by Monday evening. At the same time frame, the GFS
and Canadian solutions barely have precipitation within the far
southwest part of the region, and the European solution has
measurable rainfall bisecting the region, wet southwest, and
dry northeast. Our forecast will reflect a consensus of these
four, which would mirror a slightly slower version of the
European solution.

On aspect that does has decent agreement among the models is a
strengthening, and persistent, southeast flow across the entire
region. This has the potential of having more precipitation
generated along the crest of the Blue Ridge thanks to a strong
upslope component on the east side of the region. Also, the flow
tends to result in gusty to very gusty winds across the far
southwest portion of the area where downsloping will be taking
place. The area between Bluefield to Richlands to near Marion will
be an area to watch for potential wind headlines Monday night.

Temperatures during the Sunday night to Monday night time period
will be a little tricky given the questions about the influence of
rainfall. For now, with an anticipation of some degree of increasing
dew points, will likewise trend low temperatures milder each night
as compared to Saturday night. However, for the high on Monday, will
trend slightly cooler as compared to Saturday given the potential
for thicker clouds and the precipitation, despite likely warm air
advection into the area.

Based upon the above timing discussion, the best chance for moderate
to heavy rainfall will come Monday afternoon and overnight. At this
point our forecast will reflect its highest amounts, around two
inches along the crest of the Blue Ridge, from near Fancy Gap, VA,
southwest into the Northern Mountains of North Carolina.


As of 300 PM EDT Friday...

For Tuesday, the question of the timing and track of the upper low
continues. However, with the NAM not a player in the solutions, the
resultant consensus between the GFS, Canadian and European solutions
are more in line. Our rain will continue over the area on Tuesday
with Wednesday looking like the point where we get on the backside
of the system with the precipitation transitioning to steep lapse
rate showers as the center of the low passes overhead, or close to
the region.

On the heels of the Monday/Tuesday system will be a fast moving
northern stream system. Again guidance varies with this system as
well. Solutions differ as to whether or not this system is a closed
low, the GFS, an open low, Canadian, or relatively weak trough,
European. Either solutions brings its associated upper system across
our region Wednesday night through Thursday night, thus keeping
showers to some degree in the forecast. Details to the extent can be
worked on closer to the time of passage next week.

Our progressive pattern doesn`t end on Thursday. Yet a third system
is progged to cross the region within the northern stream jet. This
one currently is expected to be more potent than the
Wednesday/Thursday system.

Temperatures during this portion of the forecast will again be
challenging, but there is expected to be general trend toward milder
conditions comparing Tuesday through Thursday, with slightly cooler
conditions on Friday as compared to Thursday. All days are expected
to average a little below normal for this time of year.


As of 110 AM EDT Saturday...

High pressure will control our weather through Sunday.
Great flying weather expected with high confidence for VFR
conditions and light winds.

Extended Aviation Discussion...

Clouds will begin to creep back into the area late Sunday in
advance of an approaching low in the Lower Mississippi River
Valley. However, cloud bases should remain VFR until deeper
moisture arrives Monday. SCT MVFR showers are possible in
southern Virginia and northern North Carolina Sunday night with
rain becoming more widespread across the Mid Atlantic region on
Monday. MVFR/IFR ceilings and visibilities are expected Monday
night and Tuesday due to widespread rain and an upslope very
moist southeast wind. SCT MVFR showers will continue into


VA...Freeze Warning until 9 AM EDT this morning for VAZ023-024.


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