Area forecast discussion
National Weather Service Raleigh NC
745 am EDT Wednesday may 22 2013
Synopsis...
Bermuda high pressure will extend into the middle Atlantic today. A
surface pressure trough will develop in the Lee of the Appalachians
Thursday...followed by the passage of a cold front Thursday night.
&&
Near term /through tonight/...
as of 315 am Wednesday...
The middle level trough that has promoted numerous to widespread
downpours in recent days is no longer discernible in regional vwp
data. Bermuda high pressure will consequently briefly exert the
dominant influence to weather conditions over central NC today...so
convective coverage should be much less than previous days...and
focused by the sea breeze and other smaller scale/lower
predictability differential heating or remnant outflow boundaries.
A blanket 20-30 percent pop is accordingly warranted...with relative
highest chances associated with the sea breeze over the coastal
plain and eastern sandhills and Piedmont. Despite what some of the
nwp guidance would suggest...it seems improbable that any
mountain-induced convection would affect the western
Piedmont...owing to south to south southwesterly steering flow that
would hold storm motions/propagation west of the Yadkin River. Highs
similar to those of Tuesday...generally in the middle 80s.
Height falls in association with the complex trough aloft
approaching from the central Continental U.S....on the order of 20-30 meters at
500 mb...will spread eastward across central NC tonight. While these
height falls are apt to support a band of pre-frontal showers and
storms from the Tennessee Valley to the Appalachians by this evening...it
seems probable that this convection will weaken considerably as it
emerges into an increasingly stable...nocturnally-cooled boundary
layer in the Lee of the mountains - a solution depicted well by the
00z Storm Prediction Center nmm. As such...will indicate a high chance of mainly
left-over showers into the southern and western Piedmont after 11
PM...tapering to slight chance range around the Triangle area...to
dry roughly east of I-95. A continued very moist air mass will
result in additional low stratus development through Thursday morning...
with persistence lows in the middle 60s.
&&
Short term /Thursday and Thursday night/...
as of 308 am Wednesday...
Cold front will cross the area... entering the state from the west
Thursday evening... and will reach the coast by 12z Friday.
Thunderstorms will develop in the moist airmass in advance of the
front Thursday... and modest instability of 500-1000 j/kg MLCAPE and
0-6 km bulk shear values of 25 kts will support a few multicell
clusters. Best upper level forcing remains north of the area... and
with the strongest low level forcing associated with the front not
making it through the area until late Thursday... severe potential
is low. Highs low to middle 80s... lows in the 60s.
&&
Long term /Friday through Tuesday/...
as of 305 am Wednesday...
Behind the front... northwesterly flow will usher in a much drier
airmass... and precipitation will end from west to east early
Friday. Temperatures will be several degrees below normal through
the weekend... with highs in the 70s. Coolest lows Friday night...
in the upper 40s to low 50s... then low to middle 50s Saturday night.
Moisture will increase again on Monday as the flow becomes more
southerly... with precipitable water values again climbing to near 1.5 inches.
Guidance indicates a warm front will be in the vicinity of the
midatlantic Monday evening into Tuesday... and temperatures Tuesday
will rebound to near normal... upper 70s to low 80s. Depending on
the location of the front... isentropically driven showers are
possible... and will continue with slight chance probability of precipitation.
&&
Aviation /12z Wednesday through Sunday/...
as of 745 am Wednesday...
Widespread MVFR ceilings early this morning will give way to a
scattered-broken VFR cloud bases by 10-11 am. As the air mass heats
up this afternoon and becomes slightly unstable...scattered
convection will develop...especially after 2 PM. The showers and
thunderstorms may be a little more concentrated near and east of
kfay...and near and west of kint and kgso. Expect MVFR ceilings and
visibility in vicinity of the convection. The bulk of the showers
and storms will diminish after 8 PM.
MVFR ceilings due to stratus will develop late tonight and persist
into early Thursday morning. A cold front currently across the Ohio
and Tennessee valleys will continue to drift east and should cross central
NC late Thursday and Thursday evening. Ahead of the front...numerous
showers and scattered thunderstorms and associated MVFR/IFR
conditions will occur. The highest concentration of showers and
storms will occur in the afternoon through the evening hours with
convection diminishing after midnight.
Canadian high pressure will build into our area from the northwest Friday
in the wake of the departing system. This weather system will advect
drier and more stable air into the Carolinas...resulting in a
prolonged period of VFR conditions that should last through Sunday.
&&
Rah watches/warnings/advisories...
none.
&&
$$
Synopsis...mws
near term...mws
short term...second
long term...second
aviation...wss
