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Area forecast discussion 
National Weather Service Raleigh NC 
240 PM EST Sat Nov 21 2009 


Synopsis... 
a low pressure system in the northwest Gulf of Mexico will move 
slowly northeast toward the Florida Gulf Coast tonight...before 
weakening and reforming along the Carolina coast Sunday night. This 
will bring increasing cloud cover and a good chance of rain to 
central North Carolina by Sunday night. 


&& 


Near term /through Sunday/... 
as of 1200 PM Saturday... 


16z surface analysis showed 1024 mb high pressure centered over OH/WV. 
1010 mb low pressure was centered near buoy 42361 south of la. 
Aloft...12z radiosonde observation analysis showed a split flow pattern across the 
Continental U.S.. a southern stream 500 mb low was centered over southeast Texas. 
A 90 knots southern stream jet was noted along the southern periphery 
of the upper low over northern Mexico and the northern Gomex. A 100 
knots northern stream jet was noted over the northern middle-Atlantic 
ahead of a deamplifying northern stream trough over the lower Great 
Lakes. A ridge aloft was present over the central and northern Great 
Plains. 


Today: 
the upper low currently over southeast Texas is prognosticated to move east-northeast 
into la by 00z this evening...while the associated surface low currently over 
the northwest Gomex is prognosticated to be located just south of the la 
coast by that time. Surface high pressure is expected to remain centered 
over portions of the mid-Atlantic...lower Great Lakes and Ohio Valley 
in associated/west confluent flow aloft...with a surface ridge axis extending 
south into the Carolinas. Expect a steady influx of middle/upper level 
cloud cover throughout the day today as moisture moves downstream of 
the upper low along the Gulf Coast into the southeast states and 
Carolinas. Moisture advection will generally be limited to the upper 
levels today...and cloud bases will remain at or above 15-18 kft. Lift today 
will be either very weak or non-existent across central NC. Although 
there is currently a northern stream jet entrance region north/northwest of 
central NC...this feature is prognosticated to weaken and lift northeast 
away from the area. Model isentropic fields do indicate increasing 
isentropic lift across the southeast states this after/evening as a 
925-850 mb warm front moves northward toward the central Gulf Coast 
ahead of the surface/upper low in la. However...isentropic lift will be 
weak and confined well south/SW of central NC. As a result...expect 
no chance for precipitation this aft/eve. High temperatures will be complicated 
primarily by broken skies associated/west upper level cloud cover. 
Thicknesses would argue for highs in the ~63f range with full sun... 
however...given weak low level northeast flow and broken upper level 
cloud cover...a range of 60-63f looks more reasonable...although 
even that could be warm by 1-2f depending on the opacity of cloud 
cover. 


Tonight: 
the upper low currently over southeast Texas is prognosticated to deamplify 
tonight as it moves slowly east-northeast into the deep south. The attendant 
surface low is also prognosticated to weaken a few millibars as it moves along 
a baroclinic zone located over the Central/Northeast Gulf Coast. 
Despite the weakening southern stream dynamics...isentropic lift is 
expected to strengthen over the southeast states into the southern 
Carolinas tonight as a 925-850 mb warm front lifts northward into 
Alabama/Georgia ahead of the low pressure system along the Gulf Coast. The 
best low level moisture and isentropic lift is expected to remain 
well S/SW of central NC through 12z sun...with precipitation chances 
largely confined to Alabama/Georgia and perhaps southern/western portions of 
SC. As a result...expect a dry forecast to continue through the 
overnight hours. Middle/upper level cloud cover will be pervasive 
across the area tonight in associated/west the low pressure system upstream 
of the area...and that will be the primary complicating factor 
west/regard to low temperatures. A MOS guidance blend looks pretty 
reasonable...with lows in the Lower/Middle 40s. 


