107
FXUS61 KRNK 120538
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
138 AM EDT Fri Jul 12 2024

.SYNOPSIS...
A front along the East Coast drifts back to the west tonight
and Friday as low pressure tracks along the boundary. This will
bring widespread rain to parts of central and eastern Virginia
and North Carolina Friday into Saturday. Sunday looks mostly
dry. Afternoon thunderstorm chances return Monday through
Tuesday, while another system brings more widespread rain
towards the middle of next week. Temperatures will be cooler
tomorrow but increase again for the weekend, with heat index
values approaching 100 degrees in Piedmont again for much of
next week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 750 PM EDT Thursday...

Not much change in the forecast this evening. Seeing increasing
cloud cover as low pressure strengthens along the Carolinas.
Dew points already on the rise across the Virginia and North
Carolina Piedmont and still expecting to see heavier downpours
with embedded storms develop sometime near daybreak. Heaviest
rainfall still appears it will be across the southeastern
portions of the CWA, but still not confident enough that
rainfall will be widespread enough to warrant a Flood Watch.



As of 145 PM EDT Thursday...

Key Messages:

1) Increasing confidence for rain late tonight in the Piedmont,

2) Rain spreads east and thunder becomes more likely during the
day Friday.

3) There will be a sharp QPF/moisture edge, with some eastern
counties getting 1-3", and places to the east getting much
less.

4) A Flood Watch was not needed based on the length of the
event and FFG values.

Troughing to our west tightens up as a ridge of high pressure
over the Atlantic retrogrades towards the eastern seaboard on
Friday. This will steer a north to south oriented front near the
coast into our area from the east. Currently heavy rain and
lightning were occurring in the cells along this boundary, but
the width of the fine line and its impact were relatively
small.

Clouds will spread from east to west tonight in easterly flow,
allowing deeper moisture to reach the Blue Ridge by tomorrow.
Precipitation should reach the Piedmont of NC and VA before
daybreak, and pushes towards the Blue Ridge by early afternoon.
There is some discrepancy between how far west the deeper
moisture goes and how fast it arrives. Models maintain the
better moisture/convection stays along and east of the Blue
Ridge, with lighter showers to the west. It does seem like
there will be a fairly tight gradient between those receiving an
inch or more of rain, and those receiving very little. A Flood
Watch does not appear to be needed given the fact that Flash
Flood Guidance in that area is relatively high, and the rain
will fall over an extended period of time. However, localized
flooding will be possible if cells train over the same areas,
which seems likely given the boundary and slow storm motion
noted today. Other severe weather looks unlikely with too much
cloud cover and more of an almost stratiform setup.

Confidence is lower in western extent of precipitation, but
confidence in precipitation amounts increases east of the Blue
Ridge. Other forecast parameters are high confidence.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/...
As of 1215 PM EDT Thursday...

Key Points:

1. Showers/Storms in the east exiting towards the east Friday
night.
2. Decreasing chances of showers/storms through Sunday.
3. Above normal temperatures on Saturday trending hotter for
Sunday.

A look at the 11 July 00Z NCEP Ensemble Mean 500 mb Geopotential
heights shows a nearly latitudinal ridge axis across the
southern half of the US. Closer to the US/Canadian border, a
general area of flat troughiness is expected to be over the
Great Lakes and/or the New England area through the period. At
the surface, high pressure remains centered across the western
Atlantic with an associated ridge axis extending into the
Southeast US. Closer to the coast of the mid-Atlantic region, a
small ripple exists within the isobar mean averaging, enough to
suggest a trough/low moving north of the west side of the ridge
on Friday night. Saturday through Sunday night, the Southeast US
ridge holds fast, and a persistent weak trough is depicted in
the averaging over parts of the mid-Atlantic/New England
regions.

Output from the 11 July 00Z Ensemble Situational Awareness Table
shows 850mb temperatures slowly rising through this portion of
the forecast. For Saturday, values trend from around +18C in the
morning to around +20C by the afternoon. For western sections
of the region, this value falls within the 90 to 97.5 percentile
of the 30-year climatology. For Sunday, values inch a bit
higher with values between +20C and +22C for the region. This
places the entire area within the 90 to 97.5 percentile of the
30-year climatology an portions of the northwest North Carolina
mountains with the 97.5 to 99 percentile. A look at Precipitable
Water (PW) values on Friday night shows a sharp west to east
gradient in values with numbers just under 1.25 inches in the
west to around 2.00 inches across southeast sections. Values in
the far eastern sections correspond to values with the 90 to
97.5 percent of the 30-year climatology. As we progress through
Saturday night, we keep a west to east gradient, but values fall
to numbers between 1.00 and 1.25 inch in the west and around
1.50 to 1.75 in the east. PW numbers continue to decline through
Sunday night dropping to a range of roughly 1.25 to 1.50 inch
across the region.

