481 
fxus62 krah 071956 
afdrah 


Area forecast discussion 
National Weather Service Raleigh NC 
256 PM EST Wednesday Jan 7 2009 


Synopsis... 
behind a strong cold front...very windy conditions continue through 
this evening. A weak surface trough...along with an upper-level 
disturbance...provide for a small chance of precipitation Thursday. 
High pressure builds in for Friday...with another cold front moving 
through the region Saturday. 


&& 


Near term /through Thursday/... 
as of 255 PM Wednesday... 


Although instability is modest at best...with surface-based lifted 
indices on msas only about as low as 4c and surface dew points 
falling into the upper 30s in some locations over the western 
Piedmont...a few showers had developed along the primary cold 
front in the cold-air advection aloft. Wind gusts had increased 
once again as expected...with values primarily ranging from 25kt 
to 40kt...though kbuy did report a peak wind gust of 44kt at 
1836z. The strongest wind gust observed on observations to the 
west of central North Carolina was 39kt...and given the mixing 
that will continue this evening...and 925mb winds only 
diminishing from 40-50kt to 30-40kt this evening...will certainly 
continue the Wind Advisory through the evening. As diurnal trends 
take over and 925mb winds diminish to 25-35kt overnight...although 
gustiness to 15-25mph is expected overnight do not anticipate Wind 
Advisory conditions past the late evening hours. 


Any remaining showers should diminish quickly during the late 
afternoon and early evening hours courtesy of drier air moving in 
from the west along with at least moderate subsidence as forecast 
by both the NAM and the GFS. Subsidence is forecast through 12z 
Thursday...but between 12z and 18z Thursday both the NAM and the 
GFS rapidly increase lift and upper-level divergence associated 
with a moderate middle-level shortwave moving across central North 
Carolina and the development of a surface trough. Frontogenetic 
forcing is observed Thursday especially in the layer from 850mb- 
700mb from Thursday morning through at least early Thursday 
afternoon...and the GFS in particular provides for sufficient 
moisture between 850mb and 700mb...where temperatures fall to 
around -15c near the top of that layer...to result in at least a 
chance of precipitation. While bufr soundings from both the NAM 
and the GFS are fairly dry at 12z...moisture increases from west 
to east through 18z. Both the NAM and GFS quantitative precipitation forecast fields are at least 
0.01 Thursday. 


Overnight lows are expected to range from the middle to upper 30s overall... 
with thicknesses supporting highs from the upper 40s to lower 50s 
given enough sun before clouds move in. Depending on the onset of 
any precipitation...with model soundings especially toward kgso 
showing an above freezing depth of only around 1500ft...if any 
area of precipitation would begin early enough there could be a 
few flakes mixed in with any rain. Will mention this in parts of 
the northwest Piedmont...anticipating enough boundary layer 
warming in the morning to increase the above freezing depth 
elsewhere. An earlier onset would mean a better potential for a 
mix with a few flakes...but surface temperatures well above 
freezing would make the impact none. 925mb winds of 20-30kt suggest 
the potential for a few wind gusts again Thursday...however... 
these should certainly be less than this afternoon and tempered by 
clouds. 


&& 


Short term /Thursday night through Saturday night/... 
as of 255 PM Wednesday... 


Weak surface high pressure drops over the lower Mississippi Valley 
Thursday evening then moves east and offshore after dark Friday. An 
approaching cold front will move through central North Carolina 
Saturday and Saturday evening. The 12z European model (ecmwf) continues to be the 
slower model and slightly preferred as the slower model frequently 
verifies best. The surface low associated with the front travels 
through north Virginia with the surface trough quite baggy over 
North Carolina. The system also moves at a good clip across the area 
with individual cells likely moving at 40 miles per hour. Thus expect rainfall 
amounts to be modest at one third inch or less. Freezing level will 
be at nine to ten thousand feet and an all rain event expected. 


Low temperatures of 25 to 32 Friday morning behind the cold front 
then middle 30s Saturday morning ahead of the front. High temperatures 
47 to 53 Friday rising to 55 to 65 ahead of the front Saturday. 


&& 


Long term /Sunday through Wednesday/... 
as of 255 PM Wednesday... 


Surface high pressure over the area Sunday... then the GFS deepens a 
surface low Sunday night over the Great Lakes with the European model (ecmwf) similar 
but further north. The GFS then begins to drop an impressive 1048 
millibar surface high out of central Canada Monday evening... while 
the European model (ecmwf) is significantly weaker. The strong pressure gradient on 
the GFS between the high and low blasts cold air into North 
Carolina Tuesday night giving Greensboro a 1000 to 850 millibar 
thickness of 1200 meters Wednesday morning...roughly a once a decade 
event. This would indicate Wednesday morning lows around zero. As 
this is seven days out... and a forecast rare event... will follow 
the European model (ecmwf) model which is closer to climatology with Wednesday 
morning lows 25 to 30 degrees. Forecast for this entire period will 
be dry given the distant nature of the Sunday night low. 


Afternoon highs around fifty except forties Wednesday. Lows not far 
from freezing except 25 to 30 Wednesday morning. 


&& 


Aviation /18z Wednesday through Monday/... 
as of 115 PM Wednesday... 


Drier air will continue to move into central North Carolina with 
wind gusts the primary aviation factor through tonight. With 925mb 
winds of 40-50kt this afternoon and only gradually diminishing to 
25-35kt overnight...there will be some wind gusts at the surface at 
or a few knots above 35kt at times this afternoon through early 
evening before subsiding. There should be enough cold-air advection 
allowing some wind gusts into the teens kts overnight based on the 
model sounding profiles from both the NAM and the GFS. Most ceilings 
will be VFR or high MVFR with scattered clouds...if any...tonight. 


Middle-level wave moves into central North Carolina early Thursday 
combined with a surface trough that moves through the state. With 
modest divergence aloft along with low-level convergence...enough 
lift will be generated to at least provide for the development of 
low VFR or high MVFR ceilings by 18z. In addition to the clouds... 
lift should be sufficient to generate scattered areas of rain. This 
rain may be mixed with a little snow based on above freezing layers 
on the NAM and GFS bufr soundings of only near 1500ft before 18z. 
Surface temperatures will be warm...above freezing expected. 925mb 
winds of 20-30kt will allow for the continuation of a few gusts... 
though not as gusty certainly as this afternoon. 


Beyond the 18z valid taf period...high pressure brings VFR 
conditions Thursday night through Friday. Clouds will increase and 
lower into Saturday with the approach and passage of a cold front. 
Expect moderate surface wind gusts ahead of the front from the 
southwest Saturday...and behind the front from the west and 
northwest late Saturday into Sunday. Conditions should be VFR Sunday 
and Monday under high pressure. 


&& 


Rah watches/warnings/advisories... 
Wind Advisory until midnight EST tonight for ncz007>011- 
021>028-038>043-073>078-083>086-088-089. 


&& 


$$ 
Synopsis...djf 
near term...djf 
short term...rlh 
long term...rlh 
aviation...djf 

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