Area Forecast Discussion
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FXUS62 KRAH 220120
Area Forecast Discussion
National Weather Service Raleigh NC
920 PM EDT Tue Mar 21 2017
A cold front will push slowly southward through the area tonight. A
secondary, back-door cold front will surge through NC on Wed. Cool
high pressure will build into the region from the north Wednesday
through Thursday night, then shift off the East Coast on Friday.
.NEAR TERM /Through Tonight/...
As of 705 PM Tuesday...
Quick update to nudge low precip chances a bit farther northward, in
accordance with convection-allowing model trends and depictions of
low level moisture convergence along and ahead of the incoming 850
mb trough. Still expect likely showers and storms across the extreme
southern CWA, although will need to monitor trends, given that the
low level theta-e ridge is weakening, with decreasing 850 mb winds,
which may yield lower coverage or a suppression of better coverage
to the south. -GIH
Earlier discussion as of 115 PM... Overview: In the upper levels,
small amplitude disturbances embedded in NW flow aloft are progged
to traverse the Carolinas this evening/ tonight. 16Z WV imagery,
radar imagery and RAP data show several disturbances upstream over
the central MS river valley (IA/MO) this afternoon. The northern
disturbance is progged to track through the Mid-Atlantic invof the
mason-dixon line late this eve/tonight, while the southern
disturbance/MCV is progged to track through TN into GA/SC. A cold
front progressing SE into the Appalachians this morning will track
slowly S/SE into central NC late this aft/eve. Strong insolation
/diurnal pressure falls/ will enhance a pre-frontal
trough downstream of the front (east of the mountains) and aid in
the development of a weak surface low invof the NC/SC border by
~00Z. The surface low is progged to track ESE toward Cape Fear
overnight as the cold front progresses S/SSE through central NC and
an upper level disturbance tracks SE through TN to the Southeast
Today: Expect dry conditions to prevail through mid to late
afternoon given weak forcing, marginal low-level moisture and a
strong cap associated with an unmodified elevated mixed layer (H7-H5
lapse rates ~8C/km) advecting over the region from the west. The low-
level airmass over central NC has been modified by warm advection
during the past 12-18 hrs. This, in combination with skies and
unimpeded Spring insolation will result in highs ranging from the
mid/upper 70s to lower 80s.
Tonight: Strong insolation and low-level moisture advection from TN
into northern GA and Upstate SC is expected to erode the cap and
result in convective development within the pre-frontal trough over
southeast TN and Upstate SC (perhaps as far NE as CLT) by 21-00Z
this afternoon. With the best low-level moisture expected to advect
into GA/SC this afternoon, airmass destabilization/cap erosion in
central NC late this aft/eve may be confined to the SC border
counties, primarily in the SW Piedmont/Sandhills. North of Hwy 64,
forecast soundings show a capped atmosphere in place the entire day,
with little or no destabilization in advance of FROPA. With the
above in mind, convection is expected to develop ~00Z in the SW
Piedmont, with activity progressing E/ESE invof the NC/SC border
overnight. Chances for convection will decrease from W-E after
midnight as activity progresses toward Cape Fear and, ultimately,
offshore the Carolina coast. Confidence in convective development/
coverage decreases heading north toward the Hwy 64 corridor, and
portions of the N/NW Piedmont and NE Coastal Plain may ultimately
remain dry during/after FROPA late this evening.
Severe Potential: The potential for severe weather late this aft/eve
and early tonight appears to have diminished for much of central NC,
except invof the NC/SC border where destabilization/cap erosion is
still anticipated and environmental conditions remain supportive of
organized (multicellular or supercellular) convection with an
attendant threat for damaging winds and hail up to the size of
quarters. The potential for severe weather will be greatest along
the NC/SC border between ~7 pm and ~3 am. -Vincent
.SHORT TERM /WEDNESDAY AND WEDNESDAY NIGHT/...
As of 115 PM Tuesday...
Expect a clearing trend in the morning as a drier airmass advects
into the region from the north, with clear skies during the
afternoon and evening. Cold advection is expected to persist for the
majority of the diurnal heating cycle on Wed as Canadian high
pressure builds into the Mid-Atlantic from the NW. Expect highs much
cooler than today, in the mid/upper 50s N/NE to lower 60s S/SW. Lows
Wed night may be tempered by increasing mid/upper level cloud cover
after midnight. Will indicate lows ranging from the upper 20s (rural
and low-lying areas east of I-95) to lower/mid 30s. -Vincent
.LONG TERM /THURSDAY THROUGH TUESDAY/...
As of 243 PM Tuesday...
With the cP airmass centered over the area Thursday and Thursday
night, temperatures through the period will average a good 10 to 15
degrees below normal. A potentially opaque canopy of high clouds
Thursday morning should diminish in the wake of the exiting NW flow
disturbance, allowing for afternoon sunshine. Highs 50 to 55.
High level moisture/cirrus blow-off advecting downstream from the
closed cyclone over the Central US could produce fair to partly
cloudy skies Thursday night, which could impede radiational cooling
a bit. Lows in the lower to mid 30s.
The modified Canadian parent high will shift east and offshore on
Friday, with southerly return flow allowing for a quick moderation
into the 60s on Friday and then into the 70s by Saturday. Meanwhile,
ridging aloft will keep it dry through the day on Saturday.
Strengthening warm moist air advection on the eastern periphery of
the closed mid/upper level cyclone moving east-northeast into the
Ohio Valley and Lower Great Lakes will result in increasing pops in
the form of showers and possibly a few thunderstorms Saturday night
and into the day on Sunday. The most recent 12z/21 GFS has trended
farther north than previous runs, and is significantly less wet than
previous runs. As such, will keep pops at chance, and highest in the
west. If this more northern track solution is realized, Saturday
night and Sunday could end up being mostly dry across central NC,
with mostly in the way of clouds across the area. Additionally, with
the low farther removed from the area, resultant weak forcing will
not bode well for any severe weather. Highs Sunday in the 70s.
On the heels of the dampening wave moving off the Mid-Atlantic coast
Sunday night, another shortwave trough, of lower amplitude, will
skirt quickly east across the Central US on Monday and will approach
the area late Monday night and into the day on Tuesday.
Between these two systems, expect to see a lull in precip chances on
Monday, with rain chances ramping back up again Monday night and
Continued mild with highs in the 70s. Lows in the 50s.
.AVIATION /01Z Wednesday through Sunday/...
As of 915 PM Tuesday...
A lead frontal zone now stretching from near EWN to JNX to JQF, and
associated focus for scattered showers and storms and sub-VFR
conditions mainly at FAY through 09, will settle Swd tonight.
Generally Nly winds, between 6-12 kts, will develop behind the front
overnight. A secondary cold front and leading edge of strong
continental polar high pressure will surge Swd through central NC
early Wed; and this will result in strong and gusty NEly winds in
strong surface pressure rises and cold air advection between 12-18Z
Wed, with lessening winds during the afternoon.
Outlook: There will be a chance of MVFR-IFR visibility restrictions
in radiation fog in SErn NC Fri morning, including at FAY and
perhaps RWI, then again forecast area-wide on Sat morning.