Area forecast discussion
National Weather Service Raleigh NC
255 PM EDT Tuesday may 21 2013
Synopsis...
An upper level trough will linger...but gradually weaken over the
Carolinas through Wednesday. A cold front moves through the region
Thursday...followed by cool high pressure for most of the weekend.
&&
Near term /through tonight/...
as of 218 PM Tuesday...
Upper air analysis from 12z shows that the main feature across
central NC today is a shortwave trough visible from the surface up
through 500 mb. This has been realized at the surface this afternoon
with a convergence boundary that has been the primary catalyst for
showers and thunderstorms to form on just west of the 1-95 corridor.
One or two storms have become strong and approached the lower
thresholds for severe criteria but remain primarily heavy showers to
this point. This boundary will travel off to the east at a very slow
clip this afternoon 20-30 knots. A few more weaker boundaries to the
west will slowly rotate around the shortwave trough axis this
afternoon...allowing for new storm development further to the west.
Other than the surface convergence boundaries and a fair amount of
instability...not too much else out there helping the storms at this
time. Dcape in the east continues to be very low. Lapse rates are
mediocre and shear remains very low. Precipitable water values
continue to be close to 1.75 inches at this time making heavy rain
and localized flooding the primary threat this afternoon.
Temperatures are still on track to maximum out in the middle 80s across the
County Warning Area. Back to the west conditions are a bit more favorable for severe
storms with damaging winds but there is not as big of a forcing
mechanism for storm genesis at this time so coverage should be less
in The Triad. That being said better heating and instability will
still make storms possible this afternoon.
Storms should begin to taper off after sunset this evening.
Dewpoints will remain in the middle 60s and help to initiate another
round of low ceilings and visibilities after 3z or so in the east
and after 6z in the west. Expect lower ceilings and visibilities to
hang around until after sunrise Wednesday morning. Lows tonight in
the middle 60s.
&&
Short term /Wednesday and Wednesday night/...
as of 218 PM Tuesday...
On Wednesday central North Carolina will be between the exiting
shortwave system that has brought US all of the rain over the last
several days and the approaching frontal system with the parent low
pressure system crossing the western Great Lakes. It should be the
driest day of the week so far but there will still be plenty of low
level moisture convergence between return flow from the surface high
off of the Atlantic coast and southwesterly flow ahead of the
frontal system to the west. Precipitable waters will only be
slightly less than today about 1.6 inches or so in the east. Severe
parameters outside of instability are once again lacking...very
similar to the environment seen over the last several days. A bit
more ridging and subsidence in the middle to upper levels should keep
US a little drier than earlier in the week but still expect some
convective development by late afternoon. Heavy rain will once again
be the primary threat. With southerly flow and greater insolation
across much of the area...expect high temperatures again in the middle
80s over much of the County Warning Area. Dewpoints will remain high again on
Wednesday night keeping the threat for low stratus and fog around
for one more night as lows approach the dewpoints...middle to upper
60s.
&&
Long term /Thursday through Tuesday/...
as of 255 PM Tuesday...
Thursday through Friday...a cold front approaches Thursday and moves
through Thursday night into early Friday...followed by the primary
push of colder air with a second front during the day on Friday. On
Thursday...the right entrance region of a 70kt 300mb jet gets close
to central North Carolina but mainly stays north...while 850mb lift
remains modest and 850mb Theta-E and precipitable water values...
highest in this period at 1.5 inches around 12z Thursday...gradually
fall as forecast by the GFS through Thursday and into Friday. Those
precipitable water values fall to just below an inch in the coastal
plain and below 0.75 inch in The Triad by late Friday afternoon.
1000-500mb lapse rates are maximized only to around 6.5c/km around
18z Thursday...with around 1000j/kg of mixed-layer cape mainly
toward the coastal plain Thursday afternoon.
850mb lift is modest Thursday...as it appears the best instability
is out ahead of the best forcing...with the surface boundary quite
possibly not moving into the northwest Piedmont until very late in
the day Thursday. GFS bufr soundings...and to a similar degree the
NAM though the NAM is more moist...have slender cape and cin does
start to increase over a deep layer Thursday evening. MOS chances
for showers and thunderstorms are mostly in the chance range
Thursday and right now that seems reasonable given what currently
appears to be better low-level forcing lagging better boundary layer
moisture and instability. Bufr soundings start to stabilize Thursday
night and will maintain a slight chance of showers overnight to
account for the first surface front...but it is possible that by the
overnight hours...particularly along and west of U.S. 1...any
remaining instability is so marginal in a drying boundary layer that
even an isolated shower is remote. Given the modest shear...mainly
less than 25kt 0-6km Thursday afternoon toward Interstate 95 and
less toward the west...and modest instability will continue to leave
out the mention of severe weather currently in the severe weather potential statement. Highs in the
lower to middle 80s Thursday closer to the mav MOS and 1000-850mb
thicknesses...the former which has had a good track record lately
overall compared to the met. Overnight lows Thursday night in the
upper 50s northwest to the middle 60s far southeast.