Sunday: 
latest 12z model guidance continues to show the 500 mb low 
deamplifying into a shortwave during the day Sunday as it moves north/NE 
into northern Alabama/Georgia and eastern Tennessee. The attendant Gulf Coast surface low 
is prognosticated to weaken as it remains centered near the Florida/Alabama coast. An 
inverted surface trough/coastal front is expected to sharpen off the 
southeast/Carolina coast during the day sun in associated/west weak upper 
forcing associated/west the deamplifying southern stream wave. Surface high 
pressure over the middle-Atlantic is prognosticated to strengthen and shift 
northeast into New England on Sunday...with a prominent surface ridge 
axis extending south into the Carolinas. Although upper level lift 
will be rather anemic Sunday...isentropic lift in the lower levels 
will be strengthening from the south/SW throughout the day as the 
925-850mb warm front stalls out just to the south in SC and the 
mslp/height gradient tightens between the surface ridge extending south 
into the area and weak low pressure over the deep south and 
Carolina/southeast coast. Based on the 12z model runs...the timing 
of the arrival of precipitation is quite delayed compared to previous 
runs...with the best chance for rain starting in the south/SW in the 
afternoon...spreading northward into the Triangle area by or shortly 
after sunset. Will show highest probability of precipitation (categorical/likely) in the SW 
Piedmont and sandhills...tapering down to slight chance north/NE of the 
Triangle. Rainfall amounts should be no higher than 0.10-0.25" in 
the SW Piedmont/sandhills. Temperature forecast somewhat difficult on 
sun...depending primarily on the arrival of precipitation. Latest MOS 
guidance has come in warmer in almost all locations...ranging from 
the middle/upper 50s to the lower 60s...and that looks pretty 
reasonable based on the delayed onset of precipitation. -Vincent 


&& 


Short term /Sunday night through Tuesday/... 
as of 135 PM Saturday... 


For Sunday night/monday: the parent surface high strengthens further 
to around 1033 mb and drifts from over Maine across the Canadian 
Maritimes through Monday. The inverted surface trough just west of 
the southern Appalachians weakens gradually while the secondary 
surface cyclogenesis begins off the SC coast with its center then 
passing near the Outer Banks through Monday. The subcloud layer over 
central NC finally begins to saturate from southwest to northeast 
Sunday evening with increasing moist isentropic upglide at 290k-300k 
and increasing low level mass convergence and moisture transport. 
However... deep forcing for ascent remains somewhat limited. The 
DPVA decreases rapidly late Sunday through Sunday night as the 
dampening middle level shortwave trough swings up through eastern Tennessee 
and western NC... and upper divergence (while maximized over NC at 
06z Sunday night) trends weaker as well. And by the time the lowest 
1-2 km becomes saturated... we lose the moisture aloft as the very 
dry air now over nm/West Texas/OK punches in on the back side of the 
shortwave trough. These factors should result in rather low quantitative precipitation forecast... 
although given that the low level forcing features line up well... 
we still anticipate widespread light rain and drizzle from Sunday 
night through Monday... and will retain categorical probability of precipitation... mainly 
from late evening Sunday through Monday morning before tapering off 
probability of precipitation to a chance of light rain/drizzle north to south through Monday 
afternoon. Lows 46-51. Then with thick overcast persisting through 
Monday... low level cold air advection... and the coastal low 
holding offshore... temperatures should rise very little Monday. Highs 
49-59... and even this may be optimistic within The Heart of the 
cold dome in the northern Piedmont. 


