Area forecast discussion
National Weather Service Raleigh NC
1009 am EDT Sat may 25 2013
Synopsis...
Canadian high pressure will extend across our region through the
Holiday weekend.
&&
.Near term /today/...
As of 1009 am Saturday...
Today: 12z upper air analyses show 500 mb height rises of 40-60 meters
over central NC behind the departing middle-upper low over New England.
Dry northwest flow through the column and subsidence under the right
front quadrant of the upper level jet will keep skies clear through
the afternoon. Model 500 mb vorticity plots indicate a middle level
disturbance breaking away from the surface low over the Midwest and
riding the ridge axis into the area by 00z Sunday. This disturbance
may generate some high clouds this after/evening and depending on the
timing...may impact high temperatures for today. Will leave highs unchanged
for now...low 70s NE to middle 70s SW. Continue to expect winds 10-12
kts...with gusts of 15-18 kts this afternoon and min relative humidity values
around 25 percent.
&&
Short term /tonight through Sunday night/...
as of 245 am Saturday...
Tonight: the amplification of an impulse on the west side of the
aforementioned upper low will support a weak surface
trough/reinforcing cold front...not currently evident in observation
data...which nwp guidance indicates will pass from north to south
through central NC between 00-06z. The main impact from this feature
appears to be a little enhanced northerly stirring during the
evening hours...which may initially slow otherwise good radiational
cooling. The consensus of guidance also continues to indicate
middle-high clouds will increase dramatically overnight...particularly
over the southern-western Piedmont and sandhills...but the often
times Superior Gem/Canadian (with respect to particularly high
clouds) is less aggressive. Will follow the Gem given satellite
trends of the upstream cirrus diminishing as it spreads
southeastward from the upper Midwest and Great Lakes...and past
experience. As such...temperatures should cool generally into the 43
to 49 degree range...warmest west and southwest where the signal for
at least some middle-high clouds is strongest.
Sunday and Sunday night: mainly clear conditions are expected in
continued dry northwest flow aloft this period...with periods of just some
passing cirrus. A gradual warm-up will commence...with highs
reaching the middle to upper 70s...but with continued pleasantly low
humidity values in the 20th percentile. Lows generally 50 to 55.
&&
Long term /Monday through Friday/...
as of 300 am Saturday...
Monday and Monday night...300mb and 500mb jets move east early
Monday...with limited lift for this part of the forecast period on
both the NAM and the GFS. 1000-500mb lapse rates are weak...to
around 6c/km...and both the NAM and the GFS forecast lifted indices
during Monday afternoon to around 0c at best. The differences
between the GFS and the NAM are in the approach of a weak surface
boundary reflected in increasing moisture. The NAM is faster...
pushing precipitable water values to around 1.5 inches by 00z
Tuesday...while the GFS is slower pushing those values to just over
1.25 inches toward the South Carolina border by 00z Tuesday...
keeping precipitable water values below an inch in the northeast
Piedmont and the northern coastal plain. Both models show limited
quantitative precipitation forecast and...where they have any...reflected mostly south of U.S. 64.
Conceptually...like the GFS solution more...as its quantitative precipitation forecast...moisture
and instability parameters seem to better reflect its surface
boundary depiction...as opposed to the NAM which is more aggressive
with moisture increase yet has a quantitative precipitation forecast orientation similar to the GFS.
As a result...will continue to show a slight chance of showers
Monday afternoon and early Monday evening...but with a location of
such probability of precipitation generally south of a line from kgso to kgsb...and if the
GFS is correct...that is still too far north. Taking the GFS almost
exactly...slight chances would be along and south of a line from
kvuj to kgsb. Will only mention showers...as even the more moist and
unstable NAM seems to be too warm aloft toward 500mb for deep
updrafts to take place. High temperatures Monday within a degree or
three of 80. Overnight lows are challenging depending on how fast
any moisture can move northeast...and how much middle-level clouds hang
around...more on the NAM...less on the GFS. For now...will shade the
current forecast lower especially from the Triangle north and
northeast...closer to the mex there.
Tuesday through Friday...ridging aloft builds during the middle of
the week...leading to middle-level ridging very close to central North
Carolina or overhead by the end of the week. The greatest moisture
during this part of the long-term period should be early...Tuesday
into Wednesday...as the weak surface and 850mb boundaries...possibly
reflected only with increasing surface moisture and increasing 850mb
temperature...move through the area. GFS bufr soundings and more
coarse European model (ecmwf) model soundings show a fair degree of cin...but it is
on Tuesday in particular when the combination of better moisture and
cooler middle-level temperatures exists relative to other days. The 12z
European model (ecmwf) showed little or no quantitative precipitation forecast...while the 00z GFS continues to show
some quantitative precipitation forecast in central North Carolina Tuesday afternoon. At this point
think the most likely outcome is a partly sunny day Tuesday with
isolated showers and thunderstorms.
During the rest of the work week...temperatures gradually warm aloft
and middle-level lapse rates appear to lessen...under increasing
ridging. Each day there should be some reflection of a Piedmont
surface trough...but soundings show limited potential for deep
convection. Using thicknesses as a proxy for potential storm motion
it is possible that some deep convection...if any occurs...could
develop off of higher elevations and make it close to The Triad
Wednesday...but after that it appears to be a challenge for even
that to occur. The gridded forecast will reflect a slight chance of
an afternoon and evening shower or thunderstorm in the northwest
Piedmont Wednesday...and below slight chance probability of precipitation for the rest of
the period through Friday night. Temperatures will average above
normal during this period...with 1000-850mb thicknesses supportive
of highs in the middle 80s to lower 90s by the end of the week.
Overnight lows in the 60s.
&&
Aviation /12z Saturday through Wednesday/...
as of 730 am Saturday...
Canadian high pressure will build southeastward across our region
and promote a high chance of VFR conditions through the 12z taf
period. Winds around the high will average light northwesterly...but
with a several hour period of sustained 8-11 kts with gusts 15-18
kts middle-late afternoon. Initially clear skies will yield to
increasing cirrus late in the day and especially overnight.
Outlook: a stretch of prolonged VFR conditions in continued Canadian
high pressure will persist through at least Memorial Day...with the
next chance of sub-VFR conditions in fog Tuesday morning...mainly
at krwi. Light fog...or low stratus at triad terminals...will be
possible again in increasingly moist return flow Wednesday morning.
&&
Climate...
low temperatures on the 26th are expected to fall several degrees
shy of the following records for the date:
Record min temperature/year set may 26th
Rdu 43 / 1979
Gso 42 / 1930
Fay 44 / 1988
Rah watches/warnings/advisories...
none.
&&
$$
Synopsis...mws
near term...kc
short term...mws
long term...djf
aviation...mws
climate...rah
