Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 170719

National Weather Service Raleigh NC
320 AM EDT Wed Aug 17 2022

A cool and moist northeasterly flow under a wedge of high pressure
over the Appalachians will remain in place tonight and much of
Wednesday. This high pressure will weaken on Wednesday as it slowly
moves toward the coast by Thursday.


As of 320 AM Wednesday...

The center of a 574 dm trough will be just off the coast of MA to
start the period. The mid-level trough will strengthen as it
retrogrades to the north-northwest into ME by early Thu. The
longwave trough will exert its influence over NC with WNW flow
persisting today at mid-levels. A few embedded shortwaves will
largely miss central NC, with one rotating across VA and a few over
central MS/AL/GA/SC. At the surface, high pressure will continue to
extend from the Appalachians into central NC.

Meanwhile, surface low pressure offshore along a surface boundary
will exert northeasterly flow. Increasing low-level 925 mb moisture
tied to this area of low pressure will advect back in across the
Coastal Plain counties this afternoon. A few hundred J/kg of CAPE
over this area could touch off a few isolated showers with daytime
heating. Any spotty showers would taper off by sunset. Drier air
will continue to work in from the north, with some upper 50s
dewpoints reaching along/north of US-64, making for another
comfortable day. Morning mixture of clouds and sun will give way to
afternoon stratocu. Highs warmer than Tue but still about 3-7
degrees below normal in the low to middle 80s. An upper-level jet
will increase some high clouds tonight, but expecting these to
become scattered toward daybreak Thu. With good radiational cooling
expected under light winds, temperatures were lowered to the upper
50s over the north, to the low to middle 60s elsewhere.


As of 320 AM Wednesday...

The flow will transition from westerly to southwest at mid-levels as
the longwave trough over the NE US moves into Canada and a trough
settles into the Mid-Mississippi/Tennessee Valley and portions of
the southeast. At the surface, high pressure will slowly shift
offshore. With slightly higher low-level thicknesses and less cloud
cover , temperatures will rise about another degree from Wed in the
low to middle 80s to some upper 80s over the Sandhills.

The stalled surface boundary over the southeast US will largely
remain south of our region, but inch northward somewhere over
central GA to southern SC. The 850 mb front, however, is forecast to
reach portions of central SC by early Friday, as a surface wave
tracks along the northward moving boundary. The boundary will be
characterized by higher theta-e/instability and dewpoints in the
lower 70s, while cooler and drier air will likely persist across our
area and over VA. Surface upslope and weak daytime instability
upwards of 400 J/kg could produce some widely isolated/scattered
showers or thundershowers over the western Piedmont and
Appalachians. Any of this activity should dissipate after sunset.

Another chance of isolated showers or thundershowers may occur late
Thu to early Fri morning with the aforementioned frontal boundary,
mainly across the Sandhills to southern Coastal Plain. Short-range
models are not in good agreement as to how far north convection over
GA/SC may move, with the ECMWF/NAM furthest north. Tend to favor the
slower solutions given drier air likely persisting across the area.
Overnight lows should range from the upper 50s to middle 60s,
coolest in the north.


As of 300 AM Wednesday...

...Unsettled and potentially wet period returns Friday through early
next week...

The closed low over the NE US is forecast to open and lift NE into
the Canadian maritimes on Friday. Betwixt a prominent shortwave
trough/closed low diving SE into the upper Midwest and Great Lakes
regions and subtropical ridging attempting to build back west
towards the SE US, heights aloft will steadily build/increase
through the weekend.

Meanwhile at the surface, the stationary, wavy frontal zone draped
to our south will gradually lift north into the area late Friday and
into the day on Saturday, where it is then forecast to stall and
then slowly wash out through early next week. Daytime heating amidst
the moist 1.75-2.0" PWATs pooling INVOF the front could make for an
active and unsettled Friday through Saturday period, especially if a
weak surface low forms/rides up along the front. Convective rain
chances may briefly decrease to primarily diurnally driven/climo
chances on Sunday, before increasing again Monday and Tuesday as the
upper trough and associated dynamics to our NW draws near.

The GEFS, EC, and CMC ensemble mean total precipitation during that
period all show 1 to 2 inches, with the highest amounts depicted
across eastern portions of the state, which would prove highly
beneficial in helping to alleviate if not eliminate much of the
ongoing drought/abnormally dry conditions in place across the state.

Below normal temperatures at the start of the period will slowly
rebound to near normal by the weekend and continuing into early next


As of 115 AM Wednesday...

There is medium to high confidence that VFR conditions will prevail
at all terminals through the 24-hour TAF period. A few pockets of
isolated showers have developed with some weak instability beneath a
weak area of convergence, mostly near RDU/RWI. Some light/spotty
showers may briefly reach the GSO/INT/RDU/RWI terminals between 06-
08Z, but no sub-VFR restrictions are expected. However, there is a
non-zero chance that some sub-VFR CIG/VIS may develop prior to 12Z
at RDU/RWI as nearby showers have saturated the near-surface layer.
Confidence on these sub-VFR conditions is not high enough to include
in this TAF issuance. VFR is forecast through Wed with northeasterly
surface winds between 5 and 8 kt becoming light by early Thu.

Outlook: VFR conditions are forecast to persist Thu. There will be a
chance of sub-VFR conditions in isolated/scattered showers or storms




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