Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 250154

Area Forecast Discussion
National Weather Service Raleigh NC
955 PM EDT Tue Apr 24 2018

An area of low pressure will track northeastward across the
Carolinas and VA tonight. A weak cold front will cross our region
late Wed and Wed night.


As of 955 PM Tuesday...

High rain chances will persist overnight over the W and N sections
of the forecast area, although the loss of heating has led to
lowering CAPE and a resultant slow decrease in severe risk. The
surface low is currently analyzed just SE of CLT, with an occluded
front extending to its E across far SE NC, and weak troughs
extending to its NW and SSW. Stacked cyclonic flow persists up
through the mid levels over W NC, and this will move slowly eastward
overnight, as the surface low tracks northeastward over central NC.
The earlier strong to severe storms formed and moved into areas with
effective shear over 30 kts, 200-300 m2/s2 of 0-1 km SRH, and LCLs
under 750 m across all but far southern parts of central NC, north
of the occluded front within the strong low level easterly flow.
While mid level lapse rates did not exceed 6 C/km, MUCAPE was 500-
1000 J/kg along and south of the northward-pushing front, and a few
cells which tracked NE or NNE along or at a small angle to the CAPE
gradient exhibited tight gate-to-gate circulations, feeding on the
high low level vorticity. The best low level shear has pushed
further N into VA and CAPE has decreased (although moderate values
hold over the central Coastal Plain), and the risk of severe storms
should continue to dwindle. But the approaching mid level low and
pockets of DPVA and upper divergence maxima (one of which is now
tracking N over the W Piedmont) will continue to support convection
overnight, and will maintain high chance pops across the W and N,
shifting slowly WNW through daybreak Wed. This will include a threat
for isolated strong dynamically-driven storms, with the potential
for small hail. With clouds holding overnight (albeit with breaks
across the SE sections) and high dewpoints, expect temps to drop
minimally overnight, yielding lows in the mid-upper 50s. -GIH

Previous discussion from 405 PM: Water vapor imagery depicts a well
defined mature cyclone over the TN Valley with a couple of
discernible s/ws rotating around it. One s/w is lifting newd across
eastern TN/western NC, while another was noted crossing the Deep
South.  The lead s/w will lift across our region this evening, while
the later should lift into our area late overnight/early Wednesday.
These s/ws interacting with a available moisture will
trigger/sustain numerous showers and a few thunderstorms late this
afternoon through sunset. Bulk of thunderstorm activity will remain
south-southwest of Raleigh. Narrow axis of instability and shear in
vicinity of a nwd moving warm front will maintain a slight threat
for a severe storm or two through 23Z across the southern Piedmont
and Sandhills.

May see a lull in the shower activity behind the initial s/w mid-
late evening before showers start to increase in coverage across the
western Piedmont ahead of the next s/w. This increase in shower
activity should occur after 06Z. Potential for areas of fog to
develop across the eastern counties, though most of the visibilities
should be above a mile. Overnight temperatures in the 50s.


As of 405 PM Tuesday...

Wednesday, the upper level low will migrate east across the region.
Expect showers to be concentrated near the center of this system in
the morning then expand south and east in the afternoon. Will
refrain from mention thunder at this time though an isolated rumble
of thunder cannot be ruled out, especially east and southeast of
Raleigh. With peeks of sun expected, it should be notably warmer
across the northern counties compared to this afternoon. Highs
should range from near 70 in the NW to the mid 70s south.

Subsidence behind the departing upper low and loss of heating
Wednesday evening should cause showers to quickly diminish in the
early evening. Cloud cover will be slow to erode in the evening due
to the cyclonic flow though this becomes more anti-cyclonic
overnight. This should lead to partly cloudy skies. Min temps in the
low-mid 50s.


As of 335 PM Tuesday...

As the mid- upper-level low lifts off to the northeast and gets
absorbed into the low over the Great Lakes there will be a brief
lull in precipitation chances over Central NC Thursday morning into
Thursday Afternoon. Another shortwave low aloft will swing through
the the Deep South and toward the Mid-Atlantic coast Thursday
resulting in another chance for showers and thunderstorms across the
region Thursday aft/eve into the overnight hours. As that second
shortwave low moves through the area, a third shortwave will dive
south through the ARKLATEX, elongating the parent longwave trough
aloft. This third low may result in additional precipitation over
the Carolinas, however there are some model differences in that
regard at this time. Will continue to carry at least a slight chance
for rain through Friday night, but confidence in rain occurring is
not terribly high at this time. Meanwhile at the surface, a low will
drift north-northeastward out of the Deep South and along the
Eastern Seaboard toward New England. Temperatures Thursday through
Saturday: Highs ranging from around 70 degrees in the NW to mid 70s
SE and lows decreasing from mid 70s Thursday night to mid 40s NW to
around 50 degrees SE Saturday night. Largely expect Sunday and
Monday to be dry with highs in the low to mid 70s and lows in the
mid 40s to low 50s.


As of 810 PM Tuesday...

24 hour TAF period: Showers and thunderstorms continue to push
northward through the northern two thirds of the forecast area. At
this time KFAY is the only terminal that should remain dry at this
point with all northern terminals still expecting another few hours
where precipitation will be possible. Ceilings will continue to be
at sub VFR level for much of the night. Expect mainly MVFR and IFR
conditions across the forecast area overnight with some LIFR
possible after 6z through sunrise. For the northeastern quadrant of
the forecast area some fog will also be possible down to a half mile
or less after 6z through sunrise. After sunrise expect rapid
improvement in visibility and gradual improvement in ceilings back
to VFR levels by late morning/early afternoon. Wind gusts have let
up for the evening but expect some gusts of 15-20 kts again on
Wednesday afternoon out of the northwest.

Long term: After conditions return to VFR on Wednesday, expect VFR
conditions through Thursday before another system moves in for
Friday, presenting the next chance for sub-VFR conditions.





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