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fxus62 krah 211049 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
650 am EDT sun Mar 21 2010 


Synopsis... 
a low pressure system approaches from the southwest...moving into 
the central Appalachians overnight. A cold front sweeps through the 
region late tonight and Monday morning. Upper level low pressure 
remains near the region through Tuesday morning. 


&& 


Near term /through tonight/... 
as of 300 am Sunday... 


Infrared satellite imagery shows an area of showers and isolated 
thunderstorms extending some cool cloud tops ahead of it to the 
northeast toward the Carolinas. Some of the clouds are thinning and 
warming as they approach central North Carolina...and this makes 
sense given the dry air aloft over the region on the 00z regional 
upper-air analysis...and the dry kgso 00z sounding. During the day... 
both the NAM and the GFS provide for weak lift with a weakening 
middle-level wave associated with the complex upstream as it moves 
toward central North Carolina. While the GFS may be suffering from a 
little convective feedback as it forecasts a 65kt 500mb jetlet 
moving through central North Carolina this afternoon...anticipate 
some weak lift and at least middle-level moistening as the remnants of 
the complex continue to move toward the area in the southwest flow 
aloft. The primary weather factor for central North Carolina will be 
an increase in high- and middle-level clouds...although a sprinkle or 
an isolated shower/patchy area of rain could occur...with the best 
threat of anything measurable...likely under 0.05 inch...from 
The Triad toward kfay and kctz after 18z - 20z. With dry air in 
place...it will be difficult to get anything measurable in the 
northeast. Highs today may actually be warmest in the northeast 
where filtered sun is expected longest. 


The NAM is most aggressive with deep moisture increase into central 
North Carolina tonight...while the GFS forecasts strong subsidence 
behind the modest shortwave moving through central North Carolina 
this afternoon...and only marginal return of moisture overnight. 
Given the GFS forecast of 35-45kt Atlantic-based inflow into central 
North Carolina between 06z and 12z...and a pronounced increase in 
the 850mb Theta-E on the GFS during the same time period...think the 
GFS forecast for quantitative precipitation forecast and pop may be low overnight. In deference to 
the GFS...anticipating a large part of the evening either dry or 
with just a small chance of showers...reduced overall average quantitative precipitation forecast to 
a quarter- to a half-inch for central North Carolina. Primary period 
of at least numerous showers expected to be from roughly 06z to 
12z...spreading southwest-to-northeast during that time...ahead of a 
cold front that sweeps across central North Carolina through Monday 
morning. While bufr soundings show limited potential for thunder... 
enough elevated instability should exist overnight with MUCAPE in 
the lowest km to around 500j/kg to support at least scattered 
thunder overnight. The better potential for thunder may actually 
occur late at night east as any band of showers approaches a 
combination of better instability and deeper moisture late. Severe 
weather potential should be limited by difficulties achieving 
surface-based instability during the late night hours. 


&& 


Short term /Monday through Tuesday/... 
as of 300 am Sunday... 


During the morning Monday...rapid drying takes place from the 
southwest...although lingering...numerous showers and isolated 
thunderstorms should be ongoing around sunrise toward krwi and kixa. 
Will have to be watchful for isolated...strong thunderstorms around 
sunrise in that area as diurnal temperatures start to warm...and 
lifted indices fall to near -3c early in the day. K indices plunge 
to negative values throughout central North Carolina in strong 
subsidence and rapidly falling 850mb Theta-E air by 18z. Winds will 
turn to the southwest with a few gusts into the middle-to-upper 20s kts 
possible...highest toward The Triad...in ample mixing and in a tight 
gradient with the surface low just north of the area in Virginia. 
While bufr soundings show little potential for any shower 
redevelopment central and east through 00z Tuesday due to deep...dry 
air...between 21z Monday and 00z Tuesday bufr soundings from both 
the NAM and the GFS toward The Triad support the potential for 
low-topped...isolated-to-scattered showers as temperatures aloft 
cool with the approach of the upper low. Temperatures should be able 
to recover modestly from morning lows...with highs in the lower to 
middle 60s west...and upper 60s in the coastal plain. 


