598
FXUS61 KRNK 221118
AFDRNK

Area Forecast Discussion
National Weather Service Blacksburg VA
718 AM EDT Sun Sep 22 2024

.SYNOPSIS...
A weak cold front will sink southward across the Piedmont
today, bringing a north-northeasterly wind shift to the lower
Mid-Atlantic. Another cold front will approach slowly from the
west tonight into Monday, bringing rounds of on-and-off shower
activity to region through much of the coming workweek. A return
of high pressure will bring drier conditions for the end of the
week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 650 AM EDT Sunday...

Adjustments made to increase cloud cover across the Piedmont
given the backdoor cold front beginning to make its way south
across the area. Adjusted afternoon high temperatures across
the Piedmont by a degree or two downward as a result.

As of 320 AM EDT Sunday...

Key Messages:

1) Cold front will move southward across the Piedmont late this
morning into the afternoon.

2) Mostly sunny skies are expected today, though some low
clouds are possible across the Piedmont with the cold front.

3) Clouds increase tonight after sunset, with showers moving in
from the west after midnight.

Starting the morning off under mostly clear skies, though IR
satellite imagery does indicate river valley fog developing
across the mountains, as well as over a few locations across the
Piedmont that received abundant rainfall on Saturday evening.
Temperatures remain mild, generally in the low to mid 60s across
the lower Mid-Atlantic, ahead of a back door cold front
currently situated across northern Virginia.

For today, expect the cold front to advance southward across
the Piedmont during late morning through the afternoon. Low
clouds may accompany the front across the southern Shenandoah
Valley and for areas east of the Highway 29 corridor across the
Piedmont, however the remainder of the Piedmont and mountains
can expect mostly sunny skies for much of the day. Rapid update
weather forecast models are hinting at the possibility of a few
showers developing across the mountains and foothills during
late afternoon, but believe these will isolated to widely
scattered at best, and with decreased atmospheric moisture
behind the cold front, no heavy rain is expected. Afternoon high
temperatures will range from the mid 70s to the mid 80s, with
the warmest temperatures expected for the Piedmont and foothills
of North Carolina.

Tonight, strong consensus among the weather forecast models
that low clouds will fill in quickly within a few hours after
sunset as Atlantic moisture filters in across the region. Rain
chances will increase after midnight as energy associated with
another cold front approaches from the lower Great Lakes and the
Ohio River Valley regions. Rainfall is expected to be light and
scattered in coverage, with highest probabilities across the
mountains and the foothills. Overnight lows will range from the
upper 50s to the low 60s.

Confidence in the near term is moderate.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/...
As of 130 AM EDT Sunday...

Key Messages:

1) Daily chances of mainly afternoon/evening showers/storms,
especially over the mountains.

2) Locally heavy rain possible, especially on Wednesday.

3) Slightly above normal temperatures.

A look at the 21 Sep 00Z NCEP Ensemble Mean 500 mb Geopotential
heights shows upper level ridging over the region on Monday and
a trough that will extend from the Upper Mississippi River
Valley into the Central Plains States. As we progress into
Wednesday, the trough will make progress eastward, reaching the
region of the Eastern Great Lake region, south into the
Tennessee Valley by Wednesday evening. At the surface, a ridge
of high pressure will remain anchored along the East Coast of
the US. A weak low/trough will be over the Upper Mississippi
Valley on Monday, and progress eastward, reaching the eastern
Great Lakes region by Wednesday evening.

Output from the 21 Sep 12Z Ensemble Situational Awareness Table
shows 850mb temperatures averaging +14C to +16C over the area
on Monday and Tuesday, and slightly cooler around +13C to +15C
for Wednesday. Precipitable Water values on Monday and Tuesday
are expected to average 1.50 to 1.75 inches across the region.
These values correspond to the 90 to 97.5 percentile of the
30-year climatology. For Wednesday, values are only expected
to be slightly lower, right around the 1.50 inch mark.

