Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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000
FXUS62 KRAH 200538
AFDRAH

AREA FORECAST DISCUSSION
National Weather Service Raleigh NC
1240 AM EST Thu Feb 20 2020

.SYNOPSIS...
A large area of high pressure centered over the Midwest will build
into our region tonight and Thursday. A storm system in the southern
branch of the jet will approach from the Gulf Coast states Thursday.
This will induce a low pressure to develop near the SC/GA coast late
Thursday. The storm will push out to sea ending the precipitation
Thursday night. Cold high pressure will follow the storm system
Friday and Saturday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 840 PM Wednesday...

Winter Weather Advisory from 1000 am Thursday through early Friday.

No changes to the current forecast but the latest data suggests
increasing probabilities of snow Thursday afternoon and evening.

Watching several important features this evening. A secondary surge
of very dry modified arctic air was pushing southeast and south
from PA into MD, and northern VA. Dew points have fallen into the
single digits and teens from WVA into northern VA. The parent arctic
surface high pressure was located over the Northern Plains (1043-
1044 mb). The high extended east across the Great Lakes and south
into the Southern Plains where pressures were rising through the
1020s and 1030s. The critical part for our snow potential Thursday
afternoon and evening is that this arctic high will build strongly
into the northern Mid-Atlantic, then down the eastern seaboard into
NC. This process is beginning as we see the very dry dew points and
pressure rises coming now surging down the east side of the
Appalachians from PA into MD and northern VA.

The mid/upper level system with deep subtropical moisture tap was
developing over the Plains. Showers/isolated thunderstorms have been
blossoming over western and central Texas eastward into southern
MS. Very deep moisture influx up and over the old stationary front
over the northern Gulf was aiding the blossoming of the convection.
So, this critical part of the forecast (moisture and precipitation
development already ongoing upstream) is coming to fruition.

We have both the the cold dry air advection supplied with strong
high pressure to our northwest/north along with the approaching
moist system from the southwest.

A few negative items which will not affect the snow that falls, but
will affect what accumulates continue to be the warm antecedent
conditions (top soil temperatures in the upper 40s to mid 50s NW to
SE) - not to mention that the monthly temperatures are averaging 6
to 7.5 degrees above normal (almost unheard of levels), and the fact
the sun angle and length of day have increased significantly as we
approach spring. So the snow that falls during the day will have to
overcome these negating factors. This almost ensures that for the
snow to accumulate during much of the day will have to rely on
snowfall rates and evaporative cooling. Expect to see snowfall to
begin to accumulate on elevated surfaces for several hours before
the travel is affected. Therefore the main travel issues should peak
late afternoon into the evening when the snowfall rates peak as the
sun sets. It appears that there may be a narrow window of time
between late afternoon into the evening in which the snow will
accumulate significantly as the rates peak and temperatures fall into
the lower 30s.

Rest of tonight... increasing and thickening cloudiness per the
satellite data, lows generally 30 NE to 40 far SW.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 200 PM Wednesday...

...Winter Weather Advisory in effect from 10 AM Thursday to 7 AM
Friday...

Changes: Still a good bit of uncertainty with respect to amounts,
though there will likely be a "sweet spot" and a tight gradient
somewhere across central NC. Primary changes for this forecast
issuance include an earlier start time for snow, particularly across
the northern half of the area and an uptick in amounts along the I-
85/US-64 corridors. While the overall weather pattern remains
similar to previous model runs, changes to the temperature profile
aloft have resulted in a bit more p-type uncertainty. There will
likely be a stripe of wintry mix across the area as the rain mixes
and transitions to snow across the area.

Overall pattern: A cold front will settle south of the area, laying
across SC/GA Thursday morning with cool high pressure ridging
southward and advecting cold air into central NC. While the surface
front will remain south of the NC/SC border, albeit pivoting some to
more SW/NE orientation, the H85 front will stall over NC. As a s/w
aloft rides along the H85 front from the MS Valley toward the mid-
Atlantic during the day, a surface low will develop over the Gulf
Coast/Southeast US in response. With abundant moisture advection
aloft and and strong cyclogenesis at the surface off the Carolina
Coast there will likely be an area of strong frontogenesis over NC
during the aft/eve. The coastal low will lift northeast along the NC
coast then eastward out over the Atlantic through Thursday
eve/night. Meanwhile, the parent upper trough will swing eastward
through the OH/TN Valley toward the mid-Atlantic/NE Thursday night.
Precipitation will quickly taper off as the upper trough swings
through the area, exiting the region early Friday.