The above weather scenario offers eastern portions of our area
to see showers and storms exiting the region to the east on
Friday night. On Saturday, there is some question as to how far
the front will be east of the region. It may still be in enough
proximity for eastern parts of the area to see additional
showers and and storms, especially around peak heating of the
day, but coverage is not expected to be as great as what is
anticipated for Friday. Western sections may see isolated hit or
miss showers/storms. The hit or miss pattern becomes the norm
on Sunday with the front even less of a potential lifting
mechanism, with any shower/storm activity the product of max
daytime heating combine with any differential heating or weak
orographic effects. The vast majority of the area will be
precipitation-free Saturday through Sunday night. Temperatures
will trend hotter each day with increasing humidity.

Confidence in the above weather scenario is moderate with the
biggest question mark being the influence of the East Coast
front and duration of onshore flow.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 1215 PM EDT Thursday...

Key Points:

1. Above normal temperatures Monday through Wednesday.
2. Limited shower/storm chances until Wednesday/Thursday.
3. Slightly cooler for Thursday.

A look at the 11 July 00Z NCEP Ensemble Mean 500 mb Geopotential
heights shows a buckle in the western extent of the latitudinal
ridge across the southern half of the US. This buckle results in
a building ridge over the Rockies and subsequently a bit better
troughing across the Northeast US. At the surface, a ridge of
high pressure remains over the Southeast US. A trough continues
over eastern portions of the mid-Atlantic region and parts of
New England. On Tuesday into Wednesday this trough amplifies a
bit in response to an area of low pressure moving through Quebec
towards the Canadian Maritimes. On Thursday, high pressure is
expected to be centered over the Great Lakes region, west of a
front that is expected to heading east of our area Thursday in
relation to the southeast Canada low.

Output from the 11 July 00z Ensemble Situational Awareness Table
shows 850mb temperatures for Monday through Wednesday as
expected to average around +21C to +23C. This range places the
entire region at or above the 97.5 percentile with many
locations above the 99 percentile. Wednesday night through
Thursday temperatures trend lower, reaching roughly the +18C to
+20C range by the afternoon hours Thursday. PW values are
expected to gradually increase Monday through Wednesday. On
Monday, values will average 1.50 inches, but by Thursday expect
a range of 1.50 to 2.00 inches with the high end of this range
over southeast portions of the area. On Thursday, values slide
by about 0.25 inch area-wide.

This portion of the forecast we can continue to expected above
normal temperatures at least through Wednesday. Any
shower/thunderstorm activity will be limited at least Monday and
Tuesday to isolated coverage across the mountains at best. As
we progress into Wednesday, and even more so on Thursday, shower
and storm chances will be on the increase thanks to the
approach and subsequent arrival of cold front. Increased cloud
cover on Wednesday and slightly lower 850mb temps on Wednesday
should be slightly cooler temperatures, but still with values
above normal. While timing still has it level of uncertainty,
cooler temperatures can be expected on Thursday thanks to the
passage of the front and its associated clouds/precipitation.

Confidence in the above weather scenario is moderate with the
biggest question mark being the timing of the mid-week cold front
and its impacts on our sensible weather.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
As of 130 AM EDT Friday...

Cigs should lower into the MVFR/IFR range later this morning
along with increased shower chances for the eastern sites. Have
TS potential by late morning into afternoon. Coverage of precip
should be less farther west along with VFR conditions - maybe
just some alto clouds or cu 4-7 kft. Winds should be generally
NE to east under 10 kts. Friday night into Saturday morning
could have some reduced MVFR vsbys and LIFR cigs for the east
and possible MVFR cigs or lower for the central sites.

Forecast confidence is medium on western extent of precip
coverage and cigs.

AVIATION OUTLOOK...

Friday night into Saturday morning fog will become more of a
problem with some stratus around and conditions sub-MVFR.

The weekend looks drier with morning fog possible. Afternoon
TSRA chances increasing again Monday. Sub-VFR is likely over
the mountains in the afternoon.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...None.
NC...None.
WV...None.

&&

$$

SYNOPSIS...SH
NEAR TERM...BMG/SH
SHORT TERM...DS
LONG TERM...DS
AVIATION...AB/SH