Friday...the GFS has trended toward greater height falls approaching
central North Carolina late in the day. The primary push of colder
air moves through during the day based on the latest timing...and
while there is at least modest lift with the surface boundary and
the height falls...currently GFS bufr soundings appear capped to
deep convection. Some of the guidance suggests a few showers in the
immediate vicinity of Interstate 95 moving east and developing
further toward the coast in more moist air and along the boundary
during the afternoon...but based on the forecast cap aloft and drier
air moving across the region at the surface...will leave out the
mention of showers for Friday for this forecast. Highs Friday in the
lower 70s far northwest...to around 80 southeast. Friday afternoon
could end up a little breezy behind the frontal passage with
occasional wind gusts in the 20s kts...possibly close to 30mph
briefly toward The Triad.
Friday night through Tuesday...a secondary front will be moving
offshore late Friday...with high pressure building into central
North Carolina for at least the start of the weekend. Based on the
consensus of the long-term guidance...a weak surface trough or cold
front moves toward central North Carolina from the north late in the
weekend. With the broad surface pattern made up of high pressure
from the Atlantic into the northern Gulf...convergence along the
front is expected to be weak. The flow aloft should be predominantly
northwest with periodic shortwaves in that northwest flow. Both the
GFS and the European model (ecmwf) suggest that an 850mb boundary moves south across
Virginia and is located just north of the state by 12z Monday. This
could help to enhance low-level convergence somewhat...though given
the ridging expected over the Gulf...very deep moisture is not
currently anticipated.
Mean relative humidity is forecast to increase by Monday...and
coarse model soundings are suggestive of at least an increase in
clouds by then. Some of the long-term guidance produces light quantitative precipitation forecast by
Monday especially over northern parts of the state...closer to the
850mb boundary that is forecast by much of that guidance. With
surface and 850mb ridging over the Gulf and limited surface moisture
return...any rainfall that develops in middle-level lift near weak
850mb and surface boundaries should be light. For now...plan to show
an increase in clouds late in the Holiday weekend and maintain the
slight chance of showers Monday...continuing the slight chance into
Tuesday as the orientation of any surface boundary does not appear
to change much from Monday into Tuesday. Middle-level heights do rise
from Monday into Tuesday...so in a relative sense the chances for
showers may end up being a little higher Monday compared to
Tuesday...but given many model runs before the end of next weekend
and the broad ridging in place over much of the southeast...will
maintain simply slight chances for now. Temperatures should average
a category or two cooler than normal early in this part of the
long-term period...closer to normal toward the end of the period.
&&
Aviation /18z Tuesday through Saturday/...
as of 115 PM Tuesday...
24 hour taf period:
Showers and thunderstorms beginning to pop up this afternoon along a
boundary from Louisburg to Fayetteville. This boundary and
associated showers and thunderstorms will slowly move east this
afternoon. Krwi and kfay are the two terminals most at risk for
thunderstorm activity this afternoon. Winds will continue to be
south southwesterly at 5-10 knots and should stay that way through
this evening. Overnight expect IFR/LIFR ceilings once again and MVFR
visibilities. These will be quicker to develop in the east...some
time after 4z and will hold off till after 6z in the west. Once
conditions deteriorate they will most likely remain sub-VFR through
sunrise. After sunrise expect a drier day tomorrow with VFR ceilings
predominate. Winds will be a bit gustier Wednesday afternoon...10
sustained gusting to 15-20 knots. Some diurnal convection is again
possible Wednesday but coverage is not expected to be as large.
Long term: sub VFR conditions are possible again Wednesday and
Thursday as showers and thunderstorms will continue out ahead of an
approaching frontal system that will move through the area late
Thursday into early Friday. VFR conditions should return for Friday.
&&
Rah watches/warnings/advisories...
none.
&&
$$
Synopsis...djf
near term...Ellis
short term...Ellis
long term...djf
aviation...Ellis