Precipitation: Rain will move into the area from the south-southwest
early Thursday. With the cool, dry air in place this rain will help
lower temperatures to the wet bulb temperature (which should be in
the 29-32 degree range), especially across the northern half of the
area. The tricky part of the precipitation will be determining the p-
type and transition times. There is a big potential for error in
this regard, but the most likely scenario will be a quicker
transition to snow across the north with lower liquid equivalent
precipitation totals than across the south. In between there will be
higher chances of mixed precipitation which could lower the overall
snow accumulation but potentially cause more issues travel-wise.
Based on the model simulations from this morning, there should be a
period of all snow, more brief in some areas than others, across
central NC between Thursday morning and daybreak Friday. While
generally 0.5" to 3" are most likely across central NC, the more
realistic scenario will be a very localized area of higher amounts
and a tight gradient from that to trace amounts. Of course, the
thermal profile and snow-liquid ratios will both impact the overall
snow totals when it`s all said and done. As per usual, the most
significant accumulations will occur on elevated/grassy surfaces.

Temperatures: Highs on Thursday will occur in the morning, topping
out in the upper 30s north to mid 40s south. Temperatures will lower
with the onset of rainfall, more quickly across the north with
temperatures in the mid 30s across the area expected by sunset.
Overnight lows will depend on how quickly the low moves away and the
precip ends, but generally expect a steady decrease to around
freezing until the rain ends, with more rapid cooling in its wake.
For now, expect temperatures could be in the mid 20s NW to upper 20s
SE by daybreak Friday.

Winds: Winds should be northeasterly throughout the period. While
sustained winds will remain in the 5-10 kt range in the NW, gusting
to 10-15 kts in the eve/night, winds across the southeast (in closer
proximity to the coastal low) will be quite a bit stronger and
gustier. Expect sustained winds across the Sandhills and Coastal
Plain around 15 kts with gusts into the mid 20s kts.

Friday and Friday night: The precipitation should quickly exit the
area early Friday, with cool, dry weather expected in its wake.
Winds will continue to be a bit breezy/gusty, strongest in the east
and southeast during the day and tapering off overnight. Skies
should rapidly clear during the day, but with the winds highs may
reach low 40s west while topping out in the upper 30s east.
Overnight, clear skies and light winds will result in good
radiational cooling conditions with lows in the mid 20s expected.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
As of 300 PM Wednesday...

The mid level shortwave trough will be pushing well out over the
Atlantic Sat morning, leaving broad ridging aloft and an expansive
surface high in its wake. We may see some orographically enhanced
high clouds mainly Sat and Sun morning/early afternoon, but
otherwise skies should be mostly clear through much of the weekend,
with temps running a bit below seasonal normals, peaking in the low-
mid 50s Sat and mid 50s to near 60 Sun.

The pattern looks to trend mild but a bit unsettled Mon-Wed, with a
mean longwave trough developing to our W. A Pacific-source low will
cross CA and the Desert Southwest before becoming an open wave and
crossing the Southern Plains through Sun. This will draw the surface
frontal zone back northward into the area with a surface low
approaching from the west, so expect increasing clouds Sun
evening/night followed by overcast skies and rain chances for Sun
night through early Tue as this dampening trough and surface frontal
system pass through. We may get into a brief respite late Tue into
early Wed, but a polar stream wave diving into the central/eastern
states may bring in additional rain chances late Wed. Temps will
generally peak in the mid 50s to mid 60s, with lows holding above
the freezing mark. -GIH

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
As of 1240 AM Thursday...

VFR conditions are expected through 15z on Thursday. A mixture of
rain and snow will develop over the Piedmont between 15z and
18z/Thu., then transition to snow in the KGSO/KINT areas around 18z.
The rain/snow will transition around KRDU between 18z and 20z to all
snow. CIGS and VSBYS will dip into the IFR to LIFR category between
18z and 21z, lasting through 00z-03z, before the precipitation
tapers off. A return to VFR conditions is expected between 03z and
08z/Friday in the KINT/KGSO and KRDU areas. KFAY will have MVFR rain
conditions with CIGS/VSBYS Thursday - transitioning to snow by
between 00z-03z/Fri. IFR snow will last around KFAY and KFAY through
03-06z/Friday, before ending. VFR conditions will return
on Friday.

Looking beyond 09z Fri, VFR conditions are likely to last Friday
through Sunday beneath high pressure. The next storm system will
arrive Mon, briefly a good chance of sub-VFR conditions with rain.

&&

.RAH WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 10 AM Thursday to 7 AM EST Friday
for NCZ007>011-021>028-038>043-073>078.

&&

$$
SYNOPSIS...Badgett/Hartfield
NEAR TERM...Badgett
SHORT TERM...KC
LONG TERM...Hartfield
AVIATION...CBL/Badgett/Hartfield


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