Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 230220
AFDRAH

Area Forecast Discussion
National Weather Service Raleigh NC
1020 PM EDT Sun Oct 22 2017

.SYNOPSIS...
High pressure will extend across the eastern United States through
tonight. Meanwhile, a strong frontal system will approach from the
west and cross the Appalachians and Carolinas late Mon through early
Tue.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1020 PM Sunday...

Closed upper low over the Lower/Middle Ms Valley this evening
will progress east and become increasingly negatively tilted
as it moves into the TN Valley by daybreak. Strengthening SELY 925-
850mb moist upglide in advance of the low will be maximized across
western NC, leading to the expansion and lowering of the 3 to 4 kft
stratus deck across upstate SC into western Piedmont counties
overnight, that will then spread east central and eastern NC between
12 to 18z Monday.  Can`t rule out an isolated shower/sprinkles
within the low-level warm moist air advection, but measurable rain
chances should largely remain along and west of the Yadkin River,
near the upslope areas.

Dewpoints have recovered into the upper 50s to lower 60s this
evening, which will result in significantly milder temperatures
overnight. Lows generally in the upper 50s to lower 60s, with
some mid 50s possible across the NE.

&&

.SHORT TERM /MONDAY AND MONDAY NIGHT/...
As of 335 PM Sunday...


Model difference from 24 hours ago is that the models are trending
faster with system crossing our region Monday night.

S/w over the TN Valley early Monday will lift newd into the lower OH
Valley by Monday evening. This system takes a negative tilt, thanks
to strong jet digging on the backside of the system, resulting in
upper level heights of of 60-80m over central NC. Low level jet on
the order of 40-50kts will supply abundant warm moist air, with
precipitable water values of 1.5-1.8 inches expected by late Monday.
Additionally, near term model guidance now depicting a sfc wave
lifting nwd across the foothills/western Piedmont Monday evening.

Expect shower coverage to increase Monday afternoon west of highway
1, encroaching upon the highway 1 corridor after 21Z, more likely
toward 00Z, then crossing the coastal plain in the late
evening/first half of the overnight. Now appears that the bulk of
the showers will exit our far east-ne counties prior to 09Z.
While kinematics quite impressive with bulk shear 40-50kts (highly
supportive of organized convection), model guidance remains
weak/anemic when it comes to low level instability as MLCAPE on the
GFS are below 500 J/kg.  Storm relative helicity (0-1km) on the
order of 150-300 m2/s2 by Monday evening raises concerns for
rotating updrafts.

Expect a band of heavy showers to progress east-ne across central
early Monday evening with isolated thunder probable close to the SC
border. Due to strength of the low level flow, strong/locally
damaging wind gusts expected along the band of showers, especially
where any bowing occurs. Lack of low level instability may deter
tornadic threat, though if this parameter were to increase closer to
800-1000 j/kg MLCAPE, then tornadic may be realized. If low level
instability achieved, believe tornadic threat will be greatest over
the southern and western Piedmont, since this region in closer
proximity to the sfc wave.

High temperatures Monday highly dependent upon shower coverage. If
shower coverage becomes widespread by early afternoon across the
west, then high temps may end up being a couple of degrees cooler.
Due to expectation that showers will occur late in the day/early
evening in the east, potential for temperatures along and east of
highway 1 reaching well into the 70s. Expect warm/breezy conditions
Monday evening, with cooler air not reaching the western Piedmont
until after midnight, and likely not reaching the coastal plain
until close to daybreak.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
As of 220 PM Sunday...

Low level jetting and the associated deep moisture plume will be
moving east of the Coastal Plain early Tuesday morning, with drier
and cooler air beginning to filter in as the moisture tap cuts off.
Cloudiness will be diminishing, with stratocumulus lingering
primarily across the northern tier associated with the H85 trof
which will be moving across during the afternoon/evening. Cold air
advection will be delayed until the surface front exits the area
towards sunset, allowing highs to reach mostly lower 70s, with some
upper 60s in the northwest. The cool air settles in Tuesday night
and Wednesday, with mins mostly in the mid 40s Wednesday morning
warming to mostly low and mid 60s in the afternoon.

The main long wave trof axis will cross the area Wednesday night as
it rotates across the Great Lakes into New England. The ensuing
northwest flow will reinforce the cool air and perhaps produce some
scattered very light showers in the lee of the mountains, but it
will be short-lived as the progressive pattern rapidly transitions
to a short wave ridge and rising heights in the late week. Lows
Thursday morning will be our coolest of the period, falling to the
upper 30s to lower 40s, with highs Thursday topping out 5-7 degrees
below normal in the 60 to 65 range.

The short-lived ridging shifts offshore and ensuing southerly return
flow induces a modest warming trend into the weekend, with highs
from 65 to 70 on Friday, and upper 60s to lower 70s on Saturday. Our
next front will be approaching from the west, perhaps as early as
Saturday night, with associated showers and a cool down into the 60s
likely on Sunday.

&&

.AVIATION /00Z MONDAY THROUGH SATURDAY/...
As of 805 PM Sunday...

24-Hour TAF period: Not super high confidence with respect to the
timing and height of low cigs expected to develop overnight, but for
now expect MVFR/IFR cigs at KINT and KGSO, with MVFR/VFR cigs
elsewhere through Monday morning. A few isolated showers may develop
in the Triad before daybreak, but expect the better coverage and
higher intensity showers and storms to move into that area around
noon, continuing through roughly midnight. A similar pattern will
follow at KRDU, KFAY and KRWI, with slightly later start and end
times. There is high confidence for strong to severe thunderstorms
to move through the region during the latter half of the 24 hour TAF
period, with strong gusts and heavy rain likely to result in reduced
visbys. Cigs will likely remain in the MVFR range throughout the
duration of the storms.

Winds will become strong and gusty after sunrise, mainly out of the
south-southeast at 10-12 kts sustained, gusts of 18-25. Higher gusts
are possible with the storms.

Looking ahead: Showers and thunderstorms will likely continue, at
least into Monday night, ending earliest at KINT (around midnight)
and latest at KRWI (by 10Z). Gusts from Monday aft/eve should abate
overnight, with the exception of a storm moving over a terminal.
There could be some lingering sub-vfr conditions through daybreak
Tuesday, but expect VFR conditions to dominate thereafter. The
exception will be a brief instance of MVFR ceilings Wednesday as an
upper level trough passes overhead.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SYNOPSIS...MWS
NEAR TERM...CBL
SHORT TERM...WSS
LONG TERM...mlm
AVIATION...KC


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