Monday night through tuesday: 
middle/upper low is prognosticated to track east-northeast Ward across the 
area... near the NC/Virginia border on Monday night/Tuesday morning. As 
the middle/upper low tracks across the region middle level lapse rates 
(h85-5) steepen and 500 mb temperatures drop to as low as -30 degree celsius. 
This decreasing stability in conjunction with dynamic forcing for 
ascent ahead of the upper low... and an associated low level 
trough/convergence axis... still warrants a slight chance of showers 
overnight. Partial thickness values still fall in the indeterminate 
category... which is typical of steep lapse rate environments with 
above freezing low levels. However... given we are not expecting any 
moderate to heavy precipitation (which would be necessary to get a few wet 
flakes to mix in)... will keep precipitation in the form of liquid. 
Overnight lows look to be in the upper 30s to lower 40s. 


Any remaining light showers will end on Tuesday morning before noon 
across the north... as the middle/upper low lifts off to the northeast. 
However... there should be enough lingering low level moisture 
wrapping around the departing system in the cyclonic flow aloft to 
create at least scattered to broken stratocumulus. Afternoon high 
temperatures look to generally be in the lower to middle 60s... as low level 
thickness values begin to rebound late in the days as heights aloft 
increase. 


&& 


Long term /Tuesday night through Saturday/... 
as of 300 am Sunday... 


Tuesday night and wednesday: 
medium range portion of the forecast looks to start off dry with 
afternoon high temperatures returning to back above normal levels on 
Wednesday as middle level ridging builds across the area... while 
surface high pressure settles across the southeastern U.S. This will 
yield dry and mostly clear/sunny conditions for Wednesday. Morning 
low are expected to be in the upper 30 to lower 40s. With low level 
thickness values increasing into the 1360s Wednesday afternoon... 
expect highs will top out generally in the lower 70s across the area. 


Wednesday night through Thursday night: 
a weakening backdoor cold front is prognosticated to drop into the area on 
Wednesday night/Thursday morning as a northern stream trough aloft 
helps to flatten the ridge aloft over the region. However... other 
than possibly an increase in cloud cover expect we will see little 
impact with this feature. Lows Thursday morning look to be generally 
in the middle to upper 40s. Low level flow will quickly shift back 
around to southerly on Thursday afternoon as the next system 
approaches the area from the west. Thus... with partly sunny skies 
expect high temperatures to climb into the upper 60s to lower 70s. Middle and 
high level cloud cover will thicken on Thursday night/Friday morning 
as the next shortwave approaches from the west. This should help to 
keep overnight lows up a bit. Thus... expect lows will at least be 
in the upper 40s to lower 50s. 


Friday through saturday: 
next trough aloft and attendant frontal system moves into the area 
on Friday... bring chances of rain back to the forecast. Given we 
are still six days out in the forecast chance probability of precipitation look good during 
the day on Friday... with lingering slight chance probability of precipitation on Friday 
night. Much cooler air will build into the area on Saturday... as 
cool high pressure from the north builds into the area. Thus... 
expect highs on Friday to be in the middle to upper 60s... with highs 
on Saturday ranging from the upper 50s across the north to middle 60s 
across the south. Lows Saturday morning are expected to range from 
the lower 40s across the north to upper 40s across the south. 


&& 


Aviation /12z Sunday through Thursday/... 
as of 650 am Sunday... 


VFR conditions are expected through today...with gradually 
deteriorating conditions tonight...especially after midnight. 
Surface winds will gradually back...and as the gradient increases 
with approaching low pressure...wind speeds will increase most 
noticeably just off of the surface. While conditions will be 
marginal for low level wind shear between 06z and 12z Monday...aviation interests 
should be prepared for southeast winds around 2000ft at 30-35kt by 
06z...and as high as 50kt approaching 3000ft by 09z. Ceilings should 
begin to fall at least to MVFR at kgso...kint...and kfay near and 
beyond 02z...with numerous areas of IFR ceilings throughout central 
North Carolina developing between 06z and 09z. 


While isolated sprinkles or showers are possible this afternoon and 
this evening...the greatest threat for more numerous showers should 
be after midnight tonight. Scattered thunderstorms are possible as 
well. With considerable drying expected behind a cold front that 
sweeps through central North Carolina Monday morning...numerous 
areas of IFR ceilings return to VFR conditions...though expect a 
gusty southwest-to-west wind 15-25kt. Isolated instability showers 
are possible toward kint and kgso late Monday afternoon in colder 
air aloft as the upper low approaches. Aside from a slight chance of 
showers continuing Monday night into Tuesday morning...VFR 
conditions then continue through Thursday. 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


Synopsis...djf 
near term...djf 
short term...djf/bsd 
long term...bsd 
aviation...djf 

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