The above weather scenario points at one where moisture levels
will be on the high side for this time of year. Additionally,
low level flow will be such that anticyclonic flow around the
east coast ridge, and increasing southerly flow in advance of an
approaching low/cold front, will help allow for progressively
better moisture convergence over our region. Look for increasing
chances of precipitation over the area through mid-week. An
associated cold front is expected to be over, or just west of
the area by Wednesday evening. With daily forecast of afternoon
CAPE a little over 1000 J/kg and Lifted Indices slightly below
zero, each day should have the potential for showers with a few
storms. Given what should be a convergence boundary between the
nearly stationary ridge to the west, an approaching cold front
to our west, and above normal Precipitable Water values, daily
chances of locally heavy rain will be possible, especially on
Wednesday with the cold front on our western doorstep.
Temperatures are expected to be slightly above normal, with
thanks given primarily due to above normal dew point values,
keeping minimum temperatures above normal. High temperatures
will be near normal.

Confidence in the above weather scenario is moderate to high.

&&

.LONG TERM /THURSDAY THROUGH SATURDAY/...
As of 130 AM EDT Sunday...

Key Messages:

1) Low confidence in precipitation amount, timing, and
location.

2) Temperatures slightly above normal.

A look at the 21 Sep 00Z NCEP Ensemble Mean 500 mb Geopotential
heights shows the axis of an upper trough centered over our
area Thursday evening. This feature is expected to head
northeast and be situated over the Canadian Maritimes by
Saturday evening. A shortwave ridge will be centered just west
of the Appalachians Saturday evening. At the surface, high
pressure will be centered near the Canadian Maritimes on
Thursday with an associated ridge axis extending southwest into
New England. Low pressure will be centered just west of Hudson
Bay with a trough axis extending into western Quebec. As we
progress into Saturday, there is significant differences between
the individual members of the ensemble. The overall average
solution places a weak area of high pressure over New York, with
low pressure weaknesses to the south over the Southern
Appalachians and also over Southeast Canada.

Output from the 21 Sep 12z Ensemble Situational Awareness Table
shows 850mb temperatures around +14C on Thursday and Friday.
Slightly cooler readings around +13C are expected Saturday.
Precipitable Water values are expected to average 1.25 to
slightly over 1.50 inches on Thursday, 1.25 to slightly under
1.50 inch on Friday, and 1.00 to 1.25 inch for Saturday.

The above weather scenario offers a forecast with low
confidence given the wide variety of individual members yielding
the mean solution output. As we progress through the period,
moisture levels are expected to drop, but the variety of
solutions almost forces a forecast that offers small chances of
precipitation each day, but with low confidence on
timing/location and if any time period may end up with a better
than a 50/50 chance. Temperatures may trend slightly cooler
through the period, but still average values that are slightly
above normal.

&&

.AVIATION /11Z SUNDAY THROUGH THURSDAY/...
As of 700 AM EDT Sunday...

Patchy fog will burn off quickly after 13Z/9am. A backdoor cold
front is advancing across the Piedmont this morning, and will
continue to shift south into the afternoon, making for a north
to northeasterly wind shift. May see scattered MVFR ceilings
persist in the vicinity of LYH during the afternoon, but
believe most of the service area will remain VFR through the
day. Rapid update weather models are hinting a few shower
developing during the afternoon across the mountains and
foothills but, again, low confidence on how or if these will
develop.

During the 00Z to 04Z timeframe for Monday, high confidence
that widespread IFR stratus will develop and persist through
well after 12Z as marine air filters in behind the cold front.
Will not be surprised to see pockets of drizzle with this
activity as well.


EXTENDED AVIATION OUTLOOK...

Daily chance of showers, thunderstorms, and lingering sub-VFR
conditions continue Monday afternoon through at least midweek
as a slow-moving cold front approaches from the west.

&&

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

&&

$$

SYNOPSIS...NF
NEAR TERM...NF
SHORT TERM...DS
LONG TERM...DS
AVIATION...EB/NF