For Monday night/tuesday: the coastal low is expected to track 
slowly from the northern Outer Banks to off the New Jersey coast by late 
Tuesday. The parent high continues to shift east off the Canadian 
Maritimes and loses its grip on The Wedge air mass over central 
NC... however it leaves behind a formidable cool pool. Forecast 
soundings from both the NAM and GFS show stable air and plenty of 
moisture in the lowest 2 km through Monday night... and the GFS 
holds onto low level isentropic upglide in this layer around 295k. A 
weak shortwave passes by Monday night but with no moisture aloft and 
stable/warming middle levels... it should have no sensible impact in 
this area. With weak and lightly diffluent mainly northerly surface 
winds through the night and saturated low levels... we should see 
areas of light drizzle and fog through Tuesday morning. Have brought 
lows up slightly to 45-49. On Tuesday... the GFS keeps the low 
levels nearly isothermal and saturated whereas the NAM wants to 
start drying this layer out. The models often have difficulting 
correctly depicting erosion of a damming air mass that has lost 
large scale support and no longer has any connection to a cool dry 
low level air source. Despite heating from above which may start to 
burn off the upper portion of this moist layer... given the dry 
subsidence layer aloft capping off this surface-based layer and with 
weak winds in the lowest 1 km inhibiting mixing... favor a lean 
toward the GFS (which has support from the canadian) which means 
holding onto considerable cloudiness through Tuesday. With this 
greater cloud cover... highs will accordingly be lowered... to 
58-65. -Gih 


&& 


Long term /Tuesday night through Saturday/... 
as of 240 PM Friday... 


For Tuesday night: the GFS/Canadian depict a weak 850 mb warm front 
swinging northeastward leading to low level winds becoming more 
uniform (yet still light) from the west-southwest by morning. GFS 
forecast soundings show a dissolution of the low clouds especially 
in the western and southern County Warning Area. Will show a trend toward fewer 
clouds in this area overnight but based on uncertainty with such a 
feature this far out... will not completely clear out the clouds. 
Lows 43-48. 


For Wednesday through saturday: model consensus shows deep troughing 
dominating the eastern half of the nation through at least Friday... 
with the surface baroclinic zone holding off the southeast coast. 
The GFS/ECMWF/Canadian agree fairly well on one middle level vortex 
over the Midwest early Wednesday swinging east then north through 
Quebec through Friday... while another vortex drops from near lake 
winnepeg through the Midwest to the eastern Great Lakes region and 
finally tracks up along the St Lawrence Valley into Saturday. This 
certainly looks to be an anomalously deep pattern as the gefs mean 
middle level heights drop to 1-3 Standard deviations below normal by 
Friday over the southeast. Surface details are tough to pinpoint 
within such a fast flow aloft but it appears that we'll see a 
largely dry cold front passing through Wednesday night or early 
Thanksgiving day... followed by falling low level thicknesses and 
dry but brisk northwesterly low level flow as a polar-source high 
pressure ridge builds in slowly from the west-southwest. Will keep a 
slight chance of rain mainly across the north Wednesday night into 
Thanksgiving day... then indicate dry and colder weather for 
Thursday night through Saturday. Expect above-normal temperatures 
Wednesday... slightly cooler (near normal) Thanksgiving day... then 
the bulk of the cold air arrives Thursday night with low level 
thicknesses plunging below 1310 M on the GFS and Canadian by Friday 
morning. Will hold temperatures about one to two categories below normal 
Friday through Saturday... with highs in the Lower-Middle 50s. -Gih 


&& 


Aviation /18z Saturday through Thursday/... 
as of 1245 PM Saturday... 


VFR conditions are expected through the taf period. Upper level 
cloud cover will continue to increase over the area in advance of a 
storm system currently located along the northwest Gulf Coast. 
Ceilings are expected to remain in the 15-18 kft range through 
tonight. Ceilings will begin to lower during the day Sunday... 
however...any MVFR/IFR ceilings and rain are expected to hold off 
until after the end of the current taf period. 


MVFR/IFR conditions in rain are expected to affect most or all of 
the area from 00z Monday through 00z Tuesday...with MVFR/IFR and perhaps 
LIFR conditions in low ceilings/fog/drizzle Monday night through 
Tuesday night in associated/west a saturated wedge of cool air (cad wedge) 
in the lowest several thousand feet of the atmosphere. 


VFR conditions should return by middle-week (wed) in associated/west a frontal 
passage. -Vincent 


&& 


Rah watches/warnings/advisories... 
none. 


&& 


$$ 


Synopsis...Vincent 
near term...Vincent 
short term...Hartfield 
long term...Hartfield 
aviation...Vincent 

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