Area forecast discussion
National Weather Service Raleigh NC
1000 PM EDT Sat may 18 2013
Synopsis...
a stationary front over central NC will retreat northward into Virginia on
Sunday. Meanwhile...an upper level trough will track slowly east
across the Carolinas through early next week.
&&
Near term /through Sunday night/...
as of 1000 PM Saturday...
Convection has become almost entirely confined to short line segment
arching from Burlington to Hillsborough to apex...with some signs of
building further southeast toward Fuquay-Varina. The entire segment
is pushing north-northeast ahead of a vorticity maximum (centered directly
overhead) within a negatively tilted trough aloft. As has been the
case all evening...the storms are strong and may produce some small
hail and wind gusts up to 40mph...but severe criteria has not been
attainable. This line will continue to be monitored as it moves
toward the northern Piedmont/coastal plain...where Storm Prediction Center mesoanalysis
shows around 250 j/kg of cape. However...this area was partially
worked over earlier in the evening...and the remnants of the stalled
front/outflow boundaries appears to have sunken south toward
krwi/krdu/ktdf as a result. Thus...the ongoing convection should
become a bit more elevated as it moves north. Flooding has not been
a significant problem so far...with the highest rainfall totals of
near 2 inches observed in Wayne County.
Once this round of convection is over...there should be a relative
lull in activity overnight due to subsidence behind the
aforementioned upper disturbance and nocturnal stabilization. Rap
soundings show a persistent 25-30kt southerly low level
jet...strongest in the west... which in combination with
southeasterly surface wind (veering winds) and precipitable water over 1.4
inches...may provide additional redevelopment along the front and/or
lingering boundaries. Late tonight...the final upper level
disturbance within the upper trough will dip southeast from
Kentucky/Indiana...approaching the western Piedmont after 12z.
Similar to last evening...it seems the best approach will be to have
chance probability of precipitation...highest in the west overnight...increasing toward and
into likely category beyond 12z. Temperatures should be relatively steady
or drop just a few degrees...holding in the low to middle 60s. -Smith
Sunday and Sunday night:
a lot of similarities between today and Sunday...minus the stalled
surface front which will be well north of the area. Convective
coverage Sunday morning should initially be greatest in the west with
the encroaching remnants of the middle-level trough...and should
gradually spread east through the late afternoon and through the
overnight hours...peaking during daytime heating. With the upper
trough directly overhead...convection should be able to better
sustain itself Sunday night with decent coverage expected through
the overnight hours.
Models actually suggest less instability on Sunday..hindered perhaps
by more robust cloud cover and rain chances earlier in the day with
MLCAPE values of 500-1000 j/kg. Given continued weak shear of 15kts
of less...expect disorganized pulse storms to render a non-zero
severe threat. A heavy rain/minor flooding threat will need to be
monitored in instances of training cells over urban or poor drainage
areas.
Temperatures again will depend primarily on cloud cover and precipitation.
Highs ranging from the middle 80s east/southeast (highest east of Highway 1/south of
Highway 64) to middle 70s west/northwest Piedmont where cloud cover will be more
pervasive. Lows in the middle 60s.
&&
Short term /Monday through Monday night/...
as of 245 PM Saturday...
With the middle/upper trough still lingering over the region on
Monday...the increased shower and thunderstorm activity is expected
to persist for at least one more day. The trough axis will slowly
shift eastward during the day and into the overnight hours...which
should gradually shift the best precipitation chances eastward.
Therefore...the highest probability of precipitation will be focused across the central and
eastern portions of the forecast area. Good deep layer moisture will
still be in place across the region...especially across the
east...where precipitable water values are prognosticated to be near 1.75 inches. As a
result...locally heavy rainfall will be possible once again and with
the expected amount of precipitation expected to precede the rainfall on
Monday...some minor flooding...mainly in poor drainage and urban
area will be possible. With all of the clouds and precipitation across the
area...highs should be held down somewhat...with generally lower 80s
expected.
Precipitation chances should gradually decrease Monday night as the
middle/upper level trough axis shifts east of the area. Regardless...
cloud cover should remain fairly extensive throughout the overnight
hours...keeping overnight lows mostly in the middle/upper 60s.
Long term /Tuesday through Saturday/...
as of 245 PM Saturday...
Some weak shortwave ridging will start to build over the area
Tuesday and Wednesday...while at the surface...the Bermuda high will
start to ridge westward into the Carolinas. All of this will
translate to a decrease in precipitation coverage for the middle of the
week. Also contributing to this will be some slight middle level
drying...although sufficient moisture should remain in place over
the region...with precipitable water values hovering near 1.5 inches or so.
Therefore...am expecting scattered diurnally driven precipitation each day.
With more in the way of breaks in the cloud cover and less precipitation
coverage...temperatures should be able to rebound nicely into the middle/upper
80s.
Shower and thunderstorm coverage should begin to increase once again
by the end of the week with the approach of our next middle/upper level
trough and associated cold front. There is still some disagreement
with the timing of the cold frontal passage...but for now will go
with an late Friday night/early Saturday morning passage.
Therefore...will show highest probability of precipitation Thursday and Friday/Friday night
(but will still go no higher than chance until better agreement is
reached)...with probability of precipitation decreasing from west to east on Saturday. With
increased precipitation chances/clouds on Thursday and Friday...temperatures will
mostly be in the low/middle 80s...with slightly cooler temperatures expected
on Saturday.
&&
Aviation /00z Sunday through Wednesday/...
as of 820 PM Saturday...
Showers and isolated thunderstorms will be focused by a stationary
front and multitude of thunderstorm outflow boundaries...primarily
at kgso/krdu/kfay for the next several hours. Initially VFR
conditions outside of convection will become MVFR-IFR due to
lowering/developing ceilings - lowest west (kint/kgso) - overnight.
The low clouds will gradually lift and scatter through MVFR
range...to VFR between 3000-4000 feet...Sunday afternoon. The slow
approach of a lift-inducing and moisture-focusing trough aloft from
the west will also promote the occurrence of scattered showers and
isolated storms overnight...becoming numerous again with heating on
Sunday. Despite the expected coverage of showers and storms...
confidence of occurrence at any particular terminal is too low to
include a multi-hour prevailing condition of such beyond the next
several hours...and have elected to convey the threat with vicinity
showers throughout the taf period. -Mws
Looking ahead: a slow moving upper level trough moving through the
region will result in unsettled conditions with numerous to
widespread showers and storms through Monday. Drier conditions by
Tuesday with mostly diurnal rain chances with the threat of morning
fog/stratus. -Cbl
&&
Rah watches/warnings/advisories...
none.
&&
$$
Synopsis...mws
near term...cbl/Smith
short term...krr
long term...krr
aviation...mws/